runfiles: port phst/runfiles to rules_go

Today rules_go provide //go/tools/bazel as the canonical runfiles
library for binaries to be executed with `bazel test` and `bazel run`.

However, the current implementation pre-date the recent changes in
Bazel's upstream.  Since then, all of the native runfiles library of
Bash, Java, CPP, Python have been refactored to follow a certain
convention in locating files. (1)

Although these are subjected to change with the incoming BzlMod feature,
it would be easier to maintain if we can keep rules_go's runfiles
library implementation aligned with native languages' implementation.

Today, it seems like https://github.com/phst/runfiles implemented
exactly that.  So with @fmeum suggestion and @phst permission (2), let's
port the newer, more accurate implementation to rules_go.

Future refactoring will mark the current exported APIs in
//go/tools/bazel as deprecated and/or swapping out the old implementation
underneath to use this newer package.

Changes in this PR included:
- Copy paste repository over
- Removal of .git and .gitignore and .githooks dir
- Removal of repository specific files: README, WORKSPACE
- Rename BUILD to BUILD.bazel
- Rename import path for both go and BUILD files
- Run gazelle over the packages
- Adjusted test cases to reflect new package paths
- Removed godoc related to installation instruction
- Fixed test to handle window path separator

(1): https://docs.google.com/document/d/e/2PACX-1vSDIrFnFvEYhKsCMdGdD40wZRBX3m3aZ5HhVj4CtHPmiXKDCxioTUbYsDydjKtFDAzER5eg7OjJWs3V/pub
(2): phst/runfiles#3 (comment)

go/tools/bazel/runfiles/BUILD.bazel

@@ -0,0 +1,53 @@
+# Copyright 2020 Google LLC
+#
+# 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
+#
+#     https://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.
+
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "runfiles",
+    srcs = [
+        "directory.go",
+        "fs.go",
+        "global.go",
+        "manifest.go",
+        "runfiles.go",
+    ],
+    importpath = "github.com/bazelbuild/rules_go/go/tools/bazel/runfiles",
+    visibility = ["//visibility:public"],
+)
+
+go_test(
+    name = "runfiles_test",
+    srcs = [
+        "fs_test.go",
+        "runfiles_test.go",
+    ],
+    data = [
+        "test.txt",
+        "//go/tools/bazel/runfiles/testprog",
+    ],
+    rundir = ".",
+    deps = [":runfiles"],
+)
+
+exports_files(
+    ["test.txt"],
+    visibility = ["//go/tools/bazel/runfiles/testprog:__pkg__"],
+)
+
+alias(
+    name = "go_default_library",
+    actual = ":runfiles",
+    visibility = ["//visibility:public"],
+)

go/tools/bazel/runfiles/directory.go

@@ -0,0 +1,32 @@
+// Copyright 2020, 2021, 2021 Google LLC
+//
+// 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
+//
+//     https://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 runfiles
+
+import "path/filepath"
+
+// Directory specifies the location of the runfiles directory.  You can pass
+// this as an option to New.  If unset or empty, use the value of the
+// environmental variable RUNFILES_DIR.
+type Directory string
+
+func (d Directory) new() *Runfiles {
+	return &Runfiles{d, directoryVar + "=" + string(d)}
+}
+
+func (d Directory) path(s string) (string, error) {
+	return filepath.Join(string(d), filepath.FromSlash(s)), nil
+}
+
+const directoryVar = "RUNFILES_DIR"

go/tools/bazel/runfiles/fs.go

@@ -0,0 +1,98 @@
+// Copyright 2021, 2022 Google LLC
+//
+// 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
+//
+//     https://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.
+
+// +build go1.16
+
+package runfiles
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"time"
+)
+
+// Open implements fs.FS.Open.
+func (r *Runfiles) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
+	}
+	p, err := r.Path(name)
+	if errors.Is(err, ErrEmpty) {
+		return emptyFile(name), nil
+	}
+	if err != nil {
+		return nil, pathError("open", name, err)
+	}
+	return os.Open(p)
+}
+
+// Stat implements fs.StatFS.Stat.
+func (r *Runfiles) Stat(name string) (fs.FileInfo, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "stat", Path: name, Err: fs.ErrInvalid}
+	}
+	p, err := r.Path(name)
+	if errors.Is(err, ErrEmpty) {
+		return emptyFileInfo(name), nil
+	}
+	if err != nil {
+		return nil, pathError("stat", name, err)
+	}
+	return os.Stat(p)
+}
+
+// ReadFile implements fs.ReadFileFS.ReadFile.
+func (r *Runfiles) ReadFile(name string) ([]byte, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
+	}
+	p, err := r.Path(name)
+	if errors.Is(err, ErrEmpty) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, pathError("open", name, err)
+	}
+	return os.ReadFile(p)
+}
+
+type emptyFile string
+
+func (f emptyFile) Stat() (fs.FileInfo, error) { return emptyFileInfo(f), nil }
+func (f emptyFile) Read([]byte) (int, error)   { return 0, io.EOF }
+func (emptyFile) Close() error                 { return nil }
+
+type emptyFileInfo string
+
+func (i emptyFileInfo) Name() string     { return string(i) }
+func (emptyFileInfo) Size() int64        { return 0 }
+func (emptyFileInfo) Mode() fs.FileMode  { return 0444 }
+func (emptyFileInfo) ModTime() time.Time { return time.Time{} }
+func (emptyFileInfo) IsDir() bool        { return false }
+func (emptyFileInfo) Sys() interface{}   { return nil }
+
+func pathError(op, name string, err error) error {
+	if err == nil {
+		return nil
+	}
+	var rerr Error
+	if errors.As(err, &rerr) {
+		// Unwrap the error because we don’t need the failing name
+		// twice.
+		return &fs.PathError{Op: op, Path: rerr.Name, Err: rerr.Err}
+	}
+	return &fs.PathError{Op: op, Path: name, Err: err}
+}

go/tools/bazel/runfiles/fs_test.go

@@ -0,0 +1,96 @@
+// Copyright 2021 Google LLC
+//
+// 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
+//
+//     https://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.
+
+//go:build go1.16
+// +build go1.16
+
+package runfiles_test
+
+import (
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+
+	"github.com/bazelbuild/rules_go/go/tools/bazel/runfiles"
+)
+
+func TestFS(t *testing.T) {
+	fsys, err := runfiles.New()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Ensure that the Runfiles object implements FS interfaces.
+	var _ fs.FS = fsys
+	var _ fs.StatFS = fsys
+	var _ fs.ReadFileFS = fsys
+	if err := fstest.TestFS(fsys, "io_bazel_rules_go/go/tools/bazel/runfiles/test.txt", "io_bazel_rules_go/go/tools/bazel/runfiles/testprog/testprog"); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestFS_empty(t *testing.T) {
+	dir := t.TempDir()
+	manifest := filepath.Join(dir, "manifest")
+	if err := os.WriteFile(manifest, []byte("__init__.py \n"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	fsys, err := runfiles.New(runfiles.ManifestFile(manifest), runfiles.ProgramName("/invalid"), runfiles.Directory("/invalid"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Run("Open", func(t *testing.T) {
+		fd, err := fsys.Open("__init__.py")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer fd.Close()
+		got, err := io.ReadAll(fd)
+		if err != nil {
+			t.Error(err)
+		}
+		if len(got) != 0 {
+			t.Errorf("got nonempty contents: %q", got)
+		}
+	})
+	t.Run("Stat", func(t *testing.T) {
+		got, err := fsys.Stat("__init__.py")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got.Name() != "__init__.py" {
+			t.Errorf("Name: got %q, want %q", got.Name(), "__init__.py")
+		}
+		if got.Size() != 0 {
+			t.Errorf("Size: got %d, want %d", got.Size(), 0)
+		}
+		if !got.Mode().IsRegular() {
+			t.Errorf("IsRegular: got %v, want %v", got.Mode().IsRegular(), true)
+		}
+		if got.IsDir() {
+			t.Errorf("IsDir: got %v, want %v", got.IsDir(), false)
+		}
+	})
+	t.Run("ReadFile", func(t *testing.T) {
+		got, err := fsys.ReadFile("__init__.py")
+		if err != nil {
+			t.Error(err)
+		}
+		if len(got) != 0 {
+			t.Errorf("got nonempty contents: %q", got)
+		}
+	})
+}

go/tools/bazel/runfiles/global.go

@@ -0,0 +1,60 @@
+// Copyright 2020, 2021 Google LLC
+//
+// 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
+//
+//     https://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 runfiles
+
+import "sync"
+
+// Path returns the absolute path name of a runfile.  The runfile name must be
+// a relative path, using the slash (not backslash) as directory separator.  If
+// the runfiles manifest maps s to an empty name (indicating an empty runfile
+// not present in the filesystem), Path returns an error that wraps ErrEmpty.
+func Path(s string) (string, error) {
+	r, err := g.get()
+	if err != nil {
+		return "", err
+	}
+	return r.Path(s)
+}
+
+// Env returns additional environmental variables to pass to subprocesses.
+// Each element is of the form “key=value”.  Pass these variables to
+// Bazel-built binaries so they can find their runfiles as well.  See the
+// Runfiles example for an illustration of this.
+//
+// The return value is a newly-allocated slice; you can modify it at will.
+func Env() ([]string, error) {
+	r, err := g.get()
+	if err != nil {
+		return nil, err
+	}
+	return r.Env(), nil
+}
+
+type global struct {
+	once sync.Once
+	r    *Runfiles
+	err  error
+}
+
+func (g *global) get() (*Runfiles, error) {
+	g.once.Do(g.init)
+	return g.r, g.err
+}
+
+func (g *global) init() {
+	g.r, g.err = New()
+}
+
+var g global