Add a way to get info about how flags were configured (#11282)

I've wanted to add this in a few cases in the past, but have avoided it
because I've been able to find workarounds. Recently though I ran into a
use case where I can't think of a good workaround (mutually exclusive
flag groups where zero-value checks aren't sufficient). So I think maybe
it's time to add this?

Author: bduffany

server/util/flag/flag.go

@@ -18,6 +18,8 @@ var Deprecated = flagtags.DeprecatedTag
 var YAMLIgnore = flagtags.YAMLIgnoreTag
 var Internal = flagtags.InternalTag
 
+type Meta = flagtags.MetaTag
+
 var NewFlagSet = flag.NewFlagSet
 var ContinueOnError = flag.ContinueOnError
 var ExitOnError = flag.ExitOnError

server/util/flagutil/types/autoflags/tags/tags.go

@@ -389,3 +389,52 @@ func (_ *yamlIgnoreTag) Tag(flagset *flag.FlagSet, name string, f Tagged) flag.V
 }
 
 var YAMLIgnoreTag = &yamlIgnoreTag{}
+
+// MetaTag allows tracking additional metadata about a flag. It's useful for
+// checking whether a flag was explicitly configured.
+//
+// Example: enforce mutually exclusive flag configuration in cases where
+// zero-value checks aren't possible (e.g. booleans):
+//
+//	var (
+//		compressMeta flag.Meta // Info about --compress flag configuration.
+//		compress = flag.Bool("compress", true, "Enable compression.", &compressMeta)
+//		resourceName = flag.String("resource_name", "", "Blob resource name.")
+//	)
+//
+//	// --resource_name and --compress flags should be mutually exclusive, since
+//	// the resource name specifies the compression level within the string.
+//	if *resourceName != "" && compressMeta.IsConfigured() {
+//		log.Fatalf("Cannot specify both 'compress' and 'resource_name' options.")
+//	}
+type MetaTag struct {
+	isSetInCommandLine    bool
+	isSetInYAML           bool
+	isSetProgrammatically bool
+}
+
+func (m *MetaTag) Tag(flagset *flag.FlagSet, name string, tagged Tagged) flag.Value {
+	tagged.DesignateSetFunc(func(value string) error {
+		m.isSetInCommandLine = true
+		return tagged.Value().Set(value)
+	})
+	tagged.DesignateYAMLSetValueHookFunc(func() {
+		m.isSetInYAML = true
+	})
+	tagged.DesignateSetValueForFlagNameHookFunc(func() {
+		m.isSetProgrammatically = true
+	})
+	return tagged
+}
+
+// IsConfigured returns true if the flag has been explicitly configured via the
+// flag API: either via the command line, via YAML, or via the
+// "SetValueForFlagName" API.
+//
+// Note: It does not return true if the flag was set via direct assignment (e.g.
+// *fooFlag = "bar").
+func (m *MetaTag) IsConfigured() bool {
+	// isSetProgrammatically is checked here because in tests we set flags
+	// programmatically to simulate flags being intentionally enabled by users.
+	return m.isSetInCommandLine || m.isSetInYAML || m.isSetProgrammatically
+}

server/util/flagutil/types/autoflags/tags/tags_test.go

@@ -304,3 +304,52 @@ func TestYAMLIgnoreAndSecret(t *testing.T) {
 	assert.Equal(t, "default", *v1)
 	assert.Equal(t, "default2", *v2)
 }
+
+func TestMeta_SetProgrammatically(t *testing.T) {
+	flagset := replaceFlagsForTesting(t)
+
+	var meta MetaTag
+	v := flagset.String("testflag", "test-default-value", "Test flag help")
+	Tag[string, flag.Value](flagset, "testflag", &meta)
+
+	require.False(t, meta.IsConfigured(), "flag should not yet be configured")
+
+	require.NoError(t, common.SetValueForFlagName(flagset, "testflag", "test-value", nil, true))
+	require.Equal(t, "test-value", *v)
+
+	require.True(t, meta.IsConfigured(), "flag should now be configured")
+}
+
+func TestMeta_SetInCommandLine(t *testing.T) {
+	flagset := replaceFlagsForTesting(t)
+
+	var meta MetaTag
+	v := flagset.String("testflag", "test-default-value", "Test flag help")
+	Tag[string, flag.Value](flagset, "testflag", &meta)
+
+	require.False(t, meta.IsConfigured(), "flag should not yet be configured")
+
+	require.NoError(t, flagset.Parse([]string{"--testflag=test-value"}))
+	require.Equal(t, "test-value", *v)
+
+	require.True(t, meta.IsConfigured(), "flag should now be configured")
+}
+
+func TestMeta_SetInYAML(t *testing.T) {
+	flagset := replaceFlagsForTesting(t)
+	_ = replaceIgnoreSetForTesting(t)
+
+	var meta MetaTag
+	v := flagset.String("testflag", "test-default-value", "Test flag help")
+	Tag[string, flag.Value](flagset, "testflag", &meta)
+
+	require.False(t, meta.IsConfigured(), "flag should not yet be configured")
+
+	yamlData := `
+testflag: test-value
+`
+	require.NoError(t, flagyaml.PopulateFlagsFromData(yamlData))
+	require.Equal(t, "test-value", *v)
+
+	require.True(t, meta.IsConfigured(), "flag should now be configured")
+}