Use %w to wrap upstream errors

Wrapping errors instead of merely embedding error messages allows
calling code to use `errors.Is` and `errors.As` to more precisely check
the reasons for various errors instead of having to rely only on
substring searches.

Fixes #503

Author: rkennedy

internal/run.go

@@ -52,7 +52,7 @@ func OutputDebug(cmd string, args ...string) (string, error) {
 	if err := c.Run(); err != nil {
 		errMsg := strings.TrimSpace(errbuf.String())
 		debug.Print("error running '", cmd, strings.Join(args, " "), "': ", err, ": ", errMsg)
-		return "", fmt.Errorf("error running \"%s %s\": %s\n%s", cmd, strings.Join(args, " "), err, errMsg)
+		return "", fmt.Errorf("error running \"%s %s\": %w\n%s", cmd, strings.Join(args, " "), err, errMsg)
 	}
 	return strings.TrimSpace(buf.String()), nil
 }

mage/main.go

@@ -473,14 +473,14 @@ func listGoFiles(magePath, goCmd, tag string, envStr []string) ([]string, error)
 	if !filepath.IsAbs(magePath) {
 		cwd, err := os.Getwd()
 		if err != nil {
-			return nil, fmt.Errorf("can't get current working directory: %v", err)
+			return nil, fmt.Errorf("can't get current working directory: %w", err)
 		}
 		magePath = filepath.Join(cwd, magePath)
 	}
 
 	env, err := internal.SplitEnv(envStr)
 	if err != nil {
-		return nil, fmt.Errorf("error parsing environment variables: %v", err)
+		return nil, fmt.Errorf("error parsing environment variables: %w", err)
 	}
 
 	bctx := build.Default
@@ -502,7 +502,7 @@ func listGoFiles(magePath, goCmd, tag string, envStr []string) ([]string, error)
 
 		// Allow multiple packages in the same directory
 		if _, ok := err.(*build.MultiplePackageError); !ok {
-			return nil, fmt.Errorf("failed to parse go source files: %v", err)
+			return nil, fmt.Errorf("failed to parse go source files: %w", err)
 		}
 	}
 
@@ -530,7 +530,7 @@ func Magefiles(magePath, goos, goarch, goCmd string, stderr io.Writer, isMagefil
 	debug.Println("getting all files including those with mage tag in", magePath)
 	mageFiles, err := listGoFiles(magePath, goCmd, "mage", env)
 	if err != nil {
-		return nil, fmt.Errorf("listing mage files: %v", err)
+		return nil, fmt.Errorf("listing mage files: %w", err)
 	}
 
 	if isMagefilesDirectory {
@@ -546,7 +546,7 @@ func Magefiles(magePath, goos, goarch, goCmd string, stderr io.Writer, isMagefil
 	debug.Println("getting all files without mage tag in", magePath)
 	nonMageFiles, err := listGoFiles(magePath, goCmd, "", env)
 	if err != nil {
-		return nil, fmt.Errorf("listing non-mage files: %v", err)
+		return nil, fmt.Errorf("listing non-mage files: %w", err)
 	}
 
 	// convert non-Mage list to a map of files to exclude.
@@ -612,7 +612,7 @@ func GenerateMainfile(binaryName, path string, info *parse.PkgInfo) error {
 
 	f, err := os.Create(path)
 	if err != nil {
-		return fmt.Errorf("error creating generated mainfile: %v", err)
+		return fmt.Errorf("error creating generated mainfile: %w", err)
 	}
 	defer f.Close()
 	data := mainfileTemplateData{
@@ -629,16 +629,16 @@ func GenerateMainfile(binaryName, path string, info *parse.PkgInfo) error {
 
 	debug.Println("writing new file at", path)
 	if err := mainfileTemplate.Execute(f, data); err != nil {
-		return fmt.Errorf("can't execute mainfile template: %v", err)
+		return fmt.Errorf("can't execute mainfile template: %w", err)
 	}
 	if err := f.Close(); err != nil {
-		return fmt.Errorf("error closing generated mainfile: %v", err)
+		return fmt.Errorf("error closing generated mainfile: %w", err)
 	}
 	// we set an old modtime on the generated mainfile so that the go tool
 	// won't think it has changed more recently than the compiled binary.
 	longAgo := time.Now().Add(-time.Hour * 24 * 365 * 10)
 	if err := os.Chtimes(path, longAgo, longAgo); err != nil {
-		return fmt.Errorf("error setting old modtime on generated mainfile: %v", err)
+		return fmt.Errorf("error setting old modtime on generated mainfile: %w", err)
 	}
 	return nil
 }
@@ -675,13 +675,13 @@ func ExeName(goCmd, cacheDir string, files []string) (string, error) {
 func hashFile(fn string) (string, error) {
 	f, err := os.Open(fn)
 	if err != nil {
-		return "", fmt.Errorf("can't open input file for hashing: %#v", err)
+		return "", fmt.Errorf("can't open input file for hashing: %w", err)
 	}
 	defer f.Close()
 
 	h := sha1.New()
 	if _, err := io.Copy(h, f); err != nil {
-		return "", fmt.Errorf("can't write data to hash: %v", err)
+		return "", fmt.Errorf("can't write data to hash: %w", err)
 	}
 	return fmt.Sprintf("%x", h.Sum(nil)), nil
 }
@@ -690,12 +690,12 @@ func generateInit(dir string) error {
 	debug.Println("generating default magefile in", dir)
 	f, err := os.Create(filepath.Join(dir, initFile))
 	if err != nil {
-		return fmt.Errorf("could not create mage template: %v", err)
+		return fmt.Errorf("could not create mage template: %w", err)
 	}
 	defer f.Close()
 
 	if err := initOutput.Execute(f, nil); err != nil {
-		return fmt.Errorf("can't execute magefile template: %v", err)
+		return fmt.Errorf("can't execute magefile template: %w", err)
 	}
 
 	return nil

mage/main_test.go

@@ -6,6 +6,7 @@ import (
 	"debug/macho"
 	"debug/pe"
 	"encoding/hex"
+	"errors"
 	"flag"
 	"fmt"
 	"go/build"
@@ -1270,7 +1271,7 @@ func TestCompiledFlags(t *testing.T) {
 		cmd.Stderr = stderr
 		cmd.Stdout = stdout
 		if err := cmd.Run(); err != nil {
-			return fmt.Errorf("running '%s %s' failed with: %v\nstdout: %s\nstderr: %s",
+			return fmt.Errorf("running '%s %s' failed with: %w\nstdout: %s\nstderr: %s",
 				filename, strings.Join(args, " "), err, stdout, stderr)
 		}
 		return nil
@@ -1315,6 +1316,10 @@ func TestCompiledFlags(t *testing.T) {
 	if err == nil {
 		t.Fatalf("expected an error because of timeout")
 	}
+	var exitError *exec.ExitError
+	if !errors.As(err, &exitError) {
+		t.Errorf("Expected wrapped ExitError from process; got %#v", err)
+	}
 	got = stderr.String()
 	want = "context deadline exceeded"
 	if strings.Contains(got, want) == false {
@@ -1357,7 +1362,7 @@ func TestCompiledEnvironmentVars(t *testing.T) {
 		cmd.Stderr = stderr
 		cmd.Stdout = stdout
 		if err := cmd.Run(); err != nil {
-			return fmt.Errorf("running '%s %s' failed with: %v\nstdout: %s\nstderr: %s",
+			return fmt.Errorf("running '%s %s' failed with: %w\nstdout: %s\nstderr: %s",
 				filename, strings.Join(args, " "), err, stdout, stderr)
 		}
 		return nil
@@ -1511,7 +1516,7 @@ func TestSignals(t *testing.T) {
 		cmd.Stderr = stderr
 		cmd.Stdout = stdout
 		if err := cmd.Start(); err != nil {
-			return fmt.Errorf("running '%s %s' failed with: %v\nstdout: %s\nstderr: %s",
+			return fmt.Errorf("running '%s %s' failed with: %w\nstdout: %s\nstderr: %s",
 				filename, target, err, stdout, stderr)
 		}
 		pid := cmd.Process.Pid
@@ -1523,7 +1528,7 @@ func TestSignals(t *testing.T) {
 			}
 		}()
 		if err := cmd.Wait(); err != nil {
-			return fmt.Errorf("running '%s %s' failed with: %v\nstdout: %s\nstderr: %s",
+			return fmt.Errorf("running '%s %s' failed with: %w\nstdout: %s\nstderr: %s",
 				filename, target, err, stdout, stderr)
 		}
 		return nil

magefile.go

@@ -43,20 +43,20 @@ func Install() error {
 	// in GOPATH environment string
 	bin, err := sh.Output(gocmd, "env", "GOBIN")
 	if err != nil {
-		return fmt.Errorf("can't determine GOBIN: %v", err)
+		return fmt.Errorf("can't determine GOBIN: %w", err)
 	}
 	if bin == "" {
 		gopath, err := sh.Output(gocmd, "env", "GOPATH")
 		if err != nil {
-			return fmt.Errorf("can't determine GOPATH: %v", err)
+			return fmt.Errorf("can't determine GOPATH: %w", err)
 		}
 		paths := strings.Split(gopath, string([]rune{os.PathListSeparator}))
 		bin = filepath.Join(paths[0], "bin")
 	}
 	// specifically don't mkdirall, if you have an invalid gopath in the first
 	// place, that's not on us to fix.
 	if err := os.Mkdir(bin, 0700); err != nil && !os.IsExist(err) {
-		return fmt.Errorf("failed to create %q: %v", bin, err)
+		return fmt.Errorf("failed to create %q: %w", bin, err)
 	}
 	path := filepath.Join(bin, name)
 

mg/fn.go

@@ -37,7 +37,7 @@ func F(target interface{}, args ...interface{}) Fn {
 	}
 	id, err := json.Marshal(args)
 	if err != nil {
-		panic(fmt.Errorf("can't convert args into a mage-compatible id for mg.Deps: %s", err))
+		panic(fmt.Errorf("can't convert args into a mage-compatible id for mg.Deps: %w", err))
 	}
 	return fn{
 		name: funcName(target),

parse/parse.go

@@ -735,7 +735,7 @@ func getPackage(path string, files []string, fset *token.FileSet) (*ast.Package,
 
 	pkgs, err := parser.ParseDir(fset, path, filter, parser.ParseComments)
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse directory: %v", err)
+		return nil, fmt.Errorf("failed to parse directory: %w", err)
 	}
 
 	switch len(pkgs) {

sh/cmd.go

@@ -120,7 +120,7 @@ func Exec(env map[string]string, stdout, stderr io.Writer, cmd string, args ...s
 	if ran {
 		return ran, mg.Fatalf(code, `running "%s %s" failed with exit code %d`, cmd, strings.Join(args, " "), code)
 	}
-	return ran, fmt.Errorf(`failed to run "%s %s: %v"`, cmd, strings.Join(args, " "), err)
+	return ran, fmt.Errorf(`failed to run "%s %s: %w"`, cmd, strings.Join(args, " "), err)
 }
 
 func run(env map[string]string, stdout, stderr io.Writer, cmd string, args ...string) (ran bool, code int, err error) {

sh/cmd_test.go

@@ -2,6 +2,8 @@ package sh
 
 import (
 	"bytes"
+	"errors"
+	"io/fs"
 	"os"
 	"testing"
 )
@@ -70,3 +72,13 @@ func TestAutoExpand(t *testing.T) {
 	}
 
 }
+
+func TestWrappedError(t *testing.T) {
+	_, err := Exec(nil, nil, nil, os.Args[0]+"-doesnotexist", "-printArgs", "foo")
+	if err == nil {
+		t.Fatalf("Expected to fail running %s", os.Args[0]+"-doesnotexist")
+	}
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("Expected error to be like ErrNotExist, got %#v", err)
+	}
+}

sh/helpers.go

@@ -13,28 +13,28 @@ func Rm(path string) error {
 	if err == nil || os.IsNotExist(err) {
 		return nil
 	}
-	return fmt.Errorf(`failed to remove %s: %v`, path, err)
+	return fmt.Errorf(`failed to remove %s: %w`, path, err)
 }
 
 // Copy robustly copies the source file to the destination, overwriting the destination if necessary.
 func Copy(dst string, src string) error {
 	from, err := os.Open(src)
 	if err != nil {
-		return fmt.Errorf(`can't copy %s: %v`, src, err)
+		return fmt.Errorf(`can't copy %s: %w`, src, err)
 	}
 	defer from.Close()
 	finfo, err := from.Stat()
 	if err != nil {
-		return fmt.Errorf(`can't stat %s: %v`, src, err)
+		return fmt.Errorf(`can't stat %s: %w`, src, err)
 	}
 	to, err := os.OpenFile(dst, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, finfo.Mode())
 	if err != nil {
-		return fmt.Errorf(`can't copy to %s: %v`, dst, err)
+		return fmt.Errorf(`can't copy to %s: %w`, dst, err)
 	}
 	defer to.Close()
 	_, err = io.Copy(to, from)
 	if err != nil {
-		return fmt.Errorf(`error copying %s to %s: %v`, src, dst, err)
+		return fmt.Errorf(`error copying %s to %s: %w`, src, dst, err)
 	}
 	return nil
 }

sh/helpers_test.go

@@ -17,11 +17,11 @@ import (
 func compareFiles(file1 string, file2 string) error {
 	s1, err := os.Stat(file1)
 	if err != nil {
-		return fmt.Errorf("can't stat %s: %v", file1, err)
+		return fmt.Errorf("can't stat %s: %w", file1, err)
 	}
 	s2, err := os.Stat(file2)
 	if err != nil {
-		return fmt.Errorf("can't stat %s: %v", file2, err)
+		return fmt.Errorf("can't stat %s: %w", file2, err)
 	}
 	if s1.Size() != s2.Size() {
 		return fmt.Errorf("files %s and %s have different sizes: %d vs %d", file1, file2, s1.Size(), s2.Size())
@@ -31,11 +31,11 @@ func compareFiles(file1 string, file2 string) error {
 	}
 	f1bytes, err := ioutil.ReadFile(file1)
 	if err != nil {
-		return fmt.Errorf("can't read %s: %v", file1, err)
+		return fmt.Errorf("can't read %s: %w", file1, err)
 	}
 	f2bytes, err := ioutil.ReadFile(file2)
 	if err != nil {
-		return fmt.Errorf("can't read %s: %v", file2, err)
+		return fmt.Errorf("can't read %s: %w", file2, err)
 	}
 	if !bytes.Equal(f1bytes, f2bytes) {
 		return fmt.Errorf("files %s and %s have different contents", file1, file2)