Datadog Dd-trace-js MySQL2/Promise AppSec Bug Fix

Hey there, tech enthusiasts and fellow developers! Ever found yourself scratching your head, staring at an error message that just doesn't make sense, especially when dealing with powerful tools like Datadog's dd-trace-js and the robust mysql2/promise library? Well, you're not alone, and today we're diving deep into a tricky bug that can pop up when these two awesome technologies interact, particularly when Datadog's AppSec (Application Security) module decides to kick in. We're talking about a scenario where your beautifully crafted await pool.query() suddenly gets a curveball thrown its way, returning a callback-style object instead of the Promise you were expecting, all thanks to an abortController.signal.aborted check. This isn't just an annoyance; it's a full-blown programming error that can halt your application dead in its tracks. But don't you worry, because we're going to break down exactly what's happening, why it's happening, and most importantly, how we can fix it to ensure your applications run smoothly and securely, especially when integrating with crucial services like Datadog for monitoring and security.

Understanding the Core Problem: Datadog dd-trace-js and mysql2/promise Clash

Alright, let's get straight to the heart of the matter, guys. The core problem here revolves around how dd-trace-js, Datadog's tracing library for Node.js applications, interacts with mysql2/promise, which is a fantastic module providing a promise-based API for MySQL. Normally, these two play really well together, offering incredible insights into your database operations and application performance. You get detailed traces of your queries, connection metrics, and all that good stuff, making debugging and optimization a breeze. However, a specific situation arises when Datadog's Application Security (AppSec) feature is enabled. What happens is that the mysql2 instrumentation within dd-trace can, under certain conditions, return a traditional callback-style Query object instead of the expected Promise. This occurs specifically when an abortController.signal.aborted check evaluates to true within the tracing logic. Now, for those of us leveraging mysql2/promise, we're expecting a Promise from our pool.query() calls, something we can await or chain .then() and .catch() methods onto. When a callback-style object is returned instead, it throws a wrench into the whole promise-based paradigm. The mysql2 library itself includes a safety mechanism: its Query class has a .then() method that is designed to throw an error if you try to await or .then() an object that isn't actually a promise. This is a crucial detail because it's what ultimately triggers the visible error in your application. So, instead of your code gracefully handling an aborted operation, you're met with a jarring error message, interrupting your application flow. Imagine writing const [rows] = await pool.query('SELECT * FROM table'); and instead of getting your data, or even a rejected promise, you get an error telling you that what you're awaiting isn't a promise at all. That's exactly the headache this bug introduces, especially in environments where network disruptions or client-side request timeouts are more common, like during development. It's a subtle yet significant departure from the expected behavior, undermining the reliability of your database interactions when dd-trace is involved with AppSec enabled. This unexpected type mismatch is not just an inconvenience; it can lead to application crashes and make debugging incredibly frustrating, as the error message might initially point away from the tracing library itself. Understanding this fundamental conflict between expected promise behavior and the actual return type during an AppSec abort is the first step towards resolving this challenging dd-trace-js mysql2/promise bug.

Deep Dive into the Code: Where the Bug Hides

To really understand this, folks, we need to roll up our sleeves and look at the actual code where this glitch originates. The culprit is hiding within the datadog-instrumentations package, specifically in the src/mysql2.js file, around lines 167-204. This is where dd-trace wraps the Pool.prototype.query method to inject its tracing logic. Shimmer, a library used by dd-trace for monkey-patching, is employed here to extend the functionality of the original query method without directly modifying the source code of mysql2. The idea is brilliant: wrap the function, add tracing, and then call the original. However, things go sideways when the AppSec feature comes into play. Inside this wrapped query function, there's a conditional block that checks if (abortController.signal.aborted). This check is a crucial part of AppSec, designed to handle situations where an operation should be aborted, perhaps due to a security policy violation or a client-side timeout. When this condition is met, the code deviates from its normal path. Instead of returning a Promise (which is what mysql2/promise users expect), it returns queryCommand. Now, queryCommand is an internal object that represents the query in a callback-style manner, not a Promise-like object. For applications built with mysql2/promise, every query call is expected to return a Promise. When abortController.signal.aborted is true, the tracing code incorrectly returns queryCommand directly. This is the root cause! If you're using mysql2/promise and have AppSec enabled, and an abort signal is triggered, your await call will receive this queryCommand object. Since it's not a Promise, mysql2's built-in .then() method (which is specifically designed to catch these kinds of misuses) correctly flags it as an error. It's effectively yelling, "Hey! You just tried to await something that isn't a promise!" This isn't mysql2 being difficult; it's mysql2 protecting you from a fundamental type mismatch. The dd-trace instrumentation, while well-intentioned, momentarily forgets that mysql2/promise users are in a promise-land, not callback-land, when an abort event occurs, leading to this ECONNRESET error and the confusing .then() exception. The core issue, then, is a mismatch in expected return types when AppSec's abort mechanism is activated within the dd-trace-js instrumentation for mysql2/promise. It's a classic example of an unintended side effect when two powerful libraries, each designed for a specific purpose, don't perfectly align their asynchronous interfaces under every edge case. This deep understanding of the problematic code path is absolutely essential for crafting a correct and lasting solution to this dd-trace-js mysql2/promise AppSec interaction bug.

The Real-World Impact: Next.js 15+ and Development Woes

So, what does this bug actually look like in a live application, especially for those of us working with modern frameworks like Next.js 15+? Well, it can be a real headache, particularly during development. Imagine you're building a sleek new feature, using mysql2/promise for your database interactions, and suddenly, boom! Your application crashes with a cryptic error message like, "You have tried to call .then(), .catch(), or invoked await on the result of query that is not a promise, which is a programming error." Alongside that, you might see an ECONNRESET code, indicating a connection reset. This isn't just a hypothetical scenario; it's a common occurrence for developers who run into this Datadog dd-trace-js MySQL2/Promise AppSec bug. The reproduction steps outlined by users are quite specific and point to a clear pattern: it often happens on the first request after a server restart and when a connection pool is being created. Why then, you ask? Let's break it down. Modern Next.js applications, especially with features like React Server Components and Strict Mode (common in dev), often result in duplicate requests or component re-renders. Combine this with the often longer compilation times on the first request after a cold start in a development environment, and you've got a recipe for client-side request timeouts or network disruptions. When a client-side request times out or is aborted, it can trigger that abortController.signal.aborted condition in dd-trace's AppSec, leading to the problematic queryCommand return. Furthermore, Hot Module Replacement (HMR) during development can interrupt in-flight database connections, creating more opportunities for ECONNRESET errors and thus activating the AppSec abort logic. So, you're in this vicious cycle: developing, code changes, HMR, connection reset, AppSec aborts, and then your await pool.query() blows up because it received a non-Promise object. It’s frustrating because it interrupts your flow and can make you question your own database code, even though the problem lies within the instrumentation. The debug logs might even show DEBUG: Creating pool for: [schema_id] right before the error, further emphasizing that this happens early in the lifecycle when connections are being established and are more susceptible to these kinds of interruptions. The operating system (macOS or Linux) doesn't seem to matter; the bug is consistent across environments, making it a widespread concern for anyone using this particular stack. This kind of unexpected behavior definitely impacts developer productivity and overall application stability, making a robust fix absolutely essential for a smooth development and production experience. It essentially creates a brittle interaction where modern async patterns are undermined by an unexpected type return, forcing developers to either disable critical security features or grapple with unreliable database operations. This is why addressing the Datadog dd-trace-js MySQL2/Promise AppSec bug is not just a nicety, but a necessity for robust application development.

Temporary Relief: The appsec: { enabled: false } Workaround

Alright, so you're in the middle of a sprint, and this Datadog dd-trace-js MySQL2/Promise AppSec bug is throwing a wrench in your plans. You need an immediate solution, right? Good news, there's a workaround that can get you back on track quickly, even if it's not the ideal long-term fix. The temporary workaround involves simply disabling Datadog's Application Security (AppSec) module. You can achieve this by modifying your dd-trace initialization configuration. Instead of just calling tracer.default.init({ runtimeMetrics: true }), you'll add the appsec: { enabled: false } option, like so:

tracer.default.init({
  runtimeMetrics: true,
  appsec: { enabled: false }, // <-- This is the key!
});

Once you make this change and restart your application, you should find that the pesky .then() error and the ECONNRESET issues related to AppSec aborts disappear. Why does this work? Well, by setting appsec: { enabled: false }, you're effectively telling dd-trace to skip all the AppSec-related logic, including the abortController.signal.aborted check that was causing the queryCommand (callback-style object) to be returned instead of a Promise. This prevents the problematic code path from being executed, allowing your mysql2/promise calls to behave as expected, consistently returning Promises. However, and this is a big however, disabling AppSec comes with a significant trade-off. Datadog AppSec is a powerful feature designed to provide critical security visibility into your application, helping to detect and prevent various types of attacks, from SQL injection to cross-site scripting. By turning it off, you're essentially losing that layer of protection and insight that Datadog provides, potentially leaving your application more vulnerable. This might be acceptable for a local development environment where you're primarily focused on functionality, but it's generally not recommended for production environments unless you have alternative robust security measures in place. It's crucial to understand that while this workaround provides immediate relief, it's a temporary measure. It buys you time to continue development or deploy a critical fix, but it doesn't solve the underlying bug within the dd-trace instrumentation itself. So, while it's a helpful trick to keep in your toolbox for urgent situations, keep in mind its implications for your application's security posture and actively seek out the permanent solution once it becomes available. Your security team might not be too happy if AppSec is off in production, so use this workaround wisely and with caution.

The Path Forward: A Robust Solution for dd-trace-js

Now that we've understood the problem and explored a temporary workaround, let's talk about the proper, robust solution for this Datadog dd-trace-js MySQL2/Promise AppSec issue. The key here is to ensure that dd-trace's instrumentation for mysql2 behaves consistently with the mysql2/promise API, especially when the AppSec abort signal is triggered. The suggested fix is elegant and aligns perfectly with the promise-based nature of mysql2/promise. When the abortController.signal.aborted condition is met within the wrapPool function, instead of returning the raw queryCommand object, the instrumentation should detect if it's dealing with a promise-based pool and, if so, return a rejected Promise. Here's how that would look in essence:

// Inside the wrapped Pool.prototype.query function
if (abortController.signal.aborted) {
  // If this is a promise-based pool (i.e., from mysql2/promise),
  // we MUST return a rejected promise.
  if (this.promise) { // 'this' refers to the pool instance
    return Promise.reject(abortController.signal.reason);
  }
  // For traditional callback-based mysql2 (if it exists, though the issue is with /promise),
  // the existing callback handling logic might apply here, or a different rejection.
  // This part of the code needs careful consideration for both scenarios
  // if dd-trace handles both callback and promise versions.
  // For this specific bug, the focus is on mysql2/promise.
  // ... original callback handling (if applicable for non-promise contexts) ...
  return queryCommand; // This would only be for non-promise contexts.
}
// ... otherwise, continue with the original query.apply(this, arguments)

By adding the check if (this.promise), the dd-trace instrumentation can intelligently determine whether it's dealing with a mysql2/promise pool. The mysql2/promise module internally adds a promise property to its Pool instances, which makes this detection straightforward and reliable. If this.promise is true, then the correct behavior when an operation is aborted by AppSec is to return Promise.reject(abortController.signal.reason). This ensures that any await or .then().catch() chain downstream will properly catch the rejection, allowing the application to handle the aborted operation gracefully, rather than crashing with a type error. This approach maintains the integrity of the promise chain, which is absolutely critical for modern asynchronous JavaScript codebases. It treats the abort signal as a legitimate reason to reject the database operation, providing a consistent and predictable outcome for developers. This fix doesn't just paper over the problem; it fundamentally corrects the type mismatch at the source, making dd-trace-js and mysql2/promise truly compatible, even when AppSec is actively monitoring and potentially aborting requests. Implementing this fix would significantly improve the reliability of applications using this stack, reduce developer frustration, and allow users to leverage Datadog's full suite of features, including AppSec, without encountering unexpected crashes or requiring security compromises. It’s about making these powerful tools work together seamlessly, as they were always intended to, providing both performance insights and robust security without introducing runtime errors. This thoughtful adjustment ensures that Datadog's powerful tracing and security features enhance, rather than hinder, the modern asynchronous developer experience, making it a critical improvement for the entire dd-trace-js ecosystem.

Summing It Up: Ensuring Smooth Sailing with Datadog and MySQL2

Alright, folks, we've covered quite a bit today about the Datadog dd-trace-js MySQL2/Promise AppSec Bug. We started by highlighting the core issue: how dd-trace-js, when its AppSec module aborts an operation, can mistakenly return a callback-style object instead of a Promise to mysql2/promise users. This leads to that dreaded "You have tried to call .then()..." error, bringing your promise-based database interactions to a screeching halt. We then took a closer look at the actual code within packages/datadog-instrumentations/src/mysql2.js, pinpointing the exact lines where return queryCommand; was causing the type mismatch when abortController.signal.aborted was true. This deep dive helped us understand why mysql2's safety mechanism was kicking in, correctly identifying the returned object as non-Promise. From there, we explored the real-world impact, especially for developers working with Next.js 15+ in development environments. We saw how duplicate requests, long compilation times, and Hot Module Replacement could exacerbate the problem, making ECONNRESET errors and AppSec aborts more frequent. It's clear that this bug doesn't just cause errors; it significantly impacts developer productivity and application stability. We also discussed the immediate, albeit temporary, workaround of disabling AppSec (appsec: { enabled: false }) in your dd-trace configuration. While this can provide quick relief, we emphasized the security trade-offs involved, advising caution, especially in production settings. Finally, and most importantly, we detailed the robust, permanent solution: modifying the dd-trace instrumentation to intelligently detect mysql2/promise pools using this.promise and returning a Promise.reject(abortController.signal.reason) when an AppSec abort occurs. This fix ensures that the promise chain remains intact, allowing applications to gracefully handle aborted operations without encountering fundamental type errors. This isn't just about fixing a bug; it's about ensuring that developers can leverage the full power of Datadog's tracing and security features alongside modern Node.js database clients like mysql2/promise without compromising on reliability or expected asynchronous patterns. By implementing this suggested fix, we can achieve true compatibility, providing both deep observability and robust security for our applications. So, if you're experiencing this issue, consider contributing to the dd-trace-js project or keeping an eye out for updates that incorporate this crucial fix. Your smooth sailing in the world of Node.js and Datadog depends on it! Happy coding, everyone!