db_bench: in uncompress benchmark, get Snappy size from compressed stream

Summary: Now in benchmark "uncompress" in db_bench, we get size from compressed stream for all other compression types except Snappy, where we allocate memory based on parameter. Change it to match to behavior of other compression types.

Test Plan: Run ./db_bench --benchmarks=uncompress with snappy and other compression types.

Reviewers: yhchiang, kradhakrishnan, anthony, IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51681

db/db_bench.cc

@@ -2267,12 +2267,19 @@ class Benchmark {
     while (ok && bytes < 1024 * 1048576) {
       char *uncompressed = nullptr;
       switch (FLAGS_compression_type_e) {
-      case rocksdb::kSnappyCompression:
-        // allocate here to make comparison fair
-        uncompressed = new char[input.size()];
-        ok = Snappy_Uncompress(compressed.data(), compressed.size(),
-                               uncompressed);
-        break;
+        case rocksdb::kSnappyCompression: {
+          // get size and allocate here to make comparison fair
+          size_t ulength = 0;
+          if (!Snappy_GetUncompressedLength(compressed.data(),
+                                            compressed.size(), &ulength)) {
+            ok = false;
+            break;
+          }
+          uncompressed = new char[ulength];
+          ok = Snappy_Uncompress(compressed.data(), compressed.size(),
+                                 uncompressed);
+          break;
+        }
       case rocksdb::kZlibCompression:
         uncompressed = Zlib_Uncompress(compressed.data(), compressed.size(),
                                        &decompress_size, 2);