check return status for Sync() and Append() calls to avoid corruption

Summary:
Right now in `SyncClosedLogs`, `CopyFile`, and `AddRecord`, where `Sync` and `Append` are invoked in a loop, the error status are not checked. This could lead to potential corruption as later calls will overwrite the error status.
Closes facebook#3740

Differential Revision: D7678848

Pulled By: miasantreble

fbshipit-source-id: 4b0b412975989dfe80348f73217b9c4122a4bd77

db/db_impl_compaction_flush.cc

@@ -50,6 +50,9 @@ Status DBImpl::SyncClosedLogs(JobContext* job_context) {
                      "[JOB %d] Syncing log #%" PRIu64, job_context->job_id,
                      log->get_log_number());
       s = log->file()->Sync(immutable_db_options_.use_fsync);
+      if (!s.ok()) {
+        break;
+      }
     }
     if (s.ok()) {
       s = directories_.GetWalDir()->Fsync();

db/log_writer.cc

@@ -57,9 +57,11 @@ Status Writer::AddRecord(const Slice& slice) {
         // Fill the trailer (literal below relies on kHeaderSize and
         // kRecyclableHeaderSize being <= 11)
         assert(header_size <= 11);
-        dest_->Append(
-            Slice("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
-                  static_cast<size_t>(leftover)));
+        s = dest_->Append(Slice("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
+                                static_cast<size_t>(leftover)));
+        if (!s.ok()) {
+          break;
+        }
       }
       block_offset_ = 0;
     }

util/file_util.cc

@@ -62,8 +62,7 @@ Status CopyFile(Env* env, const std::string& source,
     }
     size -= slice.size();
   }
-  dest_writer->Sync(use_fsync);
-  return Status::OK();
+  return dest_writer->Sync(use_fsync);
 }
 
 // Utility function to create a file with the provided contents