compression: handle compression flag but no header (#763)

Co-authored-by: Quentin Perez <[email protected]>
Co-authored-by: Lucio Franco <[email protected]>

tonic/src/codec/decode.rs

@@ -205,9 +205,20 @@ impl<T> Streaming<T> {
             let is_compressed = match self.buf.get_u8() {
                 0 => false,
                 1 => {
-                    if cfg!(feature = "compression") {
-                        true
-                    } else {
+                    #[cfg(feature = "compression")]
+                    {
+                        if self.encoding.is_some() {
+                            true
+                        } else {
+                            // https://grpc.github.io/grpc/core/md_doc_compression.html
+                            // An ill-constructed message with its Compressed-Flag bit set but lacking a grpc-encoding
+                            // entry different from identity in its metadata MUST fail with INTERNAL status,
+                            // its associated description indicating the invalid Compressed-Flag condition.
+                            return Err(Status::new(Code::Internal, "protocol error: received message with compressed-flag but no grpc-encoding was specified"));
+                        }
+                    }
+                    #[cfg(not(feature = "compression"))]
+                    {
                         return Err(Status::new(
                             Code::Unimplemented,
                             "Message compressed, compression support not enabled.".to_string(),
@@ -250,6 +261,7 @@ impl<T> Streaming<T> {
 
                     if let Err(err) = decompress(
                         self.encoding.unwrap_or_else(|| {
+                            // SAFETY: The check while in State::ReadHeader would already have returned Code::Internal
                             unreachable!("message was compressed but `Streaming.encoding` was `None`. This is a bug in Tonic. Please file an issue")
                         }),
                         &mut self.buf,