From 638a610d0b7ada7b2099b6127410aafb9fae3313 Mon Sep 17 00:00:00 2001
From: Ahmed Omar <spawn.think@gmail.com>
Date: Wed, 15 Dec 2010 11:26:18 +0100
Subject: Fixes a race condition found in percept_db start/1 function.

When function start/1 finds an instance of percept_db running, it will send a message to stop it and spawn a process to start a new one, which leads to a race condition.
The function will return {restarted, pid()} but the pid() will die once it tries to create the ets table.
---
 lib/percept/src/percept_db.erl | 49 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 9 deletions(-)

(limited to 'lib/percept/src/percept_db.erl')

diff --git a/lib/percept/src/percept_db.erl b/lib/percept/src/percept_db.erl
index edb0d79a29..52e9afb78f 100644
--- a/lib/percept/src/percept_db.erl
+++ b/lib/percept/src/percept_db.erl
@@ -33,7 +33,7 @@
 	]).
 
 -include("percept.hrl").
-
+-define(STOP_TIMEOUT, 1000).
 %%==========================================================================
 %%
 %% 		Type definitions 
@@ -77,17 +77,32 @@
 start() ->
     case erlang:whereis(percept_db) of
     	undefined ->
-	    Pid = spawn( fun() -> init_percept_db() end),
-	    erlang:register(percept_db, Pid),
-	    {started, Pid};
+	    {started, do_start()};
 	PerceptDB ->
-	    erlang:unregister(percept_db),
-	    PerceptDB ! {action, stop},
-	    Pid = spawn( fun() -> init_percept_db() end),
-	    erlang:register(percept_db, Pid),
-	    {restarted, Pid}
+	    {restarted, restart(PerceptDB)}
     end.
 
+%% @spec restart(pid()) -> pid()
+%% @private
+%% @doc restarts the percept database.
+
+-spec restart(pid())-> pid().
+
+restart(PerceptDB)->
+        stop_sync(PerceptDB),
+        do_start().
+
+%% @spec do_start(pid()) -> pid()
+%% @private
+%% @doc starts the percept database.
+
+-spec do_start()-> pid().
+
+do_start()->
+    Pid = spawn( fun() -> init_percept_db() end),
+    erlang:register(percept_db, Pid),
+    Pid.
+
 %% @spec stop() -> not_started | {stopped, Pid}
 %%	Pid = pid()
 %% @doc Stops the percept database.
@@ -103,6 +118,22 @@ stop() ->
 	    {stopped, Pid}
     end.
 
+%% @spec stop_sync(pid()) -> true
+%% @private
+%% @doc Stops the percept database, with a synchronous call.
+
+-spec stop_sync(pid())-> true.
+
+stop_sync(Pid)->
+    MonitorRef = erlang:monitor(process, Pid),
+    stop(),
+    receive
+        {'DOWN', MonitorRef, _Type, Pid, _Info}->
+            true
+    after ?STOP_TIMEOUT->
+            exit(Pid, kill)
+    end.
+
 %% @spec insert(tuple()) -> ok
 %% @doc Inserts a trace or profile message to the database.  
 
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