Fix target update error handling.

Fixes https://github.com/prometheus/prometheus/issues/1378

retrieval/target.go

@@ -184,7 +184,7 @@ type Target struct {
 }
 
 // NewTarget creates a reasonably configured target for querying.
-func NewTarget(cfg *config.ScrapeConfig, baseLabels, metaLabels model.LabelSet) *Target {
+func NewTarget(cfg *config.ScrapeConfig, baseLabels, metaLabels model.LabelSet) (*Target, error) {
 	t := &Target{
 		url: &url.URL{
 			Scheme: string(baseLabels[model.SchemeLabel]),
@@ -194,8 +194,8 @@ func NewTarget(cfg *config.ScrapeConfig, baseLabels, metaLabels model.LabelSet)
 		scraperStopping: make(chan struct{}),
 		scraperStopped:  make(chan struct{}),
 	}
-	t.Update(cfg, baseLabels, metaLabels)
+	err := t.Update(cfg, baseLabels, metaLabels)
-	return t
+	return t, err
 }
 
 // Status returns the status of the target.
@@ -205,14 +205,13 @@ func (t *Target) Status() *TargetStatus {
 
 // Update overwrites settings in the target that are derived from the job config
 // it belongs to.
-func (t *Target) Update(cfg *config.ScrapeConfig, baseLabels, metaLabels model.LabelSet) {
+func (t *Target) Update(cfg *config.ScrapeConfig, baseLabels, metaLabels model.LabelSet) error {
 	t.Lock()
 	defer t.Unlock()
 
 	httpClient, err := newHTTPClient(cfg)
 	if err != nil {
-		log.Errorf("cannot create HTTP client: %v", err)
+		return fmt.Errorf("cannot create HTTP client: %v", err)
-		return
 	}
 	t.httpClient = httpClient
 
@@ -257,6 +256,7 @@ func (t *Target) Update(cfg *config.ScrapeConfig, baseLabels, metaLabels model.L
 		t.baseLabels[model.InstanceLabel] = model.LabelValue(t.InstanceIdentifier())
 	}
 	t.metricRelabelConfigs = cfg.MetricRelabelConfigs
+	return nil
 }
 
 func newHTTPClient(cfg *config.ScrapeConfig) (*http.Client, error) {

retrieval/target_test.go

@@ -423,7 +423,7 @@ func TestURLParams(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	target := NewTarget(
+	target, err := NewTarget(
 		&config.ScrapeConfig{
 			JobName:        "test_job1",
 			ScrapeInterval: model.Duration(1 * time.Minute),
@@ -439,6 +439,9 @@ func TestURLParams(t *testing.T) {
 			"__param_foo":      "bar",
 		},
 		nil)
+	if err != nil {
+		t.Fatal(err)
+	}
 	app := &collectResultAppender{}
 	if err = target.scrape(app); err != nil {
 		t.Fatal(err)
@@ -639,3 +642,18 @@ func newTLSConfig(t *testing.T) *tls.Config {
 	tlsConfig.BuildNameToCertificate()
 	return tlsConfig
 }
+
+func TestNewTargetWithBadTLSConfig(t *testing.T) {
+	cfg := &config.ScrapeConfig{
+		ScrapeTimeout: model.Duration(1 * time.Second),
+		TLSConfig: config.TLSConfig{
+			CAFile:   "testdata/nonexistent_ca.cer",
+			CertFile: "testdata/nonexistent_client.cer",
+			KeyFile:  "testdata/nonexistent_client.key",
+		},
+	}
+	_, err := NewTarget(cfg, nil, nil)
+	if err == nil {
+		t.Fatalf("Expected error, got nil.")
+	}
+}

retrieval/targetmanager.go

@@ -278,7 +278,9 @@ func (tm *TargetManager) updateTargetGroup(tgroup *config.TargetGroup, cfg *conf
 				// to build up.
 				wg.Add(1)
 				go func(t *Target) {
-					match.Update(cfg, t.fullLabels(), t.metaLabels)
+					if err := match.Update(cfg, t.fullLabels(), t.metaLabels); err != nil {
+						log.Errorf("Error updating target %v: %v", t, err)
+					}
 					wg.Done()
 				}(tnew)
 				newTargets[i] = match
@@ -516,7 +518,10 @@ func (tm *TargetManager) targetsFromGroup(tg *config.TargetGroup, cfg *config.Sc
 				delete(labels, ln)
 			}
 		}
-		tr := NewTarget(cfg, labels, preRelabelLabels)
+		tr, err := NewTarget(cfg, labels, preRelabelLabels)
+		if err != nil {
+			return nil, fmt.Errorf("error while creating instance %d in target group %s: %s", i, tg, err)
+		}
 		targets = append(targets, tr)
 	}