http2: don't abort half-closed streams on server connection close

If the server sends an END_STREAM for a stream and then closes the
connection, allow the stream to complete normally.

Fixes golang/go#48995

Change-Id: Ia876e6e6e7eb4a0563db74c931c03a44620ece08
Reviewed-on: https://go-review.googlesource.com/c/net/+/356030
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/transport.go

diff --git a/http2/transport.go b/http2/transport.go
index 9b126ea..faeea16 100644
--- a/http2/transport.go
+++ b/http2/transport.go
@@ -2000,7 +2000,13 @@
 	}
 	cc.closed = true
 	for _, cs := range cc.streams {
-		cs.abortStreamLocked(err)
+		select {
+		case <-cs.peerClosed:
+			// The server closed the stream before closing the conn,
+			// so no need to interrupt it.
+		default:
+			cs.abortStreamLocked(err)
+		}
 	}
 	cc.cond.Broadcast()
 	cc.mu.Unlock()