Index: ChangeLog
==================================================================
--- ChangeLog
+++ ChangeLog
@@ -1,5 +1,11 @@
+2013-07-17  Harald Oehlmann  <oehhar@users.sf.net>
+
+	* tclUnixNotify.c Tcl_InitNotifier: Bug [a0bc856dcd]
+	  Start notifier thread again if we were forked, to solve Rivet bug
+	  55153.
+
 2013-07-05  Kevin B. Kenny  <kennykb@acm.org>
 
 	* library/tzdata/Africa/Casablanca:
 	* library/tzdata/America/Asuncion:
 	* library/tzdata/Antarctica/Macquarie:

ADDED   tests/unixForkEvent.test
Index: tests/unixForkEvent.test
==================================================================
--- /dev/null
+++ tests/unixForkEvent.test
@@ -0,0 +1,45 @@
+# This file contains a collection of tests for the procedures in the file
+# tclUnixNotify.c.  Sourcing this file into Tcl runs the tests and
+# generates output for errors.  No output means no errors were found.
+#
+# Copyright (c) 1995-1997 Sun Microsystems, Inc.
+# Copyright (c) 1998-1999 by Scriptics Corporation.
+#
+# See the file "license.terms" for information on usage and redistribution
+# of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+
+package require tcltest 2
+namespace import -force ::tcltest::*
+
+testConstraint testfork [llength [info commands testfork]]
+
+# Test if the notifier thread is well initialized in a forked interpreter
+# by Tcl_InitNotifier
+test unixforkevent-1.1 {fork and test writeable event} \
+    -constraints testfork \
+    -body {
+	set myFolder [makeDirectory unixtestfork]
+	set pid [testfork]
+	if {$pid == 0} {
+	    # we are the forked process
+	    set result initialized
+	    set h [open [file join $myFolder test.txt] w]
+	    fileevent $h writable\
+		    "set result writable;\
+		    after cancel [after 1000 {set result timeout}]"
+	    vwait result
+	    close $h
+	    makeFile $result result.txt $myFolder
+	    exit
+	}
+	# we are the original process
+	while {![file readable [file join $myFolder result.txt]]} {}
+	viewFile result.txt $myFolder
+    } \
+    -result {writable} \
+    -cleanup { 
+	catch { removeFolder $myFolder }
+    }
+
+::tcltest::cleanupTests
+return

Index: unix/tclUnixNotfy.c
==================================================================
--- unix/tclUnixNotfy.c
+++ unix/tclUnixNotfy.c
@@ -110,11 +110,11 @@
 				 * this condition variable. */
 #endif /* __CYGWIN__ */
     int eventReady;		/* True if an event is ready to be processed.
 				 * Used as condition flag together with waitCV
 				 * above. */
-#endif
+#endif /* TCL_THREADS */
 } ThreadSpecificData;
 
 static Tcl_ThreadDataKey dataKey;
 
 #ifdef TCL_THREADS
@@ -125,10 +125,19 @@
  * You must hold the notifierMutex lock before accessing this variable.
  */
 
 static int notifierCount = 0;
 
+/*
+ * The following static stores the process ID of the initialized notifier
+ * thread. If it changes, we have passed a fork and we should start a new
+ * notifier thread.
+ *
+ * You must hold the notifierMutex lock before accessing this variable.
+ */
+static pid_t processIDInitialized = 0;
+
 /*
  * The following variable points to the head of a doubly-linked list of
  * ThreadSpecificData structures for all threads that are currently waiting on
  * an event.
  *
@@ -183,19 +192,25 @@
  * This is the thread ID of the notifier thread that does select.
  */
 
 static Tcl_ThreadId notifierThread;
 
-#endif
+#endif /* TCL_THREADS */
 
 /*
  * Static routines defined in this file.
  */
 
 #ifdef TCL_THREADS
 static void	NotifierThreadProc(ClientData clientData);
-#endif
+#ifdef HAVE_PTHREAD_ATFORK
+static int	atForkInit = 0;
+static void	AtForkPrepare(void);
+static void	AtForkParent(void);
+static void	AtForkChild(void);
+#endif /* HAVE_PTHREAD_ATFORK */
+#endif /* TCL_THREADS */
 static int	FileHandlerEventProc(Tcl_Event *evPtr, int flags);
 
 /*
  *----------------------------------------------------------------------
  *
@@ -265,15 +280,42 @@
     /*
      * Start the Notifier thread if necessary.
      */
 
     Tcl_MutexLock(&notifierMutex);
+#ifdef HAVE_PTHREAD_ATFORK
+    /*
+     * Install pthread_atfork handlers to reinitialize the notifier in the
+     * child of a fork.
+     */
+
+    if (!atForkInit) {
+	int result = pthread_atfork(AtForkPrepare, AtForkParent, AtForkChild);
+
+	if (result) {
+	    Tcl_Panic("Tcl_InitNotifier: pthread_atfork failed");
+	}
+	atForkInit = 1;
+    }
+#endif
+    /*
+     * Check if my process id changed, e.g. I was forked
+     * In this case, restart the notifier thread and close the
+     * pipe to the original notifier thread
+     */
+    if (notifierCount > 0 && processIDInitialized != getpid()) {
+	notifierCount = 0;
+	processIDInitialized = 0;
+	close(triggerPipe);
+	triggerPipe = -1;
+    }
     if (notifierCount == 0) {
 	if (TclpThreadCreate(&notifierThread, NotifierThreadProc, NULL,
 		TCL_THREAD_STACK_DEFAULT, TCL_THREAD_JOINABLE) != TCL_OK) {
 	    Tcl_Panic("Tcl_InitNotifier: unable to start notifier thread");
 	}
+    processIDInitialized = getpid();
     }
     notifierCount++;
 
     /*
      * Wait for the notifier pipe to be created.
@@ -282,11 +324,11 @@
     while (triggerPipe < 0) {
 	Tcl_ConditionWait(&notifierCV, &notifierMutex, NULL);
     }
 
     Tcl_MutexUnlock(&notifierMutex);
-#endif
+#endif /* TCL_THREADS */
     return (ClientData) tsdPtr;
 }
 
 /*
  *----------------------------------------------------------------------
@@ -1228,10 +1270,79 @@
     Tcl_ConditionNotify(&notifierCV);
     Tcl_MutexUnlock(&notifierMutex);
 
     TclpThreadExit (0);
 }
+
+#ifdef HAVE_PTHREAD_ATFORK
+/*
+ *----------------------------------------------------------------------
+ *
+ * AtForkPrepare --
+ *
+ *	Lock the notifier in preparation for a fork.
+ *
+ * Results:
+ *	None.
+ *
+ * Side effects:
+ *	None.
+ *
+ *----------------------------------------------------------------------
+ */
+
+static void
+AtForkPrepare(void)
+{
+}
+
+/*
+ *----------------------------------------------------------------------
+ *
+ * AtForkParent --
+ *
+ *	Unlock the notifier in the parent after a fork.
+ *
+ * Results:
+ *	None.
+ *
+ * Side effects:
+ *	None.
+ *
+ *----------------------------------------------------------------------
+ */
+
+static void
+AtForkParent(void)
+{
+}
+
+/*
+ *----------------------------------------------------------------------
+ *
+ * AtForkChild --
+ *
+ *	Unlock and reinstall the notifier in the child after a fork.
+ *
+ * Results:
+ *	None.
+ *
+ * Side effects:
+ *	None.
+ *
+ *----------------------------------------------------------------------
+ */
+
+static void
+AtForkChild(void)
+{
+    notifierMutex = NULL;
+    notifierCV = NULL;
+    Tcl_InitNotifier();
+}
+#endif /* HAVE_PTHREAD_ATFORK */
+
 #endif /* TCL_THREADS */
 
 #endif /* HAVE_COREFOUNDATION */
 
 /*