Update buffer position when returning kIgnored (pixie-io#1760)

Summary: This PR removes the frame's contents from the buffer in the
case we return `kIgnored` when the frame is a type (opcode) we do not
parse. When running the BPF test we noticed that the program would stall
when the parser encountered a frame with an opcode it does not support.
This was due to to the parser returning `kIgnored` to `ParseFramesLoop`
and it not moving the buffer forward before calling `ParseFrame` again.
This change will update the buffer position before returning `kIgnored`
to `ParseFramesLoop` so that the remaining frames in the buffer can be
parsed.

Related issues: pixie-io#640

Type of change: /kind bug

Test Plan: Modified the existing test checking for the unsupported
opcode type.


Signed-off-by: Kartik Pattaswamy <kpattaswamy@pixielabs.ai>

src/stirling/source_connectors/socket_tracer/protocols/mongodb/parse.cc

@@ -64,9 +64,10 @@ ParseState ParseFrame(message_type_t type, std::string_view* buf, Frame* frame,
     return ParseState::kInvalid;
   }
 
-  // Parser will ignore Op Codes that have been deprecated/removed from version 5.0 onwards.
-  if (!(frame_type == Type::kOPMsg || frame_type == Type::kOPCompressed ||
-        frame_type == Type::kReserved)) {
+  // Parser will ignore Op Codes that have been deprecated/removed from version 5.0 onwards as well
+  // as kOPCompressed and kReserved which are not supported by the parser yet.
+  if (frame_type != Type::kOPMsg) {
+    buf->remove_prefix(frame->length);
     return ParseState::kIgnored;
   }
 

src/stirling/source_connectors/socket_tracer/protocols/mongodb/parse_test.cc

@@ -391,6 +391,7 @@ TEST_F(MongoDBParserTest, ParseFrameWhenUnsupportedType) {
   ParseState state = ParseFrame(message_type_t::kRequest, &frame_view, &frame, &state_order);
 
   EXPECT_EQ(state, ParseState::kIgnored);
+  EXPECT_EQ(frame_view.length(), 0);
 }
 
 TEST_F(MongoDBParserTest, ParseFrameInvalidFlagBits) {