Buildozer Parsing Issue Expressions In Set Command Discussion

Hey guys, let's talk about a quirky issue some of us have bumped into while using Buildozer. It revolves around how Buildozer handles expressions within the set command, especially when you're trying to batch multiple commands in a single invocation. This can lead to unexpected behavior, like Buildozer mistakenly removing load statements that are actually in use. So, let's break down the problem, understand why it happens, and explore a workaround to keep your Bazel builds smooth and error-free.

Understanding the Issue

So, here's the deal. Imagine you're setting up a Bazel build file and want to use Buildozer to automate some changes. You might have a scenario where you're adding a new rule, loading a symbol, and then setting an attribute of that rule to an expression that uses the loaded symbol. Now, if you try to do all of this in one go with Buildozer, you might run into a snag.

The problem arises because Buildozer's parser sometimes misinterprets expressions within the set command. Instead of correctly parsing the expression, it treats the whole thing as a single identifier. This misinterpretation throws a wrench in the works, especially when you're using commands like fix unusedLoads. This command, which is meant to clean up your build files by removing unnecessary load statements, can end up removing load statements that are actually being used. This can lead to build errors and a whole lot of head-scratching.

To illustrate, consider a scenario where you have a BUILD file and you want to add a new rule named some_name, load a function foo from a file named somewhere, and then set an attribute some_attr of the rule to an expression like foo("some_arg"). If you run a single Buildozer command with all these operations, Buildozer might parse foo("some_arg") as a single identifier instead of recognizing foo as a function call. Consequently, when fix unusedLoads is executed, it might incorrectly identify the load statement for foo as unused and remove it. This can break your build because the rule now depends on a function that's no longer loaded.

This issue stems from how Buildozer parses the input file and applies the changes. When you batch commands, Buildozer doesn't re-parse the file after each modification. This means that the initial parsing determines how all subsequent commands are interpreted. If the initial parsing is incorrect, the subsequent commands will operate on a flawed representation of the build file, leading to unexpected outcomes.

A Concrete Example

Let's walk through a concrete example to really nail this down. Suppose you have a directory x with an empty BUILD file. You want to add a rule, load a function, and set an attribute using that function. You might create a Buildozer command file like this:

new some_rule some_name|x/BUILD
new_load somewhere foo|x/BUILD
set some_attr foo("some_arg")|x/BUILD:some_name
fix unusedLoads|x/BUILD

If you run this using buildozer -f <command_file>, you might expect Buildozer to add the rule, load the function foo, set the some_attr attribute to the result of calling foo with the argument "some_arg", and then clean up any unused load statements. However, what actually happens is that Buildozer misparses the expression foo("some_arg") in the set command. It treats the entire string as a single identifier, not as a function call.

As a result, when fix unusedLoads runs, it doesn't recognize that the function foo is being used. It sees an identifier named foo("some_arg"), but it doesn't connect that to the load statement for foo. Therefore, it incorrectly removes the load statement. The final BUILD file might look something like this:

some_rule(
 name = "some_name",
 some_attr = foo("some_arg"),
)

Notice that the load statement for foo is missing, even though some_attr clearly depends on it. This is a direct consequence of Buildozer's parsing issue.

Diving Deeper into the Code

For the technically inclined, let's peek under the hood a bit. The root cause of this issue lies in the way Buildozer's parser handles the right-hand side of the set command. Specifically, there's a section of code in buildozer.go that's responsible for parsing the value being set. Instead of fully parsing the expression, it treats it as an identifier. You can find this in the Buildozer source code on GitHub, specifically in the buildozer.go file around line 604-605.

This misparsing then has a cascading effect. The UsedSymbols() function, which is used to determine which symbols are actually used in the build file, relies on the parsed representation of the file. Because the expression foo("some_arg") was parsed as a single identifier, UsedSymbols() marks this entire string as used, but it doesn't recognize that the function foo is being called. Consequently, the load statement for foo is not marked as used, and fix unusedLoads removes it.

This behavior highlights a crucial aspect of Buildozer's operation: it's highly dependent on the accuracy of the initial parsing. If the parsing is off, subsequent operations can go awry, leading to unexpected and potentially harmful changes to your build files.

The Workaround: One Command at a Time

Okay, so we've identified the problem. Now, what's the fix? Fortunately, there's a straightforward workaround: run each Buildozer command in a separate invocation. In other words, instead of batching all your commands into a single file and running Buildozer once, run Buildozer multiple times, each time with a single command.

Why This Works

This workaround sidesteps the parsing issue because Buildozer re-parses the BUILD file after each command is executed. So, after you add the new rule and load the symbol, Buildozer parses the updated file. When you then set the attribute to foo("some_arg"), Buildozer correctly parses this as an expression involving the function foo. This correct parsing ensures that UsedSymbols() accurately identifies foo as being used, and fix unusedLoads won't remove the load statement.

Back to Our Example

Let's revisit our earlier example to see this in action. Instead of running Buildozer with the entire command file, you would run the following commands:

buildozer 'new some_rule some_name|x/BUILD'
buildozer 'new_load somewhere foo|x/BUILD'
buildozer 'set some_attr foo("some_arg")|x/BUILD:some_name'
buildozer 'fix unusedLoads|x/BUILD'

Each command is executed separately, and Buildozer re-parses the BUILD file after each one. This ensures that the expression foo("some_arg") is correctly parsed, and the load statement for foo is preserved.

The Trade-off

While this workaround effectively addresses the parsing issue, it does come with a trade-off. Running Buildozer multiple times can be slower than running it once with a batch of commands. The overhead of parsing the file multiple times adds up. However, the increased reliability and correctness of the changes often outweigh the performance cost. It's better to have a slightly slower process that produces the correct result than a faster process that can potentially break your build.

Long-Term Solutions and Best Practices

While the workaround of running Buildozer commands one at a time is effective, it's not the ideal long-term solution. Ideally, Buildozer should be able to correctly parse expressions in the set command, even when commands are batched. This would eliminate the need for the workaround and make Buildozer more efficient and user-friendly.

Contributing to Buildozer

If you're feeling adventurous and have some Go programming skills, you might consider contributing to Buildozer itself. The issue we've discussed is a known one, and contributions to fix it would be greatly appreciated by the Bazel community. You can find the Buildozer repository on GitHub and submit a pull request with a fix. This would not only help you learn more about Buildozer's internals but also contribute to making the tool better for everyone.

Alternative Tools and Approaches

In the meantime, while we wait for a permanent fix, there are other approaches you can take to manage your Bazel build files. One option is to use a more general-purpose scripting language, like Python, to generate your BUILD files. This gives you more control over the parsing and manipulation of the files, but it also requires more effort and expertise.

Another approach is to use Bazel's built-in macros and functions to encapsulate complex logic. This can reduce the need for extensive modifications to your BUILD files and make them easier to manage.

Best Practices for Using Buildozer

Regardless of the tools you use, there are some best practices you can follow to minimize the risk of encountering issues like the one we've discussed. One important practice is to keep your BUILD files as simple and declarative as possible. Avoid complex expressions and logic within the files themselves. Instead, encapsulate that logic in macros or functions.

Another good practice is to review the changes made by Buildozer before committing them. This allows you to catch any errors or unexpected modifications before they make their way into your codebase. You can use git diff or a similar tool to examine the changes.

Finally, it's always a good idea to test your builds after making changes with Buildozer. This ensures that the changes haven't introduced any regressions or broken dependencies.

Conclusion

So, there you have it, guys. We've explored a tricky parsing issue in Buildozer's set command, understood why it happens, and learned a practical workaround. While the workaround of running commands one at a time might not be the most efficient solution, it's a reliable way to avoid the issue and keep your Bazel builds healthy. In the long run, a proper fix in Buildozer itself would be ideal, and there are ways you can contribute to making that happen. Remember to follow best practices when using Buildozer and always review your changes to ensure everything is working as expected. Happy building!