ANTLR4 Bug Impacts JDBI: A Concurrency Issue & Workaround

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ANTLR4 Bug Affecting JDBI Discussion

Hey guys,

I wanted to share something super interesting and kinda tricky that we ran into while working with JDBI. Big shoutout to @lukasz-stec for spotting this! It turns out there's a concurrency bug deep inside ANTLR4 that can mess with JDBI, especially when parsing queries. Here's the lowdown:

The ANTLR4 Concurrency Bug: A Deep Dive

So, here's the deal: ANTLR4, under the hood, uses something called an Interval(a, b) cache. This cache gets activated when a is the same as b (think of it as representing a token with a length of 1) and when a is less than or equal to 1000. Now, the sneaky part is that the Interval class has these a and b fields, but they're not marked as final. This can cause problems on architectures with what we call a weak memory model, like ARM.

On these architectures, the act of publishing cache[a] = new Interval(a, a) isn't thread-safe. What this means is that if you're parsing multiple queries at the same time, some of those queries might grab an Interval with incorrect offsets from the cache. Imagine getting an offset that's still its initial value of 0! This can lead to some seriously weird behavior. You might end up with the whole query string jammed into the place of the next token, or maybe just the first character gets repeated. The result? Syntax errors or metadata issues where columns mysteriously vanish.

This bug was a real pain to track down because it was buried deep within the ANTLR4 code and only popped up on specific ARM processors, like Graviton, after tons and tons of iterations. Seriously, hats off to @lukasz-stec for his detective work on this one!

How the Bug Surfaces in JDBI

In the case of JDBI, this bug rears its head when the DefinedAttributeTemplateEngine is parsing a query string. This engine parses character by character for unquoted literals, and it uses Interval.of(pos, pos) to represent the position of each single-character token. The race condition occurs when multiple queries are parsed in parallel, leading to corruption.

The Workaround: Priming the Cache

To sidestep this issue, we came up with a workaround that essentially primes the Interval.of(a, a) cache before JDBI even gets a chance to use it. Here's the code snippet we used:

 private static void antlr4901Workaround()
 {
 // TODO: Remove this workaround for antlr concurrency issue for tokens
 // with length 1 and offset between 0 and INTERVAL_POOL_MAX_VALUE:
 // https://github.com/antlr/antlr4/issues/4901
 for (int i = 0; i <= $Interval.INTERVAL_POOL_MAX_VALUE; i++) {
 if ($Interval.of(i, i).length() != 1) {
 throw new IllegalStateException("Expected Interval.of(%d, %d) to be correctly cached".formatted(i, i));
 }
 }
 }

Essentially, this code iterates through the range of possible values for a and pre-populates the Interval cache. This ensures that when JDBI goes to use the cache, it's already filled with the correct values, avoiding the concurrency issue.

Key Takeaways: ANTLR4, JDBI, and Concurrency

This whole episode highlights a few important things:

  • Concurrency Bugs are Tricky: They can be incredibly difficult to track down, especially when they're buried deep within the dependencies of your dependencies.
  • Weak Memory Models Matter: Architectures like ARM have different memory models than, say, x86. This can lead to subtle concurrency issues that only manifest on specific hardware.
  • Dependencies Can Surprise You: You might not expect a low-level library like ANTLR4 to cause issues in a higher-level library like JDBI, but that's the reality of software development.
  • Workarounds are Sometimes Necessary: While it's always ideal to fix the root cause of a bug, sometimes a workaround is the most practical solution, especially when dealing with external dependencies.

Diving Deeper into JDBI and ANTLR4 Interaction

To truly appreciate the significance of this bug, it's important to understand how JDBI utilizes ANTLR4, and how the DefinedAttributeTemplateEngine plays a pivotal role. JDBI, at its core, is a library that simplifies database interactions in Java. It provides a convenient way to execute SQL queries and map the results to Java objects. One of the key features of JDBI is its templating engine, which allows you to dynamically generate SQL queries based on various parameters.

The DefinedAttributeTemplateEngine is one such templating engine. It's responsible for parsing and interpreting SQL queries that contain attributes defined within the query itself. When this engine encounters an unquoted literal, it relies on ANTLR4 to break down the query into individual tokens. For each character in the literal, Interval.of(pos, pos) is invoked to represent the position of that character.

This seemingly innocuous operation becomes a hotbed for concurrency issues when multiple queries are being parsed in parallel. The shared Interval cache in ANTLR4 becomes a point of contention, leading to the corruption of token positions.

The Role of Interval Class

The Interval class, residing within the ANTLR4 library, serves as a fundamental component for defining and managing character ranges within the input text. Each Interval instance encapsulates a starting and ending index, effectively delineating a specific section of the input string. These intervals play a critical role in tokenization and parsing, enabling the ANTLR4 parser to accurately identify and process different components of the input.

The Interval.of(a, a) method is particularly significant in this context. It's used to create an interval that represents a single character at position a. This method leverages a caching mechanism to optimize performance by reusing existing Interval instances for common values of a. However, the lack of synchronization around this cache introduces the potential for race conditions when multiple threads access it concurrently. This is where the concurrency bug manifests, leading to incorrect Interval instances being returned from the cache and ultimately causing parsing errors.

Implications of the Concurrency Bug

The consequences of this concurrency bug can be far-reaching, particularly in applications that heavily rely on JDBI for database interactions. The corruption of token positions during query parsing can lead to a variety of issues, including:

  • Syntax Errors: Incorrect token positions can cause the ANTLR4 parser to misinterpret the SQL query, resulting in syntax errors.
  • Metadata Issues: Corruption of column positions can lead to the parser failing to locate the correct columns in the database, resulting in errors when retrieving data.
  • Data Corruption: In extreme cases, the bug can even lead to data corruption if the incorrect query is executed against the database.

These issues can be difficult to diagnose and debug, as they may only manifest intermittently under heavy load. The fact that the bug is specific to ARM processors further complicates matters, as it may not be reproducible on development machines.

Lessons Learned

This experience has taught us valuable lessons about the importance of concurrency testing, the challenges of debugging issues in complex systems, and the need to be aware of the potential pitfalls of shared mutable state. It also highlights the importance of staying up-to-date with the latest versions of our dependencies, as bug fixes and performance improvements are constantly being released.

Contributing to the Community

By sharing our experience with this ANTLR4 bug, we hope to contribute to the wider JDBI and ANTLR4 communities. We encourage other developers to be aware of this issue and to consider implementing the workaround described above in their own applications. We also hope that the ANTLR4 team will address this bug in a future release, so that we can remove the workaround and rely on the library's built-in concurrency safety.

Thanks for reading, and happy coding!