cmd/go: error on space-separated list with comma

Using 'go build -tags "foo,bar"' might seem to work when you wanted
-tags "foo bar", since they make up a single tag that doesn't exist and
the build is unaffected.

Instead, error on any tag that contains a comma.

Fixes #18800.

Change-Id: I6641e03e2ae121c8878d6301c4311aef97026b73
Reviewed-on: https://go-review.googlesource.com/41951
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -3885,3 +3885,15 @@ func main() {
 	tg.creatingTemp(exe)
 	tg.run("build", "-o", exe, "p")
 }
+
+func TestBuildTagsNoComma(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("go"))
+	tg.run("install", "-tags", "tag1 tag2", "math")
+	tg.runFail("install", "-tags", "tag1,tag2", "math")
+	tg.grepBoth("space-separated list contains comma", "-tags with a comma-separated list didn't error")
+	tg.runFail("build", "-tags", "tag1,tag2", "math")
+	tg.grepBoth("space-separated list contains comma", "-tags with a comma-separated list didn't error")
+}

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

@@ -1102,6 +1102,12 @@ func (b *Builder) Do(root *Action) {
 		fmt.Fprintf(os.Stderr, "cmd/go: unsupported GOOS/GOARCH pair %s/%s\n", cfg.Goos, cfg.Goarch)
 		os.Exit(2)
 	}
+	for _, tag := range cfg.BuildContext.BuildTags {
+		if strings.Contains(tag, ",") {
+			fmt.Fprintf(os.Stderr, "cmd/go: -tags space-separated list contains comma\n")
+			os.Exit(2)
+		}
+	}
 
 	// Build list of all actions, assigning depth-first post-order priority.
 	// The original implementation here was a true queue