src/backend/port/sysv_shmem.c

/*-------------------------------------------------------------------------
 *
 * sysv_shmem.c
 *	  Implement shared memory using SysV facilities
 *
 * These routines represent a fairly thin layer on top of SysV shared
 * memory functionality.
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.54 2009/01/01 17:23:46 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/stat.h>
#ifdef HAVE_SYS_IPC_H
#include <sys/ipc.h>
#endif
#ifdef HAVE_SYS_SHM_H
#include <sys/shm.h>
#endif
#ifdef HAVE_KERNEL_OS_H
#include <kernel/OS.h>
#endif

#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"


typedef key_t IpcMemoryKey;		/* shared memory key passed to shmget(2) */
typedef int IpcMemoryId;		/* shared memory ID returned by shmget(2) */

#define IPCProtection	(0600)	/* access/modify by user only */

#ifdef SHM_SHARE_MMU			/* use intimate shared memory on Solaris */
#define PG_SHMAT_FLAGS			SHM_SHARE_MMU
#else
#define PG_SHMAT_FLAGS			0
#endif


unsigned long UsedShmemSegID = 0;
void	   *UsedShmemSegAddr = NULL;

static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
static void IpcMemoryDetach(int status, Datum shmaddr);
static void IpcMemoryDelete(int status, Datum shmId);
static PGShmemHeader *PGSharedMemoryAttach(IpcMemoryKey key,
					 IpcMemoryId *shmid);


/*
 *	InternalIpcMemoryCreate(memKey, size)
 *
 * Attempt to create a new shared memory segment with the specified key.
 * Will fail (return NULL) if such a segment already exists.  If successful,
 * attach the segment to the current process and return its attached address.
 * On success, callbacks are registered with on_shmem_exit to detach and
 * delete the segment when on_shmem_exit is called.
 *
 * If we fail with a failure code other than collision-with-existing-segment,
 * print out an error and abort.  Other types of errors are not recoverable.
 */
static void *
InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
{
	IpcMemoryId shmid;
	void	   *memAddress;

	shmid = shmget(memKey, size, IPC_CREAT | IPC_EXCL | IPCProtection);

	if (shmid < 0)
	{
		int			shmget_errno = errno;

		/*
		 * Fail quietly if error indicates a collision with existing segment.
		 * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
		 * we could get a permission violation instead?  Also, EIDRM might
		 * occur if an old seg is slated for destruction but not gone yet.
		 */
		if (shmget_errno == EEXIST || shmget_errno == EACCES
#ifdef EIDRM
			|| shmget_errno == EIDRM
#endif
			)
			return NULL;

		/*
		 * Some BSD-derived kernels are known to return EINVAL, not EEXIST, if
		 * there is an existing segment but it's smaller than "size" (this is
		 * a result of poorly-thought-out ordering of error tests). To
		 * distinguish between collision and invalid size in such cases, we
		 * make a second try with size = 0.  These kernels do not test size
		 * against SHMMIN in the preexisting-segment case, so we will not get
		 * EINVAL a second time if there is such a segment.
		 */
		if (shmget_errno == EINVAL)
		{
			shmid = shmget(memKey, 0, IPC_CREAT | IPC_EXCL | IPCProtection);

			if (shmid < 0)
			{
				/* As above, fail quietly if we verify a collision */
				if (errno == EEXIST || errno == EACCES
#ifdef EIDRM
					|| errno == EIDRM
#endif
					)
					return NULL;
				/* Otherwise, fall through to report the original error */
			}
			else
			{
				/*
				 * On most platforms we cannot get here because SHMMIN is
				 * greater than zero.  However, if we do succeed in creating a
				 * zero-size segment, free it and then fall through to report
				 * the original error.
				 */
				if (shmctl(shmid, IPC_RMID, NULL) < 0)
					elog(LOG, "shmctl(%d, %d, 0) failed: %m",
						 (int) shmid, IPC_RMID);
			}
		}

		/*
		 * Else complain and abort.
		 *
		 * Note: at this point EINVAL should mean that either SHMMIN or SHMMAX
		 * is violated.  SHMALL violation might be reported as either ENOMEM
		 * (BSDen) or ENOSPC (Linux); the Single Unix Spec fails to say which
		 * it should be.  SHMMNI violation is ENOSPC, per spec.  Just plain
		 * not-enough-RAM is ENOMEM.
		 */
		errno = shmget_errno;
		ereport(FATAL,
				(errmsg("could not create shared memory segment: %m"),
		  errdetail("Failed system call was shmget(key=%lu, size=%lu, 0%o).",
					(unsigned long) memKey, (unsigned long) size,
					IPC_CREAT | IPC_EXCL | IPCProtection),
				 (shmget_errno == EINVAL) ?
				 errhint("This error usually means that PostgreSQL's request for a shared memory "
		  "segment exceeded your kernel's SHMMAX parameter.  You can either "
						 "reduce the request size or reconfigure the kernel with larger SHMMAX.  "
				  "To reduce the request size (currently %lu bytes), reduce "
			   "PostgreSQL's shared_buffers parameter (currently %d) and/or "
						 "its max_connections parameter (currently %d).\n"
						 "If the request size is already small, it's possible that it is less than "
						 "your kernel's SHMMIN parameter, in which case raising the request size or "
						 "reconfiguring SHMMIN is called for.\n"
		"The PostgreSQL documentation contains more information about shared "
						 "memory configuration.",
						 (unsigned long) size, NBuffers, MaxBackends) : 0,
				 (shmget_errno == ENOMEM) ?
				 errhint("This error usually means that PostgreSQL's request for a shared "
				   "memory segment exceeded available memory or swap space. "
				  "To reduce the request size (currently %lu bytes), reduce "
			   "PostgreSQL's shared_buffers parameter (currently %d) and/or "
						 "its max_connections parameter (currently %d).\n"
		"The PostgreSQL documentation contains more information about shared "
						 "memory configuration.",
						 (unsigned long) size, NBuffers, MaxBackends) : 0,
				 (shmget_errno == ENOSPC) ?
				 errhint("This error does *not* mean that you have run out of disk space. "
						 "It occurs either if all available shared memory IDs have been taken, "
						 "in which case you need to raise the SHMMNI parameter in your kernel, "
		  "or because the system's overall limit for shared memory has been "
				 "reached.  If you cannot increase the shared memory limit, "
		  "reduce PostgreSQL's shared memory request (currently %lu bytes), "
			"by reducing its shared_buffers parameter (currently %d) and/or "
						 "its max_connections parameter (currently %d).\n"
		"The PostgreSQL documentation contains more information about shared "
						 "memory configuration.",
						 (unsigned long) size, NBuffers, MaxBackends) : 0));
	}

	/* Register on-exit routine to delete the new segment */
	on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));

	/* OK, should be able to attach to the segment */
	memAddress = shmat(shmid, NULL, PG_SHMAT_FLAGS);

	if (memAddress == (void *) -1)
		elog(FATAL, "shmat(id=%d) failed: %m", shmid);

	/* Register on-exit routine to detach new segment before deleting */
	on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));

	/* Record key and ID in lockfile for data directory. */
	RecordSharedMemoryInLockFile((unsigned long) memKey,
								 (unsigned long) shmid);

	return memAddress;
}

/****************************************************************************/
/*	IpcMemoryDetach(status, shmaddr)	removes a shared memory segment		*/
/*										from process' address spaceq		*/
/*	(called as an on_shmem_exit callback, hence funny argument list)		*/
/****************************************************************************/
static void
IpcMemoryDetach(int status, Datum shmaddr)
{
	if (shmdt(DatumGetPointer(shmaddr)) < 0)
		elog(LOG, "shmdt(%p) failed: %m", DatumGetPointer(shmaddr));
}

/****************************************************************************/
/*	IpcMemoryDelete(status, shmId)		deletes a shared memory segment		*/
/*	(called as an on_shmem_exit callback, hence funny argument list)		*/
/****************************************************************************/
static void
IpcMemoryDelete(int status, Datum shmId)
{
	if (shmctl(DatumGetInt32(shmId), IPC_RMID, NULL) < 0)
		elog(LOG, "shmctl(%d, %d, 0) failed: %m",
			 DatumGetInt32(shmId), IPC_RMID);
}

/*
 * PGSharedMemoryIsInUse
 *
 * Is a previously-existing shmem segment still existing and in use?
 *
 * The point of this exercise is to detect the case where a prior postmaster
 * crashed, but it left child backends that are still running.	Therefore
 * we only care about shmem segments that are associated with the intended
 * DataDir.  This is an important consideration since accidental matches of
 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
	IpcMemoryId shmId = (IpcMemoryId) id2;
	struct shmid_ds shmStat;
	struct stat statbuf;
	PGShmemHeader *hdr;

	/*
	 * We detect whether a shared memory segment is in use by seeing whether
	 * it (a) exists and (b) has any processes attached to it.
	 */
	if (shmctl(shmId, IPC_STAT, &shmStat) < 0)
	{
		/*
		 * EINVAL actually has multiple possible causes documented in the
		 * shmctl man page, but we assume it must mean the segment no longer
		 * exists.
		 */
		if (errno == EINVAL)
			return false;

		/*
		 * EACCES implies that the segment belongs to some other userid, which
		 * means it is not a Postgres shmem segment (or at least, not one that
		 * is relevant to our data directory).
		 */
		if (errno == EACCES)
			return false;

		/*
		 * Some Linux kernel versions (in fact, all of them as of July 2007)
		 * sometimes return EIDRM when EINVAL is correct.  The Linux kernel
		 * actually does not have any internal state that would justify
		 * returning EIDRM, so we can get away with assuming that EIDRM is
		 * equivalent to EINVAL on that platform.
		 */
#ifdef HAVE_LINUX_EIDRM_BUG
		if (errno == EIDRM)
			return false;
#endif

		/*
		 * Otherwise, we had better assume that the segment is in use. The
		 * only likely case is EIDRM, which implies that the segment has been
		 * IPC_RMID'd but there are still processes attached to it.
		 */
		return true;
	}

	/* If it has no attached processes, it's not in use */
	if (shmStat.shm_nattch == 0)
		return false;

	/*
	 * Try to attach to the segment and see if it matches our data directory.
	 * This avoids shmid-conflict problems on machines that are running
	 * several postmasters under the same userid.
	 */
	if (stat(DataDir, &statbuf) < 0)
		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);

	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;
	struct stat statbuf;

	/* 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;

	/* 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;

	/*
	 * 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;
}