net/http: make Transport respect {X-,}Idempotency-Key header

Fixes #19943

Change-Id: I5e0fefe44791d7b3556095d726c2a753ec551ef2
Reviewed-on: https://go-review.googlesource.com/c/147457
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/net/http/export_test.go

@@ -242,3 +242,5 @@ func ExportSetH2GoawayTimeout(d time.Duration) (restore func()) {
 	http2goAwayTimeout = d
 	return func() { http2goAwayTimeout = old }
 }
+
+func (r *Request) ExportIsReplayable() bool { return r.isReplayable() }

src/net/http/header.go

@@ -52,6 +52,13 @@ func (h Header) get(key string) string {
 	return ""
 }
 
+// has reports whether h has the provided key defined, even if it's
+// set to 0-length slice.
+func (h Header) has(key string) bool {
+	_, ok := h[key]
+	return ok
+}
+
 // Del deletes the values associated with key.
 // The key is case insensitive; it is canonicalized by
 // textproto.CanonicalMIMEHeaderKey.

src/net/http/request.go

@@ -579,7 +579,7 @@ func (r *Request) write(w io.Writer, usingProxy bool, extraHeaders Header, waitF
 	// Use the defaultUserAgent unless the Header contains one, which
 	// may be blank to not send the header.
 	userAgent := defaultUserAgent
-	if _, ok := r.Header["User-Agent"]; ok {
+	if r.Header.has("User-Agent") {
 		userAgent = r.Header.Get("User-Agent")
 	}
 	if userAgent != "" {
@@ -1345,6 +1345,12 @@ func (r *Request) isReplayable() bool {
 		case "GET", "HEAD", "OPTIONS", "TRACE":
 			return true
 		}
+		// The Idempotency-Key, while non-standard, is widely used to
+		// mean a POST or other request is idempotent. See
+		// https://golang.org/issue/19943#issuecomment-421092421
+		if r.Header.has("Idempotency-Key") || r.Header.has("X-Idempotency-Key") {
+			return true
+		}
 	}
 	return false
 }

src/net/http/requestwrite_test.go

@@ -544,6 +544,38 @@ var reqWriteTests = []reqWriteTest{
 			"User-Agent: Go-http-client/1.1\r\n" +
 			"\r\n",
 	},
+
+	// Verify that a nil header value doesn't get written.
+	23: {
+		Req: Request{
+			Method: "GET",
+			URL:    mustParseURL("/foo"),
+			Header: Header{
+				"X-Foo":             []string{"X-Bar"},
+				"X-Idempotency-Key": nil,
+			},
+		},
+
+		WantWrite: "GET /foo HTTP/1.1\r\n" +
+			"Host: \r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"X-Foo: X-Bar\r\n\r\n",
+	},
+	24: {
+		Req: Request{
+			Method: "GET",
+			URL:    mustParseURL("/foo"),
+			Header: Header{
+				"X-Foo":             []string{"X-Bar"},
+				"X-Idempotency-Key": []string{},
+			},
+		},
+
+		WantWrite: "GET /foo HTTP/1.1\r\n" +
+			"Host: \r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"X-Foo: X-Bar\r\n\r\n",
+	},
 }
 
 func TestRequestWrite(t *testing.T) {

src/net/http/server.go

@@ -1390,7 +1390,7 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 		}
 	}
 
-	if _, ok := header["Date"]; !ok {
+	if !header.has("Date") {
 		setHeader.date = appendTime(cw.res.dateBuf[:0], time.Now())
 	}
 

src/net/http/transport.go

@@ -91,6 +91,15 @@ func init() {
 // considered a terminal status and returned by RoundTrip. To see the
 // ignored 1xx responses, use the httptrace trace package's
 // ClientTrace.Got1xxResponse.
+//
+// Transport only retries a request upon encountering a network error
+// if the request is idempotent and either has no body or has its
+// Request.GetBody defined. HTTP requests are considered idempotent if
+// they have HTTP methods GET, HEAD, OPTIONS, or TRACE; or if their
+// Header map contains an "Idempotency-Key" or "X-Idempotency-Key"
+// entry. If the idempotency key value is an zero-length slice, the
+// request is treated as idempotent but the header is not sent on the
+// wire.
 type Transport struct {
 	idleMu     sync.Mutex
 	wantIdle   bool                                // user has requested to close all idle conns

src/net/http/transport_test.go

@@ -4952,3 +4952,56 @@ func TestTransportCONNECTBidi(t *testing.T) {
 		}
 	}
 }
+
+func TestTransportRequestReplayable(t *testing.T) {
+	someBody := ioutil.NopCloser(strings.NewReader(""))
+	tests := []struct {
+		name string
+		req  *Request
+		want bool
+	}{
+		{
+			name: "GET",
+			req:  &Request{Method: "GET"},
+			want: true,
+		},
+		{
+			name: "GET_http.NoBody",
+			req:  &Request{Method: "GET", Body: NoBody},
+			want: true,
+		},
+		{
+			name: "GET_body",
+			req:  &Request{Method: "GET", Body: someBody},
+			want: false,
+		},
+		{
+			name: "POST",
+			req:  &Request{Method: "POST"},
+			want: false,
+		},
+		{
+			name: "POST_idempotency-key",
+			req:  &Request{Method: "POST", Header: Header{"Idempotency-Key": {"x"}}},
+			want: true,
+		},
+		{
+			name: "POST_x-idempotency-key",
+			req:  &Request{Method: "POST", Header: Header{"X-Idempotency-Key": {"x"}}},
+			want: true,
+		},
+		{
+			name: "POST_body",
+			req:  &Request{Method: "POST", Header: Header{"Idempotency-Key": {"x"}}, Body: someBody},
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.req.ExportIsReplayable()
+			if got != tt.want {
+				t.Errorf("replyable = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}