VM: Fix memory leak during shutdown

runtime/platform/hashmap.cc

 // Copyright (c) 2012, 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.
 
 #include "platform/hashmap.h"
 
 #include "platform/utils.h"
 
 namespace dart {
 
@@ skipping 89 matching lines @@
     if ((next > candidate && (start <= candidate || start > next)) ||
         (next < candidate && (start <= candidate && start > next))) {
       *candidate = *next;
       candidate = next;
     }
   }
 
   // Clear the candidate which will not break searching the hash table.
   candidate->key = NULL;
   candidate->value = NULL;
+  candidate->hash = 0;

[Florian Schneider, 2016/11/30 18:08:41]

Why clear the hash code?

[kustermann, 2016/11/30 21:27:15]

For consistency:

The [key] *needs* to be cleared because a NULL key serves as the "free spot"
marker.

The [value] and [hash] don't need to be cleared. But since [value] is cleared,
why not [hash] as well? We could avoid the work and not clear any of
[value]/[hash] (though it's a little weird in the debugger if one examples the
hashmap entries and sees left-over/garbage values in there)

(I thought maybe the reason is ASAN, but then again, [hash] could be the 32-bit
lower bits of [key], so [hash] can contain a valid pointer as well.)

Removed the two assignments now.

[Florian Schneider, 2016/11/30 21:51:48]

Agree, on the other hand, it may be useful to examine historical entries in the
hashmap.

Sounds good to remove them - performance should be better as well this way.

   occupancy_--;
 }
 
 
+HashMap::Entry* HashMap::Remove(Entry* entry) {
+  // Save the key of the entry we would like to remove, since the removal itself
+  // might do a left-rotation of elements following this one, thereby overriding
+  // `entry`.
+  void* key = entry->key;

[Florian Schneider, 2016/11/30 18:08:41]

Where is the saved key used after calling Remove?

[kustermann, 2016/11/30 21:27:15]

Sorry that was a left-over. The indication whether removal made a free spot is
whether 'key == NULL' if it's not the case, a left rotation has happened.

+  Remove(key, entry->hash);
+
+  // A key can only exist once in the map and we just removed `key`. This means
+  // that either a left-rotation has happened (in which case `entry` points
+  // already to the next element (in terms of iteration order)) or alternatively
+  // we can use the normal `Next()` call to move in iteration order.
+  if (entry->key != NULL) {
+    // A left-rotation happened. `entry` points already to the next element in
+    // iteration order.
+    return entry;
+  } else {
+    return Next(entry);
+  }
+}
+
+
 void HashMap::Clear(ClearFun clear) {
   // Mark all entries as empty.
   const Entry* end = map_end();
   for (Entry* p = map_; p < end; p++) {
     if ((clear != NULL) && (p->key != NULL)) {
       clear(p->value);
     }
     p->value = NULL;
     p->key = NULL;
+    p->hash = 0;

[Florian Schneider, 2016/11/30 18:08:41]

Same here. Seems wasted work.

[kustermann, 2016/11/30 21:27:15]

Same answer.

   }
   occupancy_ = 0;
 }
 
 
 HashMap::Entry* HashMap::Start() const {
   return Next(map_ - 1);
 }
 
 
@@ skipping 53 matching lines @@
       Lookup(p->key, p->hash, true)->value = p->value;
       n--;
     }
   }
 
   // Delete old map.
   delete[] map;
 }
 
 }  // namespace dart