cmd/go: change -tags to a comma-separated list

Using commas makes it possible to put multiple tags into GOFLAGS.
The space-separated form is still recognized and will be maintained.

Alleviates #26849 somewhat.
Fixes #18800 (again).

Change-Id: I6f4cf28ea31e53e21ccbdad6ef1a0aee63b007d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/173438
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

src/cmd/go/alldocs.go

@@ -151,10 +151,12 @@
 // 		install and load all packages from dir instead of the usual locations.
 // 		For example, when building with a non-standard configuration,
 // 		use -pkgdir to keep generated packages in a separate location.
-// 	-tags 'tag list'
-// 		a space-separated list of build tags to consider satisfied during the
+// 	-tags tag,list
+// 		a comma-separated list of build tags to consider satisfied during the
 // 		build. For more information about build tags, see the description of
 // 		build constraints in the documentation for the go/build package.
+// 		(Earlier versions of Go used a space-separated list, and that form
+// 		is deprecated but still recognized.)
 // 	-trimpath
 // 		remove all file system paths from the resulting executable.
 // 		Instead of absolute file system paths, the recorded file names

src/cmd/go/go_test.go

@@ -4644,7 +4644,7 @@ func TestBuildTagsNoComma(t *testing.T) {
 	tg.makeTempdir()
 	tg.setenv("GOPATH", tg.path("go"))
 	tg.run("build", "-tags", "tag1 tag2", "math")
-	tg.runFail("build", "-tags", "tag1,tag2", "math")
+	tg.runFail("build", "-tags", "tag1,tag2 tag3", "math")
 	tg.grepBoth("space-separated list contains comma", "-tags with a comma-separated list didn't error")
 }
 

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

@@ -104,10 +104,12 @@ and test commands:
 		install and load all packages from dir instead of the usual locations.
 		For example, when building with a non-standard configuration,
 		use -pkgdir to keep generated packages in a separate location.
-	-tags 'tag list'
-		a space-separated list of build tags to consider satisfied during the
+	-tags tag,list
+		a comma-separated list of build tags to consider satisfied during the
 		build. For more information about build tags, see the description of
 		build constraints in the documentation for the go/build package.
+		(Earlier versions of Go used a space-separated list, and that form
+		is deprecated but still recognized.)
 	-trimpath
 		remove all file system paths from the resulting executable.
 		Instead of absolute file system paths, the recorded file names
@@ -233,7 +235,7 @@ func AddBuildFlags(cmd *base.Command) {
 	cmd.Flag.StringVar(&cfg.BuildPkgdir, "pkgdir", "", "")
 	cmd.Flag.BoolVar(&cfg.BuildRace, "race", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildMSan, "msan", false, "")
-	cmd.Flag.Var((*base.StringsFlag)(&cfg.BuildContext.BuildTags), "tags", "")
+	cmd.Flag.Var((*tagsFlag)(&cfg.BuildContext.BuildTags), "tags", "")
 	cmd.Flag.Var((*base.StringsFlag)(&cfg.BuildToolexec), "toolexec", "")
 	cmd.Flag.BoolVar(&cfg.BuildTrimpath, "trimpath", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildWork, "work", false, "")
@@ -242,6 +244,29 @@ func AddBuildFlags(cmd *base.Command) {
 	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
 }
 
+// tagsFlag is the implementation of the -tags flag.
+type tagsFlag []string
+
+func (v *tagsFlag) Set(s string) error {
+	// For compatibility with Go 1.12 and earlier, allow "-tags='a b c'" or even just "-tags='a'".
+	if strings.Contains(s, " ") || strings.Contains(s, "'") {
+		return (*base.StringsFlag)(v).Set(s)
+	}
+
+	// Split on commas, ignore empty strings.
+	*v = []string{}
+	for _, s := range strings.Split(s, ",") {
+		if s != "" {
+			*v = append(*v, s)
+		}
+	}
+	return nil
+}
+
+func (v *tagsFlag) String() string {
+	return "<TagsFlag>"
+}
+
 // fileExtSplit expects a filename and returns the name
 // and ext (without the dot). If the file has no
 // extension, ext will be empty.

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

@@ -16,6 +16,9 @@ stdout '\[y\.go\]'
 go list -f {{.GoFiles}} -tags 'tag1 tag2'
 stdout '\[x\.go y\.go\]'
 
+go list -f {{.GoFiles}} -tags tag1,tag2 # commas allowed as of Go 1.13
+stdout '\[x\.go y\.go\]'
+
 -- x/go.mod --
 module x