net/http: add errors.As support for x/net/http2.StreamError

To make it easier to extract the HTTP/2 error code (if any) from
net/http errors, implement an As method on the vendored copy of
golang.org/x/net/http2.StreamError. The new As method lets users work
with the vendored error type as though it were the x/net/http2
StreamError.

Fixes #53896.

Change-Id: Ib18eb428adc05a3c0e19a946ece936e2378e1c7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/425104
Run-TryBot: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/net/http/h2_error.go

@@ -0,0 +1,38 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !nethttpomithttp2
+// +build !nethttpomithttp2
+
+package http
+
+import (
+	"reflect"
+)
+
+func (e http2StreamError) As(target any) bool {
+	dst := reflect.ValueOf(target).Elem()
+	dstType := dst.Type()
+	if dstType.Kind() != reflect.Struct {
+		return false
+	}
+	src := reflect.ValueOf(e)
+	srcType := src.Type()
+	numField := srcType.NumField()
+	if dstType.NumField() != numField {
+		return false
+	}
+	for i := 0; i < numField; i++ {
+		sf := srcType.Field(i)
+		df := dstType.Field(i)
+		if sf.Name != df.Name || !sf.Type.ConvertibleTo(df.Type) {
+			return false
+		}
+	}
+	for i := 0; i < numField; i++ {
+		df := dst.Field(i)
+		df.Set(src.Field(i).Convert(df.Type()))
+	}
+	return true
+}

src/net/http/h2_error_test.go

@@ -0,0 +1,44 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !nethttpomithttp2
+// +build !nethttpomithttp2
+
+package http
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+type externalStreamErrorCode uint32
+
+type externalStreamError struct {
+	StreamID uint32
+	Code     externalStreamErrorCode
+	Cause    error
+}
+
+func (e externalStreamError) Error() string {
+	return fmt.Sprintf("ID %v, code %v", e.StreamID, e.Code)
+}
+
+func TestStreamError(t *testing.T) {
+	var target externalStreamError
+	streamErr := http2streamError(42, http2ErrCodeProtocol)
+	ok := errors.As(streamErr, &target)
+	if !ok {
+		t.Fatalf("errors.As failed")
+	}
+	if target.StreamID != streamErr.StreamID {
+		t.Errorf("got StreamID %v, expected %v", target.StreamID, streamErr.StreamID)
+	}
+	if target.Cause != streamErr.Cause {
+		t.Errorf("got Cause %v, expected %v", target.Cause, streamErr.Cause)
+	}
+	if uint32(target.Code) != uint32(streamErr.Code) {
+		t.Errorf("got Code %v, expected %v", target.Code, streamErr.Code)
+	}
+}