Avoid calling proc_exit() in processes forked by system().

The SIGTERM handler for the startup process immediately calls
proc_exit() for the duration of the restore_command, i.e., a call
to system().  This system() call forks a new process to execute the
shell command, and this child process inherits the parent's signal
handlers.  If both the parent and child processes receive SIGTERM,
both will attempt to call proc_exit().  This can end badly.  For
example, both processes will try to remove themselves from the
PGPROC shared array.

To fix this problem, this commit adds a check in
StartupProcShutdownHandler() to see whether MyProcPid == getpid().
If they match, this is the parent process, and we can proc_exit()
like before.  If they do not match, this is a child process, and we
just emit a message to STDERR (in a signal safe manner) and
_exit(), thereby skipping any problematic exit callbacks.

This commit also adds checks in proc_exit(), ProcKill(), and
AuxiliaryProcKill() that verify they are not being called within
such child processes.

Suggested-by: Andres Freund
Reviewed-by: Thomas Munro, Andres Freund
Discussion: https://postgr.es/m/Y9nGDSgIm83FHcad%40paquier.xyz
Discussion: https://postgr.es/m/20230223231503.GA743455%40nathanxps13
Backpatch-through: 11

diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c
index 69077bd207516ea08d69cf2f2dec52eb6ff26425..c3f5e18b9c933f3fd5bb14fe7e6334ad5b71f7dd 100644 (file)
--- a/src/backend/postmaster/startup.c
+++ b/src/backend/postmaster/startup.c
@@ -19,6 +19,8 @@
  */
 #include "postgres.h"
 
+#include <unistd.h>
+
 #include "access/xlog.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
@@ -102,7 +104,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
    int         save_errno = errno;
 
    if (in_restore_command)
-       proc_exit(1);
+   {
+       /*
+        * If we are in a child process (e.g., forked by system() in
+        * RestoreArchivedFile()), we don't want to call any exit callbacks.
+        * The parent will take care of that.
+        */
+       if (MyProcPid == (int) getpid())
+           proc_exit(1);
+       else
+       {
+           write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n");
+           _exit(1);
+       }
+   }
    else
        shutdown_requested = true;
    WakeupRecovery();

diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c
index 4045d7d68a0a3db2ac3da549f1b5a2c1f02e9127..df89a28d2ffb07e4560f3766f7c0f8455078aba6 100644 (file)
--- a/src/backend/storage/ipc/ipc.c
+++ b/src/backend/storage/ipc/ipc.c
@@ -103,6 +103,10 @@ static int on_proc_exit_index,
 void
 proc_exit(int code)
 {
+   /* not safe if forked by system(), etc. */
+   if (MyProcPid != (int) getpid())
+       elog(PANIC, "proc_exit() called in child process");
+
    /* Clean up everything that must be cleaned up */
    proc_exit_prepare(code);
 

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index f323228602f40016967fc1a774b0ec2fe53ef51f..1e06d6580ce02b3be82e1be9764981aa28f83920 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -826,6 +826,10 @@ ProcKill(int code, Datum arg)
 
    Assert(MyProc != NULL);
 
+   /* not safe if forked by system(), etc. */
+   if (MyProc->pid != (int) getpid())
+       elog(PANIC, "ProcKill() called in child process");
+
    /* Make sure we're out of the sync rep lists */
    SyncRepCleanupAtProcExit();
 
@@ -955,6 +959,10 @@ AuxiliaryProcKill(int code, Datum arg)
 
    Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);
 
+   /* not safe if forked by system(), etc. */
+   if (MyProc->pid != (int) getpid())
+       elog(PANIC, "AuxiliaryProcKill() called in child process");
+
    auxproc = &AuxiliaryProcs[proctype];
 
    Assert(MyProc == auxproc);

diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index a3e1c59a829940847c90b63f5bc76eea05174288..001b87ea999b81454db32552f05fbc8abbe4653b 100644 (file)
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3590,6 +3590,34 @@ write_stderr(const char *fmt,...)
 }
 
 
+/*
+ * Write a message to STDERR using only async-signal-safe functions.  This can
+ * be used to safely emit a message from a signal handler.
+ *
+ * TODO: It is likely possible to safely do a limited amount of string
+ * interpolation (e.g., %s and %d), but that is not presently supported.
+ */
+void
+write_stderr_signal_safe(const char *str)
+{
+   int         nwritten = 0;
+   int         ntotal = strlen(str);
+
+   while (nwritten < ntotal)
+   {
+       int         rc;
+
+       rc = write(STDERR_FILENO, str + nwritten, ntotal - nwritten);
+
+       /* Just give up on error.  There isn't much else we can do. */
+       if (rc == -1)
+           return;
+
+       nwritten += rc;
+   }
+}
+
+
 /*
  * Adjust the level of a recovery-related message per trace_recovery_messages.
  *

diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index f53607e12eb81de91cd4e933a210dddf7588a980..f98a1e8b629b17c99c517edbf852e35e8ebf8119 100644 (file)
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -453,4 +453,10 @@ extern void set_syslog_parameters(const char *ident, int facility);
  */
 extern void write_stderr(const char *fmt,...) pg_attribute_printf(1, 2);
 
+/*
+ * Write a message to STDERR using only async-signal-safe functions.  This can
+ * be used to safely emit a message from a signal handler.
+ */
+extern void write_stderr_signal_safe(const char *fmt);
+
 #endif                         /* ELOG_H */