text: /*-------------------------------------------------------------------------
   *
   * sysv_shmem.c
   *	  Implement shared memory using SysV facilities
   *
   * These routines used to be a fairly thin layer on top of SysV shared
   * memory functionality.  With the addition of anonymous-shmem logic,
   * they're a bit fatter now.  We still require a SysV shmem block to
   * exist, though, because mmap'd shmem provides no way to find out how
   * many processes are attached, which we need for interlocking purposes.
   *
   * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
   *	  src/backend/port/sysv_shmem.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <signal.h>
  #include <unistd.h>
  #include <sys/file.h>
  #include <sys/mman.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
  
  #include "miscadmin.h"
  #include "port/pg_bitutils.h"
  #include "portability/mem.h"
  #include "storage/dsm.h"
  #include "storage/fd.h"
  #include "storage/ipc.h"
  #include "storage/pg_shmem.h"
  #include "utils/guc.h"
  #include "utils/pidfile.h"
  
  
  /*
   * As of PostgreSQL 9.3, we normally allocate only a very small amount of
   * System V shared memory, and only for the purposes of providing an
   * interlock to protect the data directory.  The real shared memory block
   * is allocated using mmap().  This works around the problem that many
   * systems have very low limits on the amount of System V shared memory
   * that can be allocated.  Even a limit of a few megabytes will be enough
   * to run many copies of PostgreSQL without needing to adjust system settings.
   *
   * We assume that no one will attempt to run PostgreSQL 9.3 or later on
   * systems that are ancient enough that anonymous shared memory is not
   * supported, such as pre-2.4 versions of Linux.  If that turns out to be
   * false, we might need to add compile and/or run-time tests here and do this
   * only if the running kernel supports it.
   *
   * However, we must always disable this logic in the EXEC_BACKEND case, and
   * fall back to the old method of allocating the entire segment using System V
   * shared memory, because there's no way to attach an anonymous mmap'd segment
   * to a process after exec().  Since EXEC_BACKEND is intended only for
   * developer use, this shouldn't be a big problem.  Because of this, we do
   * not worry about supporting anonymous shmem in the EXEC_BACKEND cases below.
   *
   * As of PostgreSQL 12, we regained the ability to use a large System V shared
   * memory region even in non-EXEC_BACKEND builds, if shared_memory_type is set
   * to sysv (though this is not the default).
   */
  
  
  typedef key_t IpcMemoryKey;		/* shared memory key passed to shmget(2) */
  typedef int IpcMemoryId;		/* shared memory ID returned by shmget(2) */
  
  /*
   * How does a given IpcMemoryId relate to this PostgreSQL process?
   *
   * One could recycle unattached segments of different data directories if we
   * distinguished that case from other SHMSTATE_FOREIGN cases.  Doing so would
   * cause us to visit less of the key space, making us less likely to detect a
   * SHMSTATE_ATTACHED key.  It would also complicate the concurrency analysis,
   * in that postmasters of different data directories could simultaneously
   * attempt to recycle a given key.  We'll waste keys longer in some cases, but
   * avoiding the problems of the alternative justifies that loss.
   */
  typedef enum
  {
  	SHMSTATE_ANALYSIS_FAILURE,	/* unexpected failure to analyze the ID */
  	SHMSTATE_ATTACHED,			/* pertinent to DataDir, has attached PIDs */
  	SHMSTATE_ENOENT,			/* no segment of that ID */
  	SHMSTATE_FOREIGN,			/* exists, but not pertinent to DataDir */
  	SHMSTATE_UNATTACHED			/* pertinent to DataDir, no attached PIDs */
  } IpcMemoryState;
  
  
  unsigned long UsedShmemSegID = 0;
  void	   *UsedShmemSegAddr = NULL;
  
  static Size AnonymousShmemSize;
  static void *AnonymousShmem = NULL;
  
  static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
  static void IpcMemoryDetach(int status, Datum shmaddr);
  static void IpcMemoryDelete(int status, Datum shmId);
  static IpcMemoryState PGSharedMemoryAttach(IpcMemoryId shmId,
  										   void *attachAt,
  										   PGShmemHeader **addr);
  
  
  /*
   *	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	   *requestedAddress = NULL;
  	void	   *memAddress;
  
  	/*
  	 * Normally we just pass requestedAddress = NULL to shmat(), allowing the
  	 * system to choose where the segment gets mapped.  But in an EXEC_BACKEND
  	 * build, it's possible for whatever is chosen in the postmaster to not
  	 * work for backends, due to variations in address space layout.  As a
  	 * rather klugy workaround, allow the user to specify the address to use
  	 * via setting the environment variable PG_SHMEM_ADDR.  (If this were of
  	 * interest for anything except debugging, we'd probably create a cleaner
  	 * and better-documented way to set it, such as a GUC.)
  	 */
  #ifdef EXEC_BACKEND
  	{
  		char	   *pg_shmem_addr = getenv("PG_SHMEM_ADDR");
  
  		if (pg_shmem_addr)
  			requestedAddress = (void *) strtoul(pg_shmem_addr, NULL, 0);
  		else
  		{
  #if defined(__darwin__) && SIZEOF_VOID_P == 8
  			/*
  			 * Provide a default value that is believed to avoid problems with
  			 * ASLR on the current macOS release.
  			 */
  			requestedAddress = (void *) 0x80000000000;
  #endif
  		}
  	}
  #endif
  
  	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=%zu, 0%o).",
  						   (unsigned long) memKey, 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, or possibly that "
  						 "it is less than "
  						 "your kernel's SHMMIN parameter.\n"
  						 "The PostgreSQL documentation contains more information about shared "
  						 "memory configuration.") : 0,
  				 (shmget_errno == ENOMEM) ?
  				 errhint("This error usually means that PostgreSQL's request for a shared "
  						 "memory segment exceeded your kernel's SHMALL parameter.  You might need "
  						 "to reconfigure the kernel with larger SHMALL.\n"
  						 "The PostgreSQL documentation contains more information about shared "
  						 "memory configuration.") : 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.\n"
  						 "The PostgreSQL documentation contains more information about shared "
  						 "memory configuration.") : 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, requestedAddress, PG_SHMAT_FLAGS);
  
  	if (memAddress == (void *) -1)
  		elog(FATAL, "shmat(id=%d, addr=%p, flags=0x%x) failed: %m",
  			 shmid, requestedAddress, PG_SHMAT_FLAGS);
  
  	/* Register on-exit routine to detach new segment before deleting */
  	on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));
  
  	/*
  	 * Store shmem key and ID in data directory lockfile.  Format to try to
  	 * keep it the same length always (trailing junk in the lockfile won't
  	 * hurt, but might confuse humans).
  	 */
  	{
  		char		line[64];
  
  		sprintf(line, "%9lu %9lu",
  				(unsigned long) memKey, (unsigned long) shmid);
  		AddToDataDirLockFile(LOCK_FILE_LINE_SHMEM_KEY, line);
  	}
  
  	return memAddress;
  }
  
  /****************************************************************************/
  /*	IpcMemoryDetach(status, shmaddr)	removes a shared memory segment		*/
  /*										from process' address space			*/
  /*	(called as an on_shmem_exit callback, hence funny argument list)		*/
  /****************************************************************************/
  static void
  IpcMemoryDetach(int status, Datum shmaddr)
  {
  	/* Detach System V shared memory block. */
  	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)
  {
  	PGShmemHeader *memAddress;
  	IpcMemoryState state;
  
  	state = PGSharedMemoryAttach((IpcMemoryId) id2, NULL, &memAddress);
  	if (memAddress && shmdt(memAddress) < 0)
  		elog(LOG, "shmdt(%p) failed: %m", memAddress);
  	switch (state)
  	{
  		case SHMSTATE_ENOENT:
  		case SHMSTATE_FOREIGN:
  		case SHMSTATE_UNATTACHED:
  			return false;
  		case SHMSTATE_ANALYSIS_FAILURE:
  		case SHMSTATE_ATTACHED:
  			return true;
  	}
  	return true;
  }
  
  /*
   * Test for a segment with id shmId; see comment at IpcMemoryState.
   *
   * If the segment exists, we'll attempt to attach to it, using attachAt
   * if that's not NULL (but it's best to pass NULL if possible).
   *
   * *addr is set to the segment memory address if we attached to it, else NULL.
   */
  static IpcMemoryState
  PGSharedMemoryAttach(IpcMemoryId shmId,
  					 void *attachAt,
  					 PGShmemHeader **addr)
  {
  	struct shmid_ds shmStat;
  	struct stat statbuf;
  	PGShmemHeader *hdr;
  
  	*addr = NULL;
  
  	/*
  	 * First, try to stat the shm segment ID, to see if it exists at all.
  	 */
  	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 SHMSTATE_ENOENT;
  
  		/*
  		 * EACCES implies we have no read permission, 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 SHMSTATE_FOREIGN;
  
  		/*
  		 * 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 SHMSTATE_ENOENT;
  #endif
  
  		/*
  		 * Otherwise, we had better assume that the segment is in use.  The
  		 * only likely case is (non-Linux, assumed spec-compliant) EIDRM,
  		 * which implies that the segment has been IPC_RMID'd but there are
  		 * still processes attached to it.
  		 */
  		return SHMSTATE_ANALYSIS_FAILURE;
  	}
  
  	/*
  	 * Try to attach to the segment and see if it matches our data directory.
  	 * This avoids any risk of duplicate-shmem-key conflicts on machines that
  	 * are running several postmasters under the same userid.
  	 *
  	 * (When we're called from PGSharedMemoryCreate, this stat call is
  	 * duplicative; but since this isn't a high-traffic case it's not worth
  	 * trying to optimize.)
  	 */
  	if (stat(DataDir, &statbuf) < 0)
  		return SHMSTATE_ANALYSIS_FAILURE;	/* can't stat; be conservative */
  
  	hdr = (PGShmemHeader *) shmat(shmId, attachAt, PG_SHMAT_FLAGS);
  	if (hdr == (PGShmemHeader *) -1)
  	{
  		/*
  		 * Attachment failed.  The cases we're interested in are the same as
  		 * for the shmctl() call above.  In particular, note that the owning
  		 * postmaster could have terminated and removed the segment between
  		 * shmctl() and shmat().
  		 *
  		 * If attachAt isn't NULL, it's possible that EINVAL reflects a
  		 * problem with that address not a vanished segment, so it's best to
  		 * pass NULL when probing for conflicting segments.
  		 */
  		if (errno == EINVAL)
  			return SHMSTATE_ENOENT; /* segment disappeared */
  		if (errno == EACCES)
  			return SHMSTATE_FOREIGN;	/* must be non-Postgres */
  #ifdef HAVE_LINUX_EIDRM_BUG
  		if (errno == EIDRM)
  			return SHMSTATE_ENOENT; /* segment disappeared */
  #endif
  		/* Otherwise, be conservative. */
  		return SHMSTATE_ANALYSIS_FAILURE;
  	}
  	*addr = hdr;
  
  	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.
  		 */
  		return SHMSTATE_FOREIGN;
  	}
  
  	/*
  	 * It does match our data directory, so now test whether any processes are
  	 * still attached to it.  (We are, now, but the shm_nattch result is from
  	 * before we attached to it.)
  	 */
  	return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
  }
  
  #ifdef MAP_HUGETLB
  
  /*
   * Identify the huge page size to use, and compute the related mmap flags.
   *
   * Some Linux kernel versions have a bug causing mmap() to fail on requests
   * that are not a multiple of the hugepage size.  Versions without that bug
   * instead silently round the request up to the next hugepage multiple ---
   * and then munmap() fails when we give it a size different from that.
   * So we have to round our request up to a multiple of the actual hugepage
   * size to avoid trouble.
   *
   * Doing the round-up ourselves also lets us make use of the extra memory,
   * rather than just wasting it.  Currently, we just increase the available
   * space recorded in the shmem header, which will make the extra usable for
   * purposes such as additional locktable entries.  Someday, for very large
   * hugepage sizes, we might want to think about more invasive strategies,
   * such as increasing shared_buffers to absorb the extra space.
   *
   * Returns the (real, assumed or config provided) page size into *hugepagesize,
   * and the hugepage-related mmap flags to use into *mmap_flags.
   */
  static void
  GetHugePageSize(Size *hugepagesize, int *mmap_flags)
  {
  	Size		default_hugepagesize = 0;
  
  	/*
  	 * System-dependent code to find out the default huge page size.
  	 *
  	 * On Linux, read /proc/meminfo looking for a line like "Hugepagesize:
  	 * nnnn kB".  Ignore any failures, falling back to the preset default.
  	 */
  #ifdef __linux__
  
  	{
  		FILE	   *fp = AllocateFile("/proc/meminfo", "r");
  		char		buf[128];
  		unsigned int sz;
  		char		ch;
  
  		if (fp)
  		{
  			while (fgets(buf, sizeof(buf), fp))
  			{
  				if (sscanf(buf, "Hugepagesize: %u %c", &sz, &ch) == 2)
  				{
  					if (ch == 'k')
  					{
  						default_hugepagesize = sz * (Size) 1024;
  						break;
  					}
  					/* We could accept other units besides kB, if needed */
  				}
  			}
  			FreeFile(fp);
  		}
  	}
  #endif							/* __linux__ */
  
  	if (huge_page_size != 0)
  	{
  		/* If huge page size is requested explicitly, use that. */
  		*hugepagesize = (Size) huge_page_size * 1024;
  	}
  	else if (default_hugepagesize != 0)
  	{
  		/* Otherwise use the system default, if we have it. */
  		*hugepagesize = default_hugepagesize;
  	}
  	else
  	{
  		/*
  		 * If we fail to find out the system's default huge page size, or no
  		 * huge page size is requested explicitly, assume it is 2MB. This will
  		 * work fine when the actual size is less.  If it's more, we might get
  		 * mmap() or munmap() failures due to unaligned requests; but at this
  		 * writing, there are no reports of any non-Linux systems being picky
  		 * about that.
  		 */
  		*hugepagesize = 2 * 1024 * 1024;
  	}
  
  	*mmap_flags = MAP_HUGETLB;
  
  	/*
  	 * On recent enough Linux, also include the explicit page size, if
  	 * necessary.
  	 */
  #if defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT)
  	if (*hugepagesize != default_hugepagesize)
  	{
  		int			shift = pg_ceil_log2_64(*hugepagesize);
  
  		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
  	}
  #endif
  }
  
  #endif							/* MAP_HUGETLB */
  
  /*
   * Creates an anonymous mmap()ed shared memory segment.
   *
   * Pass the requested size in *size.  This function will modify *size to the
   * actual size of the allocation, if it ends up allocating a segment that is
   * larger than requested.
   */
  static void *
  CreateAnonymousSegment(Size *size)
  {
  	Size		allocsize = *size;
  	void	   *ptr = MAP_FAILED;
  	int			mmap_errno = 0;
  
  #ifndef MAP_HUGETLB
  	/* PGSharedMemoryCreate should have dealt with this case */
  	Assert(huge_pages != HUGE_PAGES_ON);
  #else
  	if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
  	{
  		/*
  		 * Round up the request size to a suitable large value.
  		 */
  		Size		hugepagesize;
  		int			mmap_flags;
  
  		GetHugePageSize(&hugepagesize, &mmap_flags);
  
  		if (allocsize % hugepagesize != 0)
  			allocsize += hugepagesize - (allocsize % hugepagesize);
  
  		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
  				   PG_MMAP_FLAGS | mmap_flags, -1, 0);
  		mmap_errno = errno;
  		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
  			elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
  				 allocsize);
  	}
  #endif
  
  	if (ptr == MAP_FAILED && huge_pages != HUGE_PAGES_ON)
  	{
  		/*
  		 * Use the original size, not the rounded-up value, when falling back
  		 * to non-huge pages.
  		 */
  		allocsize = *size;
  		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
  				   PG_MMAP_FLAGS, -1, 0);
  		mmap_errno = errno;
  	}
  
  	if (ptr == MAP_FAILED)
  	{
  		errno = mmap_errno;
  		ereport(FATAL,
  				(errmsg("could not map anonymous shared memory: %m"),
  				 (mmap_errno == ENOMEM) ?
  				 errhint("This error usually means that PostgreSQL's request "
  						 "for a shared memory segment exceeded available memory, "
  						 "swap space, or huge pages. To reduce the request size "
  						 "(currently %zu bytes), reduce PostgreSQL's shared "
  						 "memory usage, perhaps by reducing shared_buffers or "
  						 "max_connections.",
  						 allocsize) : 0));
  	}
  
  	*size = allocsize;
  	return ptr;
  }
  
  /*
   * AnonymousShmemDetach --- detach from an anonymous mmap'd block
   * (called as an on_shmem_exit callback, hence funny argument list)
   */
  static void
  AnonymousShmemDetach(int status, Datum arg)
  {
  	/* Release anonymous shared memory block, if any. */
  	if (AnonymousShmem != NULL)
  	{
  		if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
  			elog(LOG, "munmap(%p, %zu) failed: %m",
  				 AnonymousShmem, AnonymousShmemSize);
  		AnonymousShmem = NULL;
  	}
  }
  
  /*
   * 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 pertinent to this DataDir are recycled if found, but
   * we do not fail upon collision with foreign shmem segments.  The idea here
   * is to detect and re-use keys that may have been assigned by a crashed
   * postmaster or backend.
   */
  PGShmemHeader *
  PGSharedMemoryCreate(Size size,
  					 PGShmemHeader **shim)
  {
  	IpcMemoryKey NextShmemSegID;
  	void	   *memAddress;
  	PGShmemHeader *hdr;
  	struct stat statbuf;
  	Size		sysvsize;
  
  	/*
  	 * We use the data directory's ID info (inode and device numbers) to
  	 * positively identify shmem segments associated with this data dir, and
  	 * also as seeds for searching for a free shmem key.
  	 */
  	if (stat(DataDir, &statbuf) < 0)
  		ereport(FATAL,
  				(errcode_for_file_access(),
  				 errmsg("could not stat data directory \"%s\": %m",
  						DataDir)));
  
  	/* Complain if hugepages demanded but we can't possibly support them */
  #if !defined(MAP_HUGETLB)
  	if (huge_pages == HUGE_PAGES_ON)
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("huge pages not supported on this platform")));
  #endif
  
  	/* For now, we don't support huge pages in SysV memory */
  	if (huge_pages == HUGE_PAGES_ON && shared_memory_type != SHMEM_TYPE_MMAP)
  		ereport(ERROR,
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("huge pages not supported with the current shared_memory_type setting")));
  
  	/* Room for a header? */
  	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
  
  	if (shared_memory_type == SHMEM_TYPE_MMAP)
  	{
  		AnonymousShmem = CreateAnonymousSegment(&size);
  		AnonymousShmemSize = size;
  
  		/* Register on-exit routine to unmap the anonymous segment */
  		on_shmem_exit(AnonymousShmemDetach, (Datum) 0);
  
  		/* Now we need only allocate a minimal-sized SysV shmem block. */
  		sysvsize = sizeof(PGShmemHeader);
  	}
  	else
  		sysvsize = size;
  
  	/*
  	 * Loop till we find a free IPC key.  Trust CreateDataDirLockFile() to
  	 * ensure no more than one postmaster per data directory can enter this
  	 * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
  	 * that, but prefer fixing it over coping here.)
  	 */
  	NextShmemSegID = statbuf.st_ino;
  
  	for (;;)
  	{
  		IpcMemoryId shmid;
  		PGShmemHeader *oldhdr;
  		IpcMemoryState state;
  
  		/* Try to create new segment */
  		memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
  		if (memAddress)
  			break;				/* successful create and attach */
  
  		/* Check shared memory and possibly remove and recreate */
  
  		/*
  		 * shmget() failure is typically EACCES, hence SHMSTATE_FOREIGN.
  		 * ENOENT, a narrow possibility, implies SHMSTATE_ENOENT, but one can
  		 * safely treat SHMSTATE_ENOENT like SHMSTATE_FOREIGN.
  		 */
  		shmid = shmget(NextShmemSegID, sizeof(PGShmemHeader), 0);
  		if (shmid < 0)
  		{
  			oldhdr = NULL;
  			state = SHMSTATE_FOREIGN;
  		}
  		else
  			state = PGSharedMemoryAttach(shmid, NULL, &oldhdr);
  
  		switch (state)
  		{
  			case SHMSTATE_ANALYSIS_FAILURE:
  			case SHMSTATE_ATTACHED:
  				ereport(FATAL,
  						(errcode(ERRCODE_LOCK_FILE_EXISTS),
  						 errmsg("pre-existing shared memory block (key %lu, ID %lu) is still in use",
  								(unsigned long) NextShmemSegID,
  								(unsigned long) shmid),
  						 errhint("Terminate any old server processes associated with data directory \"%s\".",
  								 DataDir)));
  				break;
  			case SHMSTATE_ENOENT:
  
  				/*
  				 * To our surprise, some other process deleted since our last
  				 * InternalIpcMemoryCreate().  Moments earlier, we would have
  				 * seen SHMSTATE_FOREIGN.  Try that same ID again.
  				 */
  				elog(LOG,
  					 "shared memory block (key %lu, ID %lu) deleted during startup",
  					 (unsigned long) NextShmemSegID,
  					 (unsigned long) shmid);
  				break;
  			case SHMSTATE_FOREIGN:
  				NextShmemSegID++;
  				break;
  			case SHMSTATE_UNATTACHED:
  
  				/*
  				 * The segment pertains to DataDir, and every process that had
  				 * used it has died or detached.  Zap it, if possible, and any
  				 * associated dynamic shared memory segments, as well.  This
  				 * shouldn't fail, but if it does, assume the segment belongs
  				 * to someone else after all, and try the next candidate.
  				 * Otherwise, try again to create the segment.  That may fail
  				 * if some other process creates the same shmem key before we
  				 * do, in which case we'll try the next key.
  				 */
  				if (oldhdr->dsm_control != 0)
  					dsm_cleanup_using_control_segment(oldhdr->dsm_control);
  				if (shmctl(shmid, IPC_RMID, NULL) < 0)
  					NextShmemSegID++;
  				break;
  		}
  
  		if (oldhdr && shmdt(oldhdr) < 0)
  			elog(LOG, "shmdt(%p) failed: %m", oldhdr);
  	}
  
  	/* Initialize new segment. */
  	hdr = (PGShmemHeader *) memAddress;
  	hdr->creatorPID = getpid();
  	hdr->magic = PGShmemMagic;
  	hdr->dsm_control = 0;
  
  	/* Fill in the data directory ID info, too */
  	hdr->device = statbuf.st_dev;
  	hdr->inode = statbuf.st_ino;
  
  	/*
  	 * Initialize space allocation status for segment.
  	 */
  	hdr->totalsize = size;
  	hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
  	*shim = hdr;
  
  	/* Save info for possible future use */
  	UsedShmemSegAddr = memAddress;
  	UsedShmemSegID = (unsigned long) NextShmemSegID;
  
  	/*
  	 * If AnonymousShmem is NULL here, then we're not using anonymous shared
  	 * memory, and should return a pointer to the System V shared memory
  	 * block. Otherwise, the System V shared memory block is only a shim, and
  	 * we must return a pointer to the real block.
  	 */
  	if (AnonymousShmem == NULL)
  		return hdr;
  	memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
  	return (PGShmemHeader *) AnonymousShmem;
  }
  
  #ifdef EXEC_BACKEND
  
  /*
   * PGSharedMemoryReAttach
   *
   * This is called during startup of a postmaster child process to re-attach to
   * an already existing shared memory segment.  This is needed only in the
   * EXEC_BACKEND case; otherwise 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;
  	PGShmemHeader *hdr;
  	IpcMemoryState state;
  	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);
  	shmid = shmget(UsedShmemSegID, sizeof(PGShmemHeader), 0);
  	if (shmid < 0)
  		state = SHMSTATE_FOREIGN;
  	else
  		state = PGSharedMemoryAttach(shmid, UsedShmemSegAddr, &hdr);
  	if (state != SHMSTATE_ATTACHED)
  		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);
  	dsm_set_control_handle(hdr->dsm_control);
  
  	UsedShmemSegAddr = hdr;		/* probably redundant */
  }
  
  /*
   * PGSharedMemoryNoReAttach
   *
   * This is called during startup of a postmaster child process when we choose
   * *not* to re-attach to the existing shared memory segment.  We must clean up
   * to leave things in the appropriate state.  This is not used in the non
   * EXEC_BACKEND case, either.
   *
   * The child process startup logic might or might not call PGSharedMemoryDetach
   * after this; make sure that it will be a no-op if called.
   *
   * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
   * routine.  The caller must have already restored them to the postmaster's
   * values.
   */
  void
  PGSharedMemoryNoReAttach(void)
  {
  	Assert(UsedShmemSegAddr != NULL);
  	Assert(IsUnderPostmaster);
  
  #ifdef __CYGWIN__
  	/* cygipc (currently) appears to not detach on exec. */
  	PGSharedMemoryDetach();
  #endif
  
  	/* For cleanliness, reset UsedShmemSegAddr to show we're not attached. */
  	UsedShmemSegAddr = NULL;
  	/* And the same for UsedShmemSegID. */
  	UsedShmemSegID = 0;
  }
  
  #endif							/* EXEC_BACKEND */
  
  /*
   * PGSharedMemoryDetach
   *
   * Detach from the shared memory segment, if still attached.  This is not
   * intended to be called explicitly by the process that originally created the
   * segment (it will have on_shmem_exit callback(s) registered to do that).
   * Rather, this is for subprocesses that have inherited an attachment and want
   * to get rid of it.
   *
   * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
   * routine, also AnonymousShmem and AnonymousShmemSize.
   */
  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;
  	}
  
  	if (AnonymousShmem != NULL)
  	{
  		if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
  			elog(LOG, "munmap(%p, %zu) failed: %m",
  				 AnonymousShmem, AnonymousShmemSize);
  		AnonymousShmem = NULL;
  	}
  }
  