config/module: handle absolute file paths on Windows

Using url.Parse to parse an absolute file path on Windows yields
a URL type where the Path element is prefixed by a slash.

For example, parsing "file:///C:/Users/user" gives a URL type
with Path:"/C:/Users/user".

According to golang.org/issue/6027, the parsing is correct as is.

The leading slash on the Path must be eliminated before any file
operations.

This commit introduces a urlParse function which wraps the url.Parse
functionality and removes the leading slash in Path for absolute file
paths on Windows.

Fixes config/module test failures on Windows.

config/module/detect_file.go

@@ -21,13 +21,5 @@ func (d *FileDetector) Detect(src, pwd string) (string, bool, error) {
 
 		src = filepath.Join(pwd, src)
 	}
-	// Make sure we're using "/" even on Windows. URLs are "/"-based.
-	src = filepath.ToSlash(src)
-
-	// Make sure that we don't start with "/" since we add that below
-	if src[0] == '/' {
-		src = src[1:]
-	}
-
-	return fmt.Sprintf("file:///%s", src), true, nil
+	return fmtFileURL(src), true, nil
 }

config/module/get.go

@@ -72,7 +72,7 @@ func Get(dst, src string) error {
 		dst = tmpDir
 	}
 
-	u, err := url.Parse(src)
+	u, err := urlParse(src)
 	if err != nil {
 		return err
 	}

config/module/module_test.go

@@ -39,15 +39,11 @@ func testModule(n string) string {
 	if err != nil {
 		panic(err)
 	}
-
-	var url url.URL
-	url.Scheme = "file"
-	url.Path = filepath.ToSlash(p)
-	return url.String()
+	return fmtFileURL(p)
 }
 
 func testModuleURL(n string) *url.URL {
-	u, err := url.Parse(testModule(n))
+	u, err := urlParse(testModule(n))
 	if err != nil {
 		panic(err)
 	}

config/module/url_helper.go

@@ -0,0 +1,50 @@
+package module
+
+import (
+	"fmt"
+	"net/url"
+	"path/filepath"
+	"runtime"
+)
+
+func urlParse(rawURL string) (*url.URL, error) {
+	if runtime.GOOS == "windows" {
+		// Make sure we're using "/" on Windows. URLs are "/"-based.
+		rawURL = filepath.ToSlash(rawURL)
+	}
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return nil, err
+	}
+
+	if runtime.GOOS != "windows" {
+		return u, err
+	}
+
+	if u.Scheme != "file" {
+		return u, err
+	}
+
+	// Remove leading slash for absolute file paths on Windows.
+	// For example, url.Parse yields u.Path = "/C:/Users/user" for
+	// rawurl = "file:///C:/Users/user", which is an incorrect syntax.
+	if len(u.Path) > 2 && u.Path[0] == '/' && u.Path[2] == ':' {
+		u.Path = u.Path[1:]
+	}
+
+	return u, err
+}
+
+func fmtFileURL(path string) string {
+	if runtime.GOOS == "windows" {
+		// Make sure we're using "/" on Windows. URLs are "/"-based.
+		path = filepath.ToSlash(path)
+	}
+
+	// Make sure that we don't start with "/" since we add that below.
+	if path[0] == '/' {
+		path = path[1:]
+	}
+
+	return fmt.Sprintf("file:///%s", path)
+}