cmd/go: support -modcacherw in 'go mod' subcommands

The -modcacherw flag is now registered in work.AddModCommonFlags,
which is called from work.AddBuildFlags, where it was registered
before. 'go mod' subcommands register the flag by calling
work.AddModCommonFlags directly.

Also, build commands now exit with an error if -modcacherw is set
explicitly (not in GOFLAGS) in GOPATH mode.

Updates #31481

Change-Id: I461e59a51ed31b006fff4d5c57c2a866be0bbf38
Reviewed-on: https://go-review.googlesource.com/c/go/+/202563
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/cfg/cfg.go

@@ -33,7 +33,6 @@ var (
 	BuildN                 bool               // -n flag
 	BuildO                 string             // -o flag
 	BuildP                 = runtime.NumCPU() // -p flag
-	BuildModcacheRW        bool               // -modcacherw flag
 	BuildPkgdir            string             // -pkgdir flag
 	BuildRace              bool               // -race flag
 	BuildToolexec          []string           // -toolexec flag
@@ -45,6 +44,8 @@ var (
 	BuildWork              bool // -work flag
 	BuildX                 bool // -x flag
 
+	ModCacheRW bool // -modcacherw flag
+
 	CmdName string // "build", "install", "list", "mod tidy", etc.
 
 	DebugActiongraph string // -debug-actiongraph flag (undocumented, unstable)

src/cmd/go/internal/get/get.go

@@ -108,7 +108,7 @@ var (
 )
 
 func init() {
-	work.AddBuildFlags(CmdGet, work.OmitModFlag)
+	work.AddBuildFlags(CmdGet, work.OmitModFlag|work.OmitModCommonFlags)
 	CmdGet.Run = runGet // break init loop
 	CmdGet.Flag.BoolVar(&Insecure, "insecure", Insecure, "")
 }

src/cmd/go/internal/modcmd/download.go

@@ -14,6 +14,7 @@ import (
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
+	"cmd/go/internal/work"
 )
 
 var cmdDownload = &base.Command{
@@ -53,6 +54,8 @@ var downloadJSON = cmdDownload.Flag.Bool("json", false, "")
 
 func init() {
 	cmdDownload.Run = runDownload // break init cycle
+
+	work.AddModCommonFlags(cmdDownload)
 }
 
 type moduleJSON struct {

src/cmd/go/internal/modcmd/edit.go

@@ -20,6 +20,7 @@ import (
 	"cmd/go/internal/modfile"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
+	"cmd/go/internal/work"
 )
 
 var cmdEdit = &base.Command{
@@ -130,6 +131,7 @@ func init() {
 	cmdEdit.Flag.Var(flagFunc(flagReplace), "replace", "")
 	cmdEdit.Flag.Var(flagFunc(flagDropExclude), "dropexclude", "")
 
+	work.AddModCommonFlags(cmdEdit)
 	base.AddBuildFlagsNX(&cmdEdit.Flag)
 }
 

src/cmd/go/internal/modcmd/graph.go

@@ -16,6 +16,7 @@ import (
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
+	"cmd/go/internal/work"
 )
 
 var cmdGraph = &base.Command{
@@ -30,6 +31,10 @@ path@version, except for the main module, which has no @version suffix.
 	Run: runGraph,
 }
 
+func init() {
+	work.AddModCommonFlags(cmdGraph)
+}
+
 func runGraph(cmd *base.Command, args []string) {
 	if len(args) > 0 {
 		base.Fatalf("go mod graph: graph takes no arguments")

src/cmd/go/internal/modcmd/init.go

@@ -9,6 +9,7 @@ package modcmd
 import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/work"
 	"os"
 	"strings"
 )
@@ -27,6 +28,10 @@ To override this guess, supply the module path as an argument.
 	Run: runInit,
 }
 
+func init() {
+	work.AddModCommonFlags(cmdInit)
+}
+
 func runInit(cmd *base.Command, args []string) {
 	modload.CmdModInit = true
 	if len(args) > 1 {

src/cmd/go/internal/modcmd/tidy.go

@@ -15,6 +15,7 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
+	"cmd/go/internal/work"
 )
 
 var cmdTidy = &base.Command{
@@ -35,6 +36,7 @@ to standard error.
 func init() {
 	cmdTidy.Run = runTidy // break init cycle
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	work.AddModCommonFlags(cmdTidy)
 }
 
 func runTidy(cmd *base.Command, args []string) {

src/cmd/go/internal/modcmd/vendor.go

@@ -20,6 +20,7 @@ import (
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 	"cmd/go/internal/semver"
+	"cmd/go/internal/work"
 )
 
 var cmdVendor = &base.Command{
@@ -38,6 +39,7 @@ modules and packages to standard error.
 
 func init() {
 	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	work.AddModCommonFlags(cmdVendor)
 }
 
 func runVendor(cmd *base.Command, args []string) {

src/cmd/go/internal/modcmd/verify.go

@@ -16,6 +16,7 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
+	"cmd/go/internal/work"
 )
 
 var cmdVerify = &base.Command{
@@ -32,6 +33,10 @@ non-zero status.
 	Run: runVerify,
 }
 
+func init() {
+	work.AddModCommonFlags(cmdVerify)
+}
+
 func runVerify(cmd *base.Command, args []string) {
 	if len(args) != 0 {
 		// NOTE(rsc): Could take a module pattern.

src/cmd/go/internal/modcmd/why.go

@@ -8,6 +8,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
+	"cmd/go/internal/work"
 	"fmt"
 	"strings"
 )
@@ -54,6 +55,7 @@ var (
 
 func init() {
 	cmdWhy.Run = runWhy // break init cycle
+	work.AddModCommonFlags(cmdWhy)
 }
 
 func runWhy(cmd *base.Command, args []string) {

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

@@ -125,7 +125,7 @@ func download(mod module.Version, dir string) (err error) {
 		return err
 	}
 
-	if !cfg.BuildModcacheRW {
+	if !cfg.ModCacheRW {
 		// Make dir read-only only *after* renaming it.
 		// os.Rename was observed to fail for read-only directories on macOS.
 		makeDirsReadOnly(dir)

src/cmd/go/internal/work/build.go

@@ -224,9 +224,10 @@ type BuildFlagMask int
 const (
 	DefaultBuildFlags BuildFlagMask = 0
 	OmitModFlag       BuildFlagMask = 1 << iota
+	OmitModCommonFlags
 )
 
-// addBuildFlags adds the flags common to the build, clean, get,
+// AddBuildFlags adds the flags common to the build, clean, get,
 // install, list, run, and test commands.
 func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
@@ -243,10 +244,12 @@ func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	if mask&OmitModFlag == 0 {
 		cmd.Flag.StringVar(&cfg.BuildMod, "mod", "", "")
 	}
+	if mask&OmitModCommonFlags == 0 {
+		AddModCommonFlags(cmd)
+	}
 	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
 	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
 	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")
-	cmd.Flag.BoolVar(&cfg.BuildModcacheRW, "modcacherw", false, "")
 	cmd.Flag.StringVar(&cfg.BuildPkgdir, "pkgdir", "", "")
 	cmd.Flag.BoolVar(&cfg.BuildRace, "race", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildMSan, "msan", false, "")
@@ -259,6 +262,12 @@ func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
 }
 
+// AddModCommonFlags adds the module-related flags common to build commands
+// and 'go mod' subcommands.
+func AddModCommonFlags(cmd *base.Command) {
+	cmd.Flag.BoolVar(&cfg.ModCacheRW, "modcacherw", false, "")
+}
+
 // tagsFlag is the implementation of the -tags flag.
 type tagsFlag []string
 

src/cmd/go/internal/work/init.go

@@ -248,12 +248,17 @@ func buildModeInit() {
 	case "":
 		// ok
 	case "readonly", "vendor", "mod":
-		if load.ModLookup == nil && !inGOFLAGS("-mod") {
+		if !cfg.ModulesEnabled && !inGOFLAGS("-mod") {
 			base.Fatalf("build flag -mod=%s only valid when using modules", cfg.BuildMod)
 		}
 	default:
 		base.Fatalf("-mod=%s not supported (can be '', 'mod', 'readonly', or 'vendor')", cfg.BuildMod)
 	}
+	if !cfg.ModulesEnabled {
+		if cfg.ModCacheRW && !inGOFLAGS("-modcacherw") {
+			base.Fatalf("build flag -modcacherw only valid when using modules")
+		}
+	}
 }
 
 func inGOFLAGS(flag string) bool {

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

@@ -33,6 +33,16 @@ go get -d rsc.io/quote@latest
 [!windows] [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 ! exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 
+
+# Repeat part of the test with 'go mod download' instead of 'go get' to verify
+# -modcacherw is supported on 'go mod' subcommands.
+go clean -modcache
+go mod download -modcacherw rsc.io/quote
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
+! go mod verify
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+
+
 -- $WORK/extraneous.txt --
 module oops
 -- go.mod --