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>

14 files changed, 57 insertions, 6 deletions

diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index ea909b5b37..36b2348888 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/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)
diff --git a/src/cmd/go/internal/get/get.go b/src/cmd/go/internal/get/get.go
index 421f1bab75..500e3e0da6 100644
--- a/src/cmd/go/internal/get/get.go
+++ b/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, "")
 }
diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index 0d432e9549..3a86d8ac06 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/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 {
diff --git a/src/cmd/go/internal/modcmd/edit.go b/src/cmd/go/internal/modcmd/edit.go
index 1be8b7cb2f..239f88bdc2 100644
--- a/src/cmd/go/internal/modcmd/edit.go
+++ b/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)
 }
 
diff --git a/src/cmd/go/internal/modcmd/graph.go b/src/cmd/go/internal/modcmd/graph.go
index 8fcb84f280..5dbbf332fb 100644
--- a/src/cmd/go/internal/modcmd/graph.go
+++ b/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")
diff --git a/src/cmd/go/internal/modcmd/init.go b/src/cmd/go/internal/modcmd/init.go
index b94453bab0..2858a46c4e 100644
--- a/src/cmd/go/internal/modcmd/init.go
+++ b/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 {
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index 789e936608..1f5a18e05e 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/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) {
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index bb1cecdbf5..71246b2f68 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/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) {
diff --git a/src/cmd/go/internal/modcmd/verify.go b/src/cmd/go/internal/modcmd/verify.go
index 81fc44dc97..9155fcb358 100644
--- a/src/cmd/go/internal/modcmd/verify.go
+++ b/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.
diff --git a/src/cmd/go/internal/modcmd/why.go b/src/cmd/go/internal/modcmd/why.go
index 03e0a039bc..93d64dcb59 100644
--- a/src/cmd/go/internal/modcmd/why.go
+++ b/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) {
diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index 438c6212b5..7d20f66041 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/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)
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 9b74963f43..d2b4bd3c65 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/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
 
diff --git a/src/cmd/go/internal/work/init.go b/src/cmd/go/internal/work/init.go
index f3055b6293..2f9fde4cb8 100644
--- a/src/cmd/go/internal/work/init.go
+++ b/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 {
diff --git a/src/cmd/go/testdata/script/mod_cache_rw.txt b/src/cmd/go/testdata/script/mod_cache_rw.txt
index 9c19494214..b4a3a456e8 100644
--- a/src/cmd/go/testdata/script/mod_cache_rw.txt
+++ b/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 --