fix(promtail): validate scrape_config job name

clients/pkg/promtail/config/config.go

@@ -40,6 +40,28 @@ type Config struct {
 	WAL             wal.Config            `yaml:"wal"`
 }
 
+// UnmarshalYAML implements the yaml.Unmarshaler interface.
+func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	*c = Config{}
+	// We want to set c to the defaults and then overwrite it with the input.
+	// To make unmarshal fill the plain data struct rather than calling UnmarshalYAML
+	// again, we have to hide it using a type indirection.
+	type plain Config
+	if err := unmarshal((*plain)(c)); err != nil {
+		return err
+	}
+
+	// Validate unique names.
+	jobNames := map[string]struct{}{}
+	for _, j := range c.ScrapeConfig {
+		if _, ok := jobNames[j.JobName]; ok {
+			return fmt.Errorf("found multiple scrape configs with job name %q", j.JobName)
+		}
+		jobNames[j.JobName] = struct{}{}
+	}
+	return nil
+}
+
 // RegisterFlags with prefix registers flags where every name is prefixed by
 // prefix. If prefix is a non-empty string, prefix should end with a period.
 func (c *Config) RegisterFlagsWithPrefix(prefix string, f *flag.FlagSet) {

clients/pkg/promtail/config/config_test.go

@@ -47,12 +47,47 @@ clients:
       name: value
 `
 
+const testDuplicateJobsName = `
+clients:
+  - external_labels:
+      cluster: dev1
+    url: https://1:[email protected]/loki/api/v1/push
+  - external_labels:
+      cluster: prod1
+    url: https://1:[email protected]/loki/api/v1/push
+scrape_configs:
+  - job_name: kubernetes-pods-name
+    kubernetes_sd_configs:
+      - role: pod
+  - job_name: system
+    static_configs:
+    - targets:
+      - localhost
+      labels:
+        job: varlogs
+  - job_name: system
+    static_configs:
+    - targets:
+      - localhost
+      labels:
+        job: varlogs2
+limits_config:
+  readline_rate: 100
+  readline_burst: 200
+`
+
 func Test_Load(t *testing.T) {
 	var dst Config
 	err := yaml.Unmarshal([]byte(testFile), &dst)
 	require.Nil(t, err)
 }
 
+func Test_Load_DuplicateJobsName(t *testing.T) {
+	var dst Config
+	err := yaml.Unmarshal([]byte(testDuplicateJobsName), &dst)
+	require.ErrorContains(t, err, `found multiple scrape configs with job name "system"`)
+}
+
 func TestHeadersConfigLoad(t *testing.T) {
 	var dst Config
 	err := yaml.Unmarshal([]byte(headersTestFile), &dst)