remotes: always try to establish tls connection when tls configured

When a endpoint is configured for http and has a tls configuration,
always try to the tls connection and fallback to http when the tls
connections fails from receiving an http response. This fixes an issue
with default localhost endpoints which get defaulted to http with
insecure tls also configured but are using tls.

Signed-off-by: Derek McGowan <derek@mcg.dev>

remotes/docker/resolver.go

@@ -18,6 +18,7 @@ package docker
 
 import (
 	"context"
+	"crypto/tls"
 	"errors"
 	"fmt"
 	"io"
@@ -707,3 +708,27 @@ func IsLocalhost(host string) bool {
 	ip := net.ParseIP(host)
 	return ip.IsLoopback()
 }
+
+// HTTPFallback is an http.RoundTripper which allows fallback from https to http
+// for registry endpoints with configurations for both http and TLS, such as
+// defaulted localhost endpoints.
+type HTTPFallback struct {
+	http.RoundTripper
+}
+
+func (f HTTPFallback) RoundTrip(r *http.Request) (*http.Response, error) {
+	resp, err := f.RoundTripper.RoundTrip(r)
+	var tlsErr tls.RecordHeaderError
+	if errors.As(err, &tlsErr) && string(tlsErr.RecordHeader[:]) == "HTTP/" {
+		// server gave HTTP response to HTTPS client
+		plainHTTPUrl := *r.URL
+		plainHTTPUrl.Scheme = "http"
+
+		plainHTTPRequest := *r
+		plainHTTPRequest.URL = &plainHTTPUrl
+
+		return f.RoundTripper.RoundTrip(&plainHTTPRequest)
+	}
+
+	return resp, err
+}