Swift: skip QL code generation on untouched files #11331
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This is a developer QoL improvement, where running codegen will skip writing (and especially formatting) any files that were not changed. **Why?** While code generation in itself was pretty much instant, QL formatting of generated code was starting to take a long time. This made unconditionally running codegen quite annoying, for example before each test run as part of an IDE workflow or as part of the pre-commit hook. **How?** This was not completely straightforward as we could not work with the contents of the file prior to code generation as that was already post-processed by the QL formatting, so we had no chance of comparing the output of template rendering with that. We therefore store the hashes of the files _prior_ to QL formatting in a checked-in file (`swift/ql/.generated.list`). We can therefore load those hashes at the beginning of code generation, use them to compare the template rendering output and update them in this special registry file. **What else?** We also extend this mechanism to detect accidental modification of generated files in a more robust way. Before this patch, we were doing it with a rough regexp based heuristic. Now, we just store the hashes of the files _after_ QL formatting in the same checked file, so we can check that and stop generation if a generated file was modified, or a stub was modified without removing the `// generated` header.
AlexDenisov
approved these changes
Nov 21, 2022
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This is a developer QoL improvement, where running codegen will skip writing (and especially formatting) any files that were not changed.
Why? While code generation in itself was pretty much instant, QL formatting of generated code was starting to take a long time. This made unconditionally running codegen quite annoying, for example before each test run as part of an IDE workflow or as part of the pre-commit hook.
How? This was not completely straightforward as we could not work with the contents of the file prior to code generation as that was already post-processed by the QL formatting, so we had no chance of comparing the output of template rendering with that. We therefore store the hashes of the files prior to QL formatting in a checked-in file (
swift/ql/.generated.list). We can therefore load those hashes at the beginning of code generation, use them to compare the template rendering output and update them in this special registry file.What else? We also extend this mechanism to detect accidental modification of generated files in a more robust way. Before this patch, we were doing it with a rough regexp based heuristic. Now, we just store the hashes of the files after QL formatting in the same checked file, so we can check that and stop generation if a generated file was modified, or a stub was modified without removing the
// generatedheader.