Code review for pgstat_get_crashed_backend_activity patch.

Avoid possibly dumping core when pgstat_track_activity_query_size has a
less-than-default value; avoid uselessly searching for the query string
of a successfully-exited backend; don't bother putting out an ERRDETAIL if
we don't have a query to show; some other minor stylistic improvements.

Author: tglsfdc

src/backend/postmaster/pgstat.c

@@ -2760,27 +2760,33 @@ pgstat_get_backend_current_activity(int pid, bool checkUser)
  * pgstat_get_crashed_backend_activity() -
  *
  *	Return a string representing the current activity of the backend with
- *	the specified PID. Like the function above, but reads shared memory with
- *	the expectation that it may be corrupt. Returns either a pointer to a
- *	constant string, or writes into the 'buffer' argument and returns it.
+ *	the specified PID.  Like the function above, but reads shared memory with
+ *	the expectation that it may be corrupt.  On success, copy the string
+ *	into the "buffer" argument and return that pointer.  On failure,
+ *	return NULL.
  *
- *	This function is only intended to be used by postmaster to report the
- *	query that crashed the backend. In particular, no attempt is made to
+ *	This function is only intended to be used by the postmaster to report the
+ *	query that crashed a backend.  In particular, no attempt is made to
  *	follow the correct concurrency protocol when accessing the
- *	BackendStatusArray. But that's OK, in the worst case we'll return a
- *	corrupted message. We also must take care not to trip on ereport(ERROR).
- *
- *	Note: return strings for special cases match pg_stat_get_backend_activity.
+ *	BackendStatusArray.  But that's OK, in the worst case we'll return a
+ *	corrupted message.  We also must take care not to trip on ereport(ERROR).
  * ----------
  */
 const char *
-pgstat_get_crashed_backend_activity(int pid, char *buffer,
-									int len)
+pgstat_get_crashed_backend_activity(int pid, char *buffer, int buflen)
 {
 	volatile PgBackendStatus *beentry;
 	int			i;
 
 	beentry = BackendStatusArray;
+
+	/*
+	 * We probably shouldn't get here before shared memory has been set up,
+	 * but be safe.
+	 */
+	if (beentry == NULL || BackendActivityBuffer == NULL)
+		return NULL;
+
 	for (i = 1; i <= MaxBackends; i++)
 	{
 		if (beentry->st_procpid == pid)
@@ -2790,26 +2796,29 @@ pgstat_get_crashed_backend_activity(int pid, char *buffer,
 			const char *activity_last;
 
 			/*
-			 * We can't access activity pointer before we verify that it
-			 * falls into BackendActivityBuffer. To make sure that the entire
-			 * string including its ending is contained within the buffer,
-			 * we subtract one activity length from it.
+			 * We mustn't access activity string before we verify that it
+			 * falls within the BackendActivityBuffer. To make sure that the
+			 * entire string including its ending is contained within the
+			 * buffer, subtract one activity length from the buffer size.
 			 */
 			activity_last = BackendActivityBuffer + BackendActivityBufferSize
-							- pgstat_track_activity_query_size;
+				- pgstat_track_activity_query_size;
 
 			if (activity < BackendActivityBuffer ||
 				activity > activity_last)
-				return "<command string corrupt>";
+				return NULL;
 
-			if (*(activity) == '\0')
-				return "<command string empty>";
+			/* If no string available, no point in a report */
+			if (activity[0] == '\0')
+				return NULL;
 
 			/*
 			 * Copy only ASCII-safe characters so we don't run into encoding
-			 * problems when reporting the message.
+			 * problems when reporting the message; and be sure not to run
+			 * off the end of memory.
 			 */
-			ascii_safe_strncpy(buffer, activity, len);
+			ascii_safe_strlcpy(buffer, activity,
+							   Min(buflen, pgstat_track_activity_query_size));
 
 			return buffer;
 		}
@@ -2818,9 +2827,10 @@ pgstat_get_crashed_backend_activity(int pid, char *buffer,
 	}
 
 	/* PID not found */
-	return "<backend information not available>";
+	return NULL;
 }
 
+
 /* ------------------------------------------------------------
  * Local support functions follow
  * ------------------------------------------------------------

src/backend/postmaster/postmaster.c

@@ -2777,12 +2777,17 @@ HandleChildCrash(int pid, int exitstatus, const char *procname)
 static void
 LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 {
-	char		activity_buffer[1024]; /* default track_activity_query_size */
-	const char *activity;
+	/*
+	 * size of activity_buffer is arbitrary, but set equal to default
+	 * track_activity_query_size
+	 */
+	char		activity_buffer[1024];
+	const char *activity = NULL;
 
-	activity = pgstat_get_crashed_backend_activity(pid,
-												   activity_buffer,
-												   sizeof(activity_buffer));
+	if (!EXIT_STATUS_0(exitstatus))
+		activity = pgstat_get_crashed_backend_activity(pid,
+													   activity_buffer,
+													   sizeof(activity_buffer));
 
 	if (WIFEXITED(exitstatus))
 		ereport(lev,
@@ -2792,7 +2797,7 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 		  "server process" */
 				(errmsg("%s (PID %d) exited with exit code %d",
 						procname, pid, WEXITSTATUS(exitstatus)),
-				 errdetail("Running query: %s", activity)));
+				 activity ? errdetail("Failed process was running: %s", activity) : 0));
 	else if (WIFSIGNALED(exitstatus))
 #if defined(WIN32)
 		ereport(lev,
@@ -2803,7 +2808,7 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 				(errmsg("%s (PID %d) was terminated by exception 0x%X",
 						procname, pid, WTERMSIG(exitstatus)),
 				 errhint("See C include file \"ntstatus.h\" for a description of the hexadecimal value."),
-				 errdetail("Running query: %s", activity)));
+				 activity ? errdetail("Failed process was running: %s", activity) : 0));
 #elif defined(HAVE_DECL_SYS_SIGLIST) && HAVE_DECL_SYS_SIGLIST
 	ereport(lev,
 
@@ -2814,7 +2819,7 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 					procname, pid, WTERMSIG(exitstatus),
 					WTERMSIG(exitstatus) < NSIG ?
 					sys_siglist[WTERMSIG(exitstatus)] : "(unknown)"),
-			 errdetail("Running query: %s", activity)));
+			 activity ? errdetail("Failed process was running: %s", activity) : 0));
 #else
 		ereport(lev,
 
@@ -2823,7 +2828,7 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 		  "server process" */
 				(errmsg("%s (PID %d) was terminated by signal %d",
 						procname, pid, WTERMSIG(exitstatus)),
-				 errdetail("Running query: %s", activity)));
+				 activity ? errdetail("Failed process was running: %s", activity) : 0));
 #endif
 	else
 		ereport(lev,
@@ -2833,7 +2838,7 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 		  "server process" */
 				(errmsg("%s (PID %d) exited with unrecognized status %d",
 						procname, pid, exitstatus),
-				 errdetail("Running query: %s", activity)));
+				 activity ? errdetail("Failed process was running: %s", activity) : 0));
 }
 
 /*

src/backend/utils/adt/ascii.c

@@ -160,35 +160,38 @@ to_ascii_default(PG_FUNCTION_ARGS)
 }
 
 /* ----------
- * "Escape" a string in unknown encoding to a valid ASCII string.
- * Replace non-ASCII bytes with '?'
- * This must not trigger ereport(ERROR), as it is called from postmaster.
+ * Copy a string in an arbitrary backend-safe encoding, converting it to a
+ * valid ASCII string by replacing non-ASCII bytes with '?'.  Otherwise the
+ * behavior is identical to strlcpy(), except that we don't bother with a
+ * return value.
  *
- * Unlike C strncpy(), the result is always terminated with exactly one null
- * byte.
+ * This must not trigger ereport(ERROR), as it is called in postmaster.
  * ----------
  */
 void
-ascii_safe_strncpy(char *dest, const char *src, int len)
+ascii_safe_strlcpy(char *dest, const char *src, size_t destsiz)
 {
-	int			i;
+	if (destsiz == 0)			/* corner case: no room for trailing nul */
+		return;
 
-	for (i = 0; i < (len - 1); i++)
+	while (--destsiz > 0)
 	{
-		unsigned char ch = src[i];	/* use unsigned char here to avoid compiler warning */
+		/* use unsigned char here to avoid compiler warning */
+		unsigned char ch = *src++;
 
 		if (ch == '\0')
 			break;
 		/* Keep printable ASCII characters */
 		if (32 <= ch && ch <= 127)
-			dest[i] = ch;
+			*dest = ch;
 		/* White-space is also OK */
 		else if (ch == '\n' || ch == '\r' || ch == '\t')
-			dest[i] = ch;
+			*dest = ch;
 		/* Everything else is replaced with '?' */
 		else
-			dest[i] = '?';
+			*dest = '?';
+		dest++;
 	}
 
-	dest[i] = '\0';
+	*dest = '\0';
 }

src/include/pgstat.h

@@ -721,7 +721,7 @@ extern void pgstat_report_xact_timestamp(TimestampTz tstamp);
 extern void pgstat_report_waiting(bool waiting);
 extern const char *pgstat_get_backend_current_activity(int pid, bool checkUser);
 extern const char *pgstat_get_crashed_backend_activity(int pid, char *buffer,
-									int len);
+									int buflen);
 
 extern PgStat_TableStatus *find_tabstat_entry(Oid rel_id);
 extern PgStat_BackendFunctionEntry *find_funcstat_entry(Oid func_id);

src/include/utils/ascii.h

@@ -16,6 +16,7 @@
 extern Datum to_ascii_encname(PG_FUNCTION_ARGS);
 extern Datum to_ascii_enc(PG_FUNCTION_ARGS);
 extern Datum to_ascii_default(PG_FUNCTION_ARGS);
-extern void ascii_safe_strncpy(char *dest, const char *src, int len);
+
+extern void ascii_safe_strlcpy(char *dest, const char *src, size_t destsiz);
 
 #endif   /* _ASCII_H_ */