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Merge pull request #22600 from github/redsun82-bazel-skill
Add a Bazel skill for this repository
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‎.github/skills/bazel/SKILL.md‎

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---
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name: bazel
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description: Conventions for editing Bazel files in the github/codeql repository: the shared `misc/bazel` helpers, `codeql_platform_select` and the `{CODEQL_PLATFORM}` packaging placeholder, adding `MODULE.bazel` dependencies, and the `semmle_code` stub that keeps the standalone build working. Use when editing any `BUILD.bazel`, `*.bzl`, `MODULE.bazel` or `.bazelrc` here, and before validating such an edit.
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---
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# Bazel in the codeql repository
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A bzlmod module named `ql`, repo name `@codeql` ([`MODULE.bazel`](../../../MODULE.bazel)). It builds standalone, and is
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also consumed by an internal module; standalone builds replace that module with a stub.
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## Traps
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Things that will waste your time or produce a wrong edit here. Read this section even if you skip the rest.
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* **`//...` does not work.** `bazel build //...`, and even `bazel query //...`, fail at the repo root: the patched
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modules under [`misc/bazel/registry`](../../../misc/bazel/registry) are real packages referencing repos that are not
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visible from the main repo, and [`.bazelrc`](../../../.bazelrc) notes separately that transitions break `...` builds.
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The error names a registry directory unrelated to your edit, so it is easy to misdiagnose. **Validate the specific
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target or package you changed**, not a recursive pattern.
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* **There is no `MODULE.bazel.lock`, deliberately.** [`.bazelrc`](../../../.bazelrc) sets `--lockfile_mode=off` because
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the workspace-relative module override makes a lockfile unstable. Do not add one, and do not "fix" its absence.
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* **`linux_arm64` vs `linux-arm64`.** The keyword argument and config setting use an underscore; the platform *string*
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substituted into paths and zip names uses a hyphen. They are not interchangeable.
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* **Do not stub your way out of a missing internal dependency.** See [Building standalone](#building-standalone) for the
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one narrow case where extending the stub is correct.
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## Conventions
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* **Copy a neighboring target rather than inventing a shape.** Packaging is not uniform: some packages use the
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`codeql_*` wrappers, others still use `pkg_files` directly. Copy the closest *working* neighbor, and prefer the
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wrapper for new code.
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* **Pin anything fetched over the network** with `sha256` or `integrity`. Bazel only *warns* on an unpinned download, so
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nothing fails loudly, but the build stops being reproducible and a retagged upstream release silently changes what you
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build. `lfs_archive` is the exception: content is pinned by git object.
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* **Format with `bazel run //misc/bazel/buildifier`.** It rewrites in place, so do not hand-tune formatting. Also wired
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as a `pre-commit` hook ([`.pre-commit-config.yaml`](../../../.pre-commit-config.yaml)).
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## Where new code goes
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Bazel's own macro / rule / repository-rule distinction applies as usual. What is repo-specific:
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| Adding | Goes in |
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| --- | --- |
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| a new packaging shape | extend [`misc/bazel/pkg.bzl`](../../../misc/bazel/pkg.bzl), do not fork `pkg_files` |
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| a new OS or arch split | [`misc/bazel/os.bzl`](../../../misc/bazel/os.bzl), do not hand-roll a `select()` over `@platforms//` |
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| a fetch of something external | a repository rule ([`lfs.bzl`](../../../misc/bazel/lfs.bzl), [`ripunzip.bzl`](../../../misc/ripunzip/ripunzip.bzl)), not a `genrule` |
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| a wrapper used by one language | next to that language ([`swift/rules.bzl`](../../../swift/rules.bzl)) |
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| a wrapper used across languages | `misc/bazel/` |
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Prefer inline rules in `BUILD.bazel`. A `.bzl` file earns its `load()` only when the shape repeats across packages or a
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value must be computed: [`rust.bzl`](../../../misc/bazel/rust.bzl) is worth it because every Rust binary that ships in a
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pack must get the same universal-binary wrapper and symbols test, and forgetting either is a release bug. A local
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debugging aid opts out and declares a plain `rust_binary`; see `swift-syntax-parse` in
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[`unified/swift-syntax-rs/BUILD.bazel`](../../../unified/swift-syntax-rs/BUILD.bazel). The `_gen_binaries` list in
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[`go/BUILD.bazel`](../../../go/BUILD.bazel) does not earn a `.bzl`, because it is shared within a single file, where a
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local variable does the job.
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Macros here are typically a thin public wrapper around a private rule (`codeql_csharp_binary`, `swift_cc_binary`). Keep
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the rule narrow and the ergonomics in the macro. Each macro decorates the caller's `name` to mint its helper targets,
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for example `internal/<name>` or `single_arch/<name>`. Their visibility is that macro's choice (private, package
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default, or the caller's own), so read the macro instead of assuming. When an error names a target you cannot find in
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any source file, a macro minted it: grep the suffix under `misc/bazel/`.
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## Shared helpers
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Frequently-used pieces, so you load the existing one instead of rewriting it. Read the file for its actual exports.
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| `load()` path | Covers |
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| --- | --- |
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| `//misc/bazel:pkg.bzl` | CodeQL packs and packaging |
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| `//misc/bazel:os.bzl` | platform and architecture selection |
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| `//misc/bazel:lfs.bzl` | on-demand git-LFS repositories |
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| `//misc/bazel:rust.bzl` | Rust binary wrapper |
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| `//misc/bazel:csharp.bzl` | C# binary/library/test wrappers |
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| `//misc/bazel:utils.bzl` | `select_os`; prefer `os.bzl`'s `os_select` in new code |
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[`defs.bzl`](../../../defs.bzl) at the root exports `codeql_platform` for *dependent* modules. It is not the way to get
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the platform string here; use `os.bzl`.
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## Platform selection
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[`codeql_platform_select`](../../../misc/bazel/os.bzl) takes one keyword argument per CodeQL platform: `linux64`,
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`linux_arm64`, `osx64` and `win64`. `otherwise` supplies the value for whichever of those you leave unset; it is **not**
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a `//conditions:default`. **There is deliberately no fallback from `linux_arm64` to `linux64`.** If you only care about
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the OS, use `os_select`, which gives Linux the same value on both architectures and has a `posix` shorthand for the
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shared Linux/macOS value.
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In a macro (no `ctx`) it returns a `select()`:
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```python
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load("//misc/bazel:os.bzl", "codeql_platform_select")
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load("//misc/bazel:pkg.bzl", "codeql_pkg_files")
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codeql_pkg_files(
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name = "extractor-arch",
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exes = codeql_platform_select(
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otherwise = ["//unified/extractor"],
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win64 = ["//unified/extractor-unsupported-os:extractor"],
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),
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prefix = "tools/{CODEQL_PLATFORM}",
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)
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```
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If implementation code needs to *branch* on the value rather than pass it through, pass `ctx` and add
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`OS_DETECTION_ATTRS` to the rule's attributes. The value is then resolved eagerly instead of being an opaque `select()`:
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```python
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load("//misc/bazel:os.bzl", "OS_DETECTION_ATTRS", "os_select")
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def _impl(ctx):
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ext = os_select(ctx, windows = ".exe", posix = "")
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...
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my_rule = rule(
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implementation = _impl,
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attrs = {"src": attr.label()} | OS_DETECTION_ATTRS,
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)
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```
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## Packs
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`codeql_pack` assembles the files that become an extractor pack. See [`unified/BUILD.bazel`](../../../unified/BUILD.bazel)
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for a minimal complete example and [`pkg.bzl`](../../../misc/bazel/pkg.bzl) for the arguments. The non-obvious parts:
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* **`{CODEQL_PLATFORM}` in a destination path is the routing mechanism**, not just a substitution. A path containing it
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is *arch-specific* and lands in the per-architecture zip; every other path is *common*. So `prefix =
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"tools/{CODEQL_PLATFORM}"` both places the file and marks it arch-specific. `arch_overrides` forces named
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destinations into the arch-specific part without a placeholder.
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* **`codeql_pkg_files` splits `srcs` (plain) from `exes` (mode 755)** and **rejects `attributes =`** with an explicit
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error. Use `exes` rather than hand-rolling `pkg_attributes(mode = "755")`.
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* **`pkg_dirs` and `pkg_symlinks` are unsupported** and fail at analysis time.
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* `codeql_pack` also generates an installer and an `install` alias, hence `bazel run //unified:install`. Pass
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`installer_alias = None` if one package defines several packs.
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* `codeql_pack_group` exists for bundling packs into distribution zips, but nothing in this repo instantiates it.
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## Adding a dependency
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In order of preference:
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1. **A [Bazel Central Registry](https://registry.bazel.build/) module.** Add a `bazel_dep` in
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[`MODULE.bazel`](../../../MODULE.bazel).
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2. **A patched upstream module.** Add it under [`misc/bazel/registry`](../../../misc/bazel/registry), which `.bazelrc`
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puts ahead of the BCR. Put patches in `modules/<repo>/<version>/patches`, rename the version with a `-codeql.N`
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suffix, and run [`fix.py`](../../../misc/bazel/registry/fix.py) to realign the metadata.
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3. **A raw archive.** Use `http_archive` via `use_repo_rule`, or a repository rule. Copy an adjacent declaration and
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keep its checksum field populated.
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Vendored Rust crates under [`misc/bazel/3rdparty`](../../../misc/bazel/3rdparty) are generated. Regenerate with
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[`update_cargo_deps.sh`](../../../misc/bazel/3rdparty/update_cargo_deps.sh) rather than editing, and keep the
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`use_repo` lists in sync, which `bazel mod tidy` does for module extensions.
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## Building standalone
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[`MODULE.bazel`](../../../MODULE.bazel) declares `semmle_code` with a `local_path_override` pointing at `..`, which
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resolves when this repo is checked out inside the internal module. [`.bazelrc`](../../../.bazelrc), which Bazel reads
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when invoked in *this* workspace, overrides that with a stub. This line is the whole reason a standalone build resolves:
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```
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common --override_module=semmle_code=%workspace%/misc/bazel/semmle_code_stub
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```
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Do not change either the override path or the `local_path_override`; they work as a pair.
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[`misc/bazel/semmle_code_stub`](../../../misc/bazel/semmle_code_stub) is an otherwise empty module supplying no-op
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versions of the internal helpers that shared `.bzl` files load *unconditionally*. That is its only job, and it is small
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enough to read.
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**Extend the stub only when a `.bzl` file every standalone target loads gains a new internal `load()`**, which breaks
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package loading outright, for everyone. Prefer not needing one. **Never add a stub so that an internal-only target
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appears to build**: some targets depend on internal libraries (`grep -rl @semmle_code --include=*.bazel` finds them) and
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are correctly unbuildable here. Unlike a `load()`, such a dependency only fails when that target is actually requested.
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[`.bazelrc.internal`](../../../.bazelrc.internal) is **not** read here; it carries settings for the internal build. A
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setting needed by both has to be written in both files, with paths differing because this repo sits at a different depth
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there.

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