Allow a user to kill his own queries using pg_cancel_backend()

Allows a user to use pg_cancel_queries() to cancel queries in
other backends if they are running under the same role.
pg_terminate_backend() still requires superuser permissoins.

Short patch, many authors working on the bikeshed: Magnus Hagander,
Josh Kupershmidt, Edward Muller, Greg Smith.

doc/src/sgml/func.sgml

@@ -14262,8 +14262,8 @@ SELECT set_config('log_statement_stats', 'off', false);
    <para>
     The functions shown in <xref
     linkend="functions-admin-signal-table"> send control signals to
-    other server processes.  Use of these functions is restricted
+    other server processes.  Use of these functions is usually restricted
-    to superusers.
+    to superusers, with noted exceptions.
    </para>
 
    <table id="functions-admin-signal-table">
@@ -14280,7 +14280,10 @@ SELECT set_config('log_statement_stats', 'off', false);
         <literal><function>pg_cancel_backend(<parameter>pid</parameter> <type>int</>)</function></literal>
         </entry>
        <entry><type>boolean</type></entry>
-       <entry>Cancel a backend's current query</entry>
+       <entry>Cancel a backend's current query.  You can execute this against
+        another backend that has exactly the same role as the user calling the
+        function.  In all other cases, you must be a superuser.
+        </entry>
       </row>
       <row>
        <entry>
@@ -14322,6 +14325,10 @@ SELECT set_config('log_statement_stats', 'off', false);
     <command>postgres</command> processes on the server (using
     <application>ps</> on Unix or the <application>Task
     Manager</> on <productname>Windows</>).
+    For the less restrictive <function>pg_cancel_backend</>, the role of an
+    active backend can be found from
+    the <structfield>usename</structfield> column of the
+    <structname>pg_stat_activity</structname> view.
    </para>
 
    <para>

src/backend/utils/adt/misc.c

@@ -30,6 +30,7 @@
 #include "postmaster/syslogger.h"
 #include "storage/fd.h"
 #include "storage/pmsignal.h"
+#include "storage/proc.h"
 #include "storage/procarray.h"
 #include "tcop/tcopprot.h"
 #include "utils/builtins.h"
@@ -70,15 +71,42 @@ current_query(PG_FUNCTION_ARGS)
 }
 
 /*
- * Functions to send signals to other backends.
+ * Send a signal to another backend.
+ * The signal is delivered if the user is either a superuser or the same
+ * role as the backend being signaled. For "dangerous" signals, an explicit
+ * check for superuser needs to be done prior to calling this function.
+ *
+ * Returns 0 on success, 1 on general failure, and 2 on permission error.
+ * In the event of a general failure (returncode 1), a warning message will
+ * be emitted. For permission errors, doing that is the responsibility of
+ * the caller.
  */
-static bool
+#define SIGNAL_BACKEND_SUCCESS 0
+#define SIGNAL_BACKEND_ERROR 1
+#define SIGNAL_BACKEND_NOPERMISSION 2
+static int
 pg_signal_backend(int pid, int sig)
 {
+	PGPROC	   *proc;
+
 	if (!superuser())
-		ereport(ERROR,
+	{
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+		/*
-			(errmsg("must be superuser to signal other server processes"))));
+		 * Since the user is not superuser, check for matching roles. Trust
+		 * that BackendPidGetProc will return NULL if the pid isn't valid,
+		 * even though the check for whether it's a backend process is below.
+		 * The IsBackendPid check can't be relied on as definitive even if it
+		 * was first. The process might end between successive checks
+		 * regardless of their order. There's no way to acquire a lock on an
+		 * arbitrary process to prevent that. But since so far all the callers
+		 * of this mechanism involve some request for ending the process
+		 * anyway, that it might end on its own first is not a problem.
+		 */
+		proc = BackendPidGetProc(pid);
+
+		if (proc == NULL || proc->roleId != GetUserId())
+			return SIGNAL_BACKEND_NOPERMISSION;
+	}
 
 	if (!IsBackendPid(pid))
 	{
@@ -88,9 +116,18 @@ pg_signal_backend(int pid, int sig)
 		 */
 		ereport(WARNING,
 				(errmsg("PID %d is not a PostgreSQL server process", pid)));
-		return false;
+		return SIGNAL_BACKEND_ERROR;
 	}
 
+	/*
+	 * Can the process we just validated above end, followed by the pid being
+	 * recycled for a new process, before reaching here?  Then we'd be trying
+	 * to kill the wrong thing.  Seems near impossible when sequential pid
+	 * assignment and wraparound is used.  Perhaps it could happen on a system
+	 * where pid re-use is randomized.	That race condition possibility seems
+	 * too unlikely to worry about.
+	 */
+
 	/* If we have setsid(), signal the backend's whole process group */
 #ifdef HAVE_SETSID
 	if (kill(-pid, sig))
@@ -101,23 +138,46 @@ pg_signal_backend(int pid, int sig)
 		/* Again, just a warning to allow loops */
 		ereport(WARNING,
 				(errmsg("could not send signal to process %d: %m", pid)));
-		return false;
+		return SIGNAL_BACKEND_ERROR;
 	}
-	return true;
+	return SIGNAL_BACKEND_SUCCESS;
 }
 
+/*
+ * Signal to cancel a backend process.	This is allowed if you are superuser or
+ * have the same role as the process being canceled.
+ */
 Datum
 pg_cancel_backend(PG_FUNCTION_ARGS)
 {
-	PG_RETURN_BOOL(pg_signal_backend(PG_GETARG_INT32(0), SIGINT));
+	int			r = pg_signal_backend(PG_GETARG_INT32(0), SIGINT);
+
+	if (r == SIGNAL_BACKEND_NOPERMISSION)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 (errmsg("must be superuser or have the same role to cancel queries running in other server processes"))));
+
+	PG_RETURN_BOOL(r == SIGNAL_BACKEND_SUCCESS);
 }
 
+/*
+ * Signal to terminate a backend process.  Only allowed by superuser.
+ */
 Datum
 pg_terminate_backend(PG_FUNCTION_ARGS)
 {
-	PG_RETURN_BOOL(pg_signal_backend(PG_GETARG_INT32(0), SIGTERM));
+	if (!superuser())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+			 errmsg("must be superuser to terminate other server processes"),
+				 errhint("You can cancel your own processes with pg_cancel_backend().")));
+
+	PG_RETURN_BOOL(pg_signal_backend(PG_GETARG_INT32(0), SIGTERM) == SIGNAL_BACKEND_SUCCESS);
 }
 
+/*
+ * Signal to reload the database configuration
+ */
 Datum
 pg_reload_conf(PG_FUNCTION_ARGS)
 {