Go の http.Client が送り直す要求と返すエラー

mux.HandleFunc("/items/1", func(w http.ResponseWriter, r *http.Request) {
	code, _ := strconv.Atoi(r.URL.Query().Get("code"))
	http.Redirect(w, r, "/items", code)
})
mux.HandleFunc("/items", func(w http.ResponseWriter, r *http.Request) {
	b, _ := io.ReadAll(r.Body)
	w.Header().Set("X-Got", fmt.Sprintf("%s body=%dB", r.Method, len(b)))
})
// ...
req, _ := http.NewRequest(method, fmt.Sprintf("%s/items/1?code=%d", srv.URL, code), strings.NewReader(`{"id":1}`))
res, err := http.DefaultClient.Do(req)

$ go run ./redirect
# 既定の Client
301 DELETE => GET body=0B      status=200
301 PUT    => GET body=0B      status=200
301 POST   => GET body=0B      status=200
301 HEAD   => HEAD body=0B     status=200
302 DELETE => GET body=0B      status=200
302 PUT    => GET body=0B      status=200
302 POST   => GET body=0B      status=200
302 HEAD   => HEAD body=0B     status=200
303 DELETE => GET body=0B      status=200
303 PUT    => GET body=0B      status=200
303 POST   => GET body=0B      status=200
303 HEAD   => HEAD body=0B     status=200
307 DELETE => DELETE body=8B   status=200
307 PUT    => PUT body=8B      status=200
307 POST   => POST body=8B     status=200
307 HEAD   => HEAD body=0B     status=200
308 DELETE => DELETE body=8B   status=200
308 PUT    => PUT body=8B      status=200
308 POST   => POST body=8B     status=200
308 HEAD   => HEAD body=0B     status=200

If permitted, a 301, 302, or 303 redirect causes subsequent requests to use HTTP method GET
(or HEAD if the original request was HEAD), with no body.
A 307 or 308 redirect preserves the original HTTP method and body,
provided that the [Request.GetBody] function is defined.

noFollow := &http.Client{CheckRedirect: func(*http.Request, []*http.Request) error {
	return http.ErrUseLastResponse
}}

# CheckRedirect で http.ErrUseLastResponse を返す Client
302 DELETE => (転送しない)          status=302

// 応答を返さずに TCP を RST で切る (SetLinger(0) で Close すると FIN ではなく RST)
func rst(c net.Conn) {
	if tc, ok := c.(*tls.Conn); ok {
		c = tc.NetConn()
	}
	c.(*net.TCPConn).SetLinger(0)
	c.Close()
}

srv := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
	if calls.Add(1) == 2 { // 2 回目 = 再利用した接続の上の要求
		rst(r.Context().Value(connKey{}).(net.Conn))
		return
	}
	io.WriteString(w, "ok")
}))
srv.Config.ConnContext = func(ctx context.Context, c net.Conn) context.Context {
	return context.WithValue(ctx, connKey{}, c) // HTTP/2 は Hijack できないので接続を ctx に載せる
}
srv.EnableHTTP2 = o.h2
srv.StartTLS()

for range 2 {
	req, _ := http.NewRequest(o.method, srv.URL, nil)
	res, e := srv.Client().Do(req)
	if err = e; err == nil {
		io.Copy(io.Discard, res.Body) // 読み切らないと接続が再利用されない
		res.Body.Close()
	}
}

$ go run ./resend
HTTP/1.1 GET                       proto=HTTP/1.1 calls=3 err=<nil>
HTTP/1.1 POST                      proto=HTTP/1.1 calls=2 err=Post "https://127.0.0.1:36697": read tcp 127.0.0.1:40284->127.0.0.1:36697: read: connection reset by peer
HTTP/1.1 POST + Idempotency-Key    proto=HTTP/1.1 calls=3 err=<nil>
HTTP/1.1 POST + Idempotency-Key + 本文 proto=HTTP/1.1 calls=3 err=<nil>
HTTP/1.1 POST + Idempotency-Key + 本文(io.Reader) proto=HTTP/1.1 calls=2 err=Post "https://127.0.0.1:33239": write tcp 127.0.0.1:35862->127.0.0.1:33239: write: connection reset by peer
HTTP/2 GET                         proto=HTTP/2.0 calls=2 err=Get "https://127.0.0.1:45489": read tcp 127.0.0.1:59756->127.0.0.1:45489: read: connection reset by peer
HTTP/1.1 GET (1 回目の本文を読み切らない)      proto=HTTP/1.1 calls=2 err=Get "https://127.0.0.1:46749": read tcp 127.0.0.1:33944->127.0.0.1:46749: read: connection reset by peer

func (r *Request) isReplayable() bool {
	if r.Body == nil || r.Body == NoBody || r.GetBody != nil {
		switch valueOrDefault(r.Method, "GET") {
		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
}

If body is of type [*bytes.Buffer], [*bytes.Reader], or
[*strings.Reader], the returned request's ContentLength is set to its
exact value (instead of -1), GetBody is populated (so 307 and 308
redirects can replay the body), and Body is set to [NoBody] if the
ContentLength is 0.

$ go run ./getbody
# 本文の型ごとの GetBody と ContentLength
*strings.Reader  GetBody=true  ContentLength=8
*bytes.Reader    GetBody=true  ContentLength=8
*bytes.Buffer    GetBody=true  ContentLength=8
io.MultiReader   GetBody=false ContentLength=0

# POST /old (307 で /new へ転送)
*strings.Reader  status=200 url=/new got=POST body="{\"id\":1}"
io.MultiReader   status=307 url=/old got=

# 同じ *http.Request をもう一度 Do する (アプリ側の再試行)
*strings.Reader  そのまま                           GetConn=2 2 回目: POST body="{\"id\":1}"
*strings.Reader  GetBody を nil にする              GetConn=1 Post "http://127.0.0.1:40703/new": http: ContentLength=8 with Body length 0
*strings.Reader  Body を GetBody() で差し替える        GetConn=1 2 回目: POST body="{\"id\":1}"
io.MultiReader   そのまま                           GetConn=1 2 回目: POST body=""

	if _, ok := err.(nothingWrittenError); ok {
		// We never wrote anything, so it's safe to retry, if there's no body or we
		// can "rewind" the body with GetBody.
		return req.outgoingLength() == 0 || req.GetBody != nil
	}

$ cd "$(go env GOROOT)/src/net/http"
$ grep -c 'MaxIdleConnsPerHost\|MaxConnsPerHost' h2_bundle.go
0
$ grep -o 't\.t1\.[A-Za-z]*' h2_bundle.go | sort -u
t.t1.DisableCompression
t.t1.DisableKeepAlives
t.t1.ExpectContinueTimeout
t.t1.IdleConnTimeout
t.t1.MaxResponseHeaderBytes
t.t1.ResponseHeaderTimeout
$ grep -n 't\.MaxConnsPerHost' transport.go
351:		MaxConnsPerHost:        t.MaxConnsPerHost,
1600:	if t.MaxConnsPerHost <= 0 {
1605:	if n := t.connsPerHost[w.key]; n < t.MaxConnsPerHost {
1664:	if t.MaxConnsPerHost <= 0 {

srv.Config.ConnState = func(_ net.Conn, s http.ConnState) {
	if s == http.StateNew {
		conns.Add(1) // TCP + TLS の握手を 1 回した
	}
}
if c.serverStreams > 0 {
	srv.Config.HTTP2 = &http.HTTP2Config{MaxConcurrentStreams: c.serverStreams}
}
srv.EnableHTTP2 = c.h2
srv.StartTLS()

tr := srv.Client().Transport.(*http.Transport).Clone()
tr.MaxIdleConnsPerHost = c.maxIdle
tr.MaxConnsPerHost = c.maxConns
client := &http.Client{Transport: tr}

$ go run ./h2conns
# MaxIdleConnsPerHost=2 (idle=2)
HTTP/1.1 idle=2 conns=0 streams=0   warm=false 同時=8   x 3波 => 握手 計20  波ごと[8 6 6]
HTTP/2.0 idle=2 conns=0 streams=0   warm=false 同時=8   x 3波 => 握手 計8   波ごと[8 0 0]
HTTP/2.0 idle=2 conns=0 streams=0   warm=true  同時=160 x 3波 => 握手 計1   波ごと[0 0 0]
# MaxConnsPerHost=1 (conns=1)。HTTP/2 はサーバの同時ストリームを 2 にして接続を増やさせる
HTTP/1.1 idle=1 conns=1 streams=0   warm=false 同時=8   x 3波 => 握手 計1   波ごと[1 0 0]
HTTP/2.0 idle=1 conns=1 streams=2   warm=true  同時=8   x 3波 => 握手 計4   波ごと[3 0 0]
HTTP/2.0 idle=1 conns=0 streams=2   warm=true  同時=8   x 3波 => 握手 計11  波ごと[10 0 0]
# 接続が 1 本も無いところへ同時に送る (warm=false)
HTTP/2.0 idle=2 conns=0 streams=0   warm=false 同時=160 x 1波 => 握手 計160 波ごと[160]
HTTP/2.0 idle=2 conns=1 streams=0   warm=false 同時=160 x 1波 => 握手 計1   波ごと[1]

// 誤り: net.Error を満たせばタイムアウトとみなす
func statusWrong(err error) int {
	var ne net.Error
	if errors.As(err, &ne) {
		return http.StatusRequestTimeout
	}
	return http.StatusInternalServerError
}

// 正: Timeout() が true のときだけタイムアウトとみなす
func statusRight(err error) int {
	var ne net.Error
	if errors.As(err, &ne) && ne.Timeout() {
		return http.StatusRequestTimeout
	}
	return http.StatusInternalServerError
}

$ go run ./neterror
接続リセット         net.Error=true  Timeout()=false ECONNRESET=true  wrong=408 right=500
               Get "http://127.0.0.1:33749": read tcp 127.0.0.1:35392->127.0.0.1:33749: read: connection reset by peer
接続拒否           net.Error=true  Timeout()=false ECONNRESET=false wrong=408 right=500
               Get "http://127.0.0.1:35831": dial tcp 127.0.0.1:35831: connect: connection refused
Client.Timeout net.Error=true  Timeout()=true  ECONNRESET=false wrong=408 right=408
               Get "http://127.0.0.1:46471": context deadline exceeded (Client.Timeout exceeded while awaiting headers)

func fetch(client doer, rawURL string, hideQuery bool) error {
	req, _ := http.NewRequest("GET", rawURL, nil)
	res, err := client.Do(req)
	if err != nil {
		var uerr *url.Error
		if hideQuery && errors.As(err, &uerr) {
			uerr.URL = req.URL.Path // クエリを落としパスだけ残す
		}
		return fmt.Errorf("fetch status: %w", err)
	}
	res.Body.Close()
	return nil
}
// ...
err := fetch(http.DefaultClient, "http://"+addr+"/v1/status?token=secret-123", false)
log.Error("request failed", "path", "/v1/status", "error", err)

$ go run ./urlerror
# そのまま %w で包む
{"level":"ERROR","msg":"request failed","path":"/v1/status","error":"fetch status: Get \"http://127.0.0.1:38211/v1/status?token=secret-123\": dial tcp 127.0.0.1:38211: connect: connection refused"}
# url.Error の URL をパスに差し替えてから %w で包む
{"level":"ERROR","msg":"request failed","path":"/v1/status","error":"fetch status: Get \"/v1/status\": dial tcp 127.0.0.1:38211: connect: connection refused"}
errors.Is(err, syscall.ECONNREFUSED) = true
errors.As(err, &uerr) = true

# fake の doer (Client ごと差し替え) が errors.New を返すとき
fetch status: connection refused
token を含む: false
# fake の Transport (RoundTripper) が errors.New を返すとき
fetch status: Get "http://127.0.0.1:38211/v1/status?token=secret-123": connection refused
token を含む: true