reverseproxy: buffer requests for fastcgi by default (#6759)

* buffer requests for fastcgi by default

* fix import cycle

* fix the return value of bufferedBody

* more comments about fastcgi buffering

---------

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

Author: WeidiDeng

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -243,6 +243,19 @@ func (h *Handler) Provision(ctx caddy.Context) error {
 			return fmt.Errorf("loading transport: %v", err)
 		}
 		h.Transport = mod.(http.RoundTripper)
+		// enable request buffering for fastcgi if not configured
+		// This is because most fastcgi servers are php-fpm that require the content length to be set to read the body, golang
+		// std has fastcgi implementation that doesn't need this value to process the body, but we can safely assume that's
+		// not used.
+		// http3 requests have a negative content length for GET and HEAD requests, if that header is not sent.
+		// see: https://github.com/caddyserver/caddy/issues/6678#issuecomment-2472224182
+		// Though it appears even if CONTENT_LENGTH is invalid, php-fpm can handle just fine if the body is empty (no Stdin records sent).
+		// php-fpm will hang if there is any data in the body though, https://github.com/caddyserver/caddy/issues/5420#issuecomment-2415943516
+
+		// TODO: better default buffering for fastcgi requests without content length, in theory a value of 1 should be enough, make it bigger anyway
+		if module, ok := h.Transport.(caddy.Module); ok && module.CaddyModule().ID.Name() == "fastcgi" && h.RequestBuffers == 0 {
+			h.RequestBuffers = 4096
+		}
 	}
 	if h.LoadBalancing != nil && h.LoadBalancing.SelectionPolicyRaw != nil {
 		mod, err := ctx.LoadModule(h.LoadBalancing, "SelectionPolicyRaw")
@@ -1216,13 +1229,14 @@ func (h Handler) bufferedBody(originalBody io.ReadCloser, limit int64) (io.ReadC
 	buf := bufPool.Get().(*bytes.Buffer)
 	buf.Reset()
 	if limit > 0 {
-		n, err := io.CopyN(buf, originalBody, limit)
-		if (err != nil && err != io.EOF) || n == limit {
+		var err error
+		written, err = io.CopyN(buf, originalBody, limit)
+		if (err != nil && err != io.EOF) || written == limit {
 			return bodyReadCloser{
 				Reader: io.MultiReader(buf, originalBody),
 				buf:    buf,
 				body:   originalBody,
-			}, n
+			}, written
 		}
 	} else {
 		written, _ = io.Copy(buf, originalBody)