Implement autodetection of Windows buildkitd socket

Buildkit on Windows doesn't support rootless mode, and doesn't put the
namespace into the pipe name currently, so the Windows version is
near-trivial.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

pkg/buildkitutil/buildkitutil.go

@@ -56,28 +56,14 @@ func BuildctlBaseArgs(buildkitHost string) []string {
 }
 
 func GetBuildkitHost(namespace string) (string, error) {
-	if namespace == "" {
-		return "", fmt.Errorf("namespace must be specified")
-	}
-	// Try candidate locations of the current containerd namespace.
-	run := "/run/"
-	if rootlessutil.IsRootless() {
-		var err error
-		run, err = rootlessutil.XDGRuntimeDir()
-		if err != nil {
-			log.L.Warn(err)
-			run = fmt.Sprintf("/run/user/%d", rootlessutil.ParentEUID())
-		}
-	}
-	var hostRel []string
-	if namespace != "default" {
-		hostRel = append(hostRel, fmt.Sprintf("buildkit-%s/buildkitd.sock", namespace))
+	paths, err := getBuildkitHostCandidates(namespace)
+	if err != nil {
+		return "", err
 	}
-	hostRel = append(hostRel, "buildkit-default/buildkitd.sock", "buildkit/buildkitd.sock")
+
 	var errs []error //nolint:prealloc
-	for _, p := range hostRel {
-		log.L.Debugf("Choosing the buildkit host %q, candidates=%v (in %q)", p, hostRel, run)
-		buildkitHost := "unix://" + filepath.Join(run, p)
+	for _, buildkitHost := range paths {
+		log.L.Debugf("Choosing the buildkit host %q, candidates=%v", buildkitHost, paths)
 		_, err := pingBKDaemon(buildkitHost)
 		if err == nil {
 			log.L.Debugf("Chosen buildkit host %q", buildkitHost)
@@ -87,7 +73,7 @@ func GetBuildkitHost(namespace string) (string, error) {
 	}
 	allErr := errors.Join(errs...)
 	log.L.WithError(allErr).Error(getHint())
-	return "", fmt.Errorf("no buildkit host is available, tried %d candidates: %w", len(hostRel), allErr)
+	return "", fmt.Errorf("no buildkit host is available, tried %d candidates: %w", len(paths), allErr)
 }
 
 func GetWorkerLabels(buildkitHost string) (labels map[string]string, _ error) {

pkg/buildkitutil/buildkitutil_unix.go

@@ -0,0 +1,55 @@
+//go:build freebsd || linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package buildkitutil
+
+import (
+	"fmt"
+	"path/filepath"
+
+	"github.com/containerd/log"
+	"github.com/containerd/nerdctl/pkg/rootlessutil"
+)
+
+func getBuildkitHostCandidates(namespace string) ([]string, error) {
+	if namespace == "" {
+		return []string{}, fmt.Errorf("namespace must be specified")
+	}
+	// Try candidate locations of the current containerd namespace.
+	run := "/run/"
+	if rootlessutil.IsRootless() {
+		var err error
+		run, err = rootlessutil.XDGRuntimeDir()
+		if err != nil {
+			log.L.Warn(err)
+			run = fmt.Sprintf("/run/user/%d", rootlessutil.ParentEUID())
+		}
+	}
+	var hostRel []string
+	if namespace != "default" {
+		hostRel = append(hostRel, fmt.Sprintf("buildkit-%s/buildkitd.sock", namespace))
+	}
+	hostRel = append(hostRel, "buildkit-default/buildkitd.sock", "buildkit/buildkitd.sock")
+
+	candidates := make([]string, len(hostRel))
+	for _, p := range hostRel {
+		candidates = append(candidates, "unix://"+filepath.Join(run, p))
+	}
+
+	return candidates, nil
+}

pkg/buildkitutil/buildkitutil_windows.go

@@ -0,0 +1,21 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package buildkitutil
+
+func getBuildkitHostCandidates(namespace string) ([]string, error) {
+	return []string{"npipe:////./pipe/buildkitd"}, nil
+}