generator.go

/* Copyright 2016 The Bazel Authors. All rights reserved.

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 generator provides core functionality of
// BUILD file generation in gazelle.
package generator

import (
	"fmt"
	"go/build"
	"path/filepath"
	"sort"
	"strings"

	bzl "github.com/bazelbuild/buildtools/build"
	"github.com/bazelbuild/rules_go/go/tools/gazelle/packages"
	"github.com/bazelbuild/rules_go/go/tools/gazelle/rules"
)

const (
	// goRulesBzl is the label of the Skylark file which provides Go rules
	goRulesBzl = "@io_bazel_rules_go//go:def.bzl"
)

// Generator generates BUILD files for a Go repository.
type Generator struct {
	repoRoot      string
	goPrefix      string
	buildFileName string
	bctx          build.Context
	g             rules.Generator
}

// New returns a new Generator which is responsible for a Go repository.
//
// "repoRoot" is a path to the root directory of the repository.
// "goPrefix" is the go_prefix corresponding to the repository root directory.
// See also https://github.com/bazelbuild/rules_go#go_prefix.
// "buildFileName" is the name of the BUILD file (BUILD or BUILD.bazel).
// "buildTags" is a comma-delimited set of build tags to set in the build context.
// "external" is how external packages should be resolved.
func New(repoRoot, goPrefix, buildFileName, buildTags string, external rules.ExternalResolver) (*Generator, error) {
	bctx := build.Default
	// Ignore source files in $GOROOT and $GOPATH
	bctx.GOROOT = ""
	bctx.GOPATH = ""

	repoRoot, err := filepath.Abs(repoRoot)
	if err != nil {
		return nil, err
	}

	// If build tags are not specified, do not apply build constraints.
	// The rules filter files at compile time, so it's not usually necessary
	// to filter when generating BUILD files, too.
	if buildTags == "" {
		bctx.UseAllFiles = true
	} else {
		bctx.UseAllFiles = false
		bctx.BuildTags = strings.Split(buildTags, ",")
	}

	return &Generator{
		repoRoot:      repoRoot,
		goPrefix:      goPrefix,
		buildFileName: buildFileName,
		bctx:          bctx,
		g:             rules.NewGenerator(repoRoot, goPrefix, external),
	}, nil
}

// Generate generates a BUILD file for each Go package found under
// the given directory.
// The directory must be the repository root directory the caller
// passed to New, or its subdirectory.
func (g *Generator) Generate(dir string) ([]*bzl.File, error) {
	dir, err := filepath.Abs(dir)
	if err != nil {
		return nil, err
	}
	dir = filepath.Clean(dir)
	if !isDescendingDir(dir, g.repoRoot) {
		return nil, fmt.Errorf("dir %s is not under the repository root %s", dir, g.repoRoot)
	}

	var files []*bzl.File
	err = packages.Walk(g.bctx, g.repoRoot, g.goPrefix, dir, func(pkg *build.Package) error {
		rel, err := filepath.Rel(g.repoRoot, pkg.Dir)
		if err != nil {
			return err
		}
		if rel == "." {
			rel = ""
		}
		if len(files) == 0 && rel != "" {
			// "dir" was not a buildable Go package but still need a BUILD file
			// for go_prefix.
			files = append(files, g.emptyToplevel())
		}

		file, err := g.generateOne(rel, pkg)
		if err != nil {
			return err
		}

		files = append(files, file)
		return nil
	})
	if err != nil {
		return nil, err
	}
	return files, nil
}

func (g *Generator) emptyToplevel() *bzl.File {
	return &bzl.File{
		Path: g.buildFileName,
		Stmt: []bzl.Expr{
			loadExpr("go_prefix"),
			&bzl.CallExpr{
				X: &bzl.LiteralExpr{Token: "go_prefix"},
				List: []bzl.Expr{
					&bzl.StringExpr{Value: g.goPrefix},
				},
			},
		},
	}
}

func (g *Generator) generateOne(rel string, pkg *build.Package) (*bzl.File, error) {
	rs, err := g.g.Generate(filepath.ToSlash(rel), pkg)
	if err != nil {
		return nil, err
	}

	file := &bzl.File{Path: filepath.Join(rel, g.buildFileName)}
	for _, r := range rs {
		file.Stmt = append(file.Stmt, r.Call)
	}
	if load := g.generateLoad(file); load != nil {
		file.Stmt = append([]bzl.Expr{load}, file.Stmt...)
	}
	return file, nil
}

func (g *Generator) generateLoad(f *bzl.File) bzl.Expr {
	var list []string
	for _, kind := range []string{
		"go_prefix",
		"go_library",
		"go_binary",
		"go_test",
		"cgo_library",
	} {
		if len(f.Rules(kind)) > 0 {
			list = append(list, kind)
		}
	}
	if len(list) == 0 {
		return nil
	}
	return loadExpr(list...)
}

func loadExpr(rules ...string) *bzl.CallExpr {
	sort.Strings(rules)

	list := []bzl.Expr{
		&bzl.StringExpr{Value: goRulesBzl},
	}
	for _, r := range rules {
		list = append(list, &bzl.StringExpr{Value: r})
	}

	return &bzl.CallExpr{
		X:            &bzl.LiteralExpr{Token: "load"},
		List:         list,
		ForceCompact: true,
	}
}

func isDescendingDir(dir, root string) bool {
	if dir == root {
		return true
	}
	return strings.HasPrefix(dir, fmt.Sprintf("%s%c", root, filepath.Separator))
}