Check relabel action at yaml unmarshal stage (#9224)

Signed-off-by: DrAuYueng <ouyang1204@gmail.com>

config/config_test.go

@@ -1302,6 +1302,10 @@ var expectedErrors = []struct {
 		filename: "http_url_bad_scheme.bad.yml",
 		errMsg:   "URL scheme must be 'http' or 'https'",
 	},
+	{
+		filename: "empty_scrape_config_action.bad.yml",
+		errMsg:   "relabel action cannot be empty",
+	},
 }
 
 func TestBadConfigs(t *testing.T) {

config/testdata/empty_scrape_config_action.bad.yml

@@ -0,0 +1,4 @@
+scrape_configs:
+  - job_name: prometheus
+    relabel_configs:
+      - action: null

pkg/relabel/relabel.go

@@ -100,6 +100,9 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if c.Regex.Regexp == nil {
 		c.Regex = MustNewRegexp("")
 	}
+	if c.Action == "" {
+		return errors.Errorf("relabel action cannot be empty")
+	}
 	if c.Modulus == 0 && c.Action == HashMod {
 		return errors.Errorf("relabel configuration for hashmod requires non-zero modulus")
 	}