cmd/go: remove the -mod flag from 'go get'

'GOFLAGS=-mod=vendor' currently causes 'go get' to always fail unless
the '-mod' flag is explicitly overwritten. Moreover, as of CL 198319
we plan to set -mod=vendor by default if a vendor directory is
present, so all users with vendor directories will be affected — not
just those who set 'GOFLAGS' explicitly.

Similarly, an explicit '-mod=readonly' argument to 'go get' is
currently ignored as a special case, but the fact that it is ignored
(rather than rejected) can be very surprising.

Rather than adding more special cases, we should remove the '-mod'
flag from 'go get' entirely.

Fixes #30345
Fixes #32502
Updates #33848

Change-Id: Iecd3233ca3ef580ca3a66bd5e6ee8d86d4cbd8a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/198438
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

doc/go1.14.html

@@ -58,6 +58,13 @@ TODO
   graphic characters and spaces.
 </p>
 
+<p><!-- golang.org/issue/32502, golang.org/issue/30345 -->
+  The <code>go</code> <code>get</code> subcommand no longer accepts
+  the <code>-mod</code> flag. Previously, the flag's setting either
+  <a href="https://golang.org/issue/30345">was ignored</a> or
+  <a href="https://golang.org/issue/32502">caused the build to fail</a>.
+</p>
+
 <h2 id="runtime">Runtime</h2>
 
 <p>

src/cmd/go/internal/clean/clean.go

@@ -102,7 +102,7 @@ func init() {
 	// mentioned explicitly in the docs but they
 	// are part of the build flags.
 
-	work.AddBuildFlags(CmdClean)
+	work.AddBuildFlags(CmdClean, work.DefaultBuildFlags)
 }
 
 func runClean(cmd *base.Command, args []string) {

src/cmd/go/internal/generate/generate.go

@@ -149,7 +149,7 @@ var (
 )
 
 func init() {
-	work.AddBuildFlags(CmdGenerate)
+	work.AddBuildFlags(CmdGenerate, work.DefaultBuildFlags)
 	CmdGenerate.Flag.StringVar(&generateRunFlag, "run", "", "")
 }
 

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

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

src/cmd/go/internal/list/list.go

@@ -287,7 +287,7 @@ For more about modules, see 'go help modules'.
 
 func init() {
 	CmdList.Run = runList // break init cycle
-	work.AddBuildFlags(CmdList)
+	work.AddBuildFlags(CmdList, work.DefaultBuildFlags)
 }
 
 var (

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

@@ -7,7 +7,6 @@ package modget
 
 import (
 	"cmd/go/internal/base"
-	"cmd/go/internal/cfg"
 	"cmd/go/internal/get"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/load"
@@ -199,7 +198,7 @@ func (v *upgradeFlag) Set(s string) error {
 func (v *upgradeFlag) String() string { return "" }
 
 func init() {
-	work.AddBuildFlags(CmdGet)
+	work.AddBuildFlags(CmdGet, work.OmitModFlag)
 	CmdGet.Run = runGet // break init loop
 	CmdGet.Flag.BoolVar(&get.Insecure, "insecure", get.Insecure, "")
 	CmdGet.Flag.Var(&getU, "u", "")
@@ -256,11 +255,6 @@ type query struct {
 }
 
 func runGet(cmd *base.Command, args []string) {
-	// -mod=readonly has no effect on "go get".
-	if cfg.BuildMod == "readonly" {
-		cfg.BuildMod = ""
-	}
-
 	switch getU {
 	case "", "upgrade", "patch":
 		// ok
@@ -278,10 +272,6 @@ func runGet(cmd *base.Command, args []string) {
 	}
 	modload.LoadTests = *getT
 
-	if cfg.BuildMod == "vendor" {
-		base.Fatalf("go get: disabled by -mod=%s", cfg.BuildMod)
-	}
-
 	buildList := modload.LoadBuildList()
 	buildList = buildList[:len(buildList):len(buildList)] // copy on append
 	versionByPath := make(map[string]string)

src/cmd/go/internal/run/run.go

@@ -49,7 +49,7 @@ See also: go build.
 func init() {
 	CmdRun.Run = runRun // break init loop
 
-	work.AddBuildFlags(CmdRun)
+	work.AddBuildFlags(CmdRun, work.DefaultBuildFlags)
 	CmdRun.Flag.Var((*base.StringsFlag)(&work.ExecCmd), "exec", "")
 }
 

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

@@ -65,7 +65,7 @@ var testFlagDefn = []*cmdflag.Defn{
 func init() {
 	cmdflag.AddKnownFlags("test", testFlagDefn)
 	var cmd base.Command
-	work.AddBuildFlags(&cmd)
+	work.AddBuildFlags(&cmd, work.DefaultBuildFlags)
 	cmd.Flag.VisitAll(func(f *flag.Flag) {
 		if f.Name == "v" {
 			// test overrides the build -v flag

src/cmd/go/internal/vet/vetflag.go

@@ -114,7 +114,7 @@ func vetFlags(usage func(), args []string) (passToVet, packageNames []string) {
 
 	// Add build flags to vetFlagDefn.
 	var cmd base.Command
-	work.AddBuildFlags(&cmd)
+	work.AddBuildFlags(&cmd, work.DefaultBuildFlags)
 	// This flag declaration is a placeholder:
 	// -vettool is actually parsed by the init function above.
 	cmd.Flag.StringVar(new(string), "vettool", "", "path to vet tool binary")

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

@@ -167,8 +167,8 @@ func init() {
 
 	CmdInstall.Flag.BoolVar(&cfg.BuildI, "i", false, "")
 
-	AddBuildFlags(CmdBuild)
-	AddBuildFlags(CmdInstall)
+	AddBuildFlags(CmdBuild, DefaultBuildFlags)
+	AddBuildFlags(CmdInstall, DefaultBuildFlags)
 }
 
 // Note that flags consulted by other parts of the code
@@ -216,9 +216,16 @@ func init() {
 	}
 }
 
+type BuildFlagMask int
+
+const (
+	DefaultBuildFlags BuildFlagMask = 0
+	OmitModFlag       BuildFlagMask = 1 << iota
+)
+
 // addBuildFlags adds the flags common to the build, clean, get,
 // install, list, run, and test commands.
-func AddBuildFlags(cmd *base.Command) {
+func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildN, "n", false, "")
 	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
@@ -230,7 +237,9 @@ func AddBuildFlags(cmd *base.Command) {
 	cmd.Flag.StringVar(&cfg.BuildBuildmode, "buildmode", "default", "")
 	cmd.Flag.Var(&load.BuildGcflags, "gcflags", "")
 	cmd.Flag.Var(&load.BuildGccgoflags, "gccgoflags", "")
-	cmd.Flag.StringVar(&cfg.BuildMod, "mod", "", "")
+	if mask&OmitModFlag == 0 {
+		cmd.Flag.StringVar(&cfg.BuildMod, "mod", "", "")
+	}
 	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
 	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
 	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")

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

@@ -13,7 +13,7 @@ stdout '^golang.org/x/text v0.0.0.* .*vendor[\\/]golang.org[\\/]x[\\/]text$'
 ! go list -mod=vendor -m rsc.io/quote@latest
 stderr 'module lookup disabled by -mod=vendor'
 ! go get -mod=vendor -u
-stderr 'go get: disabled by -mod=vendor'
+stderr 'flag provided but not defined: -mod'
 
 -- go.mod --
 module x