Fix Go 1.19 compatibility for unix socket reverse proxy URLs

Since Go 1.19 the net/url URL.Parse function is changed when there is no
host authority. This change broke the unix socket URL handling in the
reverse proxy code. This change adds auto detection for the behavior
and thus supports both old and new Go versions without having to add
build constraints.

Related: golang/go#46059

caddyhttp/proxy/reverseproxy.go

@@ -113,7 +113,7 @@ type ReverseProxy struct {
 // "unix:///var/run/www.socket", hence the ambiguous trimming.
 func socketDial(hostName string, timeout time.Duration) func(network, addr string) (conn net.Conn, err error) {
 	return func(network, addr string) (conn net.Conn, err error) {
-		return net.DialTimeout("unix", hostName[len("unix://"):], timeout)
+		return net.DialTimeout("unix", hostName[unixProtocolPrefixLength:], timeout)
 	}
 }
 
@@ -217,7 +217,7 @@ func NewSingleHostReverseProxy(target *url.URL, without string, keepalive int, t
 		// to /var/run/www.socket/test, rendering paths useless.
 		if target.Scheme == "unix" {
 			// See comment on socketDial for the trim
-			socketPrefix := target.String()[len("unix://"):]
+			socketPrefix := target.String()[unixProtocolPrefixLength:]
 			req.URL.Path = strings.TrimPrefix(req.URL.Path, socketPrefix)
 			if req.URL.Opaque != "" {
 				req.URL.Opaque = strings.TrimPrefix(req.URL.Opaque, socketPrefix)

caddyhttp/proxy/unix.go

@@ -0,0 +1,21 @@
+package proxy
+
+import (
+	"net/url"
+	"strings"
+)
+
+// unixProtocolPrefixLength defines the length of the prefix of parsed URLs
+// starting with unix: scheme. The actual value is different since Go 1.19 where
+// the prefix no longer contains a // as unix: is not a registered scheme.
+// See https://github.com/golang/go/commit/ab0f7611d739fe10d0265dbc6bdc17684423bfc8
+// for reference.
+var unixProtocolPrefixLength = func() int {
+	u, _ := url.Parse("unix:/some/path")
+
+	if strings.HasPrefix(u.String(), "unix://") {
+		// For Go < 1.19 url parse generates strings with an injected //.
+		return len("unix://")
+	}
+	return len("unix:")
+}()