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76 lines
4.5 KiB
Markdown
76 lines
4.5 KiB
Markdown
# Developing `hogql-parser`
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## Mandatory reading
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If you're new to Python C/C++ extensions, there are some things you must have in mind. The [Python/C API Reference Manual](https://docs.python.org/3/c-api/index.html) is worth a read as a whole.
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The three pages below are must-reads though. They're key to writing production-ready code:
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### [Objects, Types and Reference Counts](https://docs.python.org/3/c-api/intro.html#objects-types-and-reference-counts)
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Key takeaways:
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1. `Py_INCREF()` and `Py_DECREF()` need to be used accurately, or there'll be memory leaks (or, less likely, segfaults). This also applies to early exits, such as these caused by an error.
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1. `Py_None`, `Py_True`, and `Py_False` are singletons, but they still need to be incref'd/decref'd - the best way to do create a new reference to them is wrapping them in `Py_NewRef()`.
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1. Pretty much only `PyList_SET_ITEM()` _steals_ references (i.e. assumes ownership of objects passed into it) - if you pass an object into any other function and no longer need it after that, remember to `Py_DECREF` it!
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### [Exception Handling](https://docs.python.org/3/c-api/exceptions.html)
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Key takeaways:
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1. If a Python exception has been raised, the module method that was called from Python must stop execution and return `NULL` immediately.
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> In `HogQLParseTreeConverter`, we are able to use C++ exceptions: throwing `SyntaxException`,
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> `NotImplementedException`, or `ParsingException` results in the same exception being raised in Python as
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> expected. Note that if a `visitAsFoo` call throws an exception and there are `PyObject*`s in scope, we have to
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> remember about cleaning up their refcounts. At such call sites, a `try {} catch (...) {}` block is appropriate.
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1. For all Python/C API calls returning `PyObject*`, make sure `NULL` wasn't returned - if it was, then something failed and the Python runtime has already set an exception (e.g. a `MemoryError`). The same applies to calls returning `int` - there the error value is `-1`. Exception: in `PyArg_Foo` functions failure is signaled by `0` and success by `1`.
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> In `HogQLParseTreeConverter`, these internal Python failures are handled simply by throwing
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> `PyInternalException`.
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### [Building Values](https://docs.python.org/3/c-api/arg.html#building-values)
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Key takeaways:
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1. Use `Py_BuildValue()` for building tuples, dicts, and lists of static size. Use type-specific functions (e.g. `PyUnicode_FromString()` or `PyList_New()`) otherwise.
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1. `str`-building with `s` involves `strlen`, while `s#` doesn't - it's better to use the latter with C++ strings.
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1. `object`-passing with `O` increments the object's refcount, while doing it with `N` doesn't - we should use `N` pretty much exclusively, because the parse tree converter is about creating new objects (not borrowing).
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## Conventions
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1. Use `snake_case`. ANTLR is `camelCase`-heavy because of its Java heritage, but both the C++ stdlib and CPython are snaky.
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2. Use the `auto` type for ANTLR and ANTLR-derived types, since they can be pretty verbose. Otherwise, specify the type explicitly.
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3. Stay out of Python land as long as possible. E.g. avoid using `PyObject*`s` for bools or strings.
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Do use Python for parsing numbers though - that way we don't need to consider integer overflow.
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4. If any child rule results in an AST node, so must the parent rule - once in Python land, always in Python land.
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E.g. it doesn't make sense to create a `vector<PyObject*>`, that should just be a `PyObject*` of Python type `list`.
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## How to develop locally on macOS
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1. Install libraries:
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```bash
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brew install boost antlr4-cpp-runtime
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```
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1. Install `hogql_parser` by building from local sources:
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```bash
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pip install ./hogql_parser
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```
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> If you're getting compilation errors like this on macOS Sonoma:
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> `/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/cstring:66:5: error: <cstring> tried including <string.h> but didn't find libc++'s <string.h> header.`
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> Then you may need to remove Xcode Command Line Tools:
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> `sudo rm -rf /Library/Developer/CommandLineTools`
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1. If you now run tests, the locally-built version of `hogql_parser` will be used:
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```bash
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pytest posthog/hogql/
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```
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## How to install dependencies on Ubuntu
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Antlr runtime provided in Ubuntu packages might be of an older version, which results in compilation errors.
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In that case run commands from [this step](https://github.com/PostHog/posthog/blob/4fba6a63e351131fdb27b85e7ba436446fdb3093/.github/actions/run-backend-tests/action.yml#L100). |