fix: panic in pop_frame()

    We met the panic in our production environment, so handle this panic
    condition before panic. The stack backtrace:

       0: rust_begin_unwind
                 at
    /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/std/src/panicking.rs:517:5
       1: core::panicking::panic_fmt
                 at
    /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/panicking.rs:101:14
       2: core::panicking::panic
                 at
    /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/core/src/panicking.rs:50:5
       3: h2::proto::streams::flow_control::FlowControl::send_data
                 at
    /build/vendor/h2/src/proto/streams/flow_control.rs:176:9
       4: h2::proto::streams::prioritize::Prioritize::pop_frame::{{closure}}
                 at
    /build/vendor/h2/src/proto/streams/prioritize.rs:737:33
       5: tracing::span::Span::in_scope
                 at /build/vendor/tracing/src/span.rs:982:9
       6: h2::proto::streams::prioritize::Prioritize::pop_frame
                 at
    /build/vendor/h2/src/proto/streams/prioritize.rs:736:29
       7: h2::proto::streams::prioritize::Prioritize::poll_complete
                 at
    /build/vendor/h2/src/proto/streams/prioritize.rs:497:19
       8: h2::proto::streams::send::Send::poll_complete
                 at /build/vendor/h2/src/proto/streams/send.rs:297:9
       9: h2::proto::streams::streams::Inner::poll_complete
                 at /build/vendor/h2/src/proto/streams/streams.rs:850:16
      10: h2::proto::streams::streams::Streams<B,P>::poll_complete
                 at /build/vendor/h2/src/proto/streams/streams.rs:180:9
      11: h2::proto::connection::Connection<T,P,B>::poll
                 at /build/vendor/h2/src/proto/connection.rs:253:36
      12: <h2::client::Connection<T,B> as
    core::future::future::Future>::poll
      13: ...

src/proto/streams/flow_control.rs

@@ -173,12 +173,15 @@ impl FlowControl {
             self.available
         );
 
-        // Ensure that the argument is correct
-        assert!(self.window_size >= sz as usize);
-
-        // Update values
-        self.window_size -= sz;
-        self.available -= sz;
+        // If send size is zero it's meaningless to update flow control window
+        if sz > 0 {
+            // Ensure that the argument is correct
+            assert!(self.window_size >= sz as usize);
+
+            // Update values
+            self.window_size -= sz;
+            self.available -= sz;
+        }
     }
 }
 

src/proto/streams/prioritize.rs

@@ -744,6 +744,14 @@ impl Prioritize {
                             // capacity at this point.
                             debug_assert!(len <= self.flow.window_size());
 
+                            // Check if the stream level window the peer knows is available. In some
+                            // scenarios, maybe the window we know is available but the window which
+                            // peer knows is not.
+                            if len > 0 && len > stream.send_flow.window_size() {
+                                stream.pending_send.push_front(buffer, frame.into());
+                                continue;
+                            }
+
                             tracing::trace!(len, "sending data frame");
 
                             // Update the flow control