Allow threads to exit an isolate during rendezvous.

runtime/vm/thread_registry.h

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_THREAD_REGISTRY_H_
 #define VM_THREAD_REGISTRY_H_
 
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
 #include "vm/thread.h"
 
 namespace dart {
 
 // Unordered collection of threads relating to a particular isolate.
 class ThreadRegistry {
  public:
   ThreadRegistry()
       : monitor_(new Monitor()),
         entries_(),
         in_rendezvous_(false),
         remaining_(0),
         round_(0) {}
 
   // Bring all threads in this isolate to a safepoint. The caller is
   // expected to be implicitly at a safepoint. The threads will wait
-  // until ResumeAllThreads is called. Must be called at a safepoint,
-  // since it first waits for any already pending requests. Any thread
-  // that tries to enter/exit this isolate during rendezvous will wait
-  // in RestoreStateTo/SaveStateFrom, respectively.
+  // until ResumeAllThreads is called. First participates in any
+  // already pending rendezvous requested by another thread. Any
+  // thread that tries to enter this isolate during rendezvous will
+  // wait in RestoreStateTo. Nesting is not supported: the caller must
+  // call ResumeAllThreads before making further calls to
+  // SafepointThreads.
   void SafepointThreads();
 
   // Unblocks all threads participating in the rendezvous that was organized
   // by a prior call to SafepointThreads.
   // TODO(koda): Consider adding a scope helper to avoid omitting this call.
   void ResumeAllThreads();
 
   // Indicate that the current thread is at a safepoint, and offer to wait for
   // any pending rendezvous request (if none, returns immediately).
   void CheckSafepoint() {
@@ skipping 31 matching lines @@
 #if defined(DEBUG)
     // State field is not in use, so zap it.
     memset(&new_entry.state, 0xda, sizeof(new_entry.state));
 #endif
     entries_.Add(new_entry);
     return false;
   }
 
   void SaveStateFrom(Thread* thread, const Thread::State& state) {
     MonitorLocker ml(monitor_);
-    // Exiting an isolate must always be a safepoint.
-    CheckSafepointLocked();
     Entry* entry = FindEntry(thread);
     ASSERT(entry != NULL);
     ASSERT(entry->scheduled);
     entry->scheduled = false;
     entry->state = state;
+    if (in_rendezvous_) {
+      // Don't wait for this thread.
+      ASSERT(remaining_ > 0);
+      if (--remaining_ == 0) {
+        ml.NotifyAll();
+      }
+    }
   }
 
   bool Contains(Thread* thread) {
     MonitorLocker ml(monitor_);
     return (FindEntry(thread) != NULL);
   }
 
   void CheckNotScheduled(Isolate* isolate) {
     MonitorLocker ml(monitor_);
     for (int i = 0; i < entries_.length(); ++i) {
@@ skipping 52 matching lines @@
   intptr_t remaining_;    // Number of threads yet to reach their safepoint.
   int64_t round_;         // Counter, to prevent missing updates to remaining_
                           // (see comments in CheckSafepointLocked).
 
   DISALLOW_COPY_AND_ASSIGN(ThreadRegistry);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_REGISTRY_H_