src/backend/port/win32_sema.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * win32_sema.c
   4  *        Microsoft Windows Win32 Semaphores Emulation
   5  *
   6  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
   7  *
   8  * IDENTIFICATION
   9  *        $PostgreSQL$
  10  *
  11  *-------------------------------------------------------------------------
  12  */
  13
  14 #include "postgres.h"
  15
  16 #include "miscadmin.h"
  17 #include "storage/ipc.h"
  18 #include "storage/pg_sema.h"
  19
  20 static HANDLE *mySemSet;                /* IDs of sema sets acquired so far */
  21 static int      numSems;                        /* number of sema sets acquired so far */
  22 static int      maxSems;                        /* allocated size of mySemaSet array */
  23
  24 static void ReleaseSemaphores(int code, Datum arg);
  25
  26 /*
  27  * PGReserveSemaphores --- initialize semaphore support
  28  *
  29  * In the Win32 implementation, we acquire semaphores on-demand; the
  30  * maxSemas parameter is just used to size the array that keeps track of
  31  * acquired semas for subsequent releasing.  We use anonymous semaphores
  32  * so the semaphores are automatically freed when the last referencing
  33  * process exits.
  34  */
  35 void
  36 PGReserveSemaphores(int maxSemas, int port)
  37 {
  38         mySemSet = (HANDLE *) malloc(maxSemas * sizeof(HANDLE));
  39         if (mySemSet == NULL)
  40                 elog(PANIC, "out of memory");
  41         numSems = 0;
  42         maxSems = maxSemas;
  43
  44         on_shmem_exit(ReleaseSemaphores, 0);
  45 }
  46
  47 /*
  48  * Release semaphores at shutdown or shmem reinitialization
  49  *
  50  * (called as an on_shmem_exit callback, hence funny argument list)
  51  */
  52 static void
  53 ReleaseSemaphores(int code, Datum arg)
  54 {
  55         int                     i;
  56
  57         for (i = 0; i < numSems; i++)
  58                 CloseHandle(mySemSet[i]);
  59         free(mySemSet);
  60 }
  61
  62 /*
  63  * PGSemaphoreCreate
  64  *
  65  * Initialize a PGSemaphore structure to represent a sema with count 1
  66  */
  67 void
  68 PGSemaphoreCreate(PGSemaphore sema)
  69 {
  70         HANDLE          cur_handle;
  71         SECURITY_ATTRIBUTES sec_attrs;
  72
  73         /* Can't do this in a backend, because static state is postmaster's */
  74         Assert(!IsUnderPostmaster);
  75
  76         if (numSems >= maxSems)
  77                 elog(PANIC, "too many semaphores created");
  78
  79         ZeroMemory(&sec_attrs, sizeof(sec_attrs));
  80         sec_attrs.nLength = sizeof(sec_attrs);
  81         sec_attrs.lpSecurityDescriptor = NULL;
  82         sec_attrs.bInheritHandle = TRUE;
  83
  84         /* We don't need a named semaphore */
  85         cur_handle = CreateSemaphore(&sec_attrs, 1, 32767, NULL);
  86         if (cur_handle)
  87         {
  88                 /* Successfully done */
  89                 *sema = cur_handle;
  90                 mySemSet[numSems++] = cur_handle;
  91         }
  92         else
  93                 ereport(PANIC,
  94                                 (errmsg("could not create semaphore: error code %d", (int) GetLastError())));
  95 }
  96
  97 /*
  98  * PGSemaphoreReset
  99  *
 100  * Reset a previously-initialized PGSemaphore to have count 0
 101  */
 102 void
 103 PGSemaphoreReset(PGSemaphore sema)
 104 {
 105         /*
 106          * There's no direct API for this in Win32, so we have to ratchet the
 107          * semaphore down to 0 with repeated trylock's.
 108          */
 109         while (PGSemaphoreTryLock(sema));
 110 }
 111
 112 /*
 113  * PGSemaphoreLock
 114  *
 115  * Lock a semaphore (decrement count), blocking if count would be < 0.
 116  * Serve the interrupt if interruptOK is true.
 117  */
 118 void
 119 PGSemaphoreLock(PGSemaphore sema, bool interruptOK)
 120 {
 121         DWORD           ret;
 122         HANDLE          wh[2];
 123
 124         wh[0] = *sema;
 125         wh[1] = pgwin32_signal_event;
 126
 127         /*
 128          * As in other implementations of PGSemaphoreLock, we need to check
 129          * for cancel/die interrupts each time through the loop.  But here,
 130          * there is no hidden magic about whether the syscall will internally
 131          * service a signal --- we do that ourselves.
 132          */
 133         do
 134         {
 135                 ImmediateInterruptOK = interruptOK;
 136                 CHECK_FOR_INTERRUPTS();
 137
 138                 errno = 0;
 139                 ret = WaitForMultipleObjectsEx(2, wh, FALSE, INFINITE, TRUE);
 140
 141                 if (ret == WAIT_OBJECT_0)
 142                 {
 143                         /* We got it! */
 144                         return;
 145                 }
 146                 else if (ret == WAIT_OBJECT_0 + 1)
 147                 {
 148                         /* Signal event is set - we have a signal to deliver */
 149                         pgwin32_dispatch_queued_signals();
 150                         errno = EINTR;
 151                 }
 152                 else
 153                         /* Otherwise we are in trouble */
 154                         errno = EIDRM;
 155
 156                 ImmediateInterruptOK = false;
 157         } while (errno == EINTR);
 158
 159         if (errno != 0)
 160                 ereport(FATAL,
 161                                 (errmsg("could not lock semaphore: error code %d", (int) GetLastError())));
 162 }
 163
 164 /*
 165  * PGSemaphoreUnlock
 166  *
 167  * Unlock a semaphore (increment count)
 168  */
 169 void
 170 PGSemaphoreUnlock(PGSemaphore sema)
 171 {
 172         if (!ReleaseSemaphore(*sema, 1, NULL))
 173                 ereport(FATAL,
 174                                 (errmsg("could not unlock semaphore: error code %d", (int) GetLastError())));
 175 }
 176
 177 /*
 178  * PGSemaphoreTryLock
 179  *
 180  * Lock a semaphore only if able to do so without blocking
 181  */
 182 bool
 183 PGSemaphoreTryLock(PGSemaphore sema)
 184 {
 185         DWORD           ret;
 186
 187         ret = WaitForSingleObject(*sema, 0);
 188
 189         if (ret == WAIT_OBJECT_0)
 190         {
 191                 /* We got it! */
 192                 return true;
 193         }
 194         else if (ret == WAIT_TIMEOUT)
 195         {
 196                 /* Can't get it */
 197                 errno = EAGAIN;
 198                 return false;
 199         }
 200
 201         /* Otherwise we are in trouble */
 202         ereport(FATAL,
 203                         (errmsg("could not try-lock semaphore: error code %d", (int) GetLastError())));
 204
 205         /* keep compiler quiet */
 206         return false;
 207 }