Guard against race conditions

.editorconfig

@@ -0,0 +1,14 @@
+
+# top-most EditorConfig file
+root = true
+
+# rules for all files
+# we use tabs with indent size 4
+[*]
+indent_style = spaces
+indent_size = 4
+tab_width = 8
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+trim_trailing_whitespace = true

prioritize.c

@@ -74,14 +74,20 @@ set_backend_priority(PG_FUNCTION_ARGS)
     else if (!superuser()) {
 	/*
          * 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
+         * that BackendPidGetProcWithLock 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.
+         *
+         * We do take the ProcArrayLock manually instead of using
+         * BackendPidGetProc, because we need to be sure that the contents of
+         * proc don't change from under us while we're using them.
          */
-        proc = BackendPidGetProc(pid);
+        LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+        proc = BackendPidGetProcWithLock(pid);
 
 	if (proc == NULL) {
 	    /*
@@ -94,9 +100,12 @@ set_backend_priority(PG_FUNCTION_ARGS)
 	}
 
 	else if (proc->roleId != GetUserId())
+            LWLockRelease(ProcArrayLock);
 	    ereport(ERROR,
                 (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                  (errmsg("must be superuser to nice arbitrary backends"))));
+        LWLockRelease(ProcArrayLock);
+
 
 	/* Otherwise, the backend PID is valid and our user is allowed
 	 * to set its priority.