Prevent possible crash reading pg_stat_activity.

Also, avoid reading PGPROC's wait_event field twice, once for the wait
event and again for the wait_event_type, because the value might change
in the middle.

Petr Jelinek and Robert Haas

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 64c4cc4538d3bad03157b9bb342a5298ea6168b1..17c5cb0fb6423a59737f58eda894e1bbe68dd68d 100644 (file)
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -27,6 +27,8 @@
 #include "utils/inet.h"
 #include "utils/timestamp.h"
 
+#define UINT32_ACCESS_ONCE(var)      ((uint32)(*((volatile uint32 *)&(var))))
+
 /* bogus ... these externs should be in a header file */
 extern Datum pg_stat_get_numscans(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_tuples_returned(PG_FUNCTION_ARGS);
@@ -783,13 +785,26 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
            values[5] = CStringGetTextDatum(beentry->st_activity);
 
            proc = BackendPidGetProc(beentry->st_procpid);
-           wait_event_type = pgstat_get_wait_event_type(proc->wait_event_info);
+           if (proc != NULL)
+           {
+               uint32  raw_wait_event;
+
+               raw_wait_event = UINT32_ACCESS_ONCE(proc->wait_event_info);
+               wait_event_type = pgstat_get_wait_event_type(raw_wait_event);
+               wait_event = pgstat_get_wait_event(raw_wait_event);
+
+           }
+           else
+           {
+               wait_event_type = NULL;
+               wait_event = NULL;
+           }
+
            if (wait_event_type)
                values[6] = CStringGetTextDatum(wait_event_type);
            else
                nulls[6] = true;
 
-           wait_event = pgstat_get_wait_event(proc->wait_event_info);
            if (wait_event)
                values[7] = CStringGetTextDatum(wait_event);
            else
@@ -984,17 +999,14 @@ pg_stat_get_backend_wait_event_type(PG_FUNCTION_ARGS)
    int32       beid = PG_GETARG_INT32(0);
    PgBackendStatus *beentry;
    PGPROC     *proc;
-   const char *wait_event_type;
+   const char *wait_event_type = NULL;
 
    if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
        wait_event_type = "";
    else if (!has_privs_of_role(GetUserId(), beentry->st_userid))
        wait_event_type = "";
-   else
-   {
-       proc = BackendPidGetProc(beentry->st_procpid);
+   else if ((proc = BackendPidGetProc(beentry->st_procpid)) != NULL)
        wait_event_type = pgstat_get_wait_event_type(proc->wait_event_info);
-   }
 
    if (!wait_event_type)
        PG_RETURN_NULL();
@@ -1008,17 +1020,14 @@ pg_stat_get_backend_wait_event(PG_FUNCTION_ARGS)
    int32       beid = PG_GETARG_INT32(0);
    PgBackendStatus *beentry;
    PGPROC     *proc;
-   const char *wait_event;
+   const char *wait_event = NULL;
 
    if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
        wait_event = "";
    else if (!has_privs_of_role(GetUserId(), beentry->st_userid))
        wait_event = "";
-   else
-   {
-       proc = BackendPidGetProc(beentry->st_procpid);
+   else if ((proc = BackendPidGetProc(beentry->st_procpid)) != NULL)
        wait_event = pgstat_get_wait_event(proc->wait_event_info);
-   }
 
    if (!wait_event)
        PG_RETURN_NULL();