b/190640998: Add fuzz test for MessageReader (#61)

**Notes**
This fuzz tests the buffering, chuck reading, and gRPC delimiter parsing logic in `MessageReader`.

The flow is as follows:
1) Add fuzzed data to an input stream, split across multiple chunks
2) Call `MessagerReader` to parse the data from the input stream
3) If more buffering is needed, re-run steps 1-2

**Why fuzz this target?**
- It parses gRPC wire data
- It handles untrusted downstream client input
- It buffers data

**Testing done**
- Ran in fuzzing mode locally, no crashes

Author: nareddyt

test/BUILD

@@ -225,6 +225,19 @@ cc_test(
     ],
 )
 
+cc_fuzz_test(
+    name = "message_reader_fuzz_test",
+    testonly = 1,
+    srcs = [
+        "message_reader_fuzz_test.cc",
+    ],
+    corpus = glob(["message_reader_fuzz_test_corpus/**"]),
+    deps = [
+        ":test_common",
+        "//src:message_reader",
+    ],
+)
+
 cc_test(
     name = "response_to_json_translator_test",
     size = "small",

test/message_reader_fuzz_test.cc

@@ -0,0 +1,45 @@
+#include "grpc_transcoding/message_reader.h"
+
+#include "test_common.h"
+
+#include <fuzzer/FuzzedDataProvider.h>
+#include <cstdint>
+#include <cstddef>
+#include <string>
+
+namespace google {
+namespace grpc {
+namespace transcoding {
+namespace testing {
+namespace {
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  FuzzedDataProvider provider(data, size);
+
+  TestZeroCopyInputStream input_stream;
+  MessageReader reader(&input_stream);
+
+  while (provider.remaining_bytes() > 0) {
+
+    // Add a few chucks of data to the input stream.
+    for (int i = 0; i < provider.ConsumeIntegralInRange(0, 5); i++) {
+      input_stream.AddChunk(provider.ConsumeRandomLengthString(100));
+    }
+
+    // Run the message reader to get the next message.
+    (void) reader.NextMessageAndGrpcFrame();
+
+    // Handle end of input or error due to malformed bytes.
+    if (reader.Finished()) {
+      return 0;
+    }
+  }
+
+  return 0;
+}
+
+}
+}
+}
+}
+}

test/message_reader_fuzz_test_corpus/grpc-delimiter.txt

@@ -0,0 +1 @@
+1\0\0\0\0a2830232