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 a6f77c105c285b654b40385f88ab28f12e6ab01d..941b678451d3d4fc1480ad4578f94a86d83d7d3a 100644 (file)
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -140,6 +140,13 @@ 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 e390468463b83daa85a21696b23b46602ada1f55..69ef93b0f84a5559602bce1977161388b1464a4c 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -774,6 +774,7 @@ ProcKill(int code, Datum arg)
 {
    /* use volatile pointer to prevent code rearrangement */
    volatile PROC_HDR *procglobal = ProcGlobal;
+   PGPROC     *proc;
 
    Assert(MyProc != NULL);
 
@@ -798,31 +799,34 @@ 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 if (IsBackgroundWorker)
    {
-       MyProc->links.next = (SHM_QUEUE *) procglobal->bgworkerFreeProcs;
-       procglobal->bgworkerFreeProcs = MyProc;
+       proc->links.next = (SHM_QUEUE *) procglobal->bgworkerFreeProcs;
+       procglobal->bgworkerFreeProcs = 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);
 
@@ -851,6 +855,7 @@ 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);
 
@@ -861,16 +866,19 @@ 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);