go/buildutil: ignore path letters case in ContainingPackage on windows

Fixes golang/go#13368

Change-Id: I4cee2078bc64b6f175f206daa9609246cc1b1f85
Reviewed-on: https://go-review.googlesource.com/17211
Reviewed-by: Alan Donovan <adonovan@google.com>

go/buildutil/util.go

@@ -15,6 +15,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"runtime"
 	"strings"
 )
 
@@ -72,7 +73,7 @@ func ContainingPackage(ctxt *build.Context, dir, filename string) (*build.Packag
 	// We assume that no source root (GOPATH[i] or GOROOT) contains any other.
 	for _, srcdir := range ctxt.SrcDirs() {
 		srcdirSlash := filepath.ToSlash(srcdir) + "/"
-		if strings.HasPrefix(dirSlash, srcdirSlash) {
+		if dirHasPrefix(dirSlash, srcdirSlash) {
 			importPath := dirSlash[len(srcdirSlash) : len(dirSlash)-len("/")]
 			return ctxt.Import(importPath, dir, build.FindOnly)
 		}
@@ -81,6 +82,14 @@ func ContainingPackage(ctxt *build.Context, dir, filename string) (*build.Packag
 	return nil, fmt.Errorf("can't find package containing %s", filename)
 }
 
+// dirHasPrefix tests whether the directory dir begins with prefix.
+func dirHasPrefix(dir, prefix string) bool {
+	if runtime.GOOS != "windows" {
+		return strings.HasPrefix(dir, prefix)
+	}
+	return len(dir) >= len(prefix) && strings.EqualFold(dir[:len(prefix)], prefix)
+}
+
 // -- Effective methods of file system interface -------------------------
 
 // (go/build.Context defines these as methods, but does not export them.)

go/buildutil/util_windows_test.go

@@ -0,0 +1,48 @@
+// Copyright 2015 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 buildutil_test
+
+import (
+	"fmt"
+	"go/build"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/go/buildutil"
+)
+
+func testContainingPackageCaseFold(file, want string) error {
+	bp, err := buildutil.ContainingPackage(&build.Default, ".", file)
+	if err != nil {
+		return err
+	}
+	if got := bp.ImportPath; got != want {
+		return fmt.Errorf("ContainingPackage(%q) = %s, want %s", file, got, want)
+	}
+	return nil
+}
+
+func TestContainingPackageCaseFold(t *testing.T) {
+	path := filepath.Join(runtime.GOROOT(), `src\fmt\print.go`)
+	err := testContainingPackageCaseFold(path, "fmt")
+	if err != nil {
+		t.Error(err)
+	}
+	vol := filepath.VolumeName(path)
+	if len(vol) != 2 || vol[1] != ':' {
+		t.Fatalf("GOROOT path has unexpected volume name: %v", vol)
+	}
+	rest := path[len(vol):]
+	err = testContainingPackageCaseFold(strings.ToUpper(vol)+rest, "fmt")
+	if err != nil {
+		t.Error(err)
+	}
+	err = testContainingPackageCaseFold(strings.ToLower(vol)+rest, "fmt")
+	if err != nil {
+		t.Error(err)
+	}
+}