cmd/go: don't lock .mod and .sum files for read in overlay

On Plan 9, locking a file requires a chmod call. In general, the go
command should not modify files in the overlay, even metadata. With
this change, we won't lock these files for reading.

The go command already reported errors when attempting to write these
files if they were in the overlay, but this change moves those checks
to the point of access for clearer error
messages. cmd/go/internal/lockedfile no longer imports
cmd/go/internal/fsys.

Fixes #44700

Change-Id: Ib544459dd6cf56ca0f7a27b3285f045f08040d7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/333012
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: Jay Conrod

src/cmd/go/internal/lockedfile/lockedfile_filelock.go

@@ -11,7 +11,6 @@ import (
 	"io/fs"
 	"os"
 
-	"cmd/go/internal/fsys"
 	"cmd/go/internal/lockedfile/internal/filelock"
 )
 
@@ -21,7 +20,7 @@ func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
 	// calls for Linux and Windows anyway, so it's simpler to use that approach
 	// consistently.
 
-	f, err := fsys.OpenFile(name, flag&^os.O_TRUNC, perm)
+	f, err := os.OpenFile(name, flag&^os.O_TRUNC, perm)
 	if err != nil {
 		return nil, err
 	}

src/cmd/go/internal/lockedfile/lockedfile_plan9.go

@@ -13,8 +13,6 @@ import (
 	"os"
 	"strings"
 	"time"
-
-	"cmd/go/internal/fsys"
 )
 
 // Opening an exclusive-use file returns an error.
@@ -59,7 +57,7 @@ func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
 	// If the file was unpacked or created by some other program, it might not
 	// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
 	// can be confident that a successful OpenFile implies exclusive use.
-	if fi, err := fsys.Stat(name); err == nil {
+	if fi, err := os.Stat(name); err == nil {
 		if fi.Mode()&fs.ModeExclusive == 0 {
 			if err := os.Chmod(name, fi.Mode()|fs.ModeExclusive); err != nil {
 				return nil, err
@@ -72,7 +70,7 @@ func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
 	nextSleep := 1 * time.Millisecond
 	const maxSleep = 500 * time.Millisecond
 	for {
-		f, err := fsys.OpenFile(name, flag, perm|fs.ModeExclusive)
+		f, err := os.OpenFile(name, flag, perm|fs.ModeExclusive)
 		if err == nil {
 			return f, nil
 		}

src/cmd/go/internal/modfetch/fetch.go

@@ -22,6 +22,7 @@ import (
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
 	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/par"
 	"cmd/go/internal/robustio"
@@ -416,7 +417,18 @@ func initGoSum() (bool, error) {
 
 	goSum.m = make(map[module.Version][]string)
 	goSum.status = make(map[modSum]modSumStatus)
-	data, err := lockedfile.Read(GoSumFile)
+	var (
+		data []byte
+		err  error
+	)
+	if actualSumFile, ok := fsys.OverlayPath(GoSumFile); ok {
+		// Don't lock go.sum if it's part of the overlay.
+		// On Plan 9, locking requires chmod, and we don't want to modify any file
+		// in the overlay. See #44700.
+		data, err = os.ReadFile(actualSumFile)
+	} else {
+		data, err = lockedfile.Read(GoSumFile)
+	}
 	if err != nil && !os.IsNotExist(err) {
 		return false, err
 	}
@@ -716,6 +728,9 @@ Outer:
 	if cfg.BuildMod == "readonly" {
 		base.Fatalf("go: updates to go.sum needed, disabled by -mod=readonly")
 	}
+	if _, ok := fsys.OverlayPath(GoSumFile); ok {
+		base.Fatalf("go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay")
+	}
 
 	// Make a best-effort attempt to acquire the side lock, only to exclude
 	// previous versions of the 'go' command from making simultaneous edits.

src/cmd/go/internal/modload/init.go

@@ -412,7 +412,16 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	}
 
 	gomod := ModFilePath()
-	data, err := lockedfile.Read(gomod)
+	var data []byte
+	var err error
+	if gomodActual, ok := fsys.OverlayPath(gomod); ok {
+		// Don't lock go.mod if it's part of the overlay.
+		// On Plan 9, locking requires chmod, and we don't want to modify any file
+		// in the overlay. See #44700.
+		data, err = os.ReadFile(gomodActual)
+	} else {
+		data, err = lockedfile.Read(gomodActual)
+	}
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}
@@ -1026,6 +1035,13 @@ func commitRequirements(ctx context.Context, goVersion string, rs *Requirements)
 		}
 		return
 	}
+	gomod := ModFilePath()
+	if _, ok := fsys.OverlayPath(gomod); ok {
+		if dirty {
+			base.Fatalf("go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay")
+		}
+		return
+	}
 
 	new, err := modFile.Format()
 	if err != nil {

src/cmd/go/internal/modload/modfile.go

@@ -8,13 +8,15 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 	"strings"
 	"sync"
 	"unicode"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
 	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/par"
@@ -601,8 +603,16 @@ func rawGoModSummary(m module.Version) (*modFileSummary, error) {
 				dir = filepath.Join(ModRoot(), dir)
 			}
 			gomod := filepath.Join(dir, "go.mod")
-
-			data, err := lockedfile.Read(gomod)
+			var data []byte
+			var err error
+			if gomodActual, ok := fsys.OverlayPath(gomod); ok {
+				// Don't lock go.mod if it's part of the overlay.
+				// On Plan 9, locking requires chmod, and we don't want to modify any file
+				// in the overlay. See #44700.
+				data, err = os.ReadFile(gomodActual)
+			} else {
+				data, err = lockedfile.Read(gomodActual)
+			}
 			if err != nil {
 				return cached{nil, module.VersionError(m, fmt.Errorf("reading %s: %v", base.ShortPath(gomod), err))}
 			}

src/cmd/go/testdata/script/mod_overlay.txt

@@ -21,7 +21,7 @@ go list -deps -overlay overlay.json .
 cd $WORK/gopath/src/get-doesnt-add-dep
 cp $WORK/overlay/get_doesnt_add_dep_go_mod $WORK/want_go_mod
 ! go get -d -overlay overlay.json .
-stderr 'overlaid files can''t be opened for write'
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
 cmp $WORK/overlay/get_doesnt_add_dep_go_mod $WORK/want_go_mod
 
 # Content of overlaid go.sum is used.
@@ -41,10 +41,10 @@ go mod verify -overlay overlay.json
 # attempting to update the file
 cp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
 ! go get -d -overlay overlay.json .
-stderr 'overlaid files can''t be opened for write'
+stderr '^go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay$'
 cmp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
 ! go mod tidy -overlay overlay.json
-stderr 'overlaid files can''t be opened for write'
+stderr '^go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay$'
 cmp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
 
 # -overlay works with -modfile.
@@ -56,7 +56,7 @@ go list -modfile=alternate.mod -overlay overlay.json .
 stdout 'found.the/module'
 # Even with -modfile, overlaid files can't be opened for write.
 ! go get -modfile=alternate.mod -overlay overlay.json -d rsc.io/quote
-stderr 'overlaid files can''t be opened for write'
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
 
 # Carving out a module by adding an overlaid go.mod file
 cd $WORK/gopath/src/carve
@@ -78,7 +78,7 @@ go list -overlay overlay.json all
 stdout ^carve2/nomod$
 # Editing go.mod file fails because overlay is read only
 ! go get -overlay overlay.json -d rsc.io/quote
-stderr 'overlaid files can''t be opened for write'
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
 ! grep rsc.io/quote $WORK/overlay/carve2-nomod-go.mod
 # Editing go.mod file succeeds because we use -modfile to redirect to same file
 go get -overlay overlay.json -modfile $WORK/overlay/carve2-nomod-go.mod -d rsc.io/quote