Fix missing defaults for empty global config blocks

config/config.go

@@ -147,6 +147,12 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal((*plain)(c)); err != nil {
 		return err
 	}
+	// If a global block was open but empty the default global config is overwritten.
+	// We have to restore it here.
+	if c.GlobalConfig.isZero() {
+		c.GlobalConfig = DefaultGlobalConfig
+	}
+
 	for _, rf := range c.RuleFiles {
 		if !patRulePath.MatchString(rf) {
 			return fmt.Errorf("invalid rule file path %q", rf)
@@ -196,6 +202,14 @@ func (c *GlobalConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	return checkOverflow(c.XXX, "global config")
 }
 
+// isZero returns true iff the global config is the zero value.
+func (c *GlobalConfig) isZero() bool {
+	return c.Labels == nil &&
+		c.ScrapeInterval == 0 &&
+		c.ScrapeTimeout == 0 &&
+		c.EvaluationInterval == 0
+}
+
 // ScrapeConfig configures a scraping unit for Prometheus.
 type ScrapeConfig struct {
 	// The job name to which the job label is set by default.

config/config_test.go

@@ -261,3 +261,16 @@ func TestEmptyConfig(t *testing.T) {
 		t.Fatalf("want %v, got %v", exp, c)
 	}
 }
+
+func TestEmptyGlobalBlock(t *testing.T) {
+	c, err := Load("global:\n")
+	if err != nil {
+		t.Fatalf("Unexpected error parsing empty config file: %s", err)
+	}
+	exp := DefaultConfig
+	exp.original = "global:\n"
+
+	if !reflect.DeepEqual(*c, exp) {
+		t.Fatalf("want %v, got %v", exp, c)
+	}
+}