combocid.c

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/*-------------------------------------------------------------------------
 *
 * combocid.c
 *    Combo command ID support routines
 *
 * Before version 8.3, HeapTupleHeaderData had separate fields for cmin
 * and cmax.  To reduce the header size, cmin and cmax are now overlayed
 * in the same field in the header.  That usually works because you rarely
 * insert and delete a tuple in the same transaction, and we don't need
 * either field to remain valid after the originating transaction exits.
 * To make it work when the inserting transaction does delete the tuple,
 * we create a "combo" command ID and store that in the tuple header
 * instead of cmin and cmax. The combo command ID can be mapped to the
 * real cmin and cmax using a backend-private array, which is managed by
 * this module.
 *
 * To allow reusing existing combo cids, we also keep a hash table that
 * maps cmin,cmax pairs to combo cids.  This keeps the data structure size
 * reasonable in most cases, since the number of unique pairs used by any
 * one transaction is likely to be small.
 *
 * With a 32-bit combo command id we can represent 2^32 distinct cmin,cmax
 * combinations. In the most perverse case where each command deletes a tuple
 * generated by every previous command, the number of combo command ids
 * required for N commands is N*(N+1)/2.  That means that in the worst case,
 * that's enough for 92682 commands.  In practice, you'll run out of memory
 * and/or disk space way before you reach that limit.
 *
 * The array and hash table are kept in TopTransactionContext, and are
 * destroyed at the end of each transaction.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/time/combocid.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/htup_details.h"
#include "access/xact.h"
#include "miscadmin.h"
#include "storage/shmem.h"
#include "utils/combocid.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"

/* Hash table to lookup combo cids by cmin and cmax */
static HTAB *comboHash = NULL;

/* Key and entry structures for the hash table */
typedef struct
{
    CommandId   cmin;
    CommandId   cmax;
} ComboCidKeyData;

typedef ComboCidKeyData *ComboCidKey;

typedef struct
{
    ComboCidKeyData key;
    CommandId   combocid;
} ComboCidEntryData;

typedef ComboCidEntryData *ComboCidEntry;

/* Initial size of the hash table */
#define CCID_HASH_SIZE          100


/*
 * An array of cmin,cmax pairs, indexed by combo command id.
 * To convert a combo cid to cmin and cmax, you do a simple array lookup.
 */
static ComboCidKey comboCids = NULL;
static int  usedComboCids = 0;  /* number of elements in comboCids */
static int  sizeComboCids = 0;  /* allocated size of array */

/* Initial size of the array */
#define CCID_ARRAY_SIZE         100


/* prototypes for internal functions */
static CommandId GetComboCommandId(CommandId cmin, CommandId cmax);
static CommandId GetRealCmin(CommandId combocid);
static CommandId GetRealCmax(CommandId combocid);


/**** External API ****/

/*
 * GetCmin and GetCmax assert that they are only called in situations where
 * they make sense, that is, can deliver a useful answer.  If you have
 * reason to examine a tuple's t_cid field from a transaction other than
 * the originating one, use HeapTupleHeaderGetRawCommandId() directly.
 */

CommandId
HeapTupleHeaderGetCmin(HeapTupleHeader tup)
{
    CommandId   cid = HeapTupleHeaderGetRawCommandId(tup);

    Assert(!(tup->t_infomask & HEAP_MOVED));
    Assert(TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(tup)));

    if (tup->t_infomask & HEAP_COMBOCID)
        return GetRealCmin(cid);
    else
        return cid;
}

CommandId
HeapTupleHeaderGetCmax(HeapTupleHeader tup)
{
    CommandId   cid = HeapTupleHeaderGetRawCommandId(tup);

    Assert(!(tup->t_infomask & HEAP_MOVED));

    /*
     * Because GetUpdateXid() performs memory allocations if xmax is a
     * multixact we can't Assert() if we're inside a critical section. This
     * weakens the check, but not using GetCmax() inside one would complicate
     * things too much.
     */
    Assert(CritSectionCount > 0 ||
           TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tup)));

    if (tup->t_infomask & HEAP_COMBOCID)
        return GetRealCmax(cid);
    else
        return cid;
}

/*
 * Given a tuple we are about to delete, determine the correct value to store
 * into its t_cid field.
 *
 * If we don't need a combo CID, *cmax is unchanged and *iscombo is set to
 * false.  If we do need one, *cmax is replaced by a combo CID and *iscombo
 * is set to true.
 *
 * The reason this is separate from the actual HeapTupleHeaderSetCmax()
 * operation is that this could fail due to out-of-memory conditions.  Hence
 * we need to do this before entering the critical section that actually
 * changes the tuple in shared buffers.
 */
void
HeapTupleHeaderAdjustCmax(HeapTupleHeader tup,
                          CommandId *cmax,
                          bool *iscombo)
{
    /*
     * If we're marking a tuple deleted that was inserted by (any
     * subtransaction of) our transaction, we need to use a combo command id.
     * Test for HeapTupleHeaderXminCommitted() first, because it's cheaper
     * than a TransactionIdIsCurrentTransactionId call.
     */
    if (!HeapTupleHeaderXminCommitted(tup) &&
        TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tup)))
    {
        CommandId   cmin = HeapTupleHeaderGetCmin(tup);

        *cmax = GetComboCommandId(cmin, *cmax);
        *iscombo = true;
    }
    else
    {
        *iscombo = false;
    }
}

/*
 * Combo command ids are only interesting to the inserting and deleting
 * transaction, so we can forget about them at the end of transaction.
 */
void
AtEOXact_ComboCid(void)
{
    /*
     * Don't bother to pfree. These are allocated in TopTransactionContext, so
     * they're going to go away at the end of transaction anyway.
     */
    comboHash = NULL;

    comboCids = NULL;
    usedComboCids = 0;
    sizeComboCids = 0;
}


/**** Internal routines ****/

/*
 * Get a combo command id that maps to cmin and cmax.
 *
 * We try to reuse old combo command ids when possible.
 */
static CommandId
GetComboCommandId(CommandId cmin, CommandId cmax)
{
    CommandId   combocid;
    ComboCidKeyData key;
    ComboCidEntry entry;
    bool        found;

    /*
     * Create the hash table and array the first time we need to use combo
     * cids in the transaction.
     */
    if (comboHash == NULL)
    {
        HASHCTL     hash_ctl;

        /* Make array first; existence of hash table asserts array exists */
        comboCids = (ComboCidKeyData *)
            MemoryContextAlloc(TopTransactionContext,
                               sizeof(ComboCidKeyData) * CCID_ARRAY_SIZE);
        sizeComboCids = CCID_ARRAY_SIZE;
        usedComboCids = 0;

        hash_ctl.keysize = sizeof(ComboCidKeyData);
        hash_ctl.entrysize = sizeof(ComboCidEntryData);
        hash_ctl.hcxt = TopTransactionContext;

        comboHash = hash_create("Combo CIDs",
                                CCID_HASH_SIZE,
                                &hash_ctl,
                                HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
    }

    /*
     * Grow the array if there's not at least one free slot.  We must do this
     * before possibly entering a new hashtable entry, else failure to
     * repalloc would leave a corrupt hashtable entry behind.
     */
    if (usedComboCids >= sizeComboCids)
    {
        int         newsize = sizeComboCids * 2;

        comboCids = (ComboCidKeyData *)
            repalloc(comboCids, sizeof(ComboCidKeyData) * newsize);
        sizeComboCids = newsize;
    }

    /* Lookup or create a hash entry with the desired cmin/cmax */

    /* We assume there is no struct padding in ComboCidKeyData! */
    key.cmin = cmin;
    key.cmax = cmax;
    entry = (ComboCidEntry) hash_search(comboHash,
                                        (void *) &key,
                                        HASH_ENTER,
                                        &found);

    if (found)
    {
        /* Reuse an existing combo cid */
        return entry->combocid;
    }

    /* We have to create a new combo cid; we already made room in the array */
    combocid = usedComboCids;

    comboCids[combocid].cmin = cmin;
    comboCids[combocid].cmax = cmax;
    usedComboCids++;

    entry->combocid = combocid;

    return combocid;
}

static CommandId
GetRealCmin(CommandId combocid)
{
    Assert(combocid < usedComboCids);
    return comboCids[combocid].cmin;
}

static CommandId
GetRealCmax(CommandId combocid)
{
    Assert(combocid < usedComboCids);
    return comboCids[combocid].cmax;
}

/*
 * Estimate the amount of space required to serialize the current ComboCID
 * state.
 */
Size
EstimateComboCIDStateSpace(void)
{
    Size        size;

    /* Add space required for saving usedComboCids */
    size = sizeof(int);

    /* Add space required for saving the combocids key */
    size = add_size(size, mul_size(sizeof(ComboCidKeyData), usedComboCids));

    return size;
}

/*
 * Serialize the ComboCID state into the memory, beginning at start_address.
 * maxsize should be at least as large as the value returned by
 * EstimateComboCIDStateSpace.
 */
void
SerializeComboCIDState(Size maxsize, char *start_address)
{
    char       *endptr;

    /* First, we store the number of currently-existing ComboCIDs. */
    *(int *) start_address = usedComboCids;

    /* If maxsize is too small, throw an error. */
    endptr = start_address + sizeof(int) +
        (sizeof(ComboCidKeyData) * usedComboCids);
    if (endptr < start_address || endptr > start_address + maxsize)
        elog(ERROR, "not enough space to serialize ComboCID state");

    /* Now, copy the actual cmin/cmax pairs. */
    if (usedComboCids > 0)
        memcpy(start_address + sizeof(int), comboCids,
               (sizeof(ComboCidKeyData) * usedComboCids));
}

/*
 * Read the ComboCID state at the specified address and initialize this
 * backend with the same ComboCIDs.  This is only valid in a backend that
 * currently has no ComboCIDs (and only makes sense if the transaction state
 * is serialized and restored as well).
 */
void
RestoreComboCIDState(char *comboCIDstate)
{
    int         num_elements;
    ComboCidKeyData *keydata;
    int         i;
    CommandId   cid;

    Assert(!comboCids && !comboHash);

    /* First, we retrieve the number of ComboCIDs that were serialized. */
    num_elements = *(int *) comboCIDstate;
    keydata = (ComboCidKeyData *) (comboCIDstate + sizeof(int));

    /* Use GetComboCommandId to restore each ComboCID. */
    for (i = 0; i < num_elements; i++)
    {
        cid = GetComboCommandId(keydata[i].cmin, keydata[i].cmax);

        /* Verify that we got the expected answer. */
        if (cid != i)
            elog(ERROR, "unexpected command ID while restoring combo CIDs");
    }
}