config: address edge case where local config specifies validation mode only (#16923)

This check ensures that local ScrapeConfigs that only specify Legacy validation do not inherit the default global AllowUTF8 escaping setting, which is an invalid combination of settings.

---------

Signed-off-by: Owen Williams <owen.williams@grafana.com>

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## main / unreleased
 
 * [BUGFIX] OTLP receiver: Generate `target_info` samples between the earliest and latest samples per resource. #16737
+* [BUGFIX] Config: Infer escaping scheme when scrape config validation scheme is set.
 
 ## 3.5.0 / 2025-07-14
 

config/config.go

@@ -885,8 +885,10 @@ func (c *ScrapeConfig) Validate(globalConfig GlobalConfig) error {
 		return fmt.Errorf("unknown global name validation method specified, must be either '', 'legacy' or 'utf8', got %s", globalConfig.MetricNameValidationScheme)
 	}
 	// Scrapeconfig validation scheme matches global if left blank.
+	localValidationUnset := false
 	switch c.MetricNameValidationScheme {
 	case model.UnsetValidation:
+		localValidationUnset = true
 		c.MetricNameValidationScheme = globalConfig.MetricNameValidationScheme
 	case model.LegacyValidation, model.UTF8Validation:
 	default:
@@ -906,8 +908,20 @@ func (c *ScrapeConfig) Validate(globalConfig GlobalConfig) error {
 		return fmt.Errorf("unknown global name escaping method specified, must be one of '%s', '%s', '%s', or '%s', got %q", model.AllowUTF8, model.EscapeUnderscores, model.EscapeDots, model.EscapeValues, globalConfig.MetricNameEscapingScheme)
 	}
 
+	// Similarly, if ScrapeConfig escaping scheme is blank, infer it from the
+	// ScrapeConfig validation scheme if that was set, or the Global validation
+	// scheme if the ScrapeConfig validation scheme was also not set. This ensures
+	// that local ScrapeConfigs that only specify Legacy validation do not inherit
+	// the global AllowUTF8 escaping setting, which is an error.
 	if c.MetricNameEscapingScheme == "" {
-		c.MetricNameEscapingScheme = globalConfig.MetricNameEscapingScheme
+		//nolint:gocritic
+		if localValidationUnset {
+			c.MetricNameEscapingScheme = globalConfig.MetricNameEscapingScheme
+		} else if c.MetricNameValidationScheme == model.LegacyValidation {
+			c.MetricNameEscapingScheme = model.EscapeUnderscores
+		} else {
+			c.MetricNameEscapingScheme = model.AllowUTF8
+		}
 	}
 
 	switch c.MetricNameEscapingScheme {

config/config_test.go

@@ -2801,29 +2801,40 @@ func TestScrapeConfigDisableCompression(t *testing.T) {
 
 func TestScrapeConfigNameValidationSettings(t *testing.T) {
 	tests := []struct {
-		name         string
-		inputFile    string
-		expectScheme model.ValidationScheme
+		name           string
+		inputFile      string
+		expectScheme   model.ValidationScheme
+		expectEscaping model.EscapingScheme
 	}{
 		{
-			name:         "blank config implies default",
-			inputFile:    "scrape_config_default_validation_mode",
-			expectScheme: model.UTF8Validation,
+			name:           "blank config implies default",
+			inputFile:      "scrape_config_default_validation_mode",
+			expectScheme:   model.UTF8Validation,
+			expectEscaping: model.NoEscaping,
 		},
 		{
-			name:         "global setting implies local settings",
-			inputFile:    "scrape_config_global_validation_mode",
-			expectScheme: model.LegacyValidation,
+			name:           "global setting implies local settings",
+			inputFile:      "scrape_config_global_validation_mode",
+			expectScheme:   model.LegacyValidation,
+			expectEscaping: model.DotsEscaping,
 		},
 		{
-			name:         "local setting",
-			inputFile:    "scrape_config_local_validation_mode",
-			expectScheme: model.LegacyValidation,
+			name:           "local setting",
+			inputFile:      "scrape_config_local_validation_mode",
+			expectScheme:   model.LegacyValidation,
+			expectEscaping: model.ValueEncodingEscaping,
 		},
 		{
-			name:         "local setting overrides global setting",
-			inputFile:    "scrape_config_local_global_validation_mode",
-			expectScheme: model.UTF8Validation,
+			name:           "local setting overrides global setting",
+			inputFile:      "scrape_config_local_global_validation_mode",
+			expectScheme:   model.UTF8Validation,
+			expectEscaping: model.DotsEscaping,
+		},
+		{
+			name:           "local validation implies underscores escaping",
+			inputFile:      "scrape_config_local_infer_escaping",
+			expectScheme:   model.LegacyValidation,
+			expectEscaping: model.UnderscoreEscaping,
 		},
 	}
 
@@ -2839,6 +2850,10 @@ func TestScrapeConfigNameValidationSettings(t *testing.T) {
 			require.NoError(t, yaml.UnmarshalStrict(out, got))
 
 			require.Equal(t, tc.expectScheme, got.ScrapeConfigs[0].MetricNameValidationScheme)
+
+			escaping, err := model.ToEscapingScheme(got.ScrapeConfigs[0].MetricNameEscapingScheme)
+			require.NoError(t, err)
+			require.Equal(t, tc.expectEscaping, escaping)
 		})
 	}
 }

config/testdata/scrape_config_local_infer_escaping.yml

@@ -0,0 +1,3 @@
+scrape_configs:
+  - job_name: prometheus
+    metric_name_validation_scheme: legacy