parallel.c

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/*-------------------------------------------------------------------------
 *
 * parallel.c
 *    Infrastructure for launching parallel workers
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/transam/parallel.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/parallel.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "catalog/namespace.h"
#include "commands/async.h"
#include "executor/execParallel.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "libpq/pqmq.h"
#include "miscadmin.h"
#include "optimizer/planmain.h"
#include "pgstat.h"
#include "storage/ipc.h"
#include "storage/sinval.h"
#include "storage/spin.h"
#include "tcop/tcopprot.h"
#include "utils/combocid.h"
#include "utils/guc.h"
#include "utils/inval.h"
#include "utils/memutils.h"
#include "utils/resowner.h"
#include "utils/snapmgr.h"


/*
 * We don't want to waste a lot of memory on an error queue which, most of
 * the time, will process only a handful of small messages.  However, it is
 * desirable to make it large enough that a typical ErrorResponse can be sent
 * without blocking.  That way, a worker that errors out can write the whole
 * message into the queue and terminate without waiting for the user backend.
 */
#define PARALLEL_ERROR_QUEUE_SIZE           16384

/* Magic number for parallel context TOC. */
#define PARALLEL_MAGIC                      0x50477c7c

/*
 * Magic numbers for parallel state sharing.  Higher-level code should use
 * smaller values, leaving these very large ones for use by this module.
 */
#define PARALLEL_KEY_FIXED                  UINT64CONST(0xFFFFFFFFFFFF0001)
#define PARALLEL_KEY_ERROR_QUEUE            UINT64CONST(0xFFFFFFFFFFFF0002)
#define PARALLEL_KEY_LIBRARY                UINT64CONST(0xFFFFFFFFFFFF0003)
#define PARALLEL_KEY_GUC                    UINT64CONST(0xFFFFFFFFFFFF0004)
#define PARALLEL_KEY_COMBO_CID              UINT64CONST(0xFFFFFFFFFFFF0005)
#define PARALLEL_KEY_TRANSACTION_SNAPSHOT   UINT64CONST(0xFFFFFFFFFFFF0006)
#define PARALLEL_KEY_ACTIVE_SNAPSHOT        UINT64CONST(0xFFFFFFFFFFFF0007)
#define PARALLEL_KEY_TRANSACTION_STATE      UINT64CONST(0xFFFFFFFFFFFF0008)
#define PARALLEL_KEY_ENTRYPOINT             UINT64CONST(0xFFFFFFFFFFFF0009)

/* Fixed-size parallel state. */
typedef struct FixedParallelState
{
    /* Fixed-size state that workers must restore. */
    Oid         database_id;
    Oid         authenticated_user_id;
    Oid         current_user_id;
    Oid         temp_namespace_id;
    Oid         temp_toast_namespace_id;
    int         sec_context;
    PGPROC     *parallel_master_pgproc;
    pid_t       parallel_master_pid;
    BackendId   parallel_master_backend_id;

    /* Mutex protects remaining fields. */
    slock_t     mutex;

    /* Maximum XactLastRecEnd of any worker. */
    XLogRecPtr  last_xlog_end;
} FixedParallelState;

/*
 * Our parallel worker number.  We initialize this to -1, meaning that we are
 * not a parallel worker.  In parallel workers, it will be set to a value >= 0
 * and < the number of workers before any user code is invoked; each parallel
 * worker will get a different parallel worker number.
 */
int         ParallelWorkerNumber = -1;

/* Is there a parallel message pending which we need to receive? */
volatile bool ParallelMessagePending = false;

/* Are we initializing a parallel worker? */
bool        InitializingParallelWorker = false;

/* Pointer to our fixed parallel state. */
static FixedParallelState *MyFixedParallelState;

/* List of active parallel contexts. */
static dlist_head pcxt_list = DLIST_STATIC_INIT(pcxt_list);

/*
 * List of internal parallel worker entry points.  We need this for
 * reasons explained in LookupParallelWorkerFunction(), below.
 */
static const struct
{
    const char *fn_name;
    parallel_worker_main_type fn_addr;
}   InternalParallelWorkers[] =

{
    {
        "ParallelQueryMain", ParallelQueryMain
    }
};

/* Private functions. */
static void HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg);
static void WaitForParallelWorkersToExit(ParallelContext *pcxt);
static parallel_worker_main_type LookupParallelWorkerFunction(const char *libraryname, const char *funcname);


/*
 * Establish a new parallel context.  This should be done after entering
 * parallel mode, and (unless there is an error) the context should be
 * destroyed before exiting the current subtransaction.
 */
ParallelContext *
CreateParallelContext(const char *library_name, const char *function_name,
                      int nworkers)
{
    MemoryContext oldcontext;
    ParallelContext *pcxt;

    /* It is unsafe to create a parallel context if not in parallel mode. */
    Assert(IsInParallelMode());

    /* Number of workers should be non-negative. */
    Assert(nworkers >= 0);

    /*
     * If dynamic shared memory is not available, we won't be able to use
     * background workers.
     */
    if (dynamic_shared_memory_type == DSM_IMPL_NONE)
        nworkers = 0;

    /*
     * If we are running under serializable isolation, we can't use parallel
     * workers, at least not until somebody enhances that mechanism to be
     * parallel-aware.
     */
    if (IsolationIsSerializable())
        nworkers = 0;

    /* We might be running in a short-lived memory context. */
    oldcontext = MemoryContextSwitchTo(TopTransactionContext);

    /* Initialize a new ParallelContext. */
    pcxt = palloc0(sizeof(ParallelContext));
    pcxt->subid = GetCurrentSubTransactionId();
    pcxt->nworkers = nworkers;
    pcxt->library_name = pstrdup(library_name);
    pcxt->function_name = pstrdup(function_name);
    pcxt->error_context_stack = error_context_stack;
    shm_toc_initialize_estimator(&pcxt->estimator);
    dlist_push_head(&pcxt_list, &pcxt->node);

    /* Restore previous memory context. */
    MemoryContextSwitchTo(oldcontext);

    return pcxt;
}

/*
 * Establish the dynamic shared memory segment for a parallel context and
 * copy state and other bookkeeping information that will be needed by
 * parallel workers into it.
 */
void
InitializeParallelDSM(ParallelContext *pcxt)
{
    MemoryContext oldcontext;
    Size        library_len = 0;
    Size        guc_len = 0;
    Size        combocidlen = 0;
    Size        tsnaplen = 0;
    Size        asnaplen = 0;
    Size        tstatelen = 0;
    Size        segsize = 0;
    int         i;
    FixedParallelState *fps;
    Snapshot    transaction_snapshot = GetTransactionSnapshot();
    Snapshot    active_snapshot = GetActiveSnapshot();

    /* We might be running in a very short-lived memory context. */
    oldcontext = MemoryContextSwitchTo(TopTransactionContext);

    /* Allow space to store the fixed-size parallel state. */
    shm_toc_estimate_chunk(&pcxt->estimator, sizeof(FixedParallelState));
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /*
     * Normally, the user will have requested at least one worker process, but
     * if by chance they have not, we can skip a bunch of things here.
     */
    if (pcxt->nworkers > 0)
    {
        /* Estimate space for various kinds of state sharing. */
        library_len = EstimateLibraryStateSpace();
        shm_toc_estimate_chunk(&pcxt->estimator, library_len);
        guc_len = EstimateGUCStateSpace();
        shm_toc_estimate_chunk(&pcxt->estimator, guc_len);
        combocidlen = EstimateComboCIDStateSpace();
        shm_toc_estimate_chunk(&pcxt->estimator, combocidlen);
        tsnaplen = EstimateSnapshotSpace(transaction_snapshot);
        shm_toc_estimate_chunk(&pcxt->estimator, tsnaplen);
        asnaplen = EstimateSnapshotSpace(active_snapshot);
        shm_toc_estimate_chunk(&pcxt->estimator, asnaplen);
        tstatelen = EstimateTransactionStateSpace();
        shm_toc_estimate_chunk(&pcxt->estimator, tstatelen);
        /* If you add more chunks here, you probably need to add keys. */
        shm_toc_estimate_keys(&pcxt->estimator, 6);

        /* Estimate space need for error queues. */
        StaticAssertStmt(BUFFERALIGN(PARALLEL_ERROR_QUEUE_SIZE) ==
                         PARALLEL_ERROR_QUEUE_SIZE,
                         "parallel error queue size not buffer-aligned");
        shm_toc_estimate_chunk(&pcxt->estimator,
                               mul_size(PARALLEL_ERROR_QUEUE_SIZE,
                                        pcxt->nworkers));
        shm_toc_estimate_keys(&pcxt->estimator, 1);

        /* Estimate how much we'll need for the entrypoint info. */
        shm_toc_estimate_chunk(&pcxt->estimator, strlen(pcxt->library_name) +
                               strlen(pcxt->function_name) + 2);
        shm_toc_estimate_keys(&pcxt->estimator, 1);
    }

    /*
     * Create DSM and initialize with new table of contents.  But if the user
     * didn't request any workers, then don't bother creating a dynamic shared
     * memory segment; instead, just use backend-private memory.
     *
     * Also, if we can't create a dynamic shared memory segment because the
     * maximum number of segments have already been created, then fall back to
     * backend-private memory, and plan not to use any workers.  We hope this
     * won't happen very often, but it's better to abandon the use of
     * parallelism than to fail outright.
     */
    segsize = shm_toc_estimate(&pcxt->estimator);
    if (pcxt->nworkers > 0)
        pcxt->seg = dsm_create(segsize, DSM_CREATE_NULL_IF_MAXSEGMENTS);
    if (pcxt->seg != NULL)
        pcxt->toc = shm_toc_create(PARALLEL_MAGIC,
                                   dsm_segment_address(pcxt->seg),
                                   segsize);
    else
    {
        pcxt->nworkers = 0;
        pcxt->private_memory = MemoryContextAlloc(TopMemoryContext, segsize);
        pcxt->toc = shm_toc_create(PARALLEL_MAGIC, pcxt->private_memory,
                                   segsize);
    }

    /* Initialize fixed-size state in shared memory. */
    fps = (FixedParallelState *)
        shm_toc_allocate(pcxt->toc, sizeof(FixedParallelState));
    fps->database_id = MyDatabaseId;
    fps->authenticated_user_id = GetAuthenticatedUserId();
    GetUserIdAndSecContext(&fps->current_user_id, &fps->sec_context);
    GetTempNamespaceState(&fps->temp_namespace_id,
                          &fps->temp_toast_namespace_id);
    fps->parallel_master_pgproc = MyProc;
    fps->parallel_master_pid = MyProcPid;
    fps->parallel_master_backend_id = MyBackendId;
    SpinLockInit(&fps->mutex);
    fps->last_xlog_end = 0;
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);

    /* We can skip the rest of this if we're not budgeting for any workers. */
    if (pcxt->nworkers > 0)
    {
        char       *libraryspace;
        char       *gucspace;
        char       *combocidspace;
        char       *tsnapspace;
        char       *asnapspace;
        char       *tstatespace;
        char       *error_queue_space;
        char       *entrypointstate;
        Size        lnamelen;

        /* Serialize shared libraries we have loaded. */
        libraryspace = shm_toc_allocate(pcxt->toc, library_len);
        SerializeLibraryState(library_len, libraryspace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_LIBRARY, libraryspace);

        /* Serialize GUC settings. */
        gucspace = shm_toc_allocate(pcxt->toc, guc_len);
        SerializeGUCState(guc_len, gucspace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_GUC, gucspace);

        /* Serialize combo CID state. */
        combocidspace = shm_toc_allocate(pcxt->toc, combocidlen);
        SerializeComboCIDState(combocidlen, combocidspace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_COMBO_CID, combocidspace);

        /* Serialize transaction snapshot and active snapshot. */
        tsnapspace = shm_toc_allocate(pcxt->toc, tsnaplen);
        SerializeSnapshot(transaction_snapshot, tsnapspace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
                       tsnapspace);
        asnapspace = shm_toc_allocate(pcxt->toc, asnaplen);
        SerializeSnapshot(active_snapshot, asnapspace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, asnapspace);

        /* Serialize transaction state. */
        tstatespace = shm_toc_allocate(pcxt->toc, tstatelen);
        SerializeTransactionState(tstatelen, tstatespace);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_STATE, tstatespace);

        /* Allocate space for worker information. */
        pcxt->worker = palloc0(sizeof(ParallelWorkerInfo) * pcxt->nworkers);

        /*
         * Establish error queues in dynamic shared memory.
         *
         * These queues should be used only for transmitting ErrorResponse,
         * NoticeResponse, and NotifyResponse protocol messages.  Tuple data
         * should be transmitted via separate (possibly larger?) queues.
         */
        error_queue_space =
            shm_toc_allocate(pcxt->toc,
                             mul_size(PARALLEL_ERROR_QUEUE_SIZE,
                                      pcxt->nworkers));
        for (i = 0; i < pcxt->nworkers; ++i)
        {
            char       *start;
            shm_mq     *mq;

            start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
            mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
            shm_mq_set_receiver(mq, MyProc);
            pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
        }
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, error_queue_space);

        /*
         * Serialize entrypoint information.  It's unsafe to pass function
         * pointers across processes, as the function pointer may be different
         * in each process in EXEC_BACKEND builds, so we always pass library
         * and function name.  (We use library name "postgres" for functions
         * in the core backend.)
         */
        lnamelen = strlen(pcxt->library_name);
        entrypointstate = shm_toc_allocate(pcxt->toc, lnamelen +
                                           strlen(pcxt->function_name) + 2);
        strcpy(entrypointstate, pcxt->library_name);
        strcpy(entrypointstate + lnamelen + 1, pcxt->function_name);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_ENTRYPOINT, entrypointstate);
    }

    /* Restore previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}

/*
 * Reinitialize the dynamic shared memory segment for a parallel context such
 * that we could launch workers for it again.
 */
void
ReinitializeParallelDSM(ParallelContext *pcxt)
{
    FixedParallelState *fps;
    char       *error_queue_space;
    int         i;

    /* Wait for any old workers to exit. */
    if (pcxt->nworkers_launched > 0)
    {
        WaitForParallelWorkersToFinish(pcxt);
        WaitForParallelWorkersToExit(pcxt);
        pcxt->nworkers_launched = 0;
    }

    /* Reset a few bits of fixed parallel state to a clean state. */
    fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
    fps->last_xlog_end = 0;

    /* Recreate error queues. */
    error_queue_space =
        shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE);
    for (i = 0; i < pcxt->nworkers; ++i)
    {
        char       *start;
        shm_mq     *mq;

        start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
        mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
        shm_mq_set_receiver(mq, MyProc);
        pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
    }
}

/*
 * Launch parallel workers.
 */
void
LaunchParallelWorkers(ParallelContext *pcxt)
{
    MemoryContext oldcontext;
    BackgroundWorker worker;
    int         i;
    bool        any_registrations_failed = false;

    /* Skip this if we have no workers. */
    if (pcxt->nworkers == 0)
        return;

    /* We need to be a lock group leader. */
    BecomeLockGroupLeader();

    /* If we do have workers, we'd better have a DSM segment. */
    Assert(pcxt->seg != NULL);

    /* We might be running in a short-lived memory context. */
    oldcontext = MemoryContextSwitchTo(TopTransactionContext);

    /* Configure a worker. */
    memset(&worker, 0, sizeof(worker));
    snprintf(worker.bgw_name, BGW_MAXLEN, "parallel worker for PID %d",
             MyProcPid);
    worker.bgw_flags =
        BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION
        | BGWORKER_CLASS_PARALLEL;
    worker.bgw_start_time = BgWorkerStart_ConsistentState;
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    sprintf(worker.bgw_library_name, "postgres");
    sprintf(worker.bgw_function_name, "ParallelWorkerMain");
    worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
    worker.bgw_notify_pid = MyProcPid;

    /*
     * Start workers.
     *
     * The caller must be able to tolerate ending up with fewer workers than
     * expected, so there is no need to throw an error here if registration
     * fails.  It wouldn't help much anyway, because registering the worker in
     * no way guarantees that it will start up and initialize successfully.
     */
    for (i = 0; i < pcxt->nworkers; ++i)
    {
        memcpy(worker.bgw_extra, &i, sizeof(int));
        if (!any_registrations_failed &&
            RegisterDynamicBackgroundWorker(&worker,
                                            &pcxt->worker[i].bgwhandle))
        {
            shm_mq_set_handle(pcxt->worker[i].error_mqh,
                              pcxt->worker[i].bgwhandle);
            pcxt->nworkers_launched++;
        }
        else
        {
            /*
             * If we weren't able to register the worker, then we've bumped up
             * against the max_worker_processes limit, and future
             * registrations will probably fail too, so arrange to skip them.
             * But we still have to execute this code for the remaining slots
             * to make sure that we forget about the error queues we budgeted
             * for those workers.  Otherwise, we'll wait for them to start,
             * but they never will.
             */
            any_registrations_failed = true;
            pcxt->worker[i].bgwhandle = NULL;
            pfree(pcxt->worker[i].error_mqh);
            pcxt->worker[i].error_mqh = NULL;
        }
    }

    /* Restore previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}

/*
 * Wait for all workers to finish computing.
 *
 * Even if the parallel operation seems to have completed successfully, it's
 * important to call this function afterwards.  We must not miss any errors
 * the workers may have thrown during the parallel operation, or any that they
 * may yet throw while shutting down.
 *
 * Also, we want to update our notion of XactLastRecEnd based on worker
 * feedback.
 */
void
WaitForParallelWorkersToFinish(ParallelContext *pcxt)
{
    for (;;)
    {
        bool        anyone_alive = false;
        int         i;

        /*
         * This will process any parallel messages that are pending, which may
         * change the outcome of the loop that follows.  It may also throw an
         * error propagated from a worker.
         */
        CHECK_FOR_INTERRUPTS();

        for (i = 0; i < pcxt->nworkers_launched; ++i)
        {
            if (pcxt->worker[i].error_mqh != NULL)
            {
                anyone_alive = true;
                break;
            }
        }

        if (!anyone_alive)
            break;

        WaitLatch(&MyProc->procLatch, WL_LATCH_SET, -1,
                  WAIT_EVENT_PARALLEL_FINISH);
        ResetLatch(&MyProc->procLatch);
    }

    if (pcxt->toc != NULL)
    {
        FixedParallelState *fps;

        fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
        if (fps->last_xlog_end > XactLastRecEnd)
            XactLastRecEnd = fps->last_xlog_end;
    }
}

/*
 * Wait for all workers to exit.
 *
 * This function ensures that workers have been completely shutdown.  The
 * difference between WaitForParallelWorkersToFinish and this function is
 * that former just ensures that last message sent by worker backend is
 * received by master backend whereas this ensures the complete shutdown.
 */
static void
WaitForParallelWorkersToExit(ParallelContext *pcxt)
{
    int         i;

    /* Wait until the workers actually die. */
    for (i = 0; i < pcxt->nworkers_launched; ++i)
    {
        BgwHandleStatus status;

        if (pcxt->worker == NULL || pcxt->worker[i].bgwhandle == NULL)
            continue;

        status = WaitForBackgroundWorkerShutdown(pcxt->worker[i].bgwhandle);

        /*
         * If the postmaster kicked the bucket, we have no chance of cleaning
         * up safely -- we won't be able to tell when our workers are actually
         * dead.  This doesn't necessitate a PANIC since they will all abort
         * eventually, but we can't safely continue this session.
         */
        if (status == BGWH_POSTMASTER_DIED)
            ereport(FATAL,
                    (errcode(ERRCODE_ADMIN_SHUTDOWN),
                 errmsg("postmaster exited during a parallel transaction")));

        /* Release memory. */
        pfree(pcxt->worker[i].bgwhandle);
        pcxt->worker[i].bgwhandle = NULL;
    }
}

/*
 * Destroy a parallel context.
 *
 * If expecting a clean exit, you should use WaitForParallelWorkersToFinish()
 * first, before calling this function.  When this function is invoked, any
 * remaining workers are forcibly killed; the dynamic shared memory segment
 * is unmapped; and we then wait (uninterruptibly) for the workers to exit.
 */
void
DestroyParallelContext(ParallelContext *pcxt)
{
    int         i;

    /*
     * Be careful about order of operations here!  We remove the parallel
     * context from the list before we do anything else; otherwise, if an
     * error occurs during a subsequent step, we might try to nuke it again
     * from AtEOXact_Parallel or AtEOSubXact_Parallel.
     */
    dlist_delete(&pcxt->node);

    /* Kill each worker in turn, and forget their error queues. */
    if (pcxt->worker != NULL)
    {
        for (i = 0; i < pcxt->nworkers_launched; ++i)
        {
            if (pcxt->worker[i].error_mqh != NULL)
            {
                TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);

                pfree(pcxt->worker[i].error_mqh);
                pcxt->worker[i].error_mqh = NULL;
            }
        }
    }

    /*
     * If we have allocated a shared memory segment, detach it.  This will
     * implicitly detach the error queues, and any other shared memory queues,
     * stored there.
     */
    if (pcxt->seg != NULL)
    {
        dsm_detach(pcxt->seg);
        pcxt->seg = NULL;
    }

    /*
     * If this parallel context is actually in backend-private memory rather
     * than shared memory, free that memory instead.
     */
    if (pcxt->private_memory != NULL)
    {
        pfree(pcxt->private_memory);
        pcxt->private_memory = NULL;
    }

    /*
     * We can't finish transaction commit or abort until all of the workers
     * have exited.  This means, in particular, that we can't respond to
     * interrupts at this stage.
     */
    HOLD_INTERRUPTS();
    WaitForParallelWorkersToExit(pcxt);
    RESUME_INTERRUPTS();

    /* Free the worker array itself. */
    if (pcxt->worker != NULL)
    {
        pfree(pcxt->worker);
        pcxt->worker = NULL;
    }

    /* Free memory. */
    pfree(pcxt->library_name);
    pfree(pcxt->function_name);
    pfree(pcxt);
}

/*
 * Are there any parallel contexts currently active?
 */
bool
ParallelContextActive(void)
{
    return !dlist_is_empty(&pcxt_list);
}

/*
 * Handle receipt of an interrupt indicating a parallel worker message.
 *
 * Note: this is called within a signal handler!  All we can do is set
 * a flag that will cause the next CHECK_FOR_INTERRUPTS() to invoke
 * HandleParallelMessages().
 */
void
HandleParallelMessageInterrupt(void)
{
    InterruptPending = true;
    ParallelMessagePending = true;
    SetLatch(MyLatch);
}

/*
 * Handle any queued protocol messages received from parallel workers.
 */
void
HandleParallelMessages(void)
{
    dlist_iter  iter;
    MemoryContext oldcontext;

    static MemoryContext hpm_context = NULL;

    /*
     * This is invoked from ProcessInterrupts(), and since some of the
     * functions it calls contain CHECK_FOR_INTERRUPTS(), there is a potential
     * for recursive calls if more signals are received while this runs.  It's
     * unclear that recursive entry would be safe, and it doesn't seem useful
     * even if it is safe, so let's block interrupts until done.
     */
    HOLD_INTERRUPTS();

    /*
     * Moreover, CurrentMemoryContext might be pointing almost anywhere.  We
     * don't want to risk leaking data into long-lived contexts, so let's do
     * our work here in a private context that we can reset on each use.
     */
    if (hpm_context == NULL)    /* first time through? */
        hpm_context = AllocSetContextCreate(TopMemoryContext,
                                            "HandleParallelMessages",
                                            ALLOCSET_DEFAULT_SIZES);
    else
        MemoryContextReset(hpm_context);

    oldcontext = MemoryContextSwitchTo(hpm_context);

    /* OK to process messages.  Reset the flag saying there are more to do. */
    ParallelMessagePending = false;

    dlist_foreach(iter, &pcxt_list)
    {
        ParallelContext *pcxt;
        int         i;

        pcxt = dlist_container(ParallelContext, node, iter.cur);
        if (pcxt->worker == NULL)
            continue;

        for (i = 0; i < pcxt->nworkers_launched; ++i)
        {
            /*
             * Read as many messages as we can from each worker, but stop when
             * either (1) the worker's error queue goes away, which can happen
             * if we receive a Terminate message from the worker; or (2) no
             * more messages can be read from the worker without blocking.
             */
            while (pcxt->worker[i].error_mqh != NULL)
            {
                shm_mq_result res;
                Size        nbytes;
                void       *data;

                res = shm_mq_receive(pcxt->worker[i].error_mqh, &nbytes,
                                     &data, true);
                if (res == SHM_MQ_WOULD_BLOCK)
                    break;
                else if (res == SHM_MQ_SUCCESS)
                {
                    StringInfoData msg;

                    initStringInfo(&msg);
                    appendBinaryStringInfo(&msg, data, nbytes);
                    HandleParallelMessage(pcxt, i, &msg);
                    pfree(msg.data);
                }
                else
                    ereport(ERROR,
                          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                           errmsg("lost connection to parallel worker")));
            }
        }
    }

    MemoryContextSwitchTo(oldcontext);

    /* Might as well clear the context on our way out */
    MemoryContextReset(hpm_context);

    RESUME_INTERRUPTS();
}

/*
 * Handle a single protocol message received from a single parallel worker.
 */
static void
HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
{
    char        msgtype;

    msgtype = pq_getmsgbyte(msg);

    switch (msgtype)
    {
        case 'K':               /* BackendKeyData */
            {
                int32       pid = pq_getmsgint(msg, 4);

                (void) pq_getmsgint(msg, 4);    /* discard cancel key */
                (void) pq_getmsgend(msg);
                pcxt->worker[i].pid = pid;
                break;
            }

        case 'E':               /* ErrorResponse */
        case 'N':               /* NoticeResponse */
            {
                ErrorData   edata;
                ErrorContextCallback *save_error_context_stack;

                /* Parse ErrorResponse or NoticeResponse. */
                pq_parse_errornotice(msg, &edata);

                /* Death of a worker isn't enough justification for suicide. */
                edata.elevel = Min(edata.elevel, ERROR);

                /*
                 * If desired, add a context line to show that this is a
                 * message propagated from a parallel worker.  Otherwise, it
                 * can sometimes be confusing to understand what actually
                 * happened.  (We don't do this in FORCE_PARALLEL_REGRESS mode
                 * because it causes test-result instability depending on
                 * whether a parallel worker is actually used or not.)
                 */
                if (force_parallel_mode != FORCE_PARALLEL_REGRESS)
                {
                    if (edata.context)
                        edata.context = psprintf("%s\n%s", edata.context,
                                                 _("parallel worker"));
                    else
                        edata.context = pstrdup(_("parallel worker"));
                }

                /*
                 * Context beyond that should use the error context callbacks
                 * that were in effect when the ParallelContext was created,
                 * not the current ones.
                 */
                save_error_context_stack = error_context_stack;
                error_context_stack = pcxt->error_context_stack;

                /* Rethrow error or print notice. */
                ThrowErrorData(&edata);

                /* Not an error, so restore previous context stack. */
                error_context_stack = save_error_context_stack;

                break;
            }

        case 'A':               /* NotifyResponse */
            {
                /* Propagate NotifyResponse. */
                int32       pid;
                const char *channel;
                const char *payload;

                pid = pq_getmsgint(msg, 4);
                channel = pq_getmsgrawstring(msg);
                payload = pq_getmsgrawstring(msg);
                pq_endmessage(msg);

                NotifyMyFrontEnd(channel, payload, pid);

                break;
            }

        case 'X':               /* Terminate, indicating clean exit */
            {
                pfree(pcxt->worker[i].error_mqh);
                pcxt->worker[i].error_mqh = NULL;
                break;
            }

        default:
            {
                elog(ERROR, "unrecognized message type received from parallel worker: %c (message length %d bytes)",
                     msgtype, msg->len);
            }
    }
}

/*
 * End-of-subtransaction cleanup for parallel contexts.
 *
 * Currently, it's forbidden to enter or leave a subtransaction while
 * parallel mode is in effect, so we could just blow away everything.  But
 * we may want to relax that restriction in the future, so this code
 * contemplates that there may be multiple subtransaction IDs in pcxt_list.
 */
void
AtEOSubXact_Parallel(bool isCommit, SubTransactionId mySubId)
{
    while (!dlist_is_empty(&pcxt_list))
    {
        ParallelContext *pcxt;

        pcxt = dlist_head_element(ParallelContext, node, &pcxt_list);
        if (pcxt->subid != mySubId)
            break;
        if (isCommit)
            elog(WARNING, "leaked parallel context");
        DestroyParallelContext(pcxt);
    }
}

/*
 * End-of-transaction cleanup for parallel contexts.
 */
void
AtEOXact_Parallel(bool isCommit)
{
    while (!dlist_is_empty(&pcxt_list))
    {
        ParallelContext *pcxt;

        pcxt = dlist_head_element(ParallelContext, node, &pcxt_list);
        if (isCommit)
            elog(WARNING, "leaked parallel context");
        DestroyParallelContext(pcxt);
    }
}

/*
 * Main entrypoint for parallel workers.
 */
void
ParallelWorkerMain(Datum main_arg)
{
    dsm_segment *seg;
    shm_toc    *toc;
    FixedParallelState *fps;
    char       *error_queue_space;
    shm_mq     *mq;
    shm_mq_handle *mqh;
    char       *libraryspace;
    char       *entrypointstate;
    char       *library_name;
    char       *function_name;
    parallel_worker_main_type entrypt;
    char       *gucspace;
    char       *combocidspace;
    char       *tsnapspace;
    char       *asnapspace;
    char       *tstatespace;
    StringInfoData msgbuf;

    /* Set flag to indicate that we're initializing a parallel worker. */
    InitializingParallelWorker = true;

    /* Establish signal handlers. */
    pqsignal(SIGTERM, die);
    BackgroundWorkerUnblockSignals();

    /* Determine and set our parallel worker number. */
    Assert(ParallelWorkerNumber == -1);
    memcpy(&ParallelWorkerNumber, MyBgworkerEntry->bgw_extra, sizeof(int));

    /* Set up a memory context and resource owner. */
    Assert(CurrentResourceOwner == NULL);
    CurrentResourceOwner = ResourceOwnerCreate(NULL, "parallel toplevel");
    CurrentMemoryContext = AllocSetContextCreate(TopMemoryContext,
                                                 "Parallel worker",
                                                 ALLOCSET_DEFAULT_SIZES);

    /*
     * Now that we have a resource owner, we can attach to the dynamic shared
     * memory segment and read the table of contents.
     */
    seg = dsm_attach(DatumGetUInt32(main_arg));
    if (seg == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not map dynamic shared memory segment")));
    toc = shm_toc_attach(PARALLEL_MAGIC, dsm_segment_address(seg));
    if (toc == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("invalid magic number in dynamic shared memory segment")));

    /* Look up fixed parallel state. */
    fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
    Assert(fps != NULL);
    MyFixedParallelState = fps;

    /*
     * Now that we have a worker number, we can find and attach to the error
     * queue provided for us.  That's good, because until we do that, any
     * errors that happen here will not be reported back to the process that
     * requested that this worker be launched.
     */
    error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE);
    mq = (shm_mq *) (error_queue_space +
                     ParallelWorkerNumber * PARALLEL_ERROR_QUEUE_SIZE);
    shm_mq_set_sender(mq, MyProc);
    mqh = shm_mq_attach(mq, seg, NULL);
    pq_redirect_to_shm_mq(seg, mqh);
    pq_set_parallel_master(fps->parallel_master_pid,
                           fps->parallel_master_backend_id);

    /*
     * Send a BackendKeyData message to the process that initiated parallelism
     * so that it has access to our PID before it receives any other messages
     * from us.  Our cancel key is sent, too, since that's the way the
     * protocol message is defined, but it won't actually be used for anything
     * in this case.
     */
    pq_beginmessage(&msgbuf, 'K');
    pq_sendint(&msgbuf, (int32) MyProcPid, sizeof(int32));
    pq_sendint(&msgbuf, (int32) MyCancelKey, sizeof(int32));
    pq_endmessage(&msgbuf);

    /*
     * Hooray! Primary initialization is complete.  Now, we need to set up our
     * backend-local state to match the original backend.
     */

    /*
     * Join locking group.  We must do this before anything that could try to
     * acquire a heavyweight lock, because any heavyweight locks acquired to
     * this point could block either directly against the parallel group
     * leader or against some process which in turn waits for a lock that
     * conflicts with the parallel group leader, causing an undetected
     * deadlock.  (If we can't join the lock group, the leader has gone away,
     * so just exit quietly.)
     */
    if (!BecomeLockGroupMember(fps->parallel_master_pgproc,
                               fps->parallel_master_pid))
        return;

    /*
     * Load libraries that were loaded by original backend.  We want to do
     * this before restoring GUCs, because the libraries might define custom
     * variables.
     */
    libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY);
    Assert(libraryspace != NULL);
    RestoreLibraryState(libraryspace);

    /*
     * Identify the entry point to be called.  In theory this could result in
     * loading an additional library, though most likely the entry point is in
     * the core backend or in a library we just loaded.
     */
    entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT);
    Assert(entrypointstate != NULL);
    library_name = entrypointstate;
    function_name = entrypointstate + strlen(library_name) + 1;

    entrypt = LookupParallelWorkerFunction(library_name, function_name);

    /* Restore database connection. */
    BackgroundWorkerInitializeConnectionByOid(fps->database_id,
                                              fps->authenticated_user_id);

    /*
     * Set the client encoding to the database encoding, since that is what
     * the leader will expect.
     */
    SetClientEncoding(GetDatabaseEncoding());

    /* Restore GUC values from launching backend. */
    gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC);
    Assert(gucspace != NULL);
    StartTransactionCommand();
    RestoreGUCState(gucspace);
    CommitTransactionCommand();

    /* Crank up a transaction state appropriate to a parallel worker. */
    tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE);
    StartParallelWorkerTransaction(tstatespace);

    /* Restore combo CID state. */
    combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID);
    Assert(combocidspace != NULL);
    RestoreComboCIDState(combocidspace);

    /* Restore transaction snapshot. */
    tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT);
    Assert(tsnapspace != NULL);
    RestoreTransactionSnapshot(RestoreSnapshot(tsnapspace),
                               fps->parallel_master_pgproc);

    /* Restore active snapshot. */
    asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT);
    Assert(asnapspace != NULL);
    PushActiveSnapshot(RestoreSnapshot(asnapspace));

    /*
     * We've changed which tuples we can see, and must therefore invalidate
     * system caches.
     */
    InvalidateSystemCaches();

    /* Restore user ID and security context. */
    SetUserIdAndSecContext(fps->current_user_id, fps->sec_context);

    /* Restore temp-namespace state to ensure search path matches leader's. */
    SetTempNamespaceState(fps->temp_namespace_id,
                          fps->temp_toast_namespace_id);

    /* Set ParallelMasterBackendId so we know how to address temp relations. */
    ParallelMasterBackendId = fps->parallel_master_backend_id;

    /*
     * We've initialized all of our state now; nothing should change
     * hereafter.
     */
    InitializingParallelWorker = false;
    EnterParallelMode();

    /*
     * Time to do the real work: invoke the caller-supplied code.
     */
    entrypt(seg, toc);

    /* Must exit parallel mode to pop active snapshot. */
    ExitParallelMode();

    /* Must pop active snapshot so resowner.c doesn't complain. */
    PopActiveSnapshot();

    /* Shut down the parallel-worker transaction. */
    EndParallelWorkerTransaction();

    /* Report success. */
    pq_putmessage('X', NULL, 0);
}

/*
 * Update shared memory with the ending location of the last WAL record we
 * wrote, if it's greater than the value already stored there.
 */
void
ParallelWorkerReportLastRecEnd(XLogRecPtr last_xlog_end)
{
    FixedParallelState *fps = MyFixedParallelState;

    Assert(fps != NULL);
    SpinLockAcquire(&fps->mutex);
    if (fps->last_xlog_end < last_xlog_end)
        fps->last_xlog_end = last_xlog_end;
    SpinLockRelease(&fps->mutex);
}

/*
 * Look up (and possibly load) a parallel worker entry point function.
 *
 * For functions contained in the core code, we use library name "postgres"
 * and consult the InternalParallelWorkers array.  External functions are
 * looked up, and loaded if necessary, using load_external_function().
 *
 * The point of this is to pass function names as strings across process
 * boundaries.  We can't pass actual function addresses because of the
 * possibility that the function has been loaded at a different address
 * in a different process.  This is obviously a hazard for functions in
 * loadable libraries, but it can happen even for functions in the core code
 * on platforms using EXEC_BACKEND (e.g., Windows).
 *
 * At some point it might be worthwhile to get rid of InternalParallelWorkers[]
 * in favor of applying load_external_function() for core functions too;
 * but that raises portability issues that are not worth addressing now.
 */
static parallel_worker_main_type
LookupParallelWorkerFunction(const char *libraryname, const char *funcname)
{
    /*
     * If the function is to be loaded from postgres itself, search the
     * InternalParallelWorkers array.
     */
    if (strcmp(libraryname, "postgres") == 0)
    {
        int         i;

        for (i = 0; i < lengthof(InternalParallelWorkers); i++)
        {
            if (strcmp(InternalParallelWorkers[i].fn_name, funcname) == 0)
                return InternalParallelWorkers[i].fn_addr;
        }

        /* We can only reach this by programming error. */
        elog(ERROR, "internal function \"%s\" not found", funcname);
    }

    /* Otherwise load from external library. */
    return (parallel_worker_main_type)
        load_external_function(libraryname, funcname, true, NULL);
}