Use data directory inode number, not port, to select SysV resource keys.

This approach provides a much tighter binding between a data directory
and the associated SysV shared memory block (and SysV or named-POSIX
semaphores, if we're using those).  Key collisions are still possible,
but only between data directories stored on different filesystems,
so the situation should be negligible in practice.  More importantly,
restarting the postmaster with a different port number no longer
risks failing to identify a relevant shared memory block, even when
postmaster.pid has been removed.  A standalone backend is likewise
much more certain to detect conflicting leftover backends.

(In the longer term, we might now think about deprecating the port as
a cluster-wide value, so that one postmaster could support sockets
with varying port numbers.  But that's for another day.)

The hazards fixed here apply only on Unix systems; our Windows code
paths already use identifiers derived from the data directory path
name rather than the port.

src/test/recovery/t/017_shm.pl, which intends to test key-collision
cases, has been substantially rewritten since it can no longer use
two postmasters with identical port numbers to trigger the case.
Instead, use Perl's IPC::SharedMem module to create a conflicting
shmem segment directly.  The test script will be skipped if that
module is not available.  (This means that some older buildfarm
members won't run it, but I don't think that that results in any
meaningful coverage loss.)

Patch by me; thanks to Noah Misch and Peter Eisentraut for discussion
and review.

Discussion: https://postgr.es/m/16908.1557521200@sss.pgh.pa.us

1 files changed, 21 insertions, 17 deletions

diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 968506dd516..dab2920ad61 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -390,11 +390,12 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 
 	/*
 	 * Try to attach to the segment and see if it matches our data directory.
-	 * This avoids key-conflict problems on machines that are running several
-	 * postmasters under the same userid and port number.  (That would not
-	 * ordinarily happen in production, but it can happen during parallel
-	 * testing.  Since our test setups don't open any TCP ports on Unix, such
-	 * cases don't conflict otherwise.)
+	 * 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 */
@@ -617,12 +618,9 @@ AnonymousShmemDetach(int status, Datum arg)
  * 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.
- *
- * The port number is passed for possible use as a key (for SysV, we use
- * it to generate the starting shmem key).
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size, int port,
+PGSharedMemoryCreate(Size size,
 					 PGShmemHeader **shim)
 {
 	IpcMemoryKey NextShmemSegID;
@@ -631,6 +629,17 @@ PGSharedMemoryCreate(Size size, int port,
 	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)
@@ -659,10 +668,10 @@ PGSharedMemoryCreate(Size size, int port,
 	/*
 	 * 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 ensure that,
-	 * but prefer fixing it over coping here.)
+	 * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
+	 * that, but prefer fixing it over coping here.)
 	 */
-	NextShmemSegID = 1 + port * 1000;
+	NextShmemSegID = statbuf.st_ino;
 
 	for (;;)
 	{
@@ -748,11 +757,6 @@ PGSharedMemoryCreate(Size size, int port,
 	hdr->dsm_control = 0;
 
 	/* 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;