net/http: set Content-Length:0 for empty PATCH requests as with POST, PATCH

Sets Content-Length:0 for nil bodies in PATCH requests, as we already do for  POST and PUT requests.

RFC 2616 mentions that unless a method’s Content-Length is forbidden it can send one.
In the wild, we’ve found that Microsoft Azure’s DataLake Gen2 storage API https://docs.microsoft.com/en-us/rest/api/storageservices/datalakestoragegen2/path/update deliberately rejects PATCH requests without a Content-Length, yet there is no workaround for setting that header when trying to flush the content of a file which was uploaded in a previous request.

Fixes #40978

Change-Id: Ib0a623b907d827a1c5ee431dca3c41024fa291c5
GitHub-Last-Rev: 12a3903
GitHub-Pull-Request: #40991
Reviewed-on: https://go-review.googlesource.com/c/go/+/250039
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>

Author: Segflow

src/net/http/requestwrite_test.go

@@ -588,6 +588,26 @@ var reqWriteTests = []reqWriteTest{
 		},
 		WantError: errors.New("net/http: can't write control character in Request.URL"),
 	},
+
+	26: { // Request with nil body and PATCH method. Issue #40978
+		Req: Request{
+			Method:        "PATCH",
+			URL:           mustParseURL("/"),
+			Host:          "example.com",
+			ProtoMajor:    1,
+			ProtoMinor:    1,
+			ContentLength: 0, // as if unset by user
+		},
+		Body: nil,
+		WantWrite: "PATCH / HTTP/1.1\r\n" +
+			"Host: example.com\r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"Content-Length: 0\r\n\r\n",
+		WantProxy: "PATCH / HTTP/1.1\r\n" +
+			"Host: example.com\r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"Content-Length: 0\r\n\r\n",
+	},
 }
 
 func TestRequestWrite(t *testing.T) {

src/net/http/transfer.go

@@ -258,7 +258,7 @@ func (t *transferWriter) shouldSendContentLength() bool {
 		return false
 	}
 	// Many servers expect a Content-Length for these methods
-	if t.Method == "POST" || t.Method == "PUT" {
+	if t.Method == "POST" || t.Method == "PUT" || t.Method == "PATCH" {
 		return true
 	}
 	if t.ContentLength == 0 && isIdentity(t.TransferEncoding) {