cache_fetch: Send Content-Length: 0 for anything not GET or HEAD

otherwise keep the existing logic of sending no Content-Length header if there
is no request body.

This approach was determined to be better than the inverse of sending C-L: 0 for
specific methods to avoid backends sending 411 responses as noted in
#4331 (comment)

This code runs before vcl_backend_fetch, so should VCL gain back control over
Content-Length as proposed in #4331, it will be possible again to override this
default.

Fixes #4331 for the special case

Author: nigoroll

bin/varnishd/cache/cache_fetch.c

@@ -411,8 +411,14 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 
 	http_PrintfHeader(bo->bereq, "X-Varnish: %ju", VXID(bo->vsl->wid));
 
-	if (bo->bereq_body == NULL && bo->req == NULL)
-		http_Unset(bo->bereq, H_Content_Length);
+	if (bo->bereq_body == NULL && bo->req == NULL) {
+		const char *met = http_GetMethod(bo->bereq);
+		if (http_method_eq(met, GET) ||
+		    http_method_eq(met, HEAD))
+			http_Unset(bo->bereq, H_Content_Length);
+		else
+			http_ForceHeader(bo->bereq, H_Content_Length, "0");
+	}
 
 	VCL_backend_fetch_method(bo->vcl, wrk, NULL, bo, NULL);
 

bin/varnishtest/tests/e00036.vtc

@@ -17,7 +17,7 @@ server s1 {
 	expect req.method == XGET
 	expect req.url == "/foo/body1"
 	expect req.bodylen == 0
-	expect req.http.Content-Length == "<undef>"
+	expect req.http.Content-Length == "0"
 	expect req.http.Transfer-Encoding == "<undef>"
 	txresp -body {
 		Included file
@@ -40,7 +40,7 @@ server s1 {
 	expect req.method == XGET
 	expect req.url == "/foo/body2"
 	expect req.bodylen == 0
-	expect req.http.Content-Length == "<undef>"
+	expect req.http.Content-Length == "0"
 	expect req.http.Transfer-Encoding == "<undef>"
 	txresp -body {
 		Included file

bin/varnishtest/tests/r04340.vtc

@@ -0,0 +1,18 @@
+varnishtest "C-L with empty POST"
+
+server s1 {
+	rxreq
+	txresp
+	expect req.http.content-length == 0
+} -start
+
+varnish v1 -vcl+backend {
+
+} -start
+
+client c1 {
+	txreq -method POST -hdr "content-length: 0"
+	rxresp
+} -run
+
+server s1 -wait