Clear MyProc and MyProcSignalState before they become invalid.

Evidence from buildfarm member crake suggests that the new test_shm_mq
module is routinely crashing the server due to the arrival of a SIGUSR1
after the shared memory segment has been unmapped.  Although processes
using the new dynamic background worker facilities are more likely to
receive a SIGUSR1 around this time, the problem is also possible on older
branches, so I'm back-patching the parts of this change that apply to
older branches as far as they apply.

It's already generally the case that code checks whether these pointers
are NULL before deferencing them, so the important thing is mostly to
make sure that they do get set to NULL before they become invalid.  But
in master, there's one case in procsignal_sigusr1_handler that lacks a
NULL guard, so add that.

Patch by me; review by Tom Lane.

diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index 0f0d4dd48dffc394b368e675c5ac33f4db31bb84..b06074982ca0070d9e3db8468b939f60c089bba4 100644 (file)
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -139,6 +139,13 @@ CleanupProcSignalState(int status, Datum arg)
        slot = &ProcSignalSlots[pss_idx - 1];
        Assert(slot == MyProcSignalSlot);
 
+       /*
+        * Clear MyProcSignalSlot, so that a SIGUSR1 received after this point
+        * won't try to access it after it's no longer ours (and perhaps even
+        * after we've unmapped the shared memory segment).
+        */
+       MyProcSignalSlot = NULL;
+
        /* sanity check */
        if (slot->pss_pid != MyProcPid)
        {

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 3bfdd237a29a361bb7fca0bd56d7cbd273e0de69..c27c48fc91f2e04076ac461db8648fbc9d29e3bb 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -760,6 +760,7 @@ ProcKill(int code, Datum arg)
 {
        /* use volatile pointer to prevent code rearrangement */
        volatile PROC_HDR *procglobal = ProcGlobal;
+       PGPROC     *proc;
 
        Assert(MyProc != NULL);
 
@@ -784,26 +785,29 @@ ProcKill(int code, Datum arg)
         */
        LWLockReleaseAll();
 
-       /* Release ownership of the process's latch, too */
-       DisownLatch(&MyProc->procLatch);
+       /*
+        * Clear MyProc first; then disown the process latch.  This is so that
+        * signal handlers won't try to clear the process latch after it's no
+        * longer ours.
+        */
+       proc = MyProc;
+       MyProc = NULL;
+       DisownLatch(&proc->procLatch);
 
        SpinLockAcquire(ProcStructLock);
 
        /* Return PGPROC structure (and semaphore) to appropriate freelist */
        if (IsAnyAutoVacuumProcess())
        {
-               MyProc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
-               procglobal->autovacFreeProcs = MyProc;
+               proc->links.next = (SHM_QUEUE *) procglobal->autovacFreeProcs;
+               procglobal->autovacFreeProcs = proc;
        }
        else
        {
-               MyProc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
-               procglobal->freeProcs = MyProc;
+               proc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
+               procglobal->freeProcs = proc;
        }
 
-       /* PGPROC struct isn't mine anymore */
-       MyProc = NULL;
-
        /* Update shared estimate of spins_per_delay */
        procglobal->spins_per_delay = update_spins_per_delay(procglobal->spins_per_delay);
 
@@ -832,6 +836,7 @@ AuxiliaryProcKill(int code, Datum arg)
 {
        int                     proctype = DatumGetInt32(arg);
        PGPROC     *auxproc PG_USED_FOR_ASSERTS_ONLY;
+       PGPROC     *proc;
 
        Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);
 
@@ -842,16 +847,19 @@ AuxiliaryProcKill(int code, Datum arg)
        /* Release any LW locks I am holding (see notes above) */
        LWLockReleaseAll();
 
-       /* Release ownership of the process's latch, too */
-       DisownLatch(&MyProc->procLatch);
+       /*
+        * Clear MyProc first; then disown the process latch.  This is so that
+        * signal handlers won't try to clear the process latch after it's no
+        * longer ours.
+        */
+       proc = MyProc;
+       MyProc = NULL;
+       DisownLatch(&proc->procLatch);
 
        SpinLockAcquire(ProcStructLock);
 
        /* Mark auxiliary proc no longer in use */
-       MyProc->pid = 0;
-
-       /* PGPROC struct isn't mine anymore */
-       MyProc = NULL;
+       proc->pid = 0;
 
        /* Update shared estimate of spins_per_delay */
        ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);