Allow WriteBatch::Handler to abort iteration

Summary:
Sometimes you don't need to iterate through the whole WriteBatch. This diff makes the Handler member functions return a bool that indicates whether to abort or not. If they return true, the iteration stops.

One thing I just thought of is that this will break backwards-compability. Maybe it would be better to add a virtual member function WriteBatch::Handler::ShouldAbort() that returns false by default. Comments requested.

I still have to add a new unit test for the abort code, but let's finalize the API first.

Test Plan: make -j32 check

Reviewers: dhruba, haobo, vamsi, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12339

db/write_batch.cc

@@ -48,6 +48,10 @@ void WriteBatch::Handler::LogData(const Slice& blob) {
   // them.
 }
 
+bool WriteBatch::Handler::Continue() {
+  return true;
+}
+
 void WriteBatch::Clear() {
   rep_.clear();
   rep_.resize(kHeader);
@@ -66,7 +70,7 @@ Status WriteBatch::Iterate(Handler* handler) const {
   input.remove_prefix(kHeader);
   Slice key, value, blob;
   int found = 0;
-  while (!input.empty()) {
+  while (!input.empty() && handler->Continue()) {
     char tag = input[0];
     input.remove_prefix(1);
     switch (tag) {