cmd/fiximports/main.go - tools.git - Git at Google

 // 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.

 // The fiximports command fixes import declarations to use the canonical
 // import path for packages that have an "import comment" as defined by
 // https://golang.org/s/go14customimport.
 //
 // # Background
 //
 // The Go 1 custom import path mechanism lets the maintainer of a
 // package give it a stable name by which clients may import and "go
 // get" it, independent of the underlying version control system (such
 // as Git) or server (such as github.com) that hosts it.  Requests for
 // the custom name are redirected to the underlying name.  This allows
 // packages to be migrated from one underlying server or system to
 // another without breaking existing clients.
 //
 // Because this redirect mechanism creates aliases for existing
 // packages, it's possible for a single program to import the same
 // package by its canonical name and by an alias.  The resulting
 // executable will contain two copies of the package, which is wasteful
 // at best and incorrect at worst.
 //
 // To avoid this, "go build" reports an error if it encounters a special
 // comment like the one below, and if the import path in the comment
 // does not match the path of the enclosing package relative to
 // GOPATH/src:
 //
 //	     $ grep ^package $GOPATH/src/github.com/bob/vanity/foo/foo.go
 //		package foo // import "vanity.com/foo"
 //
 // The error from "go build" indicates that the package canonically
 // known as "vanity.com/foo" is locally installed under the
 // non-canonical name "github.com/bob/vanity/foo".
 //
 // # Usage
 //
 // When a package that you depend on introduces a custom import comment,
 // and your workspace imports it by the non-canonical name, your build
 // will stop working as soon as you update your copy of that package
 // using "go get -u".
 //
 // The purpose of the fiximports tool is to fix up all imports of the
 // non-canonical path within a Go workspace, replacing them with imports
 // of the canonical path.  Following a run of fiximports, the workspace
 // will no longer depend on the non-canonical copy of the package, so it
 // should be safe to delete.  It may be necessary to run "go get -u"
 // again to ensure that the package is locally installed under its
 // canonical path, if it was not already.
 //
 // The fiximports tool operates locally; it does not make HTTP requests
 // and does not discover new custom import comments.  It only operates
 // on non-canonical packages present in your workspace.
 //
 // The -baddomains flag is a list of domain names that should always be
 // considered non-canonical.  You can use this if you wish to make sure
 // that you no longer have any dependencies on packages from that
 // domain, even those that do not yet provide a canonical import path
 // comment.  For example, the default value of -baddomains includes the
 // moribund code hosting site code.google.com, so fiximports will report
 // an error for each import of a package from this domain remaining
 // after canonicalization.
 //
 // To see the changes fiximports would make without applying them, use
 // the -n flag.
 package main

 import (
 	"bytes"
 	"encoding/json"
 	"flag"
 	"fmt"
 	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
 	"io"
 	"log"
 	"os"
 	"path"
 	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"

 	exec "golang.org/x/sys/execabs"
 )

 // flags
 var (
 	dryrun     = flag.Bool("n", false, "dry run: show changes, but don't apply them")
 	badDomains = flag.String("baddomains", "code.google.com",
 		"a comma-separated list of domains from which packages should not be imported")
 	replaceFlag = flag.String("replace", "",
 		"a comma-separated list of noncanonical=canonical pairs of package paths.  If both items in a pair end with '...', they are treated as path prefixes.")
 )

 // seams for testing
 var (
 	stderr    io.Writer = os.Stderr
 	writeFile           = os.WriteFile
 )

 const usage = `fiximports: rewrite import paths to use canonical package names.

 Usage: fiximports [-n] package...

 The package... arguments specify a list of packages
 in the style of the go tool; see "go help packages".
 Hint: use "all" or "..." to match the entire workspace.

 For details, see https://pkg.go.dev/golang.org/x/tools/cmd/fiximports

 Flags:
   -n:	       dry run: show changes, but don't apply them
   -baddomains  a comma-separated list of domains from which packages
                should not be imported
 `

 func main() {
 	flag.Parse()

 	if len(flag.Args()) == 0 {
 		fmt.Fprint(stderr, usage)
 		os.Exit(1)
 	}
 	if !fiximports(flag.Args()...) {
 		os.Exit(1)
 	}
 }

 type canonicalName struct{ path, name string }

 // fiximports fixes imports in the specified packages.
 // Invariant: a false result implies an error was already printed.
 func fiximports(packages ...string) bool {
 	// importedBy is the transpose of the package import graph.
 	importedBy := make(map[string]map[*listPackage]bool)

 	// addEdge adds an edge to the import graph.
 	addEdge := func(from *listPackage, to string) {
 		if to == "C" || to == "unsafe" {
 			return // fake
 		}
 		pkgs := importedBy[to]
 		if pkgs == nil {
 			pkgs = make(map[*listPackage]bool)
 			importedBy[to] = pkgs
 		}
 		pkgs[from] = true
 	}

 	// List metadata for all packages in the workspace.
 	pkgs, err := list("...")
 	if err != nil {
 		fmt.Fprintf(stderr, "importfix: %v\n", err)
 		return false
 	}

 	// packageName maps each package's path to its name.
 	packageName := make(map[string]string)
 	for _, p := range pkgs {
 		packageName[p.ImportPath] = p.Name
 	}

 	// canonical maps each non-canonical package path to
 	// its canonical path and name.
 	// A present nil value indicates that the canonical package
 	// is unknown: hosted on a bad domain with no redirect.
 	canonical := make(map[string]canonicalName)
 	domains := strings.Split(*badDomains, ",")

 	type replaceItem struct {
 		old, new    string
 		matchPrefix bool
 	}
 	var replace []replaceItem
 	for _, pair := range strings.Split(*replaceFlag, ",") {
 		if pair == "" {
 			continue
 		}
 		words := strings.Split(pair, "=")
 		if len(words) != 2 {
 			fmt.Fprintf(stderr, "importfix: -replace: %q is not of the form \"canonical=noncanonical\".\n", pair)
 			return false
 		}
 		replace = append(replace, replaceItem{
 			old: strings.TrimSuffix(words[0], "..."),
 			new: strings.TrimSuffix(words[1], "..."),
 			matchPrefix: strings.HasSuffix(words[0], "...") &&
 				strings.HasSuffix(words[1], "..."),
 		})
 	}

 	// Find non-canonical packages and populate importedBy graph.
 	for _, p := range pkgs {
 		if p.Error != nil {
 			msg := p.Error.Err
 			if strings.Contains(msg, "code in directory") &&
 				strings.Contains(msg, "expects import") {
 				// don't show the very errors we're trying to fix
 			} else {
 				fmt.Fprintln(stderr, p.Error)
 			}
 		}

 		for _, imp := range p.Imports {
 			addEdge(p, imp)
 		}
 		for _, imp := range p.TestImports {
 			addEdge(p, imp)
 		}
 		for _, imp := range p.XTestImports {
 			addEdge(p, imp)
 		}

 		// Does package have an explicit import comment?
 		if p.ImportComment != "" {
 			if p.ImportComment != p.ImportPath {
 				canonical[p.ImportPath] = canonicalName{
 					path: p.ImportComment,
 					name: p.Name,
 				}
 			}
 		} else {
 			// Is package matched by a -replace item?
 			var newPath string
 			for _, item := range replace {
 				if item.matchPrefix {
 					if strings.HasPrefix(p.ImportPath, item.old) {
 						newPath = item.new + p.ImportPath[len(item.old):]
 						break
 					}
 				} else if p.ImportPath == item.old {
 					newPath = item.new
 					break
 				}
 			}
 			if newPath != "" {
 				newName := packageName[newPath]
 				if newName == "" {
 					newName = filepath.Base(newPath) // a guess
 				}
 				canonical[p.ImportPath] = canonicalName{
 					path: newPath,
 					name: newName,
 				}
 				continue
 			}

 			// Is package matched by a -baddomains item?
 			for _, domain := range domains {
 				slash := strings.Index(p.ImportPath, "/")
 				if slash < 0 {
 					continue // no slash: standard package
 				}
 				if p.ImportPath[:slash] == domain {
 					// Package comes from bad domain and has no import comment.
 					// Report an error each time this package is imported.
 					canonical[p.ImportPath] = canonicalName{}

 					// TODO(adonovan): should we make an HTTP request to
 					// see if there's an HTTP redirect, a "go-import" meta tag,
 					// or an import comment in the latest revision?
 					// It would duplicate a lot of logic from "go get".
 				}
 				break
 			}
 		}
 	}

 	// Find all clients (direct importers) of canonical packages.
 	// These are the packages that need fixing up.
 	clients := make(map[*listPackage]bool)
 	for path := range canonical {
 		for client := range importedBy[path] {
 			clients[client] = true
 		}
 	}

 	// Restrict rewrites to the set of packages specified by the user.
 	if len(packages) == 1 && (packages[0] == "all" || packages[0] == "...") {
 		// no restriction
 	} else {
 		pkgs, err := list(packages...)
 		if err != nil {
 			fmt.Fprintf(stderr, "importfix: %v\n", err)
 			return false
 		}
 		seen := make(map[string]bool)
 		for _, p := range pkgs {
 			seen[p.ImportPath] = true
 		}
 		for client := range clients {
 			if !seen[client.ImportPath] {
 				delete(clients, client)
 			}
 		}
 	}

 	// Rewrite selected client packages.
 	ok := true
 	for client := range clients {
 		if !rewritePackage(client, canonical) {
 			ok = false

 			// There were errors.
 			// Show direct and indirect imports of client.
 			seen := make(map[string]bool)
 			var direct, indirect []string
 			for p := range importedBy[client.ImportPath] {
 				direct = append(direct, p.ImportPath)
 				seen[p.ImportPath] = true
 			}

 			var visit func(path string)
 			visit = func(path string) {
 				for q := range importedBy[path] {
 					qpath := q.ImportPath
 					if !seen[qpath] {
 						seen[qpath] = true
 						indirect = append(indirect, qpath)
 						visit(qpath)
 					}
 				}
 			}

 			if direct != nil {
 				fmt.Fprintf(stderr, "\timported directly by:\n")
 				sort.Strings(direct)
 				for _, path := range direct {
 					fmt.Fprintf(stderr, "\t\t%s\n", path)
 					visit(path)
 				}

 				if indirect != nil {
 					fmt.Fprintf(stderr, "\timported indirectly by:\n")
 					sort.Strings(indirect)
 					for _, path := range indirect {
 						fmt.Fprintf(stderr, "\t\t%s\n", path)
 					}
 				}
 			}
 		}
 	}

 	return ok
 }

 // Invariant: false result => error already printed.
 func rewritePackage(client *listPackage, canonical map[string]canonicalName) bool {
 	ok := true

 	used := make(map[string]bool)
 	var filenames []string
 	filenames = append(filenames, client.GoFiles...)
 	filenames = append(filenames, client.TestGoFiles...)
 	filenames = append(filenames, client.XTestGoFiles...)
 	var first bool
 	for _, filename := range filenames {
 		if !first {
 			first = true
 			fmt.Fprintf(stderr, "%s\n", client.ImportPath)
 		}
 		err := rewriteFile(filepath.Join(client.Dir, filename), canonical, used)
 		if err != nil {
 			fmt.Fprintf(stderr, "\tERROR: %v\n", err)
 			ok = false
 		}
 	}

 	// Show which imports were renamed in this package.
 	var keys []string
 	for key := range used {
 		keys = append(keys, key)
 	}
 	sort.Strings(keys)
 	for _, key := range keys {
 		if p := canonical[key]; p.path != "" {
 			fmt.Fprintf(stderr, "\tfixed: %s -> %s\n", key, p.path)
 		} else {
 			fmt.Fprintf(stderr, "\tERROR: %s has no import comment\n", key)
 			ok = false
 		}
 	}

 	return ok
 }

 // rewriteFile reads, modifies, and writes filename, replacing all imports
 // of packages P in canonical by canonical[P].
 // It records in used which canonical packages were imported.
 // used[P]=="" indicates that P was imported but its canonical path is unknown.
 func rewriteFile(filename string, canonical map[string]canonicalName, used map[string]bool) error {
 	fset := token.NewFileSet()
 	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
 	if err != nil {
 		return err
 	}
 	var changed bool
 	for _, imp := range f.Imports {
 		impPath, err := strconv.Unquote(imp.Path.Value)
 		if err != nil {
 			log.Printf("%s: bad import spec %q: %v",
 				fset.Position(imp.Pos()), imp.Path.Value, err)
 			continue
 		}
 		canon, ok := canonical[impPath]
 		if !ok {
 			continue // import path is canonical
 		}

 		used[impPath] = true

 		if canon.path == "" {
 			// The canonical path is unknown (a -baddomain).
 			// Show the offending import.
 			// TODO(adonovan): should we show the actual source text?
 			fmt.Fprintf(stderr, "\t%s:%d: import %q\n",
 				shortPath(filename),
 				fset.Position(imp.Pos()).Line, impPath)
 			continue
 		}

 		changed = true

 		imp.Path.Value = strconv.Quote(canon.path)

 		// Add a renaming import if necessary.
 		//
 		// This is a guess at best.  We can't see whether a 'go
 		// get' of the canonical import path would have the same
 		// name or not.  Assume it's the last segment.
 		newBase := path.Base(canon.path)
 		if imp.Name == nil && newBase != canon.name {
 			imp.Name = &ast.Ident{Name: canon.name}
 		}
 	}

 	if changed && !*dryrun {
 		var buf bytes.Buffer
 		if err := format.Node(&buf, fset, f); err != nil {
 			return fmt.Errorf("%s: couldn't format file: %v", filename, err)
 		}
 		return writeFile(filename, buf.Bytes(), 0644)
 	}

 	return nil
 }

 // listPackage corresponds to the output of go list -json,
 // but only the fields we need.
 type listPackage struct {
 	Name          string
 	Dir           string
 	ImportPath    string
 	GoFiles       []string
 	TestGoFiles   []string
 	XTestGoFiles  []string
 	Imports       []string
 	TestImports   []string
 	XTestImports  []string
 	ImportComment string
 	Error         *packageError // error loading package
 }

 // A packageError describes an error loading information about a package.
 type packageError struct {
 	ImportStack []string // shortest path from package named on command line to this one
 	Pos         string   // position of error
 	Err         string   // the error itself
 }

 func (e packageError) Error() string {
 	if e.Pos != "" {
 		return e.Pos + ": " + e.Err
 	}
 	return e.Err
 }

 // list runs 'go list' with the specified arguments and returns the
 // metadata for matching packages.
 func list(args ...string) ([]*listPackage, error) {
 	cmd := exec.Command("go", append([]string{"list", "-e", "-json"}, args...)...)
 	cmd.Stdout = new(bytes.Buffer)
 	cmd.Stderr = stderr
 	if err := cmd.Run(); err != nil {
 		return nil, err
 	}

 	dec := json.NewDecoder(cmd.Stdout.(io.Reader))
 	var pkgs []*listPackage
 	for {
 		var p listPackage
 		if err := dec.Decode(&p); err == io.EOF {
 			break
 		} else if err != nil {
 			return nil, err
 		}
 		pkgs = append(pkgs, &p)
 	}
 	return pkgs, nil
 }

 // cwd contains the current working directory of the tool.
 //
 // It is initialized directly so that its value will be set for any other
 // package variables or init functions that depend on it, such as the gopath
 // variable in main_test.go.
 var cwd string = func() string {
 	cwd, err := os.Getwd()
 	if err != nil {
 		log.Fatalf("os.Getwd: %v", err)
 	}
 	return cwd
 }()

 // shortPath returns an absolute or relative name for path, whatever is shorter.
 // Plundered from $GOROOT/src/cmd/go/build.go.
 func shortPath(path string) string {
 	if rel, err := filepath.Rel(cwd, path); err == nil && len(rel) < len(path) {
 		return rel
 	}
 	return path
 }
	// 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.

	// The fiximports command fixes import declarations to use the canonical
	// import path for packages that have an "import comment" as defined by
	// https://golang.org/s/go14customimport.
	//
	// # Background
	//
	// The Go 1 custom import path mechanism lets the maintainer of a
	// package give it a stable name by which clients may import and "go
	// get" it, independent of the underlying version control system (such
	// as Git) or server (such as github.com) that hosts it. Requests for
	// the custom name are redirected to the underlying name. This allows
	// packages to be migrated from one underlying server or system to
	// another without breaking existing clients.
	//
	// Because this redirect mechanism creates aliases for existing
	// packages, it's possible for a single program to import the same
	// package by its canonical name and by an alias. The resulting
	// executable will contain two copies of the package, which is wasteful
	// at best and incorrect at worst.
	//
	// To avoid this, "go build" reports an error if it encounters a special
	// comment like the one below, and if the import path in the comment
	// does not match the path of the enclosing package relative to
	// GOPATH/src:
	//
	// $ grep ^package $GOPATH/src/github.com/bob/vanity/foo/foo.go
	// package foo // import "vanity.com/foo"
	//
	// The error from "go build" indicates that the package canonically
	// known as "vanity.com/foo" is locally installed under the
	// non-canonical name "github.com/bob/vanity/foo".
	//
	// # Usage
	//
	// When a package that you depend on introduces a custom import comment,
	// and your workspace imports it by the non-canonical name, your build
	// will stop working as soon as you update your copy of that package
	// using "go get -u".
	//
	// The purpose of the fiximports tool is to fix up all imports of the
	// non-canonical path within a Go workspace, replacing them with imports
	// of the canonical path. Following a run of fiximports, the workspace
	// will no longer depend on the non-canonical copy of the package, so it
	// should be safe to delete. It may be necessary to run "go get -u"
	// again to ensure that the package is locally installed under its
	// canonical path, if it was not already.
	//
	// The fiximports tool operates locally; it does not make HTTP requests
	// and does not discover new custom import comments. It only operates
	// on non-canonical packages present in your workspace.
	//
	// The -baddomains flag is a list of domain names that should always be
	// considered non-canonical. You can use this if you wish to make sure
	// that you no longer have any dependencies on packages from that
	// domain, even those that do not yet provide a canonical import path
	// comment. For example, the default value of -baddomains includes the
	// moribund code hosting site code.google.com, so fiximports will report
	// an error for each import of a package from this domain remaining
	// after canonicalization.
	//
	// To see the changes fiximports would make without applying them, use
	// the -n flag.
	package main

	import (
	"bytes"
	"encoding/json"
	"flag"
	"fmt"
	"go/ast"
	"go/format"
	"go/parser"
	"go/token"
	"io"
	"log"
	"os"
	"path"
	"path/filepath"
	"sort"
	"strconv"
	"strings"

	exec "golang.org/x/sys/execabs"
	)

	// flags
	var (
	dryrun = flag.Bool("n", false, "dry run: show changes, but don't apply them")
	badDomains = flag.String("baddomains", "code.google.com",
	"a comma-separated list of domains from which packages should not be imported")
	replaceFlag = flag.String("replace", "",
	"a comma-separated list of noncanonical=canonical pairs of package paths. If both items in a pair end with '...', they are treated as path prefixes.")
	)

	// seams for testing
	var (
	stderr io.Writer = os.Stderr
	writeFile = os.WriteFile
	)

	const usage = `fiximports: rewrite import paths to use canonical package names.

	Usage: fiximports [-n] package...

	The package... arguments specify a list of packages
	in the style of the go tool; see "go help packages".
	Hint: use "all" or "..." to match the entire workspace.

	For details, see https://pkg.go.dev/golang.org/x/tools/cmd/fiximports

	Flags:
	-n: dry run: show changes, but don't apply them
	-baddomains a comma-separated list of domains from which packages
	should not be imported
	`

	func main() {
	flag.Parse()

	if len(flag.Args()) == 0 {
	fmt.Fprint(stderr, usage)
	os.Exit(1)
	}
	if !fiximports(flag.Args()...) {
	os.Exit(1)
	}
	}

	type canonicalName struct{ path, name string }

	// fiximports fixes imports in the specified packages.
	// Invariant: a false result implies an error was already printed.
	func fiximports(packages ...string) bool {
	// importedBy is the transpose of the package import graph.
	importedBy := make(map[string]map[*listPackage]bool)

	// addEdge adds an edge to the import graph.
	addEdge := func(from *listPackage, to string) {
	if to == "C" \|\| to == "unsafe" {
	return // fake
	}
	pkgs := importedBy[to]
	if pkgs == nil {
	pkgs = make(map[*listPackage]bool)
	importedBy[to] = pkgs
	}
	pkgs[from] = true
	}

	// List metadata for all packages in the workspace.
	pkgs, err := list("...")
	if err != nil {
	fmt.Fprintf(stderr, "importfix: %v\n", err)
	return false
	}

	// packageName maps each package's path to its name.
	packageName := make(map[string]string)
	for _, p := range pkgs {
	packageName[p.ImportPath] = p.Name
	}

	// canonical maps each non-canonical package path to
	// its canonical path and name.
	// A present nil value indicates that the canonical package
	// is unknown: hosted on a bad domain with no redirect.
	canonical := make(map[string]canonicalName)
	domains := strings.Split(*badDomains, ",")

	type replaceItem struct {
	old, new string
	matchPrefix bool
	}
	var replace []replaceItem
	for _, pair := range strings.Split(*replaceFlag, ",") {
	if pair == "" {
	continue
	}
	words := strings.Split(pair, "=")
	if len(words) != 2 {
	fmt.Fprintf(stderr, "importfix: -replace: %q is not of the form \"canonical=noncanonical\".\n", pair)
	return false
	}
	replace = append(replace, replaceItem{
	old: strings.TrimSuffix(words[0], "..."),
	new: strings.TrimSuffix(words[1], "..."),
	matchPrefix: strings.HasSuffix(words[0], "...") &&
	strings.HasSuffix(words[1], "..."),
	})
	}

	// Find non-canonical packages and populate importedBy graph.
	for _, p := range pkgs {
	if p.Error != nil {
	msg := p.Error.Err
	if strings.Contains(msg, "code in directory") &&
	strings.Contains(msg, "expects import") {
	// don't show the very errors we're trying to fix
	} else {
	fmt.Fprintln(stderr, p.Error)
	}
	}

	for _, imp := range p.Imports {
	addEdge(p, imp)
	}
	for _, imp := range p.TestImports {
	addEdge(p, imp)
	}
	for _, imp := range p.XTestImports {
	addEdge(p, imp)
	}

	// Does package have an explicit import comment?
	if p.ImportComment != "" {
	if p.ImportComment != p.ImportPath {
	canonical[p.ImportPath] = canonicalName{
	path: p.ImportComment,
	name: p.Name,
	}
	}
	} else {
	// Is package matched by a -replace item?
	var newPath string
	for _, item := range replace {
	if item.matchPrefix {
	if strings.HasPrefix(p.ImportPath, item.old) {
	newPath = item.new + p.ImportPath[len(item.old):]
	break
	}
	} else if p.ImportPath == item.old {
	newPath = item.new
	break
	}
	}
	if newPath != "" {
	newName := packageName[newPath]
	if newName == "" {
	newName = filepath.Base(newPath) // a guess
	}
	canonical[p.ImportPath] = canonicalName{
	path: newPath,
	name: newName,
	}
	continue
	}

	// Is package matched by a -baddomains item?
	for _, domain := range domains {
	slash := strings.Index(p.ImportPath, "/")
	if slash < 0 {
	continue // no slash: standard package
	}
	if p.ImportPath[:slash] == domain {
	// Package comes from bad domain and has no import comment.
	// Report an error each time this package is imported.
	canonical[p.ImportPath] = canonicalName{}

	// TODO(adonovan): should we make an HTTP request to
	// see if there's an HTTP redirect, a "go-import" meta tag,
	// or an import comment in the latest revision?
	// It would duplicate a lot of logic from "go get".
	}
	break
	}
	}
	}

	// Find all clients (direct importers) of canonical packages.
	// These are the packages that need fixing up.
	clients := make(map[*listPackage]bool)
	for path := range canonical {
	for client := range importedBy[path] {
	clients[client] = true
	}
	}

	// Restrict rewrites to the set of packages specified by the user.
	if len(packages) == 1 && (packages[0] == "all" \|\| packages[0] == "...") {
	// no restriction
	} else {
	pkgs, err := list(packages...)
	if err != nil {
	fmt.Fprintf(stderr, "importfix: %v\n", err)
	return false
	}
	seen := make(map[string]bool)
	for _, p := range pkgs {
	seen[p.ImportPath] = true
	}
	for client := range clients {
	if !seen[client.ImportPath] {
	delete(clients, client)
	}
	}
	}

	// Rewrite selected client packages.
	ok := true
	for client := range clients {
	if !rewritePackage(client, canonical) {
	ok = false

	// There were errors.
	// Show direct and indirect imports of client.
	seen := make(map[string]bool)
	var direct, indirect []string
	for p := range importedBy[client.ImportPath] {
	direct = append(direct, p.ImportPath)
	seen[p.ImportPath] = true
	}

	var visit func(path string)
	visit = func(path string) {
	for q := range importedBy[path] {
	qpath := q.ImportPath
	if !seen[qpath] {
	seen[qpath] = true
	indirect = append(indirect, qpath)
	visit(qpath)
	}
	}
	}

	if direct != nil {
	fmt.Fprintf(stderr, "\timported directly by:\n")
	sort.Strings(direct)
	for _, path := range direct {
	fmt.Fprintf(stderr, "\t\t%s\n", path)
	visit(path)
	}

	if indirect != nil {
	fmt.Fprintf(stderr, "\timported indirectly by:\n")
	sort.Strings(indirect)
	for _, path := range indirect {
	fmt.Fprintf(stderr, "\t\t%s\n", path)
	}
	}
	}
	}
	}

	return ok
	}

	// Invariant: false result => error already printed.
	func rewritePackage(client *listPackage, canonical map[string]canonicalName) bool {
	ok := true

	used := make(map[string]bool)
	var filenames []string
	filenames = append(filenames, client.GoFiles...)
	filenames = append(filenames, client.TestGoFiles...)
	filenames = append(filenames, client.XTestGoFiles...)
	var first bool
	for _, filename := range filenames {
	if !first {
	first = true
	fmt.Fprintf(stderr, "%s\n", client.ImportPath)
	}
	err := rewriteFile(filepath.Join(client.Dir, filename), canonical, used)
	if err != nil {
	fmt.Fprintf(stderr, "\tERROR: %v\n", err)
	ok = false
	}
	}

	// Show which imports were renamed in this package.
	var keys []string
	for key := range used {
	keys = append(keys, key)
	}
	sort.Strings(keys)
	for _, key := range keys {
	if p := canonical[key]; p.path != "" {
	fmt.Fprintf(stderr, "\tfixed: %s -> %s\n", key, p.path)
	} else {
	fmt.Fprintf(stderr, "\tERROR: %s has no import comment\n", key)
	ok = false
	}
	}

	return ok
	}

	// rewriteFile reads, modifies, and writes filename, replacing all imports
	// of packages P in canonical by canonical[P].
	// It records in used which canonical packages were imported.
	// used[P]=="" indicates that P was imported but its canonical path is unknown.
	func rewriteFile(filename string, canonical map[string]canonicalName, used map[string]bool) error {
	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
	if err != nil {
	return err
	}
	var changed bool
	for _, imp := range f.Imports {
	impPath, err := strconv.Unquote(imp.Path.Value)
	if err != nil {
	log.Printf("%s: bad import spec %q: %v",
	fset.Position(imp.Pos()), imp.Path.Value, err)
	continue
	}
	canon, ok := canonical[impPath]
	if !ok {
	continue // import path is canonical
	}

	used[impPath] = true

	if canon.path == "" {
	// The canonical path is unknown (a -baddomain).
	// Show the offending import.
	// TODO(adonovan): should we show the actual source text?
	fmt.Fprintf(stderr, "\t%s:%d: import %q\n",
	shortPath(filename),
	fset.Position(imp.Pos()).Line, impPath)
	continue
	}

	changed = true

	imp.Path.Value = strconv.Quote(canon.path)

	// Add a renaming import if necessary.
	//
	// This is a guess at best. We can't see whether a 'go
	// get' of the canonical import path would have the same
	// name or not. Assume it's the last segment.
	newBase := path.Base(canon.path)
	if imp.Name == nil && newBase != canon.name {
	imp.Name = &ast.Ident{Name: canon.name}
	}
	}

	if changed && !*dryrun {
	var buf bytes.Buffer
	if err := format.Node(&buf, fset, f); err != nil {
	return fmt.Errorf("%s: couldn't format file: %v", filename, err)
	}
	return writeFile(filename, buf.Bytes(), 0644)
	}

	return nil
	}

	// listPackage corresponds to the output of go list -json,
	// but only the fields we need.
	type listPackage struct {
	Name string
	Dir string
	ImportPath string
	GoFiles []string
	TestGoFiles []string
	XTestGoFiles []string
	Imports []string
	TestImports []string
	XTestImports []string
	ImportComment string
	Error *packageError // error loading package
	}

	// A packageError describes an error loading information about a package.
	type packageError struct {
	ImportStack []string // shortest path from package named on command line to this one
	Pos string // position of error
	Err string // the error itself
	}

	func (e packageError) Error() string {
	if e.Pos != "" {
	return e.Pos + ": " + e.Err
	}
	return e.Err
	}

	// list runs 'go list' with the specified arguments and returns the
	// metadata for matching packages.
	func list(args ...string) ([]*listPackage, error) {
	cmd := exec.Command("go", append([]string{"list", "-e", "-json"}, args...)...)
	cmd.Stdout = new(bytes.Buffer)
	cmd.Stderr = stderr
	if err := cmd.Run(); err != nil {
	return nil, err
	}

	dec := json.NewDecoder(cmd.Stdout.(io.Reader))
	var pkgs []*listPackage
	for {
	var p listPackage
	if err := dec.Decode(&p); err == io.EOF {
	break
	} else if err != nil {
	return nil, err
	}
	pkgs = append(pkgs, &p)
	}
	return pkgs, nil
	}

	// cwd contains the current working directory of the tool.
	//
	// It is initialized directly so that its value will be set for any other
	// package variables or init functions that depend on it, such as the gopath
	// variable in main_test.go.
	var cwd string = func() string {
	cwd, err := os.Getwd()
	if err != nil {
	log.Fatalf("os.Getwd: %v", err)
	}
	return cwd
	}()

	// shortPath returns an absolute or relative name for path, whatever is shorter.
	// Plundered from $GOROOT/src/cmd/go/build.go.
	func shortPath(path string) string {
	if rel, err := filepath.Rel(cwd, path); err == nil && len(rel) < len(path) {
	return rel
	}
	return path
	}