0x00 | Host
当你发送一个请求给下游服务的时候，如果你发送请求的时候是IP，这个时候你想要通过Header里传递Host。但是，如果你只是在header头里设置Host: www.baidu.com，你会发现下游服务收到的Host还是IP。。
解决方案如下：
req, _ := http.NewRequest(method, url, bodyReader)
req.Host = req.Header.Get("Host") // https://github.com/golang/go/issues/7682
我们跟踪到req.Host处也可以看到它的注释
// For client requests Host optionally overrides the Host
// header to send. If empty, the Request.Write method uses
// the value of URL.Host. Host may contain an international
// domain name.
Host string
0x01 | Content-Length
这个和上一个问题类似，区别是上一个问题导致Host错误，这个问题可能会导致丢失Content-Length从而变成chunked。
这个问题很蛋疼，因为明明我在header里设置了Content-Length，但是实际却变成了chunked。经过反复的测试都没有重现，直到在github上找到了这个https://github.com/golang/go/issues/16264
后来我又查到了这段代码。
func NewRequest(method, url string, body io.Reader) (*Request, error) {
if method == "" {
// We document that "" means "GET" for Request.Method, and people have
// relied on that from NewRequest, so keep that working.
// We still enforce validMethod for non-empty methods.
method = "GET"
}
if !validMethod(method) {
return nil, fmt.Errorf("net/http: invalid method %q", method)
}
u, err := parseURL(url) // Just url.Parse (url is shadowed for godoc).
if err != nil {
return nil, err
}
rc, ok := body.(io.ReadCloser)
if !ok && body != nil {
rc = ioutil.NopCloser(body)
}
// The host's colon:port should be normalized. See Issue 14836.
u.Host = removeEmptyPort(u.Host)
req := &Request{
Method: method,
URL: u,
Proto: "HTTP/1.1",
ProtoMajor: 1,
ProtoMinor: 1,
Header: make(Header),
Body: rc,
Host: u.Host,
}
if body != nil {
switch v := body.(type) {
case *bytes.Buffer:
req.ContentLength = int64(v.Len())
buf := v.Bytes()
req.GetBody = func() (io.ReadCloser, error) {
r := bytes.NewReader(buf)
return ioutil.NopCloser(r), nil
}
case *bytes.Reader:
req.ContentLength = int64(v.Len())
snapshot := *v
req.GetBody = func() (io.ReadCloser, error) {
r := snapshot
return ioutil.NopCloser(&r), nil
}
case *strings.Reader:
req.ContentLength = int64(v.Len())
snapshot := *v
req.GetBody = func() (io.ReadCloser, error) {
r := snapshot
return ioutil.NopCloser(&r), nil
}
default:
// This is where we'd set it to -1 (at least
// if body != NoBody) to mean unknown, but
// that broke people during the Go 1.8 testing
// period. People depend on it being 0 I
// guess. Maybe retry later. See Issue 18117.
}
// For client requests, Request.ContentLength of 0
// means either actually 0, or unknown. The only way
// to explicitly say that the ContentLength is zero is
// to set the Body to nil. But turns out too much code
// depends on NewRequest returning a non-nil Body,
// so we use a well-known ReadCloser variable instead
// and have the http package also treat that sentinel
// variable to mean explicitly zero.
if req.GetBody != nil && req.ContentLength == 0 {
req.Body = NoBody
req.GetBody = func() (io.ReadCloser, error) { return NoBody, nil }
}
}
return req, nil
}
可以看到，这里面居然有个switch，当你使用bytes.Buffer,bytes.Reader或者strings.Reader作为Body的时候，它会自动给你设置req.ContentLength...
所以，问题不是当你Post一个ReadCloser的时候，就会变成chunked，而是你Post非这三种类型的body进来的时候都没有Content-Length，需要自己显式设置。代码如下：
req, _ := http.NewRequest(method, url, bodyReader)
req.ContentLength = req.Header.Get("Content-Length")
0x02 | WriteHeader
这个问题就更蛋疼了。先看注释
// WriteHeader sends an HTTP response header with status code.
// If WriteHeader is not called explicitly, the first call to Write
// will trigger an implicit WriteHeader(http.StatusOK).
// Thus explicit calls to WriteHeader are mainly used to
// send error codes.
WriteHeader(int)
注解只说了显式调用一般是发送错误码，一般不用调用，当调用Write的时候默认会设置http.StatusOK。但是这里却没有告诉我们在WriteHeader之后进行的任何Header操作都是不生效的！
这简直就是个坑。因为我们经常会根据不同的情况Write不同的Body，一般在Write body的时候才知道是不是需要进行一些特殊的header操作。本来想着，要不把WriteHeader放到最后去，但是发现最后设置的StatusCode压根不生效。翻了下源码才明白
func (w *response) WriteHeader(code int) {
if w.conn.hijacked() {
w.conn.server.logf("http: response.WriteHeader on hijacked connection")
return
}
if w.wroteHeader {
w.conn.server.logf("http: multiple response.WriteHeader calls")
return
}
w.wroteHeader = true
w.status = code
if w.calledHeader && w.cw.header == nil {
w.cw.header = w.handlerHeader.clone()
}
if cl := w.handlerHeader.get("Content-Length"); cl != "" {
v, err := strconv.ParseInt(cl, 10, 64)
if err == nil && v >= 0 {
w.contentLength = v
} else {
w.conn.server.logf("http: invalid Content-Length of %q", cl)
w.handlerHeader.Del("Content-Length")
}
}
}
// either dataB or dataS is non-zero.
func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err error) {
if w.conn.hijacked() {
if lenData > 0 {
w.conn.server.logf("http: response.Write on hijacked connection")
}
return 0, ErrHijacked
}
if !w.wroteHeader {
w.WriteHeader(StatusOK)
}
if lenData == 0 {
return 0, nil
}
if !w.bodyAllowed() {
return 0, ErrBodyNotAllowed
}
w.written += int64(lenData) // ignoring errors, for errorKludge
if w.contentLength != -1 && w.written > w.contentLength {
return 0, ErrContentLength
}
if dataB != nil {
return w.w.Write(dataB)
} else {
return w.w.WriteString(dataS)
}
}
其实就是注解中那句话，如果没有显式调用WriteHeader，当第一次调用Write的时候就会自动进行一次WriteHeader，所以，后续就不能再显式调用WriteHeader了。。
这个坑逆天就逆天在你必须先Header.Set然后才能WriteHeader，最后才能Write。这个顺序如果乱了，要不就是丢失StatusCode，要不就是丢失Header.
最终无奈之下我们只能遵循规则，在不同的判定条件里分别WriteHeader。。