tool/gocross: merge user's build tags and implicitly added build tags together

Fixes tailscale/corp#15058

Change-Id: I7e539b3324153077597f30385a2cb540846e8bdc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/gocross/autoflags.go

@@ -169,14 +169,18 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 	// Finished computing the settings we want. Generate the modified
 	// commandline and environment modifications.
 	newArgv = append(newArgv, argv[:2]...) // Program name and `go` tool subcommand
+
+	filteredArgvPostSubcmd, originalTags := extractTags(argv[2:])
+
 	newArgv = append(newArgv, buildFlags...)
+	tags = append(tags, originalTags...)
 	if len(tags) > 0 {
 		newArgv = append(newArgv, fmt.Sprintf("-tags=%s", strings.Join(tags, ",")))
 	}
 	if len(ldflags) > 0 {
 		newArgv = append(newArgv, "-ldflags", strings.Join(ldflags, " "))
 	}
-	newArgv = append(newArgv, argv[2:]...)
+	newArgv = append(newArgv, filteredArgvPostSubcmd...)
 
 	env.Set("GOOS", targetOS)
 	env.Set("GOARCH", targetArch)
@@ -198,6 +202,39 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 	return newArgv, env, nil
 }
 
+// extractTags parses out "-tags=foo,bar" (or double hyphen or "-tags",
+// "foo,bar") in its various forms and returns v filtered to remove the 0, 1 or
+// 2 build tag elements, then the tags parsed, split on commas ("foo", "bar").
+func extractTags(v []string) (filtered, tags []string) {
+	for len(v) > 0 {
+		e := v[0]
+		if strings.HasPrefix(e, "--tags=") {
+			e = e[1:] // remove one of the hyphens for the next line
+		}
+		if suf, ok := strings.CutPrefix(e, "-tags="); ok {
+			v = v[1:]
+			if suf != "" {
+				tags = strings.Split(suf, ",")
+			}
+			continue
+		}
+		if e == "-tags" || e == "--tags" {
+			v = v[1:]
+			if len(v) > 0 {
+				tagStr := v[0]
+				v = v[1:]
+				if tagStr != "" {
+					tags = strings.Split(tagStr, ",")
+				}
+			}
+			continue
+		}
+		filtered = append(filtered, e)
+		v = v[1:]
+	}
+	return filtered, tags
+}
+
 // boolStr formats v as a string 0 or 1.
 // Used because CGO_ENABLED doesn't strconv.ParseBool, so
 // strconv.FormatBool breaks.

tool/gocross/autoflags_test.go

@@ -437,3 +437,67 @@ TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
 		})
 	}
 }
+
+func TestExtractTags(t *testing.T) {
+	s := func(ss ...string) []string { return ss }
+	tests := []struct {
+		name string
+		in   []string
+		filt []string // want filtered
+		tags []string // want tags
+	}{
+		{
+			name: "one_hyphen_tags",
+			in:   s("foo", "-tags=a,b", "bar"),
+			filt: s("foo", "bar"),
+			tags: s("a", "b"),
+		},
+		{
+			name: "two_hyphen_tags",
+			in:   s("foo", "--tags=a,b", "bar"),
+			filt: s("foo", "bar"),
+			tags: s("a", "b"),
+		},
+		{
+			name: "one_hypen_separate_arg",
+			in:   s("foo", "-tags", "a,b", "bar"),
+			filt: s("foo", "bar"),
+			tags: s("a", "b"),
+		},
+		{
+			name: "two_hypen_separate_arg",
+			in:   s("foo", "--tags", "a,b", "bar"),
+			filt: s("foo", "bar"),
+			tags: s("a", "b"),
+		},
+		{
+			name: "equal_empty",
+			in:   s("foo", "--tags=", "bar"),
+			filt: s("foo", "bar"),
+			tags: s(),
+		},
+		{
+			name: "arg_empty",
+			in:   s("foo", "--tags", "", "bar"),
+			filt: s("foo", "bar"),
+			tags: s(),
+		},
+		{
+			name: "arg_empty_truncated",
+			in:   s("foo", "--tags"),
+			filt: s("foo"),
+			tags: s(),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			filt, tags := extractTags(tt.in)
+			if !reflect.DeepEqual(filt, tt.filt) {
+				t.Errorf("extractTags(%q) filtered = %q; want %q", tt.in, filt, tt.filt)
+			}
+			if !reflect.DeepEqual(tags, tt.tags) {
+				t.Errorf("extractTags(%q) tags = %q; want %q", tt.in, tags, tt.tags)
+			}
+		})
+	}
+}