1 files changed, 388 insertions, 0 deletions
diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c
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+++ b/src/backend/port/posix_sema.c
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+/*-------------------------------------------------------------------------
+ *
+ * posix_sema.c
+ *	  Implement PGSemaphores using POSIX semaphore facilities
+ *
+ * We prefer the unnamed style of POSIX semaphore (the kind made with
+ * sem_init).  We can cope with the kind made with sem_open, however.
+ *
+ * In either implementation, typedef PGSemaphore is equivalent to "sem_t *".
+ * With unnamed semaphores, the sem_t structs live in an array in shared
+ * memory.  With named semaphores, that's not true because we cannot persuade
+ * sem_open to do its allocation there.  Therefore, the named-semaphore code
+ * *does not cope with EXEC_BACKEND*.  The sem_t structs will just be in the
+ * postmaster's private memory, where they are successfully inherited by
+ * forked backends, but they could not be accessed by exec'd backends.
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/port/posix_sema.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <fcntl.h>
+#include <semaphore.h>
+#include <signal.h>
+#include <unistd.h>
+#include <sys/stat.h>
+
+#include "miscadmin.h"
+#include "storage/ipc.h"
+#include "storage/pg_sema.h"
+#include "storage/shmem.h"
+
+
+/* see file header comment */
+#if defined(USE_NAMED_POSIX_SEMAPHORES) && defined(EXEC_BACKEND)
+#error cannot use named POSIX semaphores with EXEC_BACKEND
+#endif
+
+typedef union SemTPadded
+{
+	sem_t		pgsem;
+	char		pad[PG_CACHE_LINE_SIZE];
+} SemTPadded;
+
+/* typedef PGSemaphore is equivalent to pointer to sem_t */
+typedef struct PGSemaphoreData
+{
+	SemTPadded	sem_padded;
+} PGSemaphoreData;
+
+#define PG_SEM_REF(x)	(&(x)->sem_padded.pgsem)
+
+#define IPCProtection	(0600)	/* access/modify by user only */
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+static sem_t **mySemPointers;	/* keep track of created semaphores */
+#else
+static PGSemaphore sharedSemas; /* array of PGSemaphoreData in shared memory */
+#endif
+static int	numSems;			/* number of semas acquired so far */
+static int	maxSems;			/* allocated size of above arrays */
+static int	nextSemKey;			/* next name to try */
+
+
+static void ReleaseSemaphores(int status, Datum arg);
+
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+
+/*
+ * PosixSemaphoreCreate
+ *
+ * Attempt to create a new named semaphore.
+ *
+ * If we fail with a failure code other than collision-with-existing-sema,
+ * print out an error and abort.  Other types of errors suggest nonrecoverable
+ * problems.
+ */
+static sem_t *
+PosixSemaphoreCreate(void)
+{
+	int			semKey;
+	char		semname[64];
+	sem_t	   *mySem;
+
+	for (;;)
+	{
+		semKey = nextSemKey++;
+
+		snprintf(semname, sizeof(semname), "/pgsql-%d", semKey);
+
+		mySem = sem_open(semname, O_CREAT | O_EXCL,
+						 (mode_t) IPCProtection, (unsigned) 1);
+
+#ifdef SEM_FAILED
+		if (mySem != (sem_t *) SEM_FAILED)
+			break;
+#else
+		if (mySem != (sem_t *) (-1))
+			break;
+#endif
+
+		/* Loop if error indicates a collision */
+		if (errno == EEXIST || errno == EACCES || errno == EINTR)
+			continue;
+
+		/*
+		 * Else complain and abort
+		 */
+		elog(FATAL, "sem_open(\"%s\") failed: %m", semname);
+	}
+
+	/*
+	 * Unlink the semaphore immediately, so it can't be accessed externally.
+	 * This also ensures that it will go away if we crash.
+	 */
+	sem_unlink(semname);
+
+	return mySem;
+}
+#else							/* !USE_NAMED_POSIX_SEMAPHORES */
+
+/*
+ * PosixSemaphoreCreate
+ *
+ * Attempt to create a new unnamed semaphore.
+ */
+static void
+PosixSemaphoreCreate(sem_t *sem)
+{
+	if (sem_init(sem, 1, 1) < 0)
+		elog(FATAL, "sem_init failed: %m");
+}
+#endif							/* USE_NAMED_POSIX_SEMAPHORES */
+
+
+/*
+ * PosixSemaphoreKill	- removes a semaphore
+ */
+static void
+PosixSemaphoreKill(sem_t *sem)
+{
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+	/* Got to use sem_close for named semaphores */
+	if (sem_close(sem) < 0)
+		elog(LOG, "sem_close failed: %m");
+#else
+	/* Got to use sem_destroy for unnamed semaphores */
+	if (sem_destroy(sem) < 0)
+		elog(LOG, "sem_destroy failed: %m");
+#endif
+}
+
+
+/*
+ * Report amount of shared memory needed for semaphores
+ */
+Size
+PGSemaphoreShmemSize(int maxSemas)
+{
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+	/* No shared memory needed in this case */
+	return 0;
+#else
+	/* Need a PGSemaphoreData per semaphore */
+	return mul_size(maxSemas, sizeof(PGSemaphoreData));
+#endif
+}
+
+/*
+ * PGReserveSemaphores --- initialize semaphore support
+ *
+ * This is called during postmaster start or shared memory reinitialization.
+ * It should do whatever is needed to be able to support up to maxSemas
+ * subsequent PGSemaphoreCreate calls.  Also, if any system resources
+ * are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
+ * callback to release them.
+ *
+ * In the Posix implementation, we acquire semaphores on-demand; the
+ * maxSemas parameter is just used to size the arrays.  For unnamed
+ * semaphores, there is an array of PGSemaphoreData structs in shared memory.
+ * For named semaphores, we keep a postmaster-local array of sem_t pointers,
+ * which we use for releasing the semaphores when done.
+ * (This design minimizes the dependency of postmaster shutdown on the
+ * contents of shared memory, which a failed backend might have clobbered.
+ * We can't do much about the possibility of sem_destroy() crashing, but
+ * we don't have to expose the counters to other processes.)
+ */
+void
+PGReserveSemaphores(int maxSemas)
+{
+	struct stat statbuf;
+
+	/*
+	 * We use the data directory's inode number to seed the search for free
+	 * semaphore keys.  This minimizes the odds of collision with other
+	 * postmasters, while maximizing the odds that we will detect and clean up
+	 * semaphores left over from a crashed postmaster in our own directory.
+	 */
+	if (stat(DataDir, &statbuf) < 0)
+		ereport(FATAL,
+				(errcode_for_file_access(),
+				 errmsg("could not stat data directory \"%s\": %m",
+						DataDir)));
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+	mySemPointers = (sem_t **) malloc(maxSemas * sizeof(sem_t *));
+	if (mySemPointers == NULL)
+		elog(PANIC, "out of memory");
+#else
+
+	/*
+	 * We must use ShmemAllocUnlocked(), since the spinlock protecting
+	 * ShmemAlloc() won't be ready yet.  (This ordering is necessary when we
+	 * are emulating spinlocks with semaphores.)
+	 */
+	sharedSemas = (PGSemaphore)
+		ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
+#endif
+
+	numSems = 0;
+	maxSems = maxSemas;
+	nextSemKey = statbuf.st_ino;
+
+	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 status, Datum arg)
+{
+	int			i;
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+	for (i = 0; i < numSems; i++)
+		PosixSemaphoreKill(mySemPointers[i]);
+	free(mySemPointers);
+#endif
+
+#ifdef USE_UNNAMED_POSIX_SEMAPHORES
+	for (i = 0; i < numSems; i++)
+		PosixSemaphoreKill(PG_SEM_REF(sharedSemas + i));
+#endif
+}
+
+/*
+ * PGSemaphoreCreate
+ *
+ * Allocate a PGSemaphore structure with initial count 1
+ */
+PGSemaphore
+PGSemaphoreCreate(void)
+{
+	PGSemaphore sema;
+	sem_t	   *newsem;
+
+	/* Can't do this in a backend, because static state is postmaster's */
+	Assert(!IsUnderPostmaster);
+
+	if (numSems >= maxSems)
+		elog(PANIC, "too many semaphores created");
+
+#ifdef USE_NAMED_POSIX_SEMAPHORES
+	newsem = PosixSemaphoreCreate();
+	/* Remember new sema for ReleaseSemaphores */
+	mySemPointers[numSems] = newsem;
+	sema = (PGSemaphore) newsem;
+#else
+	sema = &sharedSemas[numSems];
+	newsem = PG_SEM_REF(sema);
+	PosixSemaphoreCreate(newsem);
+#endif
+
+	numSems++;
+
+	return sema;
+}
+
+/*
+ * PGSemaphoreReset
+ *
+ * Reset a previously-initialized PGSemaphore to have count 0
+ */
+void
+PGSemaphoreReset(PGSemaphore sema)
+{
+	/*
+	 * There's no direct API for this in POSIX, so we have to ratchet the
+	 * semaphore down to 0 with repeated trywait's.
+	 */
+	for (;;)
+	{
+		if (sem_trywait(PG_SEM_REF(sema)) < 0)
+		{
+			if (errno == EAGAIN || errno == EDEADLK)
+				break;			/* got it down to 0 */
+			if (errno == EINTR)
+				continue;		/* can this happen? */
+			elog(FATAL, "sem_trywait failed: %m");
+		}
+	}
+}
+
+/*
+ * PGSemaphoreLock
+ *
+ * Lock a semaphore (decrement count), blocking if count would be < 0
+ */
+void
+PGSemaphoreLock(PGSemaphore sema)
+{
+	int			errStatus;
+
+	/* See notes in sysv_sema.c's implementation of PGSemaphoreLock. */
+	do
+	{
+		errStatus = sem_wait(PG_SEM_REF(sema));
+	} while (errStatus < 0 && errno == EINTR);
+
+	if (errStatus < 0)
+		elog(FATAL, "sem_wait failed: %m");
+}
+
+/*
+ * PGSemaphoreUnlock
+ *
+ * Unlock a semaphore (increment count)
+ */
+void
+PGSemaphoreUnlock(PGSemaphore sema)
+{
+	int			errStatus;
+
+	/*
+	 * Note: if errStatus is -1 and errno == EINTR then it means we returned
+	 * from the operation prematurely because we were sent a signal.  So we
+	 * try and unlock the semaphore again. Not clear this can really happen,
+	 * but might as well cope.
+	 */
+	do
+	{
+		errStatus = sem_post(PG_SEM_REF(sema));
+	} while (errStatus < 0 && errno == EINTR);
+
+	if (errStatus < 0)
+		elog(FATAL, "sem_post failed: %m");
+}
+
+/*
+ * PGSemaphoreTryLock
+ *
+ * Lock a semaphore only if able to do so without blocking
+ */
+bool
+PGSemaphoreTryLock(PGSemaphore sema)
+{
+	int			errStatus;
+
+	/*
+	 * Note: if errStatus is -1 and errno == EINTR then it means we returned
+	 * from the operation prematurely because we were sent a signal.  So we
+	 * try and lock the semaphore again.
+	 */
+	do
+	{
+		errStatus = sem_trywait(PG_SEM_REF(sema));
+	} while (errStatus < 0 && errno == EINTR);
+
+	if (errStatus < 0)
+	{
+		if (errno == EAGAIN || errno == EDEADLK)
+			return false;		/* failed to lock it */
+		/* Otherwise we got trouble */
+		elog(FATAL, "sem_trywait failed: %m");
+	}
+
+	return true;
+}