Put in place some defenses against being fooled by accidental match of

shared memory segment ID.  If we can't access the existing shmem segment,
it must not be relevant to our data directory.  If we can access it,
then attach to it and check for an actual match to the data directory.
This should avoid some cases of failure-to-restart-after-boot without
introducing any significant risk of failing to detect a still-running
old backend.

src/backend/port/sysv_shmem.c

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.39 2004/10/13 01:25:11 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.40 2004/11/09 21:30:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -20,6 +20,7 @@
 #include <signal.h>
 #include <unistd.h>
 #include <sys/file.h>
+#include <sys/stat.h>
 #ifdef HAVE_SYS_IPC_H
 #include <sys/ipc.h>
 #endif
@@ -40,6 +41,12 @@ 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;
@@ -135,16 +142,10 @@ InternalIpcMemoryCreate(IpcMemoryKey memKey, uint32 size)
 	on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));
 
 	/* OK, should be able to attach to the segment */
-#ifdef SHM_SHARE_MMU
-	/* use intimate shared memory on Solaris */
-	memAddress = shmat(shmid, 0, SHM_SHARE_MMU);
-#else
-
 #ifdef EXEC_BACKEND
-	memAddress = shmat(shmid, UsedShmemSegAddr, 0);
+	memAddress = shmat(shmid, UsedShmemSegAddr, PG_SHMAT_FLAGS);
 #else
-	memAddress = shmat(shmid, NULL, 0);
-#endif
+	memAddress = shmat(shmid, NULL, PG_SHMAT_FLAGS);
 #endif
 
 	if (memAddress == (void *) -1)
@@ -188,20 +189,26 @@ IpcMemoryDelete(int status, Datum shmId)
  * 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;
+#ifndef WIN32
+	struct stat statbuf;
+	PGShmemHeader *hdr;
+#endif
 
 	/*
 	 * We detect whether a shared memory segment is in use by seeing
 	 * whether it (a) exists and (b) has any processes are attached to it.
-	 *
-	 * If we are unable to perform the stat operation for a reason other than
-	 * nonexistence of the segment (most likely, because it doesn't belong
-	 * to our userid), assume it is in use.
 	 */
 	if (shmctl(shmId, IPC_STAT, &shmStat) < 0)
 	{
@@ -212,13 +219,58 @@ PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
 		 */
 		if (errno == EINVAL)
 			return false;
-		/* Else assume segment is in use */
+		/*
+		 * 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;
+		/*
+		 * 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 attached processes, it's in use */
-	if (shmStat.shm_nattch != 0)
-		return true;
-	return false;
+
+	/* 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.  On Windows, which doesn't
+	 * have useful inode numbers, we can't do this so we punt and assume there
+	 * is a conflict.
+	 */
+#ifndef WIN32
+	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);
+#endif
+
+	return true;
 }
 
 
@@ -247,6 +299,9 @@ PGSharedMemoryCreate(uint32 size, bool makePrivate, int port)
 	void	   *memAddress;
 	PGShmemHeader *hdr;
 	IpcMemoryId shmid;
+#ifndef WIN32
+	struct stat statbuf;
+#endif
 
 #ifdef EXEC_BACKEND
 	/* If Exec case, just attach and return the pointer */
@@ -345,6 +400,17 @@ PGSharedMemoryCreate(uint32 size, bool makePrivate, int port)
 	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.
 	 */
@@ -397,15 +463,7 @@ PGSharedMemoryAttach(IpcMemoryKey key, IpcMemoryId *shmid)
 	if ((*shmid = shmget(key, sizeof(PGShmemHeader), 0)) < 0)
 		return NULL;
 
-	hdr = (PGShmemHeader *) shmat(*shmid,
-								  UsedShmemSegAddr,
-#ifdef SHM_SHARE_MMU
-	/* use intimate shared memory on Solaris */
-								  SHM_SHARE_MMU
-#else
-								  0
-#endif
-		);
+	hdr = (PGShmemHeader *) shmat(*shmid, UsedShmemSegAddr, PG_SHMAT_FLAGS);
 
 	if (hdr == (PGShmemHeader *) -1)
 		return NULL;			/* failed: must be some other app's */

src/include/storage/pg_shmem.h

@@ -17,7 +17,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/pg_shmem.h,v 1.11 2004/08/29 04:13:10 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/storage/pg_shmem.h,v 1.12 2004/11/09 21:30:18 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -27,10 +27,14 @@
 typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
 {
 	int32		magic;			/* magic # to identify Postgres segments */
-#define PGShmemMagic  679834892
+#define PGShmemMagic  679834893
 	pid_t		creatorPID;		/* PID of creating process */
 	uint32		totalsize;		/* total size of segment */
 	uint32		freeoffset;		/* offset to first free space */
+#ifndef WIN32					/* Windows doesn't have useful inode#s */
+	dev_t		device;			/* device data directory is on */
+	ino_t		inode;			/* inode number of data directory */
+#endif
 } PGShmemHeader;