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 1adfd78c6f10fc78578c9f7f097852dbb8451dff..75cd458aaa0b0242ce05c478d2eafe50d08b6264 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"
@@ -91,7 +93,20 @@ 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 bdbc2c3ac4bc06c7f55bba340200d568cad56e83..72a6b31109edaf8ed559c406186d4540a740378a 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 220777546fead7da3a0bd71ea36dfc0afb9f0a26..199c0eae078cae0cd4aba040ce8e71a064770853 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -821,6 +821,10 @@ 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();
 
@@ -945,6 +949,10 @@ 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 e4b717c79a9c199cebc1e8ead949d284d56b93fc..2db3e32cb8f4c0af4466bd8add266260a813f440 100644 (file)
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3457,6 +3457,34 @@ 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;
+   }
+}
+
+
 /*
  * is_log_level_output -- is elevel logically >= log_min_level?
  *

diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index 1e09ee05418af8e0fc74960d284cf5609568655c..1b263a886dbf52555ba2807113650ea01715fe53 100644 (file)
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -436,4 +436,10 @@ 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 */