Nixpkgs Python 3.13 Proton-core Build Fix: `test_api.py` Error

Hey NixOS Users! Understanding the proton-core Build Blip

Alright, folks, let's talk about a little hiccup that's been affecting our beloved Nixpkgs unstable channel, specifically when it comes to Python 3.13 and the proton-core package. If you've been trying to build python313Packages.proton-core recently, you might have hit a wall – a rather perplexing error stemming from test_api.py. This isn't just a minor annoyance; it's a build failure that prevents the package from completing its test suite successfully, which is a big deal in the Nixpkgs ecosystem where reliability is key. The core of the issue, as we’ll dive into, is a ValueError that screams: "password cannot be longer than 72 bytes". Yep, a simple password length constraint is causing this disruption, particularly within the TestPYSRPClass tests. Understanding this password length error is crucial for anyone working with or relying on this package in NixOS. For those of us who appreciate the stability and reproducibility that Nixpkgs offers, such build failures, even in the unstable channel, are opportunities to contribute and strengthen the platform. The proton-core library is an important piece for certain applications, and ensuring its smooth integration with the latest Python versions, like Python 3.13, is vital. We're looking at a scenario where a specific security library's test data is inadvertently tripping over a hard-coded limit, leading to cascading test failures. This article aims to break down why this is happening, how you can reproduce it, and most importantly, what steps we can take as a community to resolve this test_api.py problem and get proton-core building cleanly on NixOS again. It's all about making the Nix experience as seamless as possible, even when we encounter these unexpected bumps in the road. So, buckle up, because we're about to demystify this Python 3.13 proton-core build failure!

The Nitty-Gritty: What's Breaking in test_api.py?

So, what exactly is causing our python313Packages.proton-core build to stumble? The detailed log output points directly to failures within tests/test_api.py, specifically concerning the TestPYSRPClass. This class, as its name suggests, is responsible for testing the Secure Remote Password (SRP) protocol implementation within proton-core. The SRP protocol is a cryptographic key-exchange protocol that allows clients to authenticate themselves to servers without sending the password itself, making it much more secure than traditional password-based authentication. The test_api.py script includes various test cases, such as test_compute_v, test_generate_v, and test_srp, all of which are failing with the same fundamental ValueError. This ValueError is quite explicit: "password cannot be longer than 72 bytes, truncate manually if necessary (e.g. my_password[:72])". This error originates deep within the proton/session/srp/util.py module, specifically in the hash_password_3 function, which then calls bcrypt.hashpw.

The stack trace clearly shows the path: TestPYSRPClass calls usr.compute_v or usr.process_challenge, which in turn leads to calculate_x in _pysrp.py. This calculate_x function then relies on hash_password from util.py, and finally, hash_password_3 makes the call to bcrypt.hashpw. The problem isn't with bcrypt itself, or necessarily a bug in proton-core's implementation logic, but rather an incompatibility in the test data used by TestPYSRPClass. The test cases are supplying a password string, b'PasswordThatIsLongerThan72CharsAndContainsSomeRandomStuffThatNoOneCaresAbout', which, as you can see, far exceeds the 72-byte limit enforced by the bcrypt library. When bcrypt.hashpw receives a password longer than its internal limit, it raises this precise ValueError. This means the proton-core tests, designed to thoroughly validate the SRP implementation, are inadvertently using test data that violates an underlying dependency's constraint. Because these tests are part of the pytestCheckPhase in the Nixpkgs build process, their failure halts the entire python313Packages.proton-core build, preventing the package from being successfully created and installed on NixOS. It’s a classic case of test data not aligning with external library expectations, creating a frustrating roadblock for anyone trying to get proton-core up and running with Python 3.13 via Nixpkgs. Fixing this password length error will be key to unblocking the package build. This highlights the importance of rigorous testing in package management, and also the intricate dependencies that can sometimes lead to unexpected issues, especially across different Python versions and library updates.

Diving Deeper: The bcrypt 72-Byte Password Limit

Let’s zoom in a bit on the root cause of this ValueError: the 72-byte password limit imposed by bcrypt. You might be wondering, why does bcrypt have such a specific and seemingly arbitrary limit? Well, guys, it's not arbitrary at all; it's deeply rooted in the history and design philosophy of the bcrypt algorithm itself. Bcrypt is a password-hashing function designed by Niels Provos and David Mazières, based on the Blowfish cipher. It was specifically crafted to be resistant to brute-force attacks by being computationally intensive, and it was also designed with a fixed input size in mind. The original Blowfish cipher, on which bcrypt is based, has a key schedule that operates most efficiently with keys up to 56 bytes (448 bits). While bcrypt itself processes passwords by feeding them into Blowfish, the specific 72-byte limit for passwords is a widely accepted, albeit somewhat historical, convention in many bcrypt implementations (like py-bcrypt or bcrypt for Python). This limit is not necessarily a security vulnerability but rather a design choice. If a password exceeds this length, many bcrypt libraries will either truncate it silently (which is generally considered bad practice as it reduces the effective entropy of the password) or, as in our case with the proton-core tests, raise an explicit ValueError. The latter, while breaking the build, is actually the safer behavior, as it forces the developer to acknowledge and handle the length constraint, preventing potential security misconfigurations due to silent truncation.

For proton-core, this bcrypt limit poses a challenge because its TestPYSRPClass tests were evidently designed with sample passwords that exceed this boundary, perhaps without fully anticipating the strict enforcement by the underlying bcrypt library, or perhaps due to changes in bcrypt's behavior or proton-core's dependencies over time. The particular test password, b'PasswordThatIsLongerThan72CharsAndContainsSomeRandomStuffThatNoOneCaresAbout', is clearly just a lengthy string for testing purposes, likely intended to ensure that various parts of the SRP protocol handle long inputs correctly. However, when it hits the bcrypt.hashpw function within proton/session/srp/util.py, it triggers the ValueError. This isn't a bug in bcrypt; it's bcrypt doing exactly what it's designed to do: enforce a known input size and loudly complain when that size is violated. The interaction between proton-core's test suite and the strict bcrypt implementation, especially when compiled and run within the specific environment of Nixpkgs and Python 3.13, highlights a compatibility point that needs to be addressed. It's a fantastic example of how seemingly small details in library design can have significant ripple effects across a complex software stack like Nixpkgs, leading to build failure if not carefully managed.

How to See This Bug in Action: Reproducing the proton-core Failure

For those of you who like to get your hands dirty and verify issues for yourselves, reproducing this proton-core build failure is straightforward. This is particularly useful for maintainers or contributors who want to test proposed fixes. You don't need a complex setup; NixOS's declarative nature makes this process incredibly simple, even if you're not running a full NixOS system. Just ensure you have a working Nix environment installed. The issue manifests itself when attempting to build the python313Packages.proton-core on the Nixpkgs unstable channel. The specific version of Nixpkgs that triggered this report was c5ae371, but given that it's in the unstable channel, you might encounter it with other recent commits as well, especially as Python 3.13 integration evolves. To witness this password length error firsthand, simply open your terminal and execute the following command:

nix build github:nixos/nixpkgs/c5ae371#python313Packages.proton-core

What you would expect is for the build process, including all its test phases, to complete without any errors, yielding a successfully built proton-core package for Python 3.13. However, what you will actually experience is the pytestCheckPhase failing. The build will proceed through unpacking, patching, configuring, building the wheel, installing, and even a runtime dependency check. It's only when it hits the pytestCheckPhase that things go south. You'll see pytest being invoked, collecting tests, and then, boom! Multiple FAILED messages will appear, pointing specifically to tests/test_api.py. The relevant log output, which we saw earlier, will clearly highlight TestPYSRPClass::test_compute_v, TestPYSRPClass::test_generate_v, and TestPYSRPClass::test_srp as the culprits. Each failure will be accompanied by the tell-tale ValueError: password cannot be longer than 72 bytes, truncate manually if necessary (e.g. my_password[:72]). This is the smoking gun, confirming that the bcrypt password length constraint is indeed the source of the build failure. This simple command provides a consistent and reproducible way to observe the bug, which is invaluable for diagnosis and for confirming that any potential fixes actually work. It underscores the beauty of Nix – even with a test_api.py error, the reproduction steps are clear, isolated, and highly reliable across different environments. So, go ahead, give it a shot, and join the ranks of those who've encountered this particular Python 3.13 proton-core challenge.

Charting a Course: Potential Solutions for the Python 3.13 proton-core Issue

Alright, now that we've pinpointed the password length error in python313Packages.proton-core on Nixpkgs, let's explore some actionable solutions. When facing a build failure like this, especially one involving a third-party library's test suite, we generally have a few routes to consider, each with its own pros and cons. Our primary goal is to get proton-core building cleanly for Python 3.13 users on NixOS.

1. Patching the proton-core Tests within Nixpkgs

This is often the most direct and immediate fix for a Nixpkgs maintainer. Since the ValueError originates from proton-core's test_api.py supplying an overly long password to bcrypt.hashpw, the simplest solution is to modify the test data. A patch could be applied during the Nixpkgs build process to truncate the problematic test password string (e.g., b'PasswordThatIsLongerThan72CharsAndContainsSomeRandomStuffThatNoOneCaresAbout') to a length acceptable by bcrypt (e.g., [:72]). This approach keeps the test logic intact but adjusts the test input to comply with bcrypt's requirements. This method is effective because it targets the specific point of failure without altering the core functionality of proton-core. It's a surgical strike to fix the test_api.py problem. However, it’s a downstream patch, meaning we’re fixing it in Nixpkgs rather than the original proton-core source. This requires careful maintenance, as upstream changes to proton-core might conflict with our patch in the future. Nevertheless, for a quick and reliable fix for the Python 3.13 package in Nixpkgs, this is a strong contender.

2. Advocating for an Upstream Fix in proton-core

The ideal, long-term solution is always an upstream fix. This means submitting a pull request to the original proton-core project's repository. The project maintainers could then update their test_api.py to use a shorter test password or, more robustly, add logic to truncate passwords before passing them to bcrypt.hashpw in their test suite, perhaps with a comment explaining the bcrypt limitation. This ensures the fix benefits all users of proton-core, regardless of their build system, and reduces the maintenance burden on Nixpkgs maintainers. It makes the package inherently more compatible with libraries like bcrypt. This route ensures the integrity of the original source code and helps other distributions that might encounter the same password length error. It might take longer to implement and integrate, but it's the most holistic approach to resolving the Python 3.13 proton-core build failure permanently. Engaging with the upstream project is a cornerstone of open-source collaboration and often leads to the most robust outcomes.

3. Temporarily Disabling or Skipping the Failing Tests

In urgent situations where a working proton-core package is absolutely critical and no immediate patch is available, a temporary workaround could involve modifying the pytestCheckPhase in the Nixpkgs derivation to skip the specific failing tests (e.g., using pytest's -k or --ignore flags more aggressively, or even commenting out the problematic test cases in a temporary patch). While this would allow the package to build, it's a suboptimal solution because it means a portion of proton-core's functionality goes untested. This increases the risk of undetected bugs and should only be considered a last resort or a very short-term measure until a proper fix for the test_api.py issues can be implemented. It essentially bypasses the build failure rather than fixing it, so use with extreme caution.

4. Downgrading Python or bcrypt (Less Ideal)

Another approach, though generally not recommended as a long-term fix, is to revert the Python version from 3.13 to an earlier one or downgrade the bcrypt library to a version that might behave differently (e.g., silently truncating the password, which, as discussed, is not ideal behavior). This is mostly a diagnostic step to see if the issue is version-specific or a regression. For a robust system like Nixpkgs, where packages are built against specific dependencies, forcing a downgrade of core components like Python is rarely a clean solution and often leads to other compatibility headaches. It doesn't solve the underlying problem within proton-core's tests but rather sidesteps it by changing the environment.

The most viable paths involve either patching the tests within Nixpkgs for immediate relief or, ideally, collaborating with the upstream proton-core project for a sustained resolution. Both strategies aim to make the Nixpkgs Python 3.13 proton-core build robust and reliable, ensuring our NixOS systems remain solid.

Why This Matters for Nixpkgs and Python Devs

This python313Packages.proton-core build failure might seem like a small, isolated issue—just one package in a vast repository—but trust me, guys, it carries significant implications for both the Nixpkgs ecosystem and the broader Python 3.13 development landscape. First and foremost, for Nixpkgs, consistency and reproducibility are paramount. When a package, especially one as fundamental as a core library like proton-core, fails to build in the unstable channel, it signals a potential weakness. It means that dependents of proton-core can't build, and users relying on this library in their NixOS configurations will hit a snag. The unstable channel is where new features and Python 3.13 updates are integrated, and ideally, we want a smooth transition for all packages. A test_api.py error like this, tied to a password length error in bcrypt, indicates that vigilance is needed in testing package interactions, especially as Python versions evolve. Maintainers, like our friend @SebTM, are constantly balancing thousands of packages, and every such build failure adds to the workload, diverting resources from other crucial development. This issue underscores the importance of robust CI/CD (Continuous Integration/Continuous Deployment) pipelines within Nixpkgs, ensuring that even minor changes in dependencies or test data don't break the entire system.

For Python developers, this situation highlights the subtle complexities of library dependencies and versioning. As Python 3.13 progresses, libraries like proton-core need to ensure their test suites are compatible with the latest versions of their underlying dependencies, like bcrypt. The ValueError isn't a new bug in bcrypt; it's a long-standing behavior that proton-core's tests inadvertently bumped into. This serves as a critical reminder for all Python projects: always consider the constraints and best practices of your dependencies, especially in security-sensitive areas like password hashing. Testing with various Python versions and dependency versions is crucial to catch these kinds of test_api.py incompatibilities early. Moreover, it encourages better testing practices where test data is carefully managed to avoid triggering known limitations of libraries. If proton-core relies on bcrypt (or any other hashing library with similar constraints), its internal tests should either use compliant passwords or explicitly handle the truncation, making the code more resilient. Ultimately, resolving this Nixpkgs Python 3.13 proton-core build fix is not just about one package; it's about reinforcing the reliability of our entire Nixpkgs system and ensuring a smooth journey for Python 3.13 adoption across the board. It's a testament to the open-source spirit that such issues are openly reported, discussed, and collectively solved, making the software world a better place for everyone.

Join the Hunt: How You Can Help Resolve This proton-core Bug

Alright, fellow Nixers and Python enthusiasts, here’s where you can really make a difference! This python313Packages.proton-core build failure is an open challenge, and the NixOS community thrives on collaborative problem-solving. If you've been following along, you now understand the ins and outs of the password length error stemming from test_api.py and the bcrypt limitation. Your contribution, no matter how small, can help us get this Python 3.13 proton-core package building smoothly again.

First, if you have experience with Python development and pytest, consider diving into the proton-core source code. Can you craft a minimal patch that truncates the test passwords in test_api.py before they hit the bcrypt.hashpw function? This could be a critical step towards an immediate Nixpkgs fix. You could then propose this patch as a pull request to the nixos/nixpkgs repository, tagging the original maintainer, @SebTM, and referencing this issue. This is a direct way to contribute to a build fix that benefits everyone on the unstable channel. Even if you're new to Nixpkgs contributions, this issue is a great entry point, as the problem is clearly defined and the solution path is relatively straightforward.

Second, for those who are passionate about upstream engagement, consider reaching out to the proton-core project maintainers directly. Politely explain the password length error issue with bcrypt in their test_api.py and propose an upstream fix. Collaborating with upstream projects strengthens the entire open-source ecosystem, ensuring that such issues are resolved at the source for all users. This could involve opening an issue on their GitHub page or submitting a pull request with a proposed solution. Your proactive engagement can prevent this ValueError from affecting other distributions or future Python versions.

Lastly, even if coding or direct contribution isn't your forte, simply testing proposed solutions is immensely valuable! If someone submits a pull request with a fix, you can help by checking out their branch and trying to build python313Packages.proton-core yourself using their changes. Providing feedback, even just a "works for me!" or "still fails," helps validate the solution and accelerates the merging process. Your positive :+1: reaction on the original issue or any related pull requests also helps signal its importance to maintainers.

Let’s work together to squash this test_api.py bug and ensure proton-core runs perfectly with Python 3.13 on NixOS. Every little bit of effort helps maintain the robustness and reliability that we all love about Nixpkgs. Your insights, code, and testing are invaluable in making Nixpkgs the best package manager out there. Thanks for being awesome, guys!