Re: Unexpected "shared memory block is still in use"

Noah Misch <noah@leadboat.com> writes:
> On Fri, May 10, 2019 at 04:46:40PM -0400, Tom Lane wrote:
>> Done now, but while thinking more about the issue, I had an idea: why is
>> it that we base the shmem key on the postmaster's port number, and not
>> on the data directory's inode number?  Using the port number not only
>> increases the risk of collisions (though admittedly only in testing
>> situations), but it *decreases* our ability to detect real conflicts.
>> Consider case where DBA wants to change the installation's port number,
>> and he edits postgresql.conf, but then uses "kill -9 && rm postmaster.pid"
>> rather than some saner way of stopping the old postmaster.  When he
>> starts the new one, it won't detect any remaining children of the old
>> postmaster because it'll be looking in the wrong range of shmem keys.
>> It seems like something tied to the data directory's identity would
>> be much more trustworthy.

> Good point.  Since we now ignore (SHMSTATE_FOREIGN) any segment that bears
> (st_dev,st_ino) not matching $PGDATA, the change you describe couldn't make us
> fail to detect a real conflict or miss a cleanup opportunity.  It would reduce
> the ability to test sysv_shmem.c; I suppose one could add a debug GUC to
> override the start of the key space.

Attached is a draft patch to change both shmem and sema key selection
to be based on data directory inode rather than port.

I considered using "st_ino ^ st_dev", or some such, but decided that
that would largely just make it harder to manually correlate IPC
keys with running postmasters.  It's generally easy to find out the
data directory inode number with "ls", but the extra work to find out
and XOR in the device number is not so easy, and it's not clear what
it'd buy us in typical scenarios.

The Windows code seems fine as-is: it's already using data directory
name, not port, to set up shmem, and it doesn't need anything for
semaphores.

I'm not quite sure what's going on in src/test/recovery/t/017_shm.pl.
As expected, the test for port number non-collision no longer sees
a failure.  After fixing that, the test passes, but it takes a
ridiculously long time (minutes); apparently each postmaster start/stop
cycle takes much longer than it ought to.  I suppose this patch is
breaking its assumptions, but I've not studied it.  We'd have to do
something about that before this would be committable.

I'll add this to the next commitfest.

            regards, tom lane

diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c
index 3370adf..bdd6552 100644
--- a/src/backend/port/posix_sema.c
+++ b/src/backend/port/posix_sema.c
@@ -29,6 +29,7 @@
 #include <semaphore.h>
 #include <signal.h>
 #include <unistd.h>
+#include <sys/stat.h>

 #include "miscadmin.h"
 #include "storage/ipc.h"
@@ -181,10 +182,6 @@ PGSemaphoreShmemSize(int maxSemas)
  * are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
  * callback to release them.
  *
- * The port number is passed for possible use as a key (for Posix, we use
- * it to generate the starting semaphore name).  In a standalone backend,
- * zero will be passed.
- *
  * In the Posix implementation, we acquire semaphores on-demand; the
  * maxSemas parameter is just used to size the arrays.  For unnamed
  * semaphores, there is an array of PGSemaphoreData structs in shared memory.
@@ -196,8 +193,22 @@ PGSemaphoreShmemSize(int maxSemas)
  * we don't have to expose the counters to other processes.)
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
+    struct stat statbuf;
+
+    /*
+     * We use the data directory's inode number to seed the search for free
+     * semaphore keys.  This minimizes the odds of collision with other
+     * postmasters, while maximizing the odds that we will detect and clean up
+     * semaphores left over from a crashed postmaster in our own directory.
+     */
+    if (stat(DataDir, &statbuf) < 0)
+        ereport(FATAL,
+                (errcode_for_file_access(),
+                 errmsg("could not stat data directory \"%s\": %m",
+                        DataDir)));
+
 #ifdef USE_NAMED_POSIX_SEMAPHORES
     mySemPointers = (sem_t **) malloc(maxSemas * sizeof(sem_t *));
     if (mySemPointers == NULL)
@@ -215,7 +226,7 @@ PGReserveSemaphores(int maxSemas, int port)

     numSems = 0;
     maxSems = maxSemas;
-    nextSemKey = port * 1000;
+    nextSemKey = statbuf.st_ino;

     on_shmem_exit(ReleaseSemaphores, 0);
 }
diff --git a/src/backend/port/sysv_sema.c b/src/backend/port/sysv_sema.c
index ac5106f..a1652cc 100644
--- a/src/backend/port/sysv_sema.c
+++ b/src/backend/port/sysv_sema.c
@@ -17,6 +17,7 @@
 #include <signal.h>
 #include <unistd.h>
 #include <sys/file.h>
+#include <sys/stat.h>
 #ifdef HAVE_SYS_IPC_H
 #include <sys/ipc.h>
 #endif
@@ -301,10 +302,6 @@ PGSemaphoreShmemSize(int maxSemas)
  * are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
  * callback to release them.
  *
- * The port number is passed for possible use as a key (for SysV, we use
- * it to generate the starting semaphore key).  In a standalone backend,
- * zero will be passed.
- *
  * In the SysV implementation, we acquire semaphore sets on-demand; the
  * maxSemas parameter is just used to size the arrays.  There is an array
  * of PGSemaphoreData structs in shared memory, and a postmaster-local array
@@ -314,8 +311,22 @@ PGSemaphoreShmemSize(int maxSemas)
  * have clobbered.)
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
+    struct stat statbuf;
+
+    /*
+     * We use the data directory's inode number to seed the search for free
+     * semaphore keys.  This minimizes the odds of collision with other
+     * postmasters, while maximizing the odds that we will detect and clean up
+     * semaphores left over from a crashed postmaster in our own directory.
+     */
+    if (stat(DataDir, &statbuf) < 0)
+        ereport(FATAL,
+                (errcode_for_file_access(),
+                 errmsg("could not stat data directory \"%s\": %m",
+                        DataDir)));
+
     /*
      * We must use ShmemAllocUnlocked(), since the spinlock protecting
      * ShmemAlloc() won't be ready yet.  (This ordering is necessary when we
@@ -332,7 +343,7 @@ PGReserveSemaphores(int maxSemas, int port)
     if (mySemaSets == NULL)
         elog(PANIC, "out of memory");
     numSemaSets = 0;
-    nextSemaKey = port * 1000;
+    nextSemaKey = statbuf.st_ino;
     nextSemaNumber = SEMAS_PER_SET; /* force sema set alloc on 1st call */

     on_shmem_exit(ReleaseSemaphores, 0);
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 968506d..dab2920 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;

diff --git a/src/backend/port/win32_sema.c b/src/backend/port/win32_sema.c
index 013c122..32cc697 100644
--- a/src/backend/port/win32_sema.c
+++ b/src/backend/port/win32_sema.c
@@ -44,7 +44,7 @@ PGSemaphoreShmemSize(int maxSemas)
  * process exits.
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
     mySemSet = (HANDLE *) malloc(maxSemas * sizeof(HANDLE));
     if (mySemSet == NULL)
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index ccd7b6b..6cb6328 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -194,7 +194,7 @@ EnableLockPagesPrivilege(int elevel)
  * standard header.
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size, int port,
+PGSharedMemoryCreate(Size size,
                      PGShmemHeader **shim)
 {
     void       *memAddress;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 3339804..67cdeb5 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -385,7 +385,7 @@ static void getInstallationPaths(const char *argv0);
 static void checkControlFile(void);
 static Port *ConnCreate(int serverFd);
 static void ConnFree(Port *port);
-static void reset_shared(int port);
+static void reset_shared(void);
 static void SIGHUP_handler(SIGNAL_ARGS);
 static void pmdie(SIGNAL_ARGS);
 static void reaper(SIGNAL_ARGS);
@@ -1175,7 +1175,7 @@ PostmasterMain(int argc, char *argv[])
     /*
      * Set up shared memory and semaphores.
      */
-    reset_shared(PostPortNumber);
+    reset_shared();

     /*
      * Estimate number of openable files.  This must happen after setting up
@@ -2599,17 +2599,16 @@ InitProcessGlobals(void)
  * reset_shared -- reset shared memory and semaphores
  */
 static void
-reset_shared(int port)
+reset_shared(void)
 {
     /*
      * Create or re-create shared memory and semaphores.
      *
      * Note: in each "cycle of life" we will normally assign the same IPC keys
-     * (if using SysV shmem and/or semas), since the port number is used to
-     * determine IPC keys.  This helps ensure that we will clean up dead IPC
-     * objects if the postmaster crashes and is restarted.
+     * (if using SysV shmem and/or semas).  This helps ensure that we will
+     * clean up dead IPC objects if the postmaster crashes and is restarted.
      */
-    CreateSharedMemoryAndSemaphores(port);
+    CreateSharedMemoryAndSemaphores();
 }


@@ -3919,7 +3918,7 @@ PostmasterStateMachine(void)
         /* re-read control file into local memory */
         LocalProcessControlFile(true);

-        reset_shared(PostPortNumber);
+        reset_shared();

         StartupPID = StartupDataBase();
         Assert(StartupPID != 0);
@@ -4947,7 +4946,7 @@ SubPostmasterMain(int argc, char *argv[])
         InitProcess();

         /* Attach process to shared data structures */
-        CreateSharedMemoryAndSemaphores(0);
+        CreateSharedMemoryAndSemaphores();

         /* And run the backend */
         BackendRun(&port);        /* does not return */
@@ -4961,7 +4960,7 @@ SubPostmasterMain(int argc, char *argv[])
         InitAuxiliaryProcess();

         /* Attach process to shared data structures */
-        CreateSharedMemoryAndSemaphores(0);
+        CreateSharedMemoryAndSemaphores();

         AuxiliaryProcessMain(argc - 2, argv + 2);    /* does not return */
     }
@@ -4974,7 +4973,7 @@ SubPostmasterMain(int argc, char *argv[])
         InitProcess();

         /* Attach process to shared data structures */
-        CreateSharedMemoryAndSemaphores(0);
+        CreateSharedMemoryAndSemaphores();

         AutoVacLauncherMain(argc - 2, argv + 2);    /* does not return */
     }
@@ -4987,7 +4986,7 @@ SubPostmasterMain(int argc, char *argv[])
         InitProcess();

         /* Attach process to shared data structures */
-        CreateSharedMemoryAndSemaphores(0);
+        CreateSharedMemoryAndSemaphores();

         AutoVacWorkerMain(argc - 2, argv + 2);    /* does not return */
     }
@@ -5005,7 +5004,7 @@ SubPostmasterMain(int argc, char *argv[])
         InitProcess();

         /* Attach process to shared data structures */
-        CreateSharedMemoryAndSemaphores(0);
+        CreateSharedMemoryAndSemaphores();

         /* Fetch MyBgworkerEntry from shared memory */
         shmem_slot = atoi(argv[1] + 15);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index d7d7335..8853706 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -91,7 +91,7 @@ RequestAddinShmemSpace(Size size)
  * This is a bit code-wasteful and could be cleaned up.)
  */
 void
-CreateSharedMemoryAndSemaphores(int port)
+CreateSharedMemoryAndSemaphores(void)
 {
     PGShmemHeader *shim = NULL;

@@ -163,14 +163,14 @@ CreateSharedMemoryAndSemaphores(int port)
         /*
          * Create the shmem segment
          */
-        seghdr = PGSharedMemoryCreate(size, port, &shim);
+        seghdr = PGSharedMemoryCreate(size, &shim);

         InitShmemAccess(seghdr);

         /*
          * Create semaphores
          */
-        PGReserveSemaphores(numSemas, port);
+        PGReserveSemaphores(numSemas);

         /*
          * If spinlocks are disabled, initialize emulation layer (which
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 43b9f17..29c5ec7 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -445,12 +445,10 @@ InitCommunication(void)
     if (!IsUnderPostmaster)        /* postmaster already did this */
     {
         /*
-         * We're running a postgres bootstrap process or a standalone backend.
-         * Though we won't listen on PostPortNumber, use it to select a shmem
-         * key.  This increases the chance of detecting a leftover live
-         * backend of this DataDir.
+         * We're running a postgres bootstrap process or a standalone backend,
+         * so we need to set up shmem.
          */
-        CreateSharedMemoryAndSemaphores(PostPortNumber);
+        CreateSharedMemoryAndSemaphores();
     }
 }

diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index e9b243f..d113813 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -76,6 +76,6 @@ extern void on_exit_reset(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;

-extern void CreateSharedMemoryAndSemaphores(int port);
+extern void CreateSharedMemoryAndSemaphores(void);

 #endif                            /* IPC_H */
diff --git a/src/include/storage/pg_sema.h b/src/include/storage/pg_sema.h
index b95efa1..1ac4f9a 100644
--- a/src/include/storage/pg_sema.h
+++ b/src/include/storage/pg_sema.h
@@ -41,7 +41,7 @@ typedef HANDLE PGSemaphore;
 extern Size PGSemaphoreShmemSize(int maxSemas);

 /* Module initialization (called during postmaster start or shmem reinit) */
-extern void PGReserveSemaphores(int maxSemas, int port);
+extern void PGReserveSemaphores(int maxSemas);

 /* Allocate a PGSemaphore structure with initial count 1 */
 extern PGSemaphore PGSemaphoreCreate(void);
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index ac24878..6bd5664 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -82,7 +82,7 @@ extern void PGSharedMemoryReAttach(void);
 extern void PGSharedMemoryNoReAttach(void);
 #endif

-extern PGShmemHeader *PGSharedMemoryCreate(Size size, int port,
+extern PGShmemHeader *PGSharedMemoryCreate(Size size,
                                            PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
diff --git a/src/test/recovery/t/017_shm.pl b/src/test/recovery/t/017_shm.pl
index 7f10ff5..0b988eb 100644
--- a/src/test/recovery/t/017_shm.pl
+++ b/src/test/recovery/t/017_shm.pl
@@ -152,14 +152,12 @@ print STDERR $single_stderr;
 like($single_stderr, $pre_existing_msg,
     'single-user mode detected live backend via shared memory');
 log_ipcs();
-# Fail to reject startup if shm key N has become available and we crash while
-# using key N+1.  This is unwanted, but expected.  Windows is immune, because
-# its GetSharedMemName() use DataDir strings, not numeric keys.
+# Reject startup if shm key N has become available and we crash while
+# using key N+1.
 $flea->stop;    # release first key
 is( $gnat->start(fail_ok => 1),
-    $TestLib::windows_os ? 0 : 1,
-    'key turnover fools only sysv_shmem.c');
-$gnat->stop;     # release first key (no-op on $TestLib::windows_os)
+    0,
+    'key turnover fools nobody');
 $flea->start;    # grab first key
 # cleanup
 TestLib::system_log('pg_ctl', 'kill', 'QUIT', $slow_pid);