base: Avoid calling VirtualAlloc with 0 length.

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 7449f462e68ea4936b6c381239b316da59862594..65b6513b0a211371abca07d7dda3a8603460d130 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -232,7 +232,8 @@ DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
   }
 #elif defined(OS_WIN)
   if (base::win::GetVersion() >= base::win::VERSION_WIN8) {
-    if (!VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
+    if (length &&

[penny, 2016/02/28 21:49:10]

if (!length)
  return PURGED;

Unlock is fine as is, but perhaps in Lock we should return PURGED in the rare
case we ever get a length of zero at this point.

This will just prevent the misbehaving caller from thinking the memory contents
of this mapping have been preserved (implied by SUCCESS), when there's a chance
the contents have in fact been lost and need to be recreated.

[reveman, 2016/02/29 16:36:59]

Done.

Note: memory segments are initially locked so you can still have a locked 0-byte
segment.

+        !VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
                           AlignToPageSize(sizeof(SharedState)) + offset,
                       length, MEM_RESET_UNDO, PAGE_READWRITE)) {
       return PURGED;
@@ -271,7 +272,8 @@ void DiscardableSharedMemory::Unlock(size_t offset, size_t length) {
     // function needs to match the Lock behaviour (MEM_RESET_UNDO) to properly
     // implement memory pinning.  It needs to bias towards preserving the
     // contents of memory between an Unlock and next Lock.
-    if (!VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
+    if (length &&
+        !VirtualAlloc(reinterpret_cast<char*>(shared_memory_.memory()) +
                           AlignToPageSize(sizeof(SharedState)) + offset,
                       length, MEM_RESET, PAGE_READWRITE)) {
       DPLOG(ERROR) << "VirtualAlloc() MEM_RESET failed in Unlock()";