aset.c

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/*-------------------------------------------------------------------------
 *
 * aset.c
 *    Allocation set definitions.
 *
 * AllocSet is our standard implementation of the abstract MemoryContext
 * type.
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/mmgr/aset.c
 *
 * NOTE:
 *  This is a new (Feb. 05, 1999) implementation of the allocation set
 *  routines. AllocSet...() does not use OrderedSet...() any more.
 *  Instead it manages allocations in a block pool by itself, combining
 *  many small allocations in a few bigger blocks. AllocSetFree() normally
 *  doesn't free() memory really. It just add's the free'd area to some
 *  list for later reuse by AllocSetAlloc(). All memory blocks are free()'d
 *  at once on AllocSetReset(), which happens when the memory context gets
 *  destroyed.
 *              Jan Wieck
 *
 *  Performance improvement from Tom Lane, 8/99: for extremely large request
 *  sizes, we do want to be able to give the memory back to free() as soon
 *  as it is pfree()'d.  Otherwise we risk tying up a lot of memory in
 *  freelist entries that might never be usable.  This is specially needed
 *  when the caller is repeatedly repalloc()'ing a block bigger and bigger;
 *  the previous instances of the block were guaranteed to be wasted until
 *  AllocSetReset() under the old way.
 *
 *  Further improvement 12/00: as the code stood, request sizes in the
 *  midrange between "small" and "large" were handled very inefficiently,
 *  because any sufficiently large free chunk would be used to satisfy a
 *  request, even if it was much larger than necessary.  This led to more
 *  and more wasted space in allocated chunks over time.  To fix, get rid
 *  of the midrange behavior: we now handle only "small" power-of-2-size
 *  chunks as chunks.  Anything "large" is passed off to malloc().  Change
 *  the number of freelists to change the small/large boundary.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "utils/memdebug.h"
#include "utils/memutils.h"

/* Define this to detail debug alloc information */
/* #define HAVE_ALLOCINFO */

/*--------------------
 * Chunk freelist k holds chunks of size 1 << (k + ALLOC_MINBITS),
 * for k = 0 .. ALLOCSET_NUM_FREELISTS-1.
 *
 * Note that all chunks in the freelists have power-of-2 sizes.  This
 * improves recyclability: we may waste some space, but the wasted space
 * should stay pretty constant as requests are made and released.
 *
 * A request too large for the last freelist is handled by allocating a
 * dedicated block from malloc().  The block still has a block header and
 * chunk header, but when the chunk is freed we'll return the whole block
 * to malloc(), not put it on our freelists.
 *
 * CAUTION: ALLOC_MINBITS must be large enough so that
 * 1<<ALLOC_MINBITS is at least MAXALIGN,
 * or we may fail to align the smallest chunks adequately.
 * 8-byte alignment is enough on all currently known machines.
 *
 * With the current parameters, request sizes up to 8K are treated as chunks,
 * larger requests go into dedicated blocks.  Change ALLOCSET_NUM_FREELISTS
 * to adjust the boundary point; and adjust ALLOCSET_SEPARATE_THRESHOLD in
 * memutils.h to agree.  (Note: in contexts with small maxBlockSize, we may
 * set the allocChunkLimit to less than 8K, so as to avoid space wastage.)
 *--------------------
 */

#define ALLOC_MINBITS       3   /* smallest chunk size is 8 bytes */
#define ALLOCSET_NUM_FREELISTS  11
#define ALLOC_CHUNK_LIMIT   (1 << (ALLOCSET_NUM_FREELISTS-1+ALLOC_MINBITS))
/* Size of largest chunk that we use a fixed size for */
#define ALLOC_CHUNK_FRACTION    4
/* We allow chunks to be at most 1/4 of maxBlockSize (less overhead) */

/*--------------------
 * The first block allocated for an allocset has size initBlockSize.
 * Each time we have to allocate another block, we double the block size
 * (if possible, and without exceeding maxBlockSize), so as to reduce
 * the bookkeeping load on malloc().
 *
 * Blocks allocated to hold oversize chunks do not follow this rule, however;
 * they are just however big they need to be to hold that single chunk.
 *
 * Also, if a minContextSize is specified, the first block has that size,
 * and then initBlockSize is used for the next one.
 *--------------------
 */

#define ALLOC_BLOCKHDRSZ    MAXALIGN(sizeof(AllocBlockData))
#define ALLOC_CHUNKHDRSZ    sizeof(struct AllocChunkData)

typedef struct AllocBlockData *AllocBlock;  /* forward reference */
typedef struct AllocChunkData *AllocChunk;

/*
 * AllocPointer
 *      Aligned pointer which may be a member of an allocation set.
 */
typedef void *AllocPointer;

/*
 * AllocSetContext is our standard implementation of MemoryContext.
 *
 * Note: header.isReset means there is nothing for AllocSetReset to do.
 * This is different from the aset being physically empty (empty blocks list)
 * because we will still have a keeper block.  It's also different from the set
 * being logically empty, because we don't attempt to detect pfree'ing the
 * last active chunk.
 */
typedef struct AllocSetContext
{
    MemoryContextData header;   /* Standard memory-context fields */
    /* Info about storage allocated in this context: */
    AllocBlock  blocks;         /* head of list of blocks in this set */
    AllocChunk  freelist[ALLOCSET_NUM_FREELISTS];   /* free chunk lists */
    /* Allocation parameters for this context: */
    Size        initBlockSize;  /* initial block size */
    Size        maxBlockSize;   /* maximum block size */
    Size        nextBlockSize;  /* next block size to allocate */
    Size        allocChunkLimit;    /* effective chunk size limit */
    AllocBlock  keeper;         /* keep this block over resets */
    /* freelist this context could be put in, or -1 if not a candidate: */
    int         freeListIndex;  /* index in context_freelists[], or -1 */
} AllocSetContext;

typedef AllocSetContext *AllocSet;

/*
 * AllocBlock
 *      An AllocBlock is the unit of memory that is obtained by aset.c
 *      from malloc().  It contains one or more AllocChunks, which are
 *      the units requested by palloc() and freed by pfree().  AllocChunks
 *      cannot be returned to malloc() individually, instead they are put
 *      on freelists by pfree() and re-used by the next palloc() that has
 *      a matching request size.
 *
 *      AllocBlockData is the header data for a block --- the usable space
 *      within the block begins at the next alignment boundary.
 */
typedef struct AllocBlockData
{
    AllocSet    aset;           /* aset that owns this block */
    AllocBlock  prev;           /* prev block in aset's blocks list, if any */
    AllocBlock  next;           /* next block in aset's blocks list, if any */
    char       *freeptr;        /* start of free space in this block */
    char       *endptr;         /* end of space in this block */
}           AllocBlockData;

/*
 * AllocChunk
 *      The prefix of each piece of memory in an AllocBlock
 *
 * Note: to meet the memory context APIs, the payload area of the chunk must
 * be maxaligned, and the "aset" link must be immediately adjacent to the
 * payload area (cf. GetMemoryChunkContext).  We simplify matters for this
 * module by requiring sizeof(AllocChunkData) to be maxaligned, and then
 * we can ensure things work by adding any required alignment padding before
 * the "aset" field.  There is a static assertion below that the alignment
 * is done correctly.
 */
typedef struct AllocChunkData
{
    /* size is always the size of the usable space in the chunk */
    Size        size;
#ifdef MEMORY_CONTEXT_CHECKING
    /* when debugging memory usage, also store actual requested size */
    /* this is zero in a free chunk */
    Size        requested_size;

#define ALLOCCHUNK_RAWSIZE  (SIZEOF_SIZE_T * 2 + SIZEOF_VOID_P)
#else
#define ALLOCCHUNK_RAWSIZE  (SIZEOF_SIZE_T + SIZEOF_VOID_P)
#endif                          /* MEMORY_CONTEXT_CHECKING */

    /* ensure proper alignment by adding padding if needed */
#if (ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
    char        padding[MAXIMUM_ALIGNOF - ALLOCCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
#endif

    /* aset is the owning aset if allocated, or the freelist link if free */
    void       *aset;
    /* there must not be any padding to reach a MAXALIGN boundary here! */
}           AllocChunkData;

/*
 * Only the "aset" field should be accessed outside this module.
 * We keep the rest of an allocated chunk's header marked NOACCESS when using
 * valgrind.  But note that chunk headers that are in a freelist are kept
 * accessible, for simplicity.
 */
#define ALLOCCHUNK_PRIVATE_LEN  offsetof(AllocChunkData, aset)

/*
 * AllocPointerIsValid
 *      True iff pointer is valid allocation pointer.
 */
#define AllocPointerIsValid(pointer) PointerIsValid(pointer)

/*
 * AllocSetIsValid
 *      True iff set is valid allocation set.
 */
#define AllocSetIsValid(set) PointerIsValid(set)

#define AllocPointerGetChunk(ptr)   \
                    ((AllocChunk)(((char *)(ptr)) - ALLOC_CHUNKHDRSZ))
#define AllocChunkGetPointer(chk)   \
                    ((AllocPointer)(((char *)(chk)) + ALLOC_CHUNKHDRSZ))

/*
 * Rather than repeatedly creating and deleting memory contexts, we keep some
 * freed contexts in freelists so that we can hand them out again with little
 * work.  Before putting a context in a freelist, we reset it so that it has
 * only its initial malloc chunk and no others.  To be a candidate for a
 * freelist, a context must have the same minContextSize/initBlockSize as
 * other contexts in the list; but its maxBlockSize is irrelevant since that
 * doesn't affect the size of the initial chunk.
 *
 * We currently provide one freelist for ALLOCSET_DEFAULT_SIZES contexts
 * and one for ALLOCSET_SMALL_SIZES contexts; the latter works for
 * ALLOCSET_START_SMALL_SIZES too, since only the maxBlockSize differs.
 *
 * Ordinarily, we re-use freelist contexts in last-in-first-out order, in
 * hopes of improving locality of reference.  But if there get to be too
 * many contexts in the list, we'd prefer to drop the most-recently-created
 * contexts in hopes of keeping the process memory map compact.
 * We approximate that by simply deleting all existing entries when the list
 * overflows, on the assumption that queries that allocate a lot of contexts
 * will probably free them in more or less reverse order of allocation.
 *
 * Contexts in a freelist are chained via their nextchild pointers.
 */
#define MAX_FREE_CONTEXTS 100   /* arbitrary limit on freelist length */

typedef struct AllocSetFreeList
{
    int         num_free;       /* current list length */
    AllocSetContext *first_free;    /* list header */
} AllocSetFreeList;

/* context_freelists[0] is for default params, [1] for small params */
static AllocSetFreeList context_freelists[2] =
{
    {
        0, NULL
    },
    {
        0, NULL
    }
};

/*
 * These functions implement the MemoryContext API for AllocSet contexts.
 */
static void *AllocSetAlloc(MemoryContext context, Size size);
static void AllocSetFree(MemoryContext context, void *pointer);
static void *AllocSetRealloc(MemoryContext context, void *pointer, Size size);
static void AllocSetReset(MemoryContext context);
static void AllocSetDelete(MemoryContext context);
static Size AllocSetGetChunkSpace(MemoryContext context, void *pointer);
static bool AllocSetIsEmpty(MemoryContext context);
static void AllocSetStats(MemoryContext context,
                          MemoryStatsPrintFunc printfunc, void *passthru,
                          MemoryContextCounters *totals);

#ifdef MEMORY_CONTEXT_CHECKING
static void AllocSetCheck(MemoryContext context);
#endif

/*
 * This is the virtual function table for AllocSet contexts.
 */
static const MemoryContextMethods AllocSetMethods = {
    AllocSetAlloc,
    AllocSetFree,
    AllocSetRealloc,
    AllocSetReset,
    AllocSetDelete,
    AllocSetGetChunkSpace,
    AllocSetIsEmpty,
    AllocSetStats
#ifdef MEMORY_CONTEXT_CHECKING
    ,AllocSetCheck
#endif
};

/*
 * Table for AllocSetFreeIndex
 */
#define LT16(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n

static const unsigned char LogTable256[256] =
{
    0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
    LT16(5), LT16(6), LT16(6), LT16(7), LT16(7), LT16(7), LT16(7),
    LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8), LT16(8)
};

/* ----------
 * Debug macros
 * ----------
 */
#ifdef HAVE_ALLOCINFO
#define AllocFreeInfo(_cxt, _chunk) \
            fprintf(stderr, "AllocFree: %s: %p, %zu\n", \
                (_cxt)->header.name, (_chunk), (_chunk)->size)
#define AllocAllocInfo(_cxt, _chunk) \
            fprintf(stderr, "AllocAlloc: %s: %p, %zu\n", \
                (_cxt)->header.name, (_chunk), (_chunk)->size)
#else
#define AllocFreeInfo(_cxt, _chunk)
#define AllocAllocInfo(_cxt, _chunk)
#endif

/* ----------
 * AllocSetFreeIndex -
 *
 *      Depending on the size of an allocation compute which freechunk
 *      list of the alloc set it belongs to.  Caller must have verified
 *      that size <= ALLOC_CHUNK_LIMIT.
 * ----------
 */
static inline int
AllocSetFreeIndex(Size size)
{
    int         idx;
    unsigned int t,
                tsize;

    if (size > (1 << ALLOC_MINBITS))
    {
        tsize = (size - 1) >> ALLOC_MINBITS;

        /*
         * At this point we need to obtain log2(tsize)+1, ie, the number of
         * not-all-zero bits at the right.  We used to do this with a
         * shift-and-count loop, but this function is enough of a hotspot to
         * justify micro-optimization effort.  The best approach seems to be
         * to use a lookup table.  Note that this code assumes that
         * ALLOCSET_NUM_FREELISTS <= 17, since we only cope with two bytes of
         * the tsize value.
         */
        t = tsize >> 8;
        idx = t ? LogTable256[t] + 8 : LogTable256[tsize];

        Assert(idx < ALLOCSET_NUM_FREELISTS);
    }
    else
        idx = 0;

    return idx;
}


/*
 * Public routines
 */


/*
 * AllocSetContextCreateInternal
 *      Create a new AllocSet context.
 *
 * parent: parent context, or NULL if top-level context
 * name: name of context (must be statically allocated)
 * minContextSize: minimum context size
 * initBlockSize: initial allocation block size
 * maxBlockSize: maximum allocation block size
 *
 * Most callers should abstract the context size parameters using a macro
 * such as ALLOCSET_DEFAULT_SIZES.
 *
 * Note: don't call this directly; go through the wrapper macro
 * AllocSetContextCreate.
 */
MemoryContext
AllocSetContextCreateInternal(MemoryContext parent,
                              const char *name,
                              Size minContextSize,
                              Size initBlockSize,
                              Size maxBlockSize)
{
    int         freeListIndex;
    Size        firstBlockSize;
    AllocSet    set;
    AllocBlock  block;

    /* Assert we padded AllocChunkData properly */
    StaticAssertStmt(ALLOC_CHUNKHDRSZ == MAXALIGN(ALLOC_CHUNKHDRSZ),
                     "sizeof(AllocChunkData) is not maxaligned");
    StaticAssertStmt(offsetof(AllocChunkData, aset) + sizeof(MemoryContext) ==
                     ALLOC_CHUNKHDRSZ,
                     "padding calculation in AllocChunkData is wrong");

    /*
     * First, validate allocation parameters.  Once these were regular runtime
     * test and elog's, but in practice Asserts seem sufficient because nobody
     * varies their parameters at runtime.  We somewhat arbitrarily enforce a
     * minimum 1K block size.
     */
    Assert(initBlockSize == MAXALIGN(initBlockSize) &&
           initBlockSize >= 1024);
    Assert(maxBlockSize == MAXALIGN(maxBlockSize) &&
           maxBlockSize >= initBlockSize &&
           AllocHugeSizeIsValid(maxBlockSize)); /* must be safe to double */
    Assert(minContextSize == 0 ||
           (minContextSize == MAXALIGN(minContextSize) &&
            minContextSize >= 1024 &&
            minContextSize <= maxBlockSize));

    /*
     * Check whether the parameters match either available freelist.  We do
     * not need to demand a match of maxBlockSize.
     */
    if (minContextSize == ALLOCSET_DEFAULT_MINSIZE &&
        initBlockSize == ALLOCSET_DEFAULT_INITSIZE)
        freeListIndex = 0;
    else if (minContextSize == ALLOCSET_SMALL_MINSIZE &&
             initBlockSize == ALLOCSET_SMALL_INITSIZE)
        freeListIndex = 1;
    else
        freeListIndex = -1;

    /*
     * If a suitable freelist entry exists, just recycle that context.
     */
    if (freeListIndex >= 0)
    {
        AllocSetFreeList *freelist = &context_freelists[freeListIndex];

        if (freelist->first_free != NULL)
        {
            /* Remove entry from freelist */
            set = freelist->first_free;
            freelist->first_free = (AllocSet) set->header.nextchild;
            freelist->num_free--;

            /* Update its maxBlockSize; everything else should be OK */
            set->maxBlockSize = maxBlockSize;

            /* Reinitialize its header, installing correct name and parent */
            MemoryContextCreate((MemoryContext) set,
                                T_AllocSetContext,
                                &AllocSetMethods,
                                parent,
                                name);

            ((MemoryContext) set)->mem_allocated =
                set->keeper->endptr - ((char *) set);

            return (MemoryContext) set;
        }
    }

    /* Determine size of initial block */
    firstBlockSize = MAXALIGN(sizeof(AllocSetContext)) +
        ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
    if (minContextSize != 0)
        firstBlockSize = Max(firstBlockSize, minContextSize);
    else
        firstBlockSize = Max(firstBlockSize, initBlockSize);

    /*
     * Allocate the initial block.  Unlike other aset.c blocks, it starts with
     * the context header and its block header follows that.
     */
    set = (AllocSet) malloc(firstBlockSize);
    if (set == NULL)
    {
        if (TopMemoryContext)
            MemoryContextStats(TopMemoryContext);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while creating memory context \"%s\".",
                           name)));
    }

    /*
     * Avoid writing code that can fail between here and MemoryContextCreate;
     * we'd leak the header/initial block if we ereport in this stretch.
     */

    /* Fill in the initial block's block header */
    block = (AllocBlock) (((char *) set) + MAXALIGN(sizeof(AllocSetContext)));
    block->aset = set;
    block->freeptr = ((char *) block) + ALLOC_BLOCKHDRSZ;
    block->endptr = ((char *) set) + firstBlockSize;
    block->prev = NULL;
    block->next = NULL;

    /* Mark unallocated space NOACCESS; leave the block header alone. */
    VALGRIND_MAKE_MEM_NOACCESS(block->freeptr, block->endptr - block->freeptr);

    /* Remember block as part of block list */
    set->blocks = block;
    /* Mark block as not to be released at reset time */
    set->keeper = block;

    /* Finish filling in aset-specific parts of the context header */
    MemSetAligned(set->freelist, 0, sizeof(set->freelist));

    set->initBlockSize = initBlockSize;
    set->maxBlockSize = maxBlockSize;
    set->nextBlockSize = initBlockSize;
    set->freeListIndex = freeListIndex;

    /*
     * Compute the allocation chunk size limit for this context.  It can't be
     * more than ALLOC_CHUNK_LIMIT because of the fixed number of freelists.
     * If maxBlockSize is small then requests exceeding the maxBlockSize, or
     * even a significant fraction of it, should be treated as large chunks
     * too.  For the typical case of maxBlockSize a power of 2, the chunk size
     * limit will be at most 1/8th maxBlockSize, so that given a stream of
     * requests that are all the maximum chunk size we will waste at most
     * 1/8th of the allocated space.
     *
     * We have to have allocChunkLimit a power of two, because the requested
     * and actually-allocated sizes of any chunk must be on the same side of
     * the limit, else we get confused about whether the chunk is "big".
     *
     * Also, allocChunkLimit must not exceed ALLOCSET_SEPARATE_THRESHOLD.
     */
    StaticAssertStmt(ALLOC_CHUNK_LIMIT == ALLOCSET_SEPARATE_THRESHOLD,
                     "ALLOC_CHUNK_LIMIT != ALLOCSET_SEPARATE_THRESHOLD");

    set->allocChunkLimit = ALLOC_CHUNK_LIMIT;
    while ((Size) (set->allocChunkLimit + ALLOC_CHUNKHDRSZ) >
           (Size) ((maxBlockSize - ALLOC_BLOCKHDRSZ) / ALLOC_CHUNK_FRACTION))
        set->allocChunkLimit >>= 1;

    /* Finally, do the type-independent part of context creation */
    MemoryContextCreate((MemoryContext) set,
                        T_AllocSetContext,
                        &AllocSetMethods,
                        parent,
                        name);

    ((MemoryContext) set)->mem_allocated = firstBlockSize;

    return (MemoryContext) set;
}

/*
 * AllocSetReset
 *      Frees all memory which is allocated in the given set.
 *
 * Actually, this routine has some discretion about what to do.
 * It should mark all allocated chunks freed, but it need not necessarily
 * give back all the resources the set owns.  Our actual implementation is
 * that we give back all but the "keeper" block (which we must keep, since
 * it shares a malloc chunk with the context header).  In this way, we don't
 * thrash malloc() when a context is repeatedly reset after small allocations,
 * which is typical behavior for per-tuple contexts.
 */
static void
AllocSetReset(MemoryContext context)
{
    AllocSet    set = (AllocSet) context;
    AllocBlock  block;
    Size        keepersize PG_USED_FOR_ASSERTS_ONLY
        = set->keeper->endptr - ((char *) set);

    AssertArg(AllocSetIsValid(set));

#ifdef MEMORY_CONTEXT_CHECKING
    /* Check for corruption and leaks before freeing */
    AllocSetCheck(context);
#endif

    /* Clear chunk freelists */
    MemSetAligned(set->freelist, 0, sizeof(set->freelist));

    block = set->blocks;

    /* New blocks list will be just the keeper block */
    set->blocks = set->keeper;

    while (block != NULL)
    {
        AllocBlock  next = block->next;

        if (block == set->keeper)
        {
            /* Reset the block, but don't return it to malloc */
            char       *datastart = ((char *) block) + ALLOC_BLOCKHDRSZ;

#ifdef CLOBBER_FREED_MEMORY
            wipe_mem(datastart, block->freeptr - datastart);
#else
            /* wipe_mem() would have done this */
            VALGRIND_MAKE_MEM_NOACCESS(datastart, block->freeptr - datastart);
#endif
            block->freeptr = datastart;
            block->prev = NULL;
            block->next = NULL;
        }
        else
        {
            /* Normal case, release the block */
            context->mem_allocated -= block->endptr - ((char*) block);

#ifdef CLOBBER_FREED_MEMORY
            wipe_mem(block, block->freeptr - ((char *) block));
#endif
            free(block);
        }
        block = next;
    }

    Assert(context->mem_allocated == keepersize);

    /* Reset block size allocation sequence, too */
    set->nextBlockSize = set->initBlockSize;
}

/*
 * AllocSetDelete
 *      Frees all memory which is allocated in the given set,
 *      in preparation for deletion of the set.
 *
 * Unlike AllocSetReset, this *must* free all resources of the set.
 */
static void
AllocSetDelete(MemoryContext context)
{
    AllocSet    set = (AllocSet) context;
    AllocBlock  block = set->blocks;
    Size        keepersize PG_USED_FOR_ASSERTS_ONLY
        = set->keeper->endptr - ((char *) set);

    AssertArg(AllocSetIsValid(set));

#ifdef MEMORY_CONTEXT_CHECKING
    /* Check for corruption and leaks before freeing */
    AllocSetCheck(context);
#endif

    /*
     * If the context is a candidate for a freelist, put it into that freelist
     * instead of destroying it.
     */
    if (set->freeListIndex >= 0)
    {
        AllocSetFreeList *freelist = &context_freelists[set->freeListIndex];

        /*
         * Reset the context, if it needs it, so that we aren't hanging on to
         * more than the initial malloc chunk.
         */
        if (!context->isReset)
            MemoryContextResetOnly(context);

        /*
         * If the freelist is full, just discard what's already in it.  See
         * comments with context_freelists[].
         */
        if (freelist->num_free >= MAX_FREE_CONTEXTS)
        {
            while (freelist->first_free != NULL)
            {
                AllocSetContext *oldset = freelist->first_free;

                freelist->first_free = (AllocSetContext *) oldset->header.nextchild;
                freelist->num_free--;

                /* All that remains is to free the header/initial block */
                free(oldset);
            }
            Assert(freelist->num_free == 0);
        }

        /* Now add the just-deleted context to the freelist. */
        set->header.nextchild = (MemoryContext) freelist->first_free;
        freelist->first_free = set;
        freelist->num_free++;

        return;
    }

    /* Free all blocks, except the keeper which is part of context header */
    while (block != NULL)
    {
        AllocBlock  next = block->next;

        if (block != set->keeper)
            context->mem_allocated -= block->endptr - ((char *) block);

#ifdef CLOBBER_FREED_MEMORY
        wipe_mem(block, block->freeptr - ((char *) block));
#endif

        if (block != set->keeper)
            free(block);

        block = next;
    }

    Assert(context->mem_allocated == keepersize);

    /* Finally, free the context header, including the keeper block */
    free(set);
}

/*
 * AllocSetAlloc
 *      Returns pointer to allocated memory of given size or NULL if
 *      request could not be completed; memory is added to the set.
 *
 * No request may exceed:
 *      MAXALIGN_DOWN(SIZE_MAX) - ALLOC_BLOCKHDRSZ - ALLOC_CHUNKHDRSZ
 * All callers use a much-lower limit.
 *
 * Note: when using valgrind, it doesn't matter how the returned allocation
 * is marked, as mcxt.c will set it to UNDEFINED.  In some paths we will
 * return space that is marked NOACCESS - AllocSetRealloc has to beware!
 */
static void *
AllocSetAlloc(MemoryContext context, Size size)
{
    AllocSet    set = (AllocSet) context;
    AllocBlock  block;
    AllocChunk  chunk;
    int         fidx;
    Size        chunk_size;
    Size        blksize;

    AssertArg(AllocSetIsValid(set));

    /*
     * If requested size exceeds maximum for chunks, allocate an entire block
     * for this request.
     */
    if (size > set->allocChunkLimit)
    {
        chunk_size = MAXALIGN(size);
        blksize = chunk_size + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
        block = (AllocBlock) malloc(blksize);
        if (block == NULL)
            return NULL;

        context->mem_allocated += blksize;

        block->aset = set;
        block->freeptr = block->endptr = ((char *) block) + blksize;

        chunk = (AllocChunk) (((char *) block) + ALLOC_BLOCKHDRSZ);
        chunk->aset = set;
        chunk->size = chunk_size;
#ifdef MEMORY_CONTEXT_CHECKING
        chunk->requested_size = size;
        /* set mark to catch clobber of "unused" space */
        if (size < chunk_size)
            set_sentinel(AllocChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* fill the allocated space with junk */
        randomize_mem((char *) AllocChunkGetPointer(chunk), size);
#endif

        /*
         * Stick the new block underneath the active allocation block, if any,
         * so that we don't lose the use of the space remaining therein.
         */
        if (set->blocks != NULL)
        {
            block->prev = set->blocks;
            block->next = set->blocks->next;
            if (block->next)
                block->next->prev = block;
            set->blocks->next = block;
        }
        else
        {
            block->prev = NULL;
            block->next = NULL;
            set->blocks = block;
        }

        AllocAllocInfo(set, chunk);

        /* Ensure any padding bytes are marked NOACCESS. */
        VALGRIND_MAKE_MEM_NOACCESS((char *) AllocChunkGetPointer(chunk) + size,
                                   chunk_size - size);

        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

        return AllocChunkGetPointer(chunk);
    }

    /*
     * Request is small enough to be treated as a chunk.  Look in the
     * corresponding free list to see if there is a free chunk we could reuse.
     * If one is found, remove it from the free list, make it again a member
     * of the alloc set and return its data address.
     */
    fidx = AllocSetFreeIndex(size);
    chunk = set->freelist[fidx];
    if (chunk != NULL)
    {
        Assert(chunk->size >= size);

        set->freelist[fidx] = (AllocChunk) chunk->aset;

        chunk->aset = (void *) set;

#ifdef MEMORY_CONTEXT_CHECKING
        chunk->requested_size = size;
        /* set mark to catch clobber of "unused" space */
        if (size < chunk->size)
            set_sentinel(AllocChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* fill the allocated space with junk */
        randomize_mem((char *) AllocChunkGetPointer(chunk), size);
#endif

        AllocAllocInfo(set, chunk);

        /* Ensure any padding bytes are marked NOACCESS. */
        VALGRIND_MAKE_MEM_NOACCESS((char *) AllocChunkGetPointer(chunk) + size,
                                   chunk->size - size);

        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

        return AllocChunkGetPointer(chunk);
    }

    /*
     * Choose the actual chunk size to allocate.
     */
    chunk_size = (1 << ALLOC_MINBITS) << fidx;
    Assert(chunk_size >= size);

    /*
     * If there is enough room in the active allocation block, we will put the
     * chunk into that block.  Else must start a new one.
     */
    if ((block = set->blocks) != NULL)
    {
        Size        availspace = block->endptr - block->freeptr;

        if (availspace < (chunk_size + ALLOC_CHUNKHDRSZ))
        {
            /*
             * The existing active (top) block does not have enough room for
             * the requested allocation, but it might still have a useful
             * amount of space in it.  Once we push it down in the block list,
             * we'll never try to allocate more space from it. So, before we
             * do that, carve up its free space into chunks that we can put on
             * the set's freelists.
             *
             * Because we can only get here when there's less than
             * ALLOC_CHUNK_LIMIT left in the block, this loop cannot iterate
             * more than ALLOCSET_NUM_FREELISTS-1 times.
             */
            while (availspace >= ((1 << ALLOC_MINBITS) + ALLOC_CHUNKHDRSZ))
            {
                Size        availchunk = availspace - ALLOC_CHUNKHDRSZ;
                int         a_fidx = AllocSetFreeIndex(availchunk);

                /*
                 * In most cases, we'll get back the index of the next larger
                 * freelist than the one we need to put this chunk on.  The
                 * exception is when availchunk is exactly a power of 2.
                 */
                if (availchunk != ((Size) 1 << (a_fidx + ALLOC_MINBITS)))
                {
                    a_fidx--;
                    Assert(a_fidx >= 0);
                    availchunk = ((Size) 1 << (a_fidx + ALLOC_MINBITS));
                }

                chunk = (AllocChunk) (block->freeptr);

                /* Prepare to initialize the chunk header. */
                VALGRIND_MAKE_MEM_UNDEFINED(chunk, ALLOC_CHUNKHDRSZ);

                block->freeptr += (availchunk + ALLOC_CHUNKHDRSZ);
                availspace -= (availchunk + ALLOC_CHUNKHDRSZ);

                chunk->size = availchunk;
#ifdef MEMORY_CONTEXT_CHECKING
                chunk->requested_size = 0;  /* mark it free */
#endif
                chunk->aset = (void *) set->freelist[a_fidx];
                set->freelist[a_fidx] = chunk;
            }

            /* Mark that we need to create a new block */
            block = NULL;
        }
    }

    /*
     * Time to create a new regular (multi-chunk) block?
     */
    if (block == NULL)
    {
        Size        required_size;

        /*
         * The first such block has size initBlockSize, and we double the
         * space in each succeeding block, but not more than maxBlockSize.
         */
        blksize = set->nextBlockSize;
        set->nextBlockSize <<= 1;
        if (set->nextBlockSize > set->maxBlockSize)
            set->nextBlockSize = set->maxBlockSize;

        /*
         * If initBlockSize is less than ALLOC_CHUNK_LIMIT, we could need more
         * space... but try to keep it a power of 2.
         */
        required_size = chunk_size + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
        while (blksize < required_size)
            blksize <<= 1;

        /* Try to allocate it */
        block = (AllocBlock) malloc(blksize);

        /*
         * We could be asking for pretty big blocks here, so cope if malloc
         * fails.  But give up if there's less than 1 MB or so available...
         */
        while (block == NULL && blksize > 1024 * 1024)
        {
            blksize >>= 1;
            if (blksize < required_size)
                break;
            block = (AllocBlock) malloc(blksize);
        }

        if (block == NULL)
            return NULL;

        context->mem_allocated += blksize;

        block->aset = set;
        block->freeptr = ((char *) block) + ALLOC_BLOCKHDRSZ;
        block->endptr = ((char *) block) + blksize;

        /* Mark unallocated space NOACCESS. */
        VALGRIND_MAKE_MEM_NOACCESS(block->freeptr,
                                   blksize - ALLOC_BLOCKHDRSZ);

        block->prev = NULL;
        block->next = set->blocks;
        if (block->next)
            block->next->prev = block;
        set->blocks = block;
    }

    /*
     * OK, do the allocation
     */
    chunk = (AllocChunk) (block->freeptr);

    /* Prepare to initialize the chunk header. */
    VALGRIND_MAKE_MEM_UNDEFINED(chunk, ALLOC_CHUNKHDRSZ);

    block->freeptr += (chunk_size + ALLOC_CHUNKHDRSZ);
    Assert(block->freeptr <= block->endptr);

    chunk->aset = (void *) set;
    chunk->size = chunk_size;
#ifdef MEMORY_CONTEXT_CHECKING
    chunk->requested_size = size;
    /* set mark to catch clobber of "unused" space */
    if (size < chunk->size)
        set_sentinel(AllocChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
    /* fill the allocated space with junk */
    randomize_mem((char *) AllocChunkGetPointer(chunk), size);
#endif

    AllocAllocInfo(set, chunk);

    /* Ensure any padding bytes are marked NOACCESS. */
    VALGRIND_MAKE_MEM_NOACCESS((char *) AllocChunkGetPointer(chunk) + size,
                               chunk_size - size);

    /* Disallow external access to private part of chunk header. */
    VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

    return AllocChunkGetPointer(chunk);
}

/*
 * AllocSetFree
 *      Frees allocated memory; memory is removed from the set.
 */
static void
AllocSetFree(MemoryContext context, void *pointer)
{
    AllocSet    set = (AllocSet) context;
    AllocChunk  chunk = AllocPointerGetChunk(pointer);

    /* Allow access to private part of chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);

    AllocFreeInfo(set, chunk);

#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    if (chunk->requested_size < chunk->size)
        if (!sentinel_ok(pointer, chunk->requested_size))
            elog(WARNING, "detected write past chunk end in %s %p",
                 set->header.name, chunk);
#endif

    if (chunk->size > set->allocChunkLimit)
    {
        /*
         * Big chunks are certain to have been allocated as single-chunk
         * blocks.  Just unlink that block and return it to malloc().
         */
        AllocBlock  block = (AllocBlock) (((char *) chunk) - ALLOC_BLOCKHDRSZ);

        /*
         * Try to verify that we have a sane block pointer: it should
         * reference the correct aset, and freeptr and endptr should point
         * just past the chunk.
         */
        if (block->aset != set ||
            block->freeptr != block->endptr ||
            block->freeptr != ((char *) block) +
            (chunk->size + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ))
            elog(ERROR, "could not find block containing chunk %p", chunk);

        /* OK, remove block from aset's list and free it */
        if (block->prev)
            block->prev->next = block->next;
        else
            set->blocks = block->next;
        if (block->next)
            block->next->prev = block->prev;

        context->mem_allocated -= block->endptr - ((char*) block);

#ifdef CLOBBER_FREED_MEMORY
        wipe_mem(block, block->freeptr - ((char *) block));
#endif
        free(block);
    }
    else
    {
        /* Normal case, put the chunk into appropriate freelist */
        int         fidx = AllocSetFreeIndex(chunk->size);

        chunk->aset = (void *) set->freelist[fidx];

#ifdef CLOBBER_FREED_MEMORY
        wipe_mem(pointer, chunk->size);
#endif

#ifdef MEMORY_CONTEXT_CHECKING
        /* Reset requested_size to 0 in chunks that are on freelist */
        chunk->requested_size = 0;
#endif
        set->freelist[fidx] = chunk;
    }
}

/*
 * AllocSetRealloc
 *      Returns new pointer to allocated memory of given size or NULL if
 *      request could not be completed; this memory is added to the set.
 *      Memory associated with given pointer is copied into the new memory,
 *      and the old memory is freed.
 *
 * Without MEMORY_CONTEXT_CHECKING, we don't know the old request size.  This
 * makes our Valgrind client requests less-precise, hazarding false negatives.
 * (In principle, we could use VALGRIND_GET_VBITS() to rediscover the old
 * request size.)
 */
static void *
AllocSetRealloc(MemoryContext context, void *pointer, Size size)
{
    AllocSet    set = (AllocSet) context;
    AllocChunk  chunk = AllocPointerGetChunk(pointer);
    Size        oldsize;

    /* Allow access to private part of chunk header. */
    VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);

    oldsize = chunk->size;

#ifdef MEMORY_CONTEXT_CHECKING
    /* Test for someone scribbling on unused space in chunk */
    if (chunk->requested_size < oldsize)
        if (!sentinel_ok(pointer, chunk->requested_size))
            elog(WARNING, "detected write past chunk end in %s %p",
                 set->header.name, chunk);
#endif

    if (oldsize > set->allocChunkLimit)
    {
        /*
         * The chunk must have been allocated as a single-chunk block.  Use
         * realloc() to make the containing block bigger, or smaller, with
         * minimum space wastage.
         */
        AllocBlock  block = (AllocBlock) (((char *) chunk) - ALLOC_BLOCKHDRSZ);
        Size        chksize;
        Size        blksize;
        Size        oldblksize;

        /*
         * Try to verify that we have a sane block pointer: it should
         * reference the correct aset, and freeptr and endptr should point
         * just past the chunk.
         */
        if (block->aset != set ||
            block->freeptr != block->endptr ||
            block->freeptr != ((char *) block) +
            (oldsize + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ))
            elog(ERROR, "could not find block containing chunk %p", chunk);

        /*
         * Even if the new request is less than set->allocChunkLimit, we stick
         * with the single-chunk block approach.  Therefore we need
         * chunk->size to be bigger than set->allocChunkLimit, so we don't get
         * confused about the chunk's status in future calls.
         */
        chksize = Max(size, set->allocChunkLimit + 1);
        chksize = MAXALIGN(chksize);

        /* Do the realloc */
        blksize = chksize + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
        oldblksize = block->endptr - ((char *)block);

        block = (AllocBlock) realloc(block, blksize);
        if (block == NULL)
        {
            /* Disallow external access to private part of chunk header. */
            VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
            return NULL;
        }

        /* updated separately, not to underflow when (oldblksize > blksize) */
        context->mem_allocated -= oldblksize;
        context->mem_allocated += blksize;

        block->freeptr = block->endptr = ((char *) block) + blksize;

        /* Update pointers since block has likely been moved */
        chunk = (AllocChunk) (((char *) block) + ALLOC_BLOCKHDRSZ);
        pointer = AllocChunkGetPointer(chunk);
        if (block->prev)
            block->prev->next = block;
        else
            set->blocks = block;
        if (block->next)
            block->next->prev = block;
        chunk->size = chksize;

#ifdef MEMORY_CONTEXT_CHECKING
#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* We can only fill the extra space if we know the prior request */
        if (size > chunk->requested_size)
            randomize_mem((char *) pointer + chunk->requested_size,
                          size - chunk->requested_size);
#endif

        /*
         * realloc() (or randomize_mem()) will have left any newly-allocated
         * part UNDEFINED, but we may need to adjust trailing bytes from the
         * old allocation.
         */
#ifdef USE_VALGRIND
        if (oldsize > chunk->requested_size)
            VALGRIND_MAKE_MEM_UNDEFINED((char *) pointer + chunk->requested_size,
                                        oldsize - chunk->requested_size);
#endif

        chunk->requested_size = size;

        /* set mark to catch clobber of "unused" space */
        if (size < chunk->size)
            set_sentinel(pointer, size);
#else                           /* !MEMORY_CONTEXT_CHECKING */

        /*
         * We don't know how much of the old chunk size was the actual
         * allocation; it could have been as small as one byte.  We have to be
         * conservative and just mark the entire old portion DEFINED.
         */
        VALGRIND_MAKE_MEM_DEFINED(pointer, oldsize);
#endif

        /* Ensure any padding bytes are marked NOACCESS. */
        VALGRIND_MAKE_MEM_NOACCESS((char *) pointer + size, chksize - size);

        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

        return pointer;
    }

    /*
     * Chunk sizes are aligned to power of 2 in AllocSetAlloc().  Maybe the
     * allocated area already is >= the new size.  (In particular, we will
     * fall out here if the requested size is a decrease.)
     */
    else if (oldsize >= size)
    {
#ifdef MEMORY_CONTEXT_CHECKING
        Size        oldrequest = chunk->requested_size;

#ifdef RANDOMIZE_ALLOCATED_MEMORY
        /* We can only fill the extra space if we know the prior request */
        if (size > oldrequest)
            randomize_mem((char *) pointer + oldrequest,
                          size - oldrequest);
#endif

        chunk->requested_size = size;

        /*
         * If this is an increase, mark any newly-available part UNDEFINED.
         * Otherwise, mark the obsolete part NOACCESS.
         */
        if (size > oldrequest)
            VALGRIND_MAKE_MEM_UNDEFINED((char *) pointer + oldrequest,
                                        size - oldrequest);
        else
            VALGRIND_MAKE_MEM_NOACCESS((char *) pointer + size,
                                       oldsize - size);

        /* set mark to catch clobber of "unused" space */
        if (size < oldsize)
            set_sentinel(pointer, size);
#else                           /* !MEMORY_CONTEXT_CHECKING */

        /*
         * We don't have the information to determine whether we're growing
         * the old request or shrinking it, so we conservatively mark the
         * entire new allocation DEFINED.
         */
        VALGRIND_MAKE_MEM_NOACCESS(pointer, oldsize);
        VALGRIND_MAKE_MEM_DEFINED(pointer, size);
#endif

        /* Disallow external access to private part of chunk header. */
        VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

        return pointer;
    }
    else
    {
        /*
         * Enlarge-a-small-chunk case.  We just do this by brute force, ie,
         * allocate a new chunk and copy the data.  Since we know the existing
         * data isn't huge, this won't involve any great memcpy expense, so
         * it's not worth being smarter.  (At one time we tried to avoid
         * memcpy when it was possible to enlarge the chunk in-place, but that
         * turns out to misbehave unpleasantly for repeated cycles of
         * palloc/repalloc/pfree: the eventually freed chunks go into the
         * wrong freelist for the next initial palloc request, and so we leak
         * memory indefinitely.  See pgsql-hackers archives for 2007-08-11.)
         */
        AllocPointer newPointer;

        /* allocate new chunk */
        newPointer = AllocSetAlloc((MemoryContext) set, size);

        /* leave immediately if request was not completed */
        if (newPointer == NULL)
        {
            /* Disallow external access to private part of chunk header. */
            VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
            return NULL;
        }

        /*
         * AllocSetAlloc() may have returned a region that is still NOACCESS.
         * Change it to UNDEFINED for the moment; memcpy() will then transfer
         * definedness from the old allocation to the new.  If we know the old
         * allocation, copy just that much.  Otherwise, make the entire old
         * chunk defined to avoid errors as we copy the currently-NOACCESS
         * trailing bytes.
         */
        VALGRIND_MAKE_MEM_UNDEFINED(newPointer, size);
#ifdef MEMORY_CONTEXT_CHECKING
        oldsize = chunk->requested_size;
#else
        VALGRIND_MAKE_MEM_DEFINED(pointer, oldsize);
#endif

        /* transfer existing data (certain to fit) */
        memcpy(newPointer, pointer, oldsize);

        /* free old chunk */
        AllocSetFree((MemoryContext) set, pointer);

        return newPointer;
    }
}

/*
 * AllocSetGetChunkSpace
 *      Given a currently-allocated chunk, determine the total space
 *      it occupies (including all memory-allocation overhead).
 */
static Size
AllocSetGetChunkSpace(MemoryContext context, void *pointer)
{
    AllocChunk  chunk = AllocPointerGetChunk(pointer);
    Size        result;

    VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);
    result = chunk->size + ALLOC_CHUNKHDRSZ;
    VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
    return result;
}

/*
 * AllocSetIsEmpty
 *      Is an allocset empty of any allocated space?
 */
static bool
AllocSetIsEmpty(MemoryContext context)
{
    /*
     * For now, we say "empty" only if the context is new or just reset. We
     * could examine the freelists to determine if all space has been freed,
     * but it's not really worth the trouble for present uses of this
     * functionality.
     */
    if (context->isReset)
        return true;
    return false;
}

/*
 * AllocSetStats
 *      Compute stats about memory consumption of an allocset.
 *
 * printfunc: if not NULL, pass a human-readable stats string to this.
 * passthru: pass this pointer through to printfunc.
 * totals: if not NULL, add stats about this context into *totals.
 */
static void
AllocSetStats(MemoryContext context,
              MemoryStatsPrintFunc printfunc, void *passthru,
              MemoryContextCounters *totals)
{
    AllocSet    set = (AllocSet) context;
    Size        nblocks = 0;
    Size        freechunks = 0;
    Size        totalspace;
    Size        freespace = 0;
    AllocBlock  block;
    int         fidx;

    /* Include context header in totalspace */
    totalspace = MAXALIGN(sizeof(AllocSetContext));

    for (block = set->blocks; block != NULL; block = block->next)
    {
        nblocks++;
        totalspace += block->endptr - ((char *) block);
        freespace += block->endptr - block->freeptr;
    }
    for (fidx = 0; fidx < ALLOCSET_NUM_FREELISTS; fidx++)
    {
        AllocChunk  chunk;

        for (chunk = set->freelist[fidx]; chunk != NULL;
             chunk = (AllocChunk) chunk->aset)
        {
            freechunks++;
            freespace += chunk->size + ALLOC_CHUNKHDRSZ;
        }
    }

    if (printfunc)
    {
        char        stats_string[200];

        snprintf(stats_string, sizeof(stats_string),
                 "%zu total in %zd blocks; %zu free (%zd chunks); %zu used",
                 totalspace, nblocks, freespace, freechunks,
                 totalspace - freespace);
        printfunc(context, passthru, stats_string);
    }

    if (totals)
    {
        totals->nblocks += nblocks;
        totals->freechunks += freechunks;
        totals->totalspace += totalspace;
        totals->freespace += freespace;
    }
}


#ifdef MEMORY_CONTEXT_CHECKING

/*
 * AllocSetCheck
 *      Walk through chunks and check consistency of memory.
 *
 * NOTE: report errors as WARNING, *not* ERROR or FATAL.  Otherwise you'll
 * find yourself in an infinite loop when trouble occurs, because this
 * routine will be entered again when elog cleanup tries to release memory!
 */
static void
AllocSetCheck(MemoryContext context)
{
    AllocSet    set = (AllocSet) context;
    const char *name = set->header.name;
    AllocBlock  prevblock;
    AllocBlock  block;
    Size        total_allocated = 0;

    for (prevblock = NULL, block = set->blocks;
         block != NULL;
         prevblock = block, block = block->next)
    {
        char       *bpoz = ((char *) block) + ALLOC_BLOCKHDRSZ;
        long        blk_used = block->freeptr - bpoz;
        long        blk_data = 0;
        long        nchunks = 0;

        if (set->keeper == block)
            total_allocated += block->endptr - ((char *) set);
        else
            total_allocated += block->endptr - ((char *) block);

        /*
         * Empty block - empty can be keeper-block only
         */
        if (!blk_used)
        {
            if (set->keeper != block)
                elog(WARNING, "problem in alloc set %s: empty block %p",
                     name, block);
        }

        /*
         * Check block header fields
         */
        if (block->aset != set ||
            block->prev != prevblock ||
            block->freeptr < bpoz ||
            block->freeptr > block->endptr)
            elog(WARNING, "problem in alloc set %s: corrupt header in block %p",
                 name, block);

        /*
         * Chunk walker
         */
        while (bpoz < block->freeptr)
        {
            AllocChunk  chunk = (AllocChunk) bpoz;
            Size        chsize,
                        dsize;

            /* Allow access to private part of chunk header. */
            VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);

            chsize = chunk->size;   /* aligned chunk size */
            dsize = chunk->requested_size;  /* real data */

            /*
             * Check chunk size
             */
            if (dsize > chsize)
                elog(WARNING, "problem in alloc set %s: req size > alloc size for chunk %p in block %p",
                     name, chunk, block);
            if (chsize < (1 << ALLOC_MINBITS))
                elog(WARNING, "problem in alloc set %s: bad size %zu for chunk %p in block %p",
                     name, chsize, chunk, block);

            /* single-chunk block? */
            if (chsize > set->allocChunkLimit &&
                chsize + ALLOC_CHUNKHDRSZ != blk_used)
                elog(WARNING, "problem in alloc set %s: bad single-chunk %p in block %p",
                     name, chunk, block);

            /*
             * If chunk is allocated, check for correct aset pointer. (If it's
             * free, the aset is the freelist pointer, which we can't check as
             * easily...)  Note this is an incomplete test, since palloc(0)
             * produces an allocated chunk with requested_size == 0.
             */
            if (dsize > 0 && chunk->aset != (void *) set)
                elog(WARNING, "problem in alloc set %s: bogus aset link in block %p, chunk %p",
                     name, block, chunk);

            /*
             * Check for overwrite of padding space in an allocated chunk.
             */
            if (chunk->aset == (void *) set && dsize < chsize &&
                !sentinel_ok(chunk, ALLOC_CHUNKHDRSZ + dsize))
                elog(WARNING, "problem in alloc set %s: detected write past chunk end in block %p, chunk %p",
                     name, block, chunk);

            /*
             * If chunk is allocated, disallow external access to private part
             * of chunk header.
             */
            if (chunk->aset == (void *) set)
                VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);

            blk_data += chsize;
            nchunks++;

            bpoz += ALLOC_CHUNKHDRSZ + chsize;
        }

        if ((blk_data + (nchunks * ALLOC_CHUNKHDRSZ)) != blk_used)
            elog(WARNING, "problem in alloc set %s: found inconsistent memory block %p",
                 name, block);
    }

    Assert(total_allocated == context->mem_allocated);
}

#endif                          /* MEMORY_CONTEXT_CHECKING */