Backported from GHC HEAD to fit GHC 6.12.2

Fri May  7 11:32:22 CEST 2010  Simon Marlow <marlowsd@gmail.com>
  * Fix crash in nested callbacks (#4038)
  Broken by "Split part of the Task struct into a separate struct
  InCall".
diff -rN -u old-ghc/rts/RtsAPI.c new-ghc/rts/RtsAPI.c
--- old-ghc/rts/RtsAPI.c	2010-05-17 19:45:10.811117781 +0200
+++ new-ghc/rts/RtsAPI.c	2010-05-17 19:45:11.172367293 +0200
@@ -510,7 +510,7 @@
 void
 rts_checkSchedStatus (char* site, Capability *cap)
 {
-    SchedulerStatus rc = cap->running_task->stat;
+    SchedulerStatus rc = cap->running_task->incall->stat;
     switch (rc) {
     case Success:
 	return;
@@ -529,7 +529,7 @@
 SchedulerStatus
 rts_getSchedStatus (Capability *cap)
 {
-    return cap->running_task->stat;
+    return cap->running_task->incall->stat;
 }
 
 Capability *
diff -rN -u old-ghc/rts/Schedule.c new-ghc/rts/Schedule.c
--- old-ghc/rts/Schedule.c	2010-05-17 19:45:10.814117674 +0200
+++ new-ghc/rts/Schedule.c	2010-05-17 19:45:11.175367815 +0200
@@ -1235,23 +1235,23 @@
 	  ASSERT(task->incall->tso == t);
 
 	  if (t->what_next == ThreadComplete) {
-	      if (task->ret) {
+	      if (task->incall->ret) {
 		  // NOTE: return val is tso->sp[1] (see StgStartup.hc)
-		  *(task->ret) = (StgClosure *)task->incall->tso->sp[1]; 
+		  *(task->incall->ret) = (StgClosure *)task->incall->tso->sp[1]; 
 	      }
-	      task->stat = Success;
+	      task->incall->stat = Success;
 	  } else {
-	      if (task->ret) {
-		  *(task->ret) = NULL;
+	      if (task->incall->ret) {
+		  *(task->incall->ret) = NULL;
 	      }
 	      if (sched_state >= SCHED_INTERRUPTING) {
                   if (heap_overflow) {
-                      task->stat = HeapExhausted;
+                      task->incall->stat = HeapExhausted;
                   } else {
-                      task->stat = Interrupted;
+                      task->incall->stat = Interrupted;
                   }
 	      } else {
-		  task->stat = Killed;
+		  task->incall->stat = Killed;
 	      }
 	  }
 #ifdef DEBUG
@@ -1887,8 +1887,8 @@
     tso->cap = cap;
 
     task->incall->tso = tso;
-    task->ret = ret;
-    task->stat = NoStatus;
+    task->incall->ret = ret;
+    task->incall->stat = NoStatus;
 
     appendToRunQueue(cap,tso);
 
@@ -1897,7 +1897,7 @@
 
     cap = schedule(cap,task);
 
-    ASSERT(task->stat != NoStatus);
+    ASSERT(task->incall->stat != NoStatus);
     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 
     debugTrace(DEBUG_sched, "bound thread (%lu) finished", (unsigned long)id);
diff -rN -u old-ghc/rts/Task.c new-ghc/rts/Task.c
--- old-ghc/rts/Task.c	2010-05-17 19:45:10.817118126 +0200
+++ new-ghc/rts/Task.c	2010-05-17 19:45:11.211367513 +0200
@@ -153,8 +153,6 @@
     task->cap           = NULL;
     task->worker        = worker;
     task->stopped       = rtsFalse;
-    task->stat          = NoStatus;
-    task->ret           = NULL;
     task->n_spare_incalls = 0;
     task->spare_incalls = NULL;
     task->incall        = NULL;
@@ -211,6 +209,8 @@
     incall->task = task;
     incall->suspended_tso = NULL;
     incall->suspended_cap = NULL;
+    incall->stat          = NoStatus;
+    incall->ret           = NULL;
     incall->next = NULL;
     incall->prev = NULL;
     incall->prev_stack = task->incall;
diff -rN -u old-ghc/rts/Task.h new-ghc/rts/Task.h
--- old-ghc/rts/Task.h	2010-05-17 19:45:10.817118126 +0200
+++ new-ghc/rts/Task.h	2010-05-17 19:45:11.211367513 +0200
@@ -83,6 +83,9 @@
                                 // without owning a Capability in the
                                 // first place.
 
+    SchedulerStatus  stat;      // return status
+    StgClosure **    ret;       // return value
+
     struct Task_ *task;
 
     // When a Haskell thread makes a foreign call that re-enters
@@ -137,9 +140,6 @@
     // So that we can detect when a finalizer illegally calls back into Haskell
     rtsBool running_finalizers;
 
-    SchedulerStatus  stat;      // return status
-    StgClosure **    ret;       // return value
-
     // Stats that we collect about this task
     // ToDo: we probably want to put this in a separate TaskStats
     // structure, so we can share it between multiple Tasks.  We don't