cmd/cover: invoke go command to find packages

cmd/cover has always assumed that package x/y/z can be
found in $GOPATH/src/x/y/z (roughly; by using go/build).
That won't be true for too much longer. Instead, run the
go command to find out where packages are.

This will make 'go tool cover' safe for use with Go modules
when they are in use in Go 1.11, and it continues to work
with the existing Go toolchains too.

An alternative would be to modify the cover profile format
to record file names directly, but that would require also
updating golang.org/x/tools/cover/profile and any tools
that use it, which seems not worth the trouble.
(That fork of the code does not contain any code to resolve
package names to directory locations, so it's unaffected.)

No new test here: cmd/go's TestCoverageFunc tests this code.

Fixes #25318 (when people use Go 1.11 instead of vgo).

Change-Id: I8769b15107aecf25f7aaf8692b724cf7d0f073d0
Reviewed-on: https://go-review.googlesource.com/122478
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/cover/func.go

@@ -8,13 +8,20 @@ package main
 
 import (
 	"bufio"
+	"bytes"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"go/ast"
-	"go/build"
 	"go/parser"
 	"go/token"
+	"io"
 	"os"
+	"os/exec"
+	"path"
 	"path/filepath"
+	"runtime"
+	"strings"
 	"text/tabwriter"
 )
 
@@ -36,6 +43,11 @@ func funcOutput(profile, outputFile string) error {
 		return err
 	}
 
+	dirs, err := findPkgs(profiles)
+	if err != nil {
+		return err
+	}
+
 	var out *bufio.Writer
 	if outputFile == "" {
 		out = bufio.NewWriter(os.Stdout)
@@ -55,7 +67,7 @@ func funcOutput(profile, outputFile string) error {
 	var total, covered int64
 	for _, profile := range profiles {
 		fn := profile.FileName
-		file, err := findFile(fn)
+		file, err := findFile(dirs, fn)
 		if err != nil {
 			return err
 		}
@@ -154,14 +166,72 @@ func (f *FuncExtent) coverage(profile *Profile) (num, den int64) {
 	return covered, total
 }
 
-// findFile finds the location of the named file in GOROOT, GOPATH etc.
-func findFile(file string) (string, error) {
-	dir, file := filepath.Split(file)
-	pkg, err := build.Import(dir, ".", build.FindOnly)
+// Pkg describes a single package, compatible with the JSON output from 'go list'; see 'go help list'.
+type Pkg struct {
+	ImportPath string
+	Dir        string
+	Error      *struct {
+		Err string
+	}
+}
+
+func findPkgs(profiles []*Profile) (map[string]*Pkg, error) {
+	// Run go list to find the location of every package we care about.
+	pkgs := make(map[string]*Pkg)
+	var list []string
+	for _, profile := range profiles {
+		if strings.HasPrefix(profile.FileName, ".") || filepath.IsAbs(profile.FileName) {
+			// Relative or absolute path.
+			continue
+		}
+		pkg := path.Dir(profile.FileName)
+		if _, ok := pkgs[pkg]; !ok {
+			pkgs[pkg] = nil
+			list = append(list, pkg)
+		}
+	}
+
+	// Note: usually run as "go tool cover" in which case $GOROOT is set,
+	// in which case runtime.GOROOT() does exactly what we want.
+	goTool := filepath.Join(runtime.GOROOT(), "bin/go")
+	cmd := exec.Command(goTool, append([]string{"list", "-e", "-json"}, list...)...)
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	stdout, err := cmd.Output()
 	if err != nil {
-		return "", fmt.Errorf("can't find %q: %v", file, err)
+		return nil, fmt.Errorf("cannot run go list: %v\n%s", err, stderr.Bytes())
+	}
+	dec := json.NewDecoder(bytes.NewReader(stdout))
+	for {
+		var pkg Pkg
+		err := dec.Decode(&pkg)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, fmt.Errorf("decoding go list json: %v", err)
+		}
+		pkgs[pkg.ImportPath] = &pkg
+	}
+	return pkgs, nil
+}
+
+// findFile finds the location of the named file in GOROOT, GOPATH etc.
+func findFile(pkgs map[string]*Pkg, file string) (string, error) {
+	if strings.HasPrefix(file, ".") || filepath.IsAbs(file) {
+		// Relative or absolute path.
+		return file, nil
+	}
+	pkg := pkgs[path.Dir(file)]
+	if pkg != nil {
+		if pkg.Dir != "" {
+			return filepath.Join(pkg.Dir, path.Base(file)), nil
+		}
+		if pkg.Error != nil {
+			return "", errors.New(pkg.Error.Err)
+		}
 	}
-	return filepath.Join(pkg.Dir, file), nil
+	return "", fmt.Errorf("did not find package for %s in go list output", file)
 }
 
 func percent(covered, total int64) float64 {

src/cmd/cover/html.go

@@ -29,12 +29,17 @@ func htmlOutput(profile, outfile string) error {
 
 	var d templateData
 
+	dirs, err := findPkgs(profiles)
+	if err != nil {
+		return err
+	}
+
 	for _, profile := range profiles {
 		fn := profile.FileName
 		if profile.Mode == "set" {
 			d.Set = true
 		}
-		file, err := findFile(fn)
+		file, err := findFile(dirs, fn)
 		if err != nil {
 			return err
 		}

src/cmd/go/internal/load/test.go

@@ -36,7 +36,7 @@ type TestCover struct {
 	Pkgs     []*Package
 	Paths    []string
 	Vars     []coverInfo
-	DeclVars func(string, ...string) map[string]*CoverVar
+	DeclVars func(*Package, ...string) map[string]*CoverVar
 }
 
 // TestPackagesFor returns three packages:
@@ -264,7 +264,7 @@ func TestPackagesFor(p *Package, cover *TestCover) (pmain, ptest, pxtest *Packag
 		var coverFiles []string
 		coverFiles = append(coverFiles, ptest.GoFiles...)
 		coverFiles = append(coverFiles, ptest.CgoFiles...)
-		ptest.Internal.CoverVars = cover.DeclVars(ptest.ImportPath, coverFiles...)
+		ptest.Internal.CoverVars = cover.DeclVars(ptest, coverFiles...)
 	}
 
 	for _, cp := range pmain.Internal.Imports {

src/cmd/go/internal/test/test.go

@@ -698,7 +698,7 @@ func runTest(cmd *base.Command, args []string) {
 			coverFiles = append(coverFiles, p.GoFiles...)
 			coverFiles = append(coverFiles, p.CgoFiles...)
 			coverFiles = append(coverFiles, p.TestGoFiles...)
-			p.Internal.CoverVars = declareCoverVars(p.ImportPath, coverFiles...)
+			p.Internal.CoverVars = declareCoverVars(p, coverFiles...)
 			if testCover && testCoverMode == "atomic" {
 				ensureImport(p, "sync/atomic")
 			}
@@ -966,7 +966,7 @@ func isTestFile(file string) bool {
 
 // declareCoverVars attaches the required cover variables names
 // to the files, to be used when annotating the files.
-func declareCoverVars(importPath string, files ...string) map[string]*load.CoverVar {
+func declareCoverVars(p *load.Package, files ...string) map[string]*load.CoverVar {
 	coverVars := make(map[string]*load.CoverVar)
 	coverIndex := 0
 	// We create the cover counters as new top-level variables in the package.
@@ -975,14 +975,25 @@ func declareCoverVars(importPath string, files ...string) map[string]*load.Cover
 	// so we append 12 hex digits from the SHA-256 of the import path.
 	// The point is only to avoid accidents, not to defeat users determined to
 	// break things.
-	sum := sha256.Sum256([]byte(importPath))
+	sum := sha256.Sum256([]byte(p.ImportPath))
 	h := fmt.Sprintf("%x", sum[:6])
 	for _, file := range files {
 		if isTestFile(file) {
 			continue
 		}
+		// For a package that is "local" (imported via ./ import or command line, outside GOPATH),
+		// we record the full path to the file name.
+		// Otherwise we record the import path, then a forward slash, then the file name.
+		// This makes profiles within GOPATH file system-independent.
+		// These names appear in the cmd/cover HTML interface.
+		var longFile string
+		if p.Internal.Local {
+			longFile = filepath.Join(p.Dir, file)
+		} else {
+			longFile = path.Join(p.ImportPath, file)
+		}
 		coverVars[file] = &load.CoverVar{
-			File: filepath.Join(importPath, file),
+			File: longFile,
 			Var:  fmt.Sprintf("GoCover_%d_%x", coverIndex, h),
 		}
 		coverIndex++