fix(http2): received Body::size_hint() now return 0 if implicitly empty (#2715)

An HTTP/2 stream may include a set of headers, and a flag signalling
END-STREAM, even if a `content-length` isn't included. hyper wouldn't
notice, and so the `Body` would report a size-hint of `0..MAX`. hyper
now notices that the stream is ended, and couldn't possibly include any
bytes for the body, and thus will give a size-hint of `0` exactly.

src/body/body.rs

@@ -204,9 +204,14 @@ impl Body {
     #[cfg(all(feature = "http2", any(feature = "client", feature = "server")))]
     pub(crate) fn h2(
         recv: h2::RecvStream,
-        content_length: DecodedLength,
+        mut content_length: DecodedLength,
         ping: ping::Recorder,
     ) -> Self {
+        // If the stream is already EOS, then the "unknown length" is clearly
+        // actually ZERO.
+        if !content_length.is_exact() && recv.is_end_stream() {
+            content_length = DecodedLength::ZERO;
+        }
         let body = Body::new(Kind::H2 {
             ping,
             content_length,

src/body/length.rs

@@ -68,6 +68,16 @@ impl DecodedLength {
             }
         }
     }
+
+    /// Returns whether this represents an exact length.
+    ///
+    /// This includes 0, which of course is an exact known length.
+    ///
+    /// It would return false if "chunked" or otherwise size-unknown.
+    #[cfg(feature = "http2")]
+    pub(crate) fn is_exact(&self) -> bool {
+        self.0 <= MAX_LEN
+    }
 }
 
 impl fmt::Debug for DecodedLength {

src/proto/h2/server.rs

@@ -484,12 +484,13 @@ where
                         }
                     }
 
-                    // automatically set Content-Length from body...
-                    if let Some(len) = body.size_hint().exact() {
-                        headers::set_content_length_if_missing(res.headers_mut(), len);
-                    }
 
                     if !body.is_end_stream() {
+                        // automatically set Content-Length from body...
+                        if let Some(len) = body.size_hint().exact() {
+                            headers::set_content_length_if_missing(res.headers_mut(), len);
+                        }
+
                         let body_tx = reply!(me, res, false);
                         H2StreamState::Body {
                             pipe: PipeToSendStream::new(body, body_tx),