Do not apply offsets for explicit timeouts

main.go

@@ -71,24 +71,12 @@ func probeHandler(w http.ResponseWriter, r *http.Request, c *config.Config, logg
 		return
 	}
 
-	// If a timeout is configured via the Prometheus header, add it to the request.
-	var timeoutSeconds float64
-	if v := r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds"); v != "" {
-		var err error
-		timeoutSeconds, err = strconv.ParseFloat(v, 64)
-		if err != nil {
-			http.Error(w, fmt.Sprintf("Failed to parse timeout from Prometheus header: %s", err), http.StatusInternalServerError)
-			return
-		}
-	}
-	if timeoutSeconds == 0 {
-		timeoutSeconds = 10
+	timeoutSeconds, err := getTimeout(r, module, *timeoutOffset)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("Failed to parse timeout from Prometheus header: %s", err), http.StatusInternalServerError)
+		return
 	}
 
-	if module.Timeout.Seconds() < timeoutSeconds && module.Timeout.Seconds() > 0 {
-		timeoutSeconds = module.Timeout.Seconds()
-	}
-	timeoutSeconds -= *timeoutOffset
 	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeoutSeconds*float64(time.Second)))
 	defer cancel()
 	r = r.WithContext(ctx)
@@ -355,3 +343,26 @@ func run() int {
 	}
 
 }
+
+func getTimeout(r *http.Request, module config.Module, offset float64) (timeoutSeconds float64, err error) {
+	// If a timeout is configured via the Prometheus header, add it to the request.
+	if v := r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds"); v != "" {
+		var err error
+		timeoutSeconds, err = strconv.ParseFloat(v, 64)
+		if err != nil {
+			return 0, err
+		}
+	}
+	if timeoutSeconds == 0 {
+		timeoutSeconds = 10
+	}
+
+	var maxTimeoutSeconds = timeoutSeconds - offset
+	if module.Timeout.Seconds() < maxTimeoutSeconds && module.Timeout.Seconds() > 0 {
+		timeoutSeconds = module.Timeout.Seconds()
+	} else {
+		timeoutSeconds = maxTimeoutSeconds
+	}
+
+	return timeoutSeconds, nil
+}

main_test.go

@@ -102,3 +102,38 @@ func TestDebugOutputSecretsHidden(t *testing.T) {
 		t.Errorf("Hidden secret missing from debug output: %v", out)
 	}
 }
+
+func TestTimeoutIsSetCorrectly(t *testing.T) {
+	var tests = []struct {
+		inModuleTimeout     time.Duration
+		inPrometheusTimeout string
+		inOffset            float64
+		outTimeout          float64
+	}{
+		{0 * time.Second, "15", 0.5, 14.5},
+		{0 * time.Second, "15", 0, 15},
+		{20 * time.Second, "15", 0.5, 14.5},
+		{20 * time.Second, "15", 0, 15},
+		{5 * time.Second, "15", 0, 5},
+		{5 * time.Second, "15", 0.5, 5},
+		{10 * time.Second, "", 0.5, 9.5},
+		{10 * time.Second, "10", 0.5, 9.5},
+		{9500 * time.Millisecond, "", 0.5, 9.5},
+		{9500 * time.Millisecond, "", 1, 9},
+		{0 * time.Second, "", 0.5, 9.5},
+		{0 * time.Second, "", 0, 10},
+	}
+
+	for _, v := range tests {
+		request, _ := http.NewRequest("GET", "", nil)
+		request.Header.Set("X-Prometheus-Scrape-Timeout-Seconds", v.inPrometheusTimeout)
+		module := config.Module{
+			Timeout: v.inModuleTimeout,
+		}
+
+		timeout, _ := getTimeout(request, module, v.inOffset)
+		if timeout != v.outTimeout {
+			t.Errorf("timeout is incorrect: %v, want %v", timeout, v.outTimeout)
+		}
+	}
+}