From c0c88b4597e979d1dcfcbd2a5eb5200484c3b671 Mon Sep 17 00:00:00 2001
From: Rohithmatham12 <rohithmatham@gmail.com>
Date: Sat, 18 Jul 2026 13:28:40 -0400
Subject: [PATCH] Fix empty YAML validation for explicit zero values

Signed-off-by: Rohithmatham12 <rohithmatham@gmail.com>
---
 cmd/cortex/main.go      |  5 ++++
 cmd/cortex/main_test.go |  5 ++++
 pkg/cortex/cortex.go    | 61 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+)

diff --git a/cmd/cortex/main.go b/cmd/cortex/main.go
index d8a48677523..77c3f8fab56 100644
--- a/cmd/cortex/main.go
+++ b/cmd/cortex/main.go
@@ -259,6 +259,11 @@ func LoadConfig(filename string, expandENV bool, cfg *cortex.Config) error {
 		buf = expandEnv(buf)
 	}
 
+	err = cfg.SetYAMLRootNodeStates(buf)
+	if err != nil {
+		return errors.Wrap(err, "Error parsing config file")
+	}
+
 	err = yaml.UnmarshalStrict(buf, cfg)
 	if err != nil {
 		return errors.Wrap(err, "Error parsing config file")
diff --git a/cmd/cortex/main_test.go b/cmd/cortex/main_test.go
index abcb230d7bb..30f06067754 100644
--- a/cmd/cortex/main_test.go
+++ b/cmd/cortex/main_test.go
@@ -79,6 +79,11 @@ func TestFlagParsing(t *testing.T) {
 			stderrMessage: "the Querier configuration in YAML has been specified as an empty YAML node",
 		},
 
+		"root level configuration option specified with explicit zero values": {
+			yaml:           "flusher: { exit_after_flush: false }",
+			stderrExcluded: "empty YAML node",
+		},
+
 		"version": {
 			arguments:     []string{"-version"},
 			stdoutMessage: "Cortex, version",
diff --git a/pkg/cortex/cortex.go b/pkg/cortex/cortex.go
index 658388e5ebb..8a2243746ae 100644
--- a/pkg/cortex/cortex.go
+++ b/pkg/cortex/cortex.go
@@ -98,6 +98,7 @@ type Config struct {
 	PrintConfig          bool                   `yaml:"-"`
 	HTTPPrefix           string                 `yaml:"http_prefix"`
 	NameValidationScheme model.ValidationScheme `yaml:"name_validation_scheme"`
+	yamlRootNodeStates   map[string]yamlRootNodeState
 
 	ResourceMonitor configs.ResourceMonitor `yaml:"resource_monitor"`
 
@@ -135,6 +136,54 @@ type Config struct {
 	Tracing tracing.Config `yaml:"tracing"`
 }
 
+type yamlRootNodeState int
+
+const (
+	yamlRootNodeEmpty yamlRootNodeState = iota
+	yamlRootNodeNonEmpty
+)
+
+// SetYAMLRootNodeStates records whether root-level YAML config keys were empty.
+// This lets validation distinguish "querier:" from explicit zero values such
+// as "flusher: { exit_after_flush: false }" after unmarshalling.
+func (c *Config) SetYAMLRootNodeStates(buf []byte) error {
+	var root map[interface{}]interface{}
+	if err := yaml.Unmarshal(buf, &root); err != nil {
+		return err
+	}
+
+	c.yamlRootNodeStates = map[string]yamlRootNodeState{}
+	for key, value := range root {
+		name, ok := key.(string)
+		if !ok {
+			continue
+		}
+
+		if isEmptyYAMLRootNode(value) {
+			c.yamlRootNodeStates[name] = yamlRootNodeEmpty
+		} else {
+			c.yamlRootNodeStates[name] = yamlRootNodeNonEmpty
+		}
+	}
+
+	return nil
+}
+
+func isEmptyYAMLRootNode(value interface{}) bool {
+	if value == nil {
+		return true
+	}
+
+	switch typed := value.(type) {
+	case map[interface{}]interface{}:
+		return len(typed) == 0
+	case []interface{}:
+		return len(typed) == 0
+	default:
+		return false
+	}
+}
+
 // RegisterFlags registers flag.
 func (c *Config) RegisterFlags(f *flag.FlagSet) {
 	c.Server.MetricsNamespace = "cortex"
@@ -296,6 +345,12 @@ func (c *Config) validateYAMLEmptyNodes() error {
 		// If the struct has been reset due to empty YAML value and the zero struct value
 		// doesn't match the default one, then we should warn the user about the issue.
 		if cfgStruct.Field(i).Kind() == reflect.Struct && cfgStruct.Field(i).IsZero() && !defStruct.Field(i).IsZero() {
+			if c.yamlRootNodeStates != nil {
+				yamlName := yamlFieldName(cfgStruct.Type().Field(i))
+				if c.yamlRootNodeStates[yamlName] == yamlRootNodeNonEmpty {
+					continue
+				}
+			}
 			return fmt.Errorf("the %s configuration in YAML has been specified as an empty YAML node", cfgStruct.Type().Field(i).Name)
 		}
 	}
@@ -303,6 +358,12 @@ func (c *Config) validateYAMLEmptyNodes() error {
 	return nil
 }
 
+func yamlFieldName(field reflect.StructField) string {
+	tag := field.Tag.Get("yaml")
+	name, _, _ := strings.Cut(tag, ",")
+	return name
+}
+
 func (c *Config) registerServerFlagsWithChangedDefaultValues(fs *flag.FlagSet) {
 	throwaway := flag.NewFlagSet("throwaway", flag.PanicOnError)