prevent hang when compressing Sized(0) bodies

fixes #3229

actix-http/CHANGES.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+## Fixed
+
+- Prevent hang when returning zero-sized response bodies through compression layer.
+
 ## 3.5.0
 
 ### Changed

actix-http/src/encoding/encoder.rs

@@ -50,10 +50,21 @@ impl<B: MessageBody> Encoder<B> {
         }
     }
 
+    fn empty() -> Self {
+        Encoder {
+            body: EncoderBody::Full { body: Bytes::new() },
+            encoder: None,
+            fut: None,
+            eof: true,
+        }
+    }
+
     pub fn response(encoding: ContentEncoding, head: &mut ResponseHead, body: B) -> Self {
-        // no need to compress an empty body
-        if matches!(body.size(), BodySize::None | BodySize::Sized(0)) {
-            return Self::none();
+        // no need to compress empty bodies
+        match body.size() {
+            BodySize::None => return Self::none(),
+            BodySize::Sized(0) => return Self::empty(),
+            _ => {}
         }
 
         let should_encode = !(head.headers().contains_key(&CONTENT_ENCODING)