Check return value of TableBuilder::Add() in compaction routine.

This is a prospective fix for the crash described in
crbug.com/326566884. Inspection of the crash dump suggests that
`Add()` is failing, but the compaction routine is continuing to
run. While it's not entirely clear how precisely this leads to
the observed failure mode, it seems safer to check the status
of the TableBuilder and abort early as needed.

db/db_impl.cc

@@ -998,7 +998,12 @@ Status DBImpl::DoCompactionWork(CompactionState* compact) {
         compact->current_output()->smallest.DecodeFrom(key);
       }
       compact->current_output()->largest.DecodeFrom(key);
-      compact->builder->Add(key, input->value());
+
+      status = compact->builder->Add(key, input->value());
+      if (!status.ok()) {
+        FinishCompactionOutputFile(compact, input);
+        break;
+      }
 
       // Close output file if it is big enough
       if (compact->builder->FileSize() >=

include/leveldb/table_builder.h

@@ -49,7 +49,7 @@ class LEVELDB_EXPORT TableBuilder {
   // Add key,value to the table being constructed.
   // REQUIRES: key is after any previously added key according to comparator.
   // REQUIRES: Finish(), Abandon() have not been called
-  void Add(const Slice& key, const Slice& value);
+  Status Add(const Slice& key, const Slice& value);
 
   // Advanced operation: flush any buffered key/value pairs to file.
   // Can be used to ensure that two adjacent entries never live in

table/table_builder.cc

@@ -91,10 +91,12 @@ Status TableBuilder::ChangeOptions(const Options& options) {
   return Status::OK();
 }
 
-void TableBuilder::Add(const Slice& key, const Slice& value) {
+Status TableBuilder::Add(const Slice& key, const Slice& value) {
   Rep* r = rep_;
   assert(!r->closed);
-  if (!ok()) return;
+  if (!ok()) {
+    return status();
+  }
   if (r->num_entries > 0) {
     assert(r->options.comparator->Compare(key, Slice(r->last_key)) > 0);
   }
@@ -120,6 +122,7 @@ void TableBuilder::Add(const Slice& key, const Slice& value) {
   if (estimated_block_size >= r->options.block_size) {
     Flush();
   }
+  return status();
 }
 
 void TableBuilder::Flush() {