win32_sema.c

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/*-------------------------------------------------------------------------
 *
 * win32_sema.c
 *    Microsoft Windows Win32 Semaphores Emulation
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/port/win32_sema.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/pg_sema.h"
 
static HANDLE *mySemSet;        /* IDs of sema sets acquired so far */
static int  numSems;            /* number of sema sets acquired so far */
static int  maxSems;            /* allocated size of mySemaSet array */
 
static void ReleaseSemaphores(int code, Datum arg);
 
 
/*
 * Report amount of shared memory needed for semaphores
 */
Size
PGSemaphoreShmemSize(int maxSemas)
{
    /* No shared memory needed on Windows */
    return 0;
}
 
/*
 * PGReserveSemaphores --- initialize semaphore support
 *
 * In the Win32 implementation, we acquire semaphores on-demand; the
 * maxSemas parameter is just used to size the array that keeps track of
 * acquired semas for subsequent releasing.  We use anonymous semaphores
 * so the semaphores are automatically freed when the last referencing
 * process exits.
 */
void
PGReserveSemaphores(int maxSemas)
{
    mySemSet = (HANDLE *) malloc(maxSemas * sizeof(HANDLE));
    if (mySemSet == NULL)
        elog(PANIC, "out of memory");
    numSems = 0;
    maxSems = maxSemas;
 
    on_shmem_exit(ReleaseSemaphores, 0);
}
 
/*
 * Release semaphores at shutdown or shmem reinitialization
 *
 * (called as an on_shmem_exit callback, hence funny argument list)
 */
static void
ReleaseSemaphores(int code, Datum arg)
{
    int         i;
 
    for (i = 0; i < numSems; i++)
        CloseHandle(mySemSet[i]);
    free(mySemSet);
}
 
/*
 * PGSemaphoreCreate
 *
 * Allocate a PGSemaphore structure with initial count 1
 */
PGSemaphore
PGSemaphoreCreate(void)
{
    HANDLE      cur_handle;
    SECURITY_ATTRIBUTES sec_attrs;
 
    /* Can't do this in a backend, because static state is postmaster's */
    Assert(!IsUnderPostmaster);
 
    if (numSems >= maxSems)
        elog(PANIC, "too many semaphores created");
 
    ZeroMemory(&sec_attrs, sizeof(sec_attrs));
    sec_attrs.nLength = sizeof(sec_attrs);
    sec_attrs.lpSecurityDescriptor = NULL;
    sec_attrs.bInheritHandle = TRUE;
 
    /* We don't need a named semaphore */
    cur_handle = CreateSemaphore(&sec_attrs, 1, 32767, NULL);
    if (cur_handle)
    {
        /* Successfully done */
        mySemSet[numSems++] = cur_handle;
    }
    else
        ereport(PANIC,
                (errmsg("could not create semaphore: error code %lu",
                        GetLastError())));
 
    return (PGSemaphore) cur_handle;
}
 
/*
 * PGSemaphoreReset
 *
 * Reset a previously-initialized PGSemaphore to have count 0
 */
void
PGSemaphoreReset(PGSemaphore sema)
{
    /*
     * There's no direct API for this in Win32, so we have to ratchet the
     * semaphore down to 0 with repeated trylock's.
     */
    while (PGSemaphoreTryLock(sema))
         /* loop */ ;
}
 
/*
 * PGSemaphoreLock
 *
 * Lock a semaphore (decrement count), blocking if count would be < 0.
 */
void
PGSemaphoreLock(PGSemaphore sema)
{
    HANDLE      wh[2];
    bool        done = false;
 
    /*
     * Note: pgwin32_signal_event should be first to ensure that it will be
     * reported when multiple events are set.  We want to guarantee that
     * pending signals are serviced.
     */
    wh[0] = pgwin32_signal_event;
    wh[1] = sema;
 
    /*
     * As in other implementations of PGSemaphoreLock, we need to check for
     * cancel/die interrupts each time through the loop.  But here, there is
     * no hidden magic about whether the syscall will internally service a
     * signal --- we do that ourselves.
     */
    while (!done)
    {
        DWORD       rc;
 
        CHECK_FOR_INTERRUPTS();
 
        rc = WaitForMultipleObjectsEx(2, wh, FALSE, INFINITE, TRUE);
        switch (rc)
        {
            case WAIT_OBJECT_0:
                /* Signal event is set - we have a signal to deliver */
                pgwin32_dispatch_queued_signals();
                break;
            case WAIT_OBJECT_0 + 1:
                /* We got it! */
                done = true;
                break;
            case WAIT_IO_COMPLETION:
 
                /*
                 * The system interrupted the wait to execute an I/O
                 * completion routine or asynchronous procedure call in this
                 * thread.  PostgreSQL does not provoke either of these, but
                 * atypical loaded DLLs or even other processes might do so.
                 * Now, resume waiting.
                 */
                break;
            case WAIT_FAILED:
                ereport(FATAL,
                        (errmsg("could not lock semaphore: error code %lu",
                                GetLastError())));
                break;
            default:
                elog(FATAL, "unexpected return code from WaitForMultipleObjectsEx(): %lu", rc);
                break;
        }
    }
}
 
/*
 * PGSemaphoreUnlock
 *
 * Unlock a semaphore (increment count)
 */
void
PGSemaphoreUnlock(PGSemaphore sema)
{
    if (!ReleaseSemaphore(sema, 1, NULL))
        ereport(FATAL,
                (errmsg("could not unlock semaphore: error code %lu",
                        GetLastError())));
}
 
/*
 * PGSemaphoreTryLock
 *
 * Lock a semaphore only if able to do so without blocking
 */
bool
PGSemaphoreTryLock(PGSemaphore sema)
{
    DWORD       ret;
 
    ret = WaitForSingleObject(sema, 0);
 
    if (ret == WAIT_OBJECT_0)
    {
        /* We got it! */
        return true;
    }
    else if (ret == WAIT_TIMEOUT)
    {
        /* Can't get it */
        errno = EAGAIN;
        return false;
    }
 
    /* Otherwise we are in trouble */
    ereport(FATAL,
            (errmsg("could not try-lock semaphore: error code %lu",
                    GetLastError())));
 
    /* keep compiler quiet */
    return false;
}