internal/http3: avoid potential race when aborting RoundTrip

TestRoundTripRequestBodyErrors/read_error currently has a small chance
of failing due to a race condition.

Most of the time, within writeBodyAndTrailer which runs concurrently,
when a request body returns an error on Read, rt.abort will be called
with the Read error. Since rt.abort is idempotent, the Read error will
then be propagated and the test would pass as expected.

However, before rt.abort is called with the Read error,
rt.reqBodyWriter.Close is first called, which will close the write
direction of the QUIC stream. This creates a small chance where the main
goroutine which runs RoundTrip can encounter an EOF error due to the
closed stream, and subsequently call rt.abort with a different error
than expected, causing test flakiness.

To fix this, this change makes sure that rt.abort is immediately called
when a request body Read returns an error. This should be a no-op, since
a body Read error has always been prioritized over a potential error
from rt.reqBodyWriter.Close anyways.

Fixes golang/go#77782

Change-Id: I9ec5acf82dd95ac91963ce5c8ddbf08b49af9508
Reviewed-on: https://go-review.googlesource.com/c/net/+/751280
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

Author: nicholashusin

internal/http3/roundtrip.go

@@ -222,7 +222,9 @@ func (cc *ClientConn) writeBodyAndTrailer(rt *roundTripState, req *http.Request)
 		rt.reqBody = http.NoBody
 	}
 
-	_, err := io.Copy(&rt.reqBodyWriter, rt.reqBody)
+	if _, err := io.Copy(&rt.reqBodyWriter, rt.reqBody); err != nil {
+		rt.abort(err)
+	}
 	// Get rid of any trailer that was not declared beforehand, before we
 	// close the request body which will cause the trailer headers to be
 	// written.
@@ -231,11 +233,7 @@ func (cc *ClientConn) writeBodyAndTrailer(rt *roundTripState, req *http.Request)
 			delete(req.Trailer, name)
 		}
 	}
-	if closeErr := rt.reqBodyWriter.Close(); err == nil {
-		err = closeErr
-	}
-	// Something went wrong writing the body.
-	if err != nil {
+	if err := rt.reqBodyWriter.Close(); err != nil {
 		rt.abort(err)
 	}
 }