[tools] Move getProjectRoot() out to fileutils.

Add tests.
This is going to be used by intrinsic-gen, a new tool

Bug: tint:832
Change-Id: Ib22ffa88a699be9c8dfdf3f8f6e32cff4ffc88e3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52602
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>

tools/src/fileutils/fileutils.go

@@ -17,6 +17,8 @@ package fileutils
 
 import (
 	"os"
+	"path/filepath"
+	"runtime"
 )
 
 // IsExe returns true if the file at path is an executable
@@ -27,3 +29,27 @@ func IsExe(path string) bool {
 	}
 	return s.Mode()&0100 != 0
 }
+
+// GoSourcePath returns the absolute path to the .go file that calls the
+// function
+func GoSourcePath() string {
+	_, filename, _, ok := runtime.Caller(1)
+	if !ok {
+		panic("No caller information")
+	}
+	path, err := filepath.Abs(filename)
+	if err != nil {
+		panic(err)
+	}
+	return path
+}
+
+// ProjectRoot returns the path to the tint project root
+func ProjectRoot() string {
+	toolRoot := filepath.Dir(GoSourcePath())
+	root, err := filepath.Abs(filepath.Join(toolRoot, "../../.."))
+	if err != nil {
+		panic(err)
+	}
+	return root
+}

tools/src/fileutils/fileutils_test.go

@@ -0,0 +1,38 @@
+// Copyright 2021 The Tint 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 fileutils_test
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"dawn.googlesource.com/tint/tools/src/fileutils"
+)
+
+func TestGoSourcePath(t *testing.T) {
+	p := fileutils.GoSourcePath()
+	if !strings.HasSuffix(p, "fileutils/fileutils_test.go") {
+		t.Errorf("GoSourcePath() returned %v", p)
+	}
+}
+
+func TestProjectRoot(t *testing.T) {
+	p := filepath.Join(fileutils.ProjectRoot(), "tint_overrides_with_defaults.gni")
+	if _, err := os.Stat(p); os.IsNotExist(err) {
+		t.Errorf("ProjectRoot() returned %v", p)
+	}
+}