Troubleshooting Flaky `TestMergeAlerts` In Alertmanager

Nov 14, 2025 by Admin 56 views

We've hit a snag with the TestMergeAlerts test in the Prometheus Alertmanager project, and it's showing some flaky behavior. This means the test passes fine locally, but it fails intermittently in the Continuous Integration (CI) environment, specifically during pull request checks. This kind of issue can be a real headache, as it prevents us from confidently merging code. In this article, we'll break down the problem, analyze the error logs, and explore potential solutions to stabilize this test.

Understanding the Issue

The core problem is that TestMergeAlerts is failing in the CI environment with a connection refused error. The error message Get "http://127.0.0.1:36357/api/v2/status": dial tcp 127.0.0.1:36357: connect: connection refused indicates that the test is unable to connect to the Alertmanager instance it's trying to test. This often suggests that the Alertmanager process either failed to start correctly or is not listening on the expected port.

Let's dive a bit deeper into why this might be happening. One common cause of flakiness in tests, especially integration tests, is timing issues. The test might be proceeding before Alertmanager has fully initialized and started accepting connections. Another possibility is resource contention within the CI environment. If the CI environment is under heavy load, it might take longer for Alertmanager to start up, or the test might be starved of resources, leading to a timeout.

Another potential culprit is related to how the test manages Alertmanager's lifecycle. The error message acceptance.go:172: Error sending SIGTERM to Alertmanager process: no such process hints that the test might be trying to terminate the Alertmanager process before it has actually started or after it has already terminated unexpectedly. This can leave the test in an inconsistent state, leading to further failures.

To effectively tackle this issue, we need to consider these factors and implement robust error handling and synchronization mechanisms within the test.

Analyzing the Error Logs

To get a clearer picture, let's dissect the provided error logs. Here's a breakdown of the key parts:

Connection Refused Error: The primary error, as mentioned earlier, is the connection refused error. This suggests that the test is trying to connect to Alertmanager before it's ready.
Alertmanager Startup Logs: The logs show Alertmanager starting up, loading its configuration, and joining a cluster. The line time=2025-11-14T16:14:59.573Z level=ERROR source=main.go:559 msg="Listen error" err="listen tcp 127.0.0.1:36357: bind: address already in use" is particularly telling. It indicates that Alertmanager is failing to bind to the specified port because the address is already in use. This could mean that a previous Alertmanager process wasn't properly shut down, or another process is using the same port.
Test Failure Details: The test output shows discrepancies in the number of alerts received. It expects 5 alerts but gets 0, indicating a failure in the alert merging logic.
SIGTERM Error: The error about failing to send SIGTERM suggests that the test's cleanup process is not robust and might be trying to kill a process that doesn't exist.

By piecing together these clues, we can hypothesize that the test's flakiness stems from a combination of port conflicts, timing issues, and improper process management.

Potential Solutions

Given the analysis, here are several strategies we can employ to fix the flakes in TestMergeAlerts:

Retry Logic with Timeout: Implement retry logic with a timeout when connecting to the Alertmanager API. This will allow the test to wait for Alertmanager to become fully available before proceeding. Use exponential backoff to avoid overwhelming the system with rapid retries.
Port Management: Ensure that each test run uses a unique port for Alertmanager. This can be achieved by dynamically allocating ports or using a port range specifically for testing. This will prevent port conflicts and the address already in use error.
Robust Process Management: Improve the test's process management to ensure that Alertmanager is properly shut down after each test run. Use a defer statement to ensure that the cleanup code is always executed, even if the test fails. Also, add checks to verify that the Alertmanager process is actually running before attempting to terminate it. This will address the Error sending SIGTERM to Alertmanager process: no such process error.
Synchronization Mechanisms: Introduce synchronization mechanisms, such as channels or wait groups, to ensure that the test waits for Alertmanager to reach a stable state before proceeding. This will prevent timing issues and ensure that the test doesn't start before Alertmanager is ready.
Resource Management: Investigate resource contention within the CI environment. If the CI environment is consistently overloaded, consider increasing the resources allocated to the test or optimizing the CI configuration to reduce the load.
Test Isolation: Ensure that the test is properly isolated from other tests. This can be achieved by using separate temporary directories and configurations for each test run. This will prevent interference between tests and ensure that each test runs in a clean environment.
Logging and Debugging: Add more detailed logging to the test and the Alertmanager process. This will help diagnose the root cause of the failures and identify any unexpected behavior.
Configuration Review: Carefully review the Alertmanager configuration used in the test. Ensure that the configuration is valid and doesn't contain any settings that could cause instability or conflicts. Pay close attention to the repeat_interval and group_interval settings, as the logs indicate a potential issue with these values.

Implementing the Fixes

Let's look at how we might implement some of these solutions in Go:

Retry Logic

import (
	"fmt"
	"net/http"
	"time"
)

func waitForAlertmanager(url string, timeout time.Duration) error {
	startTime := time.Now()
	for time.Since(startTime) < timeout {
		resp, err := http.Get(url)
		if err == nil && resp.StatusCode == http.StatusOK {
			return nil
		}
		fmt.Printf("Attempting to connect to Alertmanager at %s. Error: %v\n", url, err)
		time.Sleep(500 * time.Millisecond) // Exponential backoff
	}
	return fmt.Errorf("timed out waiting for Alertmanager at %s", url)
}

// Example usage:
// url := "http://127.0.0.1:36357/api/v2/status"
// timeout := 10 * time.Second
// err := waitForAlertmanager(url, timeout)
// if err != nil {
//	log.Fatalf("Failed to connect to Alertmanager: %v", err)
// }

Port Management

To avoid port conflicts, you can use the net.Listen function with address ":0" to let the OS choose an available port. Then, extract the port number and use it for Alertmanager's configuration.

Robust Process Management

import (
	"os"
	"os/exec"
	"syscall"
)

func startAlertmanager(configPath string, port int) (*exec.Cmd, error) {
	cmd := exec.Command("alertmanager",
		"--config.file=", configPath,
		fmt.Sprintf("--web.listen-address=127.0.0.1:%d", port),
	)

	// Set process group ID so we can kill the process and its children
	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}

	if err := cmd.Start(); err != nil {
		return nil, err
	}

	return cmd, nil
}

func stopAlertmanager(cmd *exec.Cmd) error {
	// Kill the process group to ensure all child processes are terminated
	if cmd.Process != nil {
		pgid, err := syscall.Getpgid(cmd.Process.Pid)
		 if err != nil {
			 return fmt.Errorf("could not get process group id: %w", err)
		 }

		// Negate the PID to kill the entire process group
		 err = syscall.Kill(-pgid, syscall.SIGTERM)
		 if err != nil {
			 return fmt.Errorf("could not kill process group: %w", err)
		 }

		// Wait for process to exit
		_, err = cmd.Process.Wait() 
		return err
	} 
	return nil
}

// Example usage:
// cmd, err := startAlertmanager("/tmp/am_test3659521531/config.yml", 36357)
// if err != nil {
//	log.Fatalf("Failed to start Alertmanager: %v", err)
// }
// defer func() {
//	 if err := stopAlertmanager(cmd); err != nil {
//		 log.Printf("Error stopping Alertmanager: %v", err)
//	 }
// }()

Conclusion

Fixing flaky tests like TestMergeAlerts requires a systematic approach. By carefully analyzing the error logs, understanding the potential causes of flakiness, and implementing robust solutions like retry logic, port management, and proper process handling, we can stabilize the test and prevent it from causing further issues in the CI environment. Remember to add detailed logging and continuously monitor the test's performance to ensure its reliability. These changes will not only make the test more stable but also increase our confidence in the Alertmanager codebase.