more comments about fastcgi buffering

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -244,7 +244,15 @@ func (h *Handler) Provision(ctx caddy.Context) error {
 		}
 		h.Transport = mod.(http.RoundTripper)
 		// enable request buffering for fastcgi if not configured
-		// TODO: better default buffering for fastcgi requests without content length
+		// 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
 		}