internal/vuln: add convertFileURLPath on Windows

This addresses bcmills's comment in CL 508503.

Change-Id: I528465feb074c220ca2fc8a1f50eb6c1376f6b75
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/755581
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
TryBot-Bypass: Michael Matloob <matloob@google.com>

5 files changed, 65 insertions, 18 deletions

diff --git a/internal/vuln/source_test.go b/internal/vuln/source_test.go
index 6eff01c8..03893e41 100644
--- a/internal/vuln/source_test.go
+++ b/internal/vuln/source_test.go
@@ -12,16 +12,12 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
-	"runtime"
 	"testing"
 
 	"golang.org/x/pkgsite/internal/osv"
 )
 
 func TestNewSource(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("windows is not supported (see convertFileURLPath")
-	}
 	t.Run("https", func(t *testing.T) {
 		url := "https://vuln.go.dev"
 		s, err := NewSource(url)
diff --git a/internal/vuln/url.go b/internal/vuln/url.go
index 60e2d1ea..67774c13 100644
--- a/internal/vuln/url.go
+++ b/internal/vuln/url.go
@@ -12,7 +12,6 @@ import (
 	"errors"
 	"net/url"
 	"path/filepath"
-	"runtime"
 )
 
 var errNotAbsolute = errors.New("path is not absolute")
@@ -43,15 +42,3 @@ func URLToFilePath(u *url.URL) (string, error) {
 	}
 	return checkAbs(path)
 }
-
-func convertFileURLPath(host, path string) (string, error) {
-	if runtime.GOOS == "windows" {
-		return "", errors.New("windows not supported")
-	}
-	switch host {
-	case "", "localhost":
-	default:
-		return "", errors.New("file URL specifies non-local host")
-	}
-	return filepath.FromSlash(path), nil
-}
diff --git a/internal/vuln/url_other.go b/internal/vuln/url_other.go
new file mode 100644
index 00000000..6694390b
--- /dev/null
+++ b/internal/vuln/url_other.go
@@ -0,0 +1,21 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package vuln
+
+import (
+	"errors"
+	"path/filepath"
+)
+
+func convertFileURLPath(host, path string) (string, error) {
+	switch host {
+	case "", "localhost":
+	default:
+		return "", errors.New("file URL specifies non-local host")
+	}
+	return filepath.FromSlash(path), nil
+}
diff --git a/internal/vuln/url_test.go b/internal/vuln/url_test.go
index 22334cb7..46908568 100644
--- a/internal/vuln/url_test.go
+++ b/internal/vuln/url_test.go
@@ -12,7 +12,7 @@ import (
 
 func TestURLToFilePath(t *testing.T) {
 	if runtime.GOOS == "windows" {
-		t.Skip("windows is not supported (see convertFileURLPath")
+		t.Skip("windows is not supported (see convertFileURLPath)")
 	}
 	for _, tc := range urlTests {
 		if tc.url == "" {
diff --git a/internal/vuln/url_windows.go b/internal/vuln/url_windows.go
new file mode 100644
index 00000000..bb649697
--- /dev/null
+++ b/internal/vuln/url_windows.go
@@ -0,0 +1,43 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vuln
+
+import (
+	"errors"
+	"path/filepath"
+	"strings"
+)
+
+func convertFileURLPath(host, path string) (string, error) {
+	if len(path) == 0 || path[0] != '/' {
+		return "", errNotAbsolute
+	}
+
+	path = filepath.FromSlash(path)
+
+	// We interpret Windows file URLs per the description in
+	// https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/.
+
+	// The host part of a file URL (if any) is the UNC volume name,
+	// but RFC 8089 reserves the authority "localhost" for the local machine.
+	if host != "" && host != "localhost" {
+		// A common "legacy" format omits the leading slash before a drive letter,
+		// encoding the drive letter as the host instead of part of the path.
+		// (See https://blogs.msdn.microsoft.com/freeassociations/2005/05/19/the-bizarre-and-unhappy-story-of-file-urls/.)
+		// We do not support that format, but we should at least emit a more
+		// helpful error message for it.
+		if filepath.VolumeName(host) != "" {
+			return "", errors.New("file URL encodes drive letter in host field (example file:///C:/path)")
+		}
+		return `\\` + host + path, nil
+	}
+
+	// If host is empty, path must contain an initial slash followed by a
+	// drive letter and path. Remove the slash and verify that the path is valid.
+	if vol := filepath.VolumeName(path[1:]); vol == "" || strings.HasPrefix(vol, `\\`) {
+		return "", errors.New("file URL missing drive letter (e.g., file:///C:/path)")
+	}
+	return filepath.Clean(path[1:]), nil
+}