command: check for wsl mount path on windows

This checks for the equivalent WSL mount path on windows. WSL will mount
the windows drives at `/mnt/c` (or whichever drive is being used).

This is done by parsing a UNC path with forward slashes from the unix
socket URL.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
(cherry picked from commit 38c3fef)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/telemetry_docker.go

@@ -5,9 +5,14 @@ package command
 
 import (
 	"context"
+	"fmt"
+	"io/fs"
 	"net/url"
 	"os"
 	"path"
+	"path/filepath"
+	"strings"
+	"unicode"
 
 	"github.com/pkg/errors"
 	"go.opentelemetry.io/otel"
@@ -77,14 +82,7 @@ func dockerExporterOTLPEndpoint(cli Cli) (endpoint string, secure bool) {
 
 	switch u.Scheme {
 	case "unix":
-		// Unix sockets are a bit weird. OTEL seems to imply they
-		// can be used as an environment variable and are handled properly,
-		// but they don't seem to be as the behavior of the environment variable
-		// is to strip the scheme from the endpoint, but the underlying implementation
-		// needs the scheme to use the correct resolver.
-		//
-		// We'll just handle this in a special way and add the unix:// back to the endpoint.
-		endpoint = "unix://" + path.Join(u.Host, u.Path)
+		endpoint = unixSocketEndpoint(u)
 	case "https":
 		secure = true
 		fallthrough
@@ -135,3 +133,109 @@ func dockerMetricExporter(ctx context.Context, cli Cli) []sdkmetric.Option {
 	}
 	return []sdkmetric.Option{sdkmetric.WithReader(newCLIReader(exp))}
 }
+
+// unixSocketEndpoint converts the unix scheme from URL to
+// an OTEL endpoint that can be used with the OTLP exporter.
+//
+// The OTLP exporter handles unix sockets in a strange way.
+// It seems to imply they can be used as an environment variable
+// and are handled properly, but they don't seem to be as the behavior
+// of the environment variable is to strip the scheme from the endpoint
+// while the underlying implementation needs the scheme to use the
+// correct resolver.
+func unixSocketEndpoint(u *url.URL) string {
+	// GRPC does not allow host to be used.
+	socketPath := u.Path
+
+	// If we are on windows and we have an absolute path
+	// that references a letter drive, check to see if the
+	// WSL equivalent path exists and we should use that instead.
+	if isWsl() {
+		if p := wslSocketPath(socketPath, os.DirFS("/")); p != "" {
+			socketPath = p
+		}
+	}
+	// Enforce that we are using forward slashes.
+	return "unix://" + filepath.ToSlash(socketPath)
+}
+
+// wslSocketPath will convert the referenced URL to a WSL-compatible
+// path and check if that path exists. If the path exists, it will
+// be returned.
+func wslSocketPath(s string, f fs.FS) string {
+	if p := toWslPath(s); p != "" {
+		if _, err := stat(p, f); err == nil {
+			return "/" + p
+		}
+	}
+	return ""
+}
+
+// toWslPath converts the referenced URL to a WSL-compatible
+// path if this looks like a Windows absolute path.
+//
+// If no drive is in the URL, defaults to the C drive.
+func toWslPath(s string) string {
+	drive, p, ok := parseUNCPath(s)
+	if !ok {
+		return ""
+	}
+	return fmt.Sprintf("mnt/%s%s", drive, p)
+}
+
+func parseUNCPath(s string) (drive, p string, ok bool) {
+	// UNC paths use backslashes but we're using forward slashes
+	// so also enforce that here.
+	//
+	// In reality, this should have been enforced much earlier
+	// than here since backslashes aren't allowed in URLs, but
+	// we're going to code defensively here.
+	s = filepath.ToSlash(s)
+
+	const uncPrefix = "//./"
+	if !strings.HasPrefix(s, uncPrefix) {
+		// Not a UNC path.
+		return "", "", false
+	}
+	s = s[len(uncPrefix):]
+
+	parts := strings.SplitN(s, "/", 2)
+	if len(parts) != 2 {
+		// Not enough components.
+		return "", "", false
+	}
+
+	drive, ok = splitWindowsDrive(parts[0])
+	if !ok {
+		// Not a windows drive.
+		return "", "", false
+	}
+	return drive, "/" + parts[1], true
+}
+
+// splitWindowsDrive checks if the string references a windows
+// drive (such as c:) and returns the drive letter if it is.
+func splitWindowsDrive(s string) (string, bool) {
+	if b := []rune(s); len(b) == 2 && unicode.IsLetter(b[0]) && b[1] == ':' {
+		return string(b[0]), true
+	}
+	return "", false
+}
+
+func stat(p string, f fs.FS) (fs.FileInfo, error) {
+	if f, ok := f.(fs.StatFS); ok {
+		return f.Stat(p)
+	}
+
+	file, err := f.Open(p)
+	if err != nil {
+		return nil, err
+	}
+
+	defer file.Close()
+	return file.Stat()
+}
+
+func isWsl() bool {
+	return os.Getenv("WSL_DISTRO_NAME") != ""
+}

cli/command/telemetry_docker_test.go

@@ -0,0 +1,29 @@
+package command
+
+import (
+	"net/url"
+	"testing"
+	"testing/fstest"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestWslSocketPath(t *testing.T) {
+	u, err := url.Parse("unix:////./c:/my/file/path")
+	assert.NilError(t, err)
+
+	// Ensure host is empty.
+	assert.Equal(t, u.Host, "")
+
+	// Use a filesystem where the WSL path exists.
+	fs := fstest.MapFS{
+		"mnt/c/my/file/path": {},
+	}
+	assert.Equal(t, wslSocketPath(u.Path, fs), "/mnt/c/my/file/path")
+
+	// Use a filesystem where the WSL path doesn't exist.
+	fs = fstest.MapFS{
+		"my/file/path": {},
+	}
+	assert.Equal(t, wslSocketPath(u.Path, fs), "")
+}