sysv_shmem.c

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

		return true;			/* if can't stat, be conservative */

	hdr = (PGShmemHeader *) shmat(shmId, NULL, PG_SHMAT_FLAGS);

	if (hdr == (PGShmemHeader *) -1)
		return true;			/* if can't attach, be conservative */

	if (hdr->magic != PGShmemMagic ||
		hdr->device != statbuf.st_dev ||
		hdr->inode != statbuf.st_ino)
	{
		/*
		 * It's either not a Postgres segment, or not one for my data
		 * directory.  In either case it poses no threat.
		 */
		shmdt((void *) hdr);
		return false;
	}

	/* Trouble --- looks a lot like there's still live backends */
	shmdt((void *) hdr);
#endif

	return true;
}


/*
 * PGSharedMemoryCreate
 *
 * Create a shared memory segment of the given size and initialize its
 * standard header.  Also, register an on_shmem_exit callback to release
 * the storage.
 *
 * Dead Postgres segments are recycled if found, but we do not fail upon
 * collision with non-Postgres shmem segments.	The idea here is to detect and
 * re-use keys that may have been assigned by a crashed postmaster or backend.
 *
 * makePrivate means to always create a new segment, rather than attach to
 * or recycle any existing segment.
 *
 * The port number is passed for possible use as a key (for SysV, we use
 * it to generate the starting shmem key).	In a standalone backend,
 * zero will be passed.
 */
PGShmemHeader *
PGSharedMemoryCreate(Size size, bool makePrivate, int port)
{
	IpcMemoryKey NextShmemSegID;
	void	   *memAddress;
	PGShmemHeader *hdr;
	IpcMemoryId shmid;

#ifndef WIN32
	struct stat statbuf;
#endif

	/* Room for a header? */
	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));

	/* Make sure PGSharedMemoryAttach doesn't fail without need */
	UsedShmemSegAddr = NULL;

	/* Loop till we find a free IPC key */
	NextShmemSegID = port * 1000;

	for (NextShmemSegID++;; NextShmemSegID++)
	{
		/* Try to create new segment */
		memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
		if (memAddress)
			break;				/* successful create and attach */

		/* Check shared memory and possibly remove and recreate */

		if (makePrivate)		/* a standalone backend shouldn't do this */
			continue;

		if ((memAddress = PGSharedMemoryAttach(NextShmemSegID, &shmid)) == NULL)
			continue;			/* can't attach, not one of mine */

		/*
		 * If I am not the creator and it belongs to an extant process,
		 * continue.
		 */
		hdr = (PGShmemHeader *) memAddress;
		if (hdr->creatorPID != getpid())
		{
			if (kill(hdr->creatorPID, 0) == 0 || errno != ESRCH)
			{
				shmdt(memAddress);
				continue;		/* segment belongs to a live process */
			}
		}

		/*
		 * The segment appears to be from a dead Postgres process, or from a
		 * previous cycle of life in this same process.  Zap it, if possible.
		 * This probably shouldn't fail, but if it does, assume the segment
		 * belongs to someone else after all, and continue quietly.
		 */
		shmdt(memAddress);
		if (shmctl(shmid, IPC_RMID, NULL) < 0)
			continue;

		/*
		 * Now try again to create the segment.
		 */
		memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
		if (memAddress)
			break;				/* successful create and attach */

		/*
		 * Can only get here if some other process managed to create the same
		 * shmem key before we did.  Let him have that one, loop around to try
		 * next key.
		 */
	}

	/*
	 * OK, we created a new segment.  Mark it as created by this process. The
	 * order of assignments here is critical so that another Postgres process
	 * can't see the header as valid but belonging to an invalid PID!
	 */
	hdr = (PGShmemHeader *) memAddress;
	hdr->creatorPID = getpid();
	hdr->magic = PGShmemMagic;

#ifndef WIN32
	/* Fill in the data directory ID info, too */
	if (stat(DataDir, &statbuf) < 0)
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not stat data directory \"%s\": %m",
						DataDir)));
	hdr->device = statbuf.st_dev;
	hdr->inode = statbuf.st_ino;
#endif

	/*
	 * Initialize space allocation status for segment.
	 */
	hdr->totalsize = size;
	hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));

	/* Save info for possible future use */
	UsedShmemSegAddr = memAddress;
	UsedShmemSegID = (unsigned long) NextShmemSegID;

	return hdr;
}

#ifdef EXEC_BACKEND

/*
 * PGSharedMemoryReAttach
 *
 * Re-attach to an already existing shared memory segment.	In the non
 * EXEC_BACKEND case this is not used, because postmaster children inherit
 * the shared memory segment attachment via fork().
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryReAttach(void)
{
	IpcMemoryId shmid;
	void	   *hdr;
	void	   *origUsedShmemSegAddr = UsedShmemSegAddr;

	Assert(UsedShmemSegAddr != NULL);
	Assert(IsUnderPostmaster);

#ifdef __CYGWIN__
	/* cygipc (currently) appears to not detach on exec. */
	PGSharedMemoryDetach();
	UsedShmemSegAddr = origUsedShmemSegAddr;
#endif

	elog(DEBUG3, "Attaching to %p", UsedShmemSegAddr);
	hdr = (void *) PGSharedMemoryAttach((IpcMemoryKey) UsedShmemSegID, &shmid);
	if (hdr == NULL)
		elog(FATAL, "could not reattach to shared memory (key=%d, addr=%p): %m",
			 (int) UsedShmemSegID, UsedShmemSegAddr);
	if (hdr != origUsedShmemSegAddr)
		elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
			 hdr, origUsedShmemSegAddr);

	UsedShmemSegAddr = hdr;		/* probably redundant */
}
#endif   /* EXEC_BACKEND */

/*
 * PGSharedMemoryDetach
 *
 * Detach from the shared memory segment, if still attached.  This is not
 * intended for use by the process that originally created the segment
 * (it will have an on_shmem_exit callback registered to do that).	Rather,
 * this is for subprocesses that have inherited an attachment and want to
 * get rid of it.
 */
void
PGSharedMemoryDetach(void)
{
	if (UsedShmemSegAddr != NULL)
	{
		if ((shmdt(UsedShmemSegAddr) < 0)
#if defined(EXEC_BACKEND) && defined(__CYGWIN__)
		/* Work-around for cygipc exec bug */
			&& shmdt(NULL) < 0
#endif
			)
			elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
		UsedShmemSegAddr = NULL;
	}
}


/*
 * Attach to shared memory and make sure it has a Postgres header
 *
 * Returns attach address if OK, else NULL
 */
static PGShmemHeader *
PGSharedMemoryAttach(IpcMemoryKey key, IpcMemoryId *shmid)
{
	PGShmemHeader *hdr;

	if ((*shmid = shmget(key, sizeof(PGShmemHeader), 0)) < 0)
		return NULL;

	hdr = (PGShmemHeader *) shmat(*shmid, UsedShmemSegAddr, PG_SHMAT_FLAGS);

	if (hdr == (PGShmemHeader *) -1)
		return NULL;			/* failed: must be some other app's */

	if (hdr->magic != PGShmemMagic)
	{
		shmdt((void *) hdr);
		return NULL;			/* segment belongs to a non-Postgres app */
	}

	return hdr;
}