Add support for julia hooks

This patch adds 2nd class support for hooks using julia as the language.
pre-commit will install any dependencies defined in the hooks repo
`Project.toml` file, with support for `additional_dependencies` as well.
Julia doesn't (yet) have a way to install binaries/scripts so for julia
hooks the `entry` value is a (relative) path to a julia script within
the hooks repository. When executing a julia hook the (globally
installed) julia interpreter is prepended to the entry.

Example `.pre-commit-hooks.yaml`:

```yaml
- id: foo
  name: ...
  language: julia
  entry: bin/foo.jl --arg1
```

Example hooks repo: https://github.com/fredrikekre/runic-pre-commit/tree/fe/julia
Accompanying pre-commit.com PR: pre-commit/pre-commit.com#998

Fixes pre-commit#2689.

.pre-commit-config.yaml

@@ -42,3 +42,7 @@ repos:
     -   id: mypy
         additional_dependencies: [types-pyyaml]
         exclude: ^testing/resources/
+-   repo: https://github.com/fredrikekre/runic-pre-commit
+    rev: fe/julia
+    hooks:
+    -   id: runic-julia

pre_commit/all_languages.py

@@ -10,6 +10,7 @@
 from pre_commit.languages import fail
 from pre_commit.languages import golang
 from pre_commit.languages import haskell
+from pre_commit.languages import julia
 from pre_commit.languages import lua
 from pre_commit.languages import node
 from pre_commit.languages import perl
@@ -33,6 +34,7 @@
     'fail': fail,
     'golang': golang,
     'haskell': haskell,
+    'julia': julia,
     'lua': lua,
     'node': node,
     'perl': perl,

pre_commit/languages/julia.py

@@ -0,0 +1,132 @@
+from __future__ import annotations
+
+import contextlib
+from collections.abc import Generator
+from collections.abc import Sequence
+
+from pre_commit import lang_base
+from pre_commit.envcontext import envcontext
+from pre_commit.envcontext import PatchesT
+from pre_commit.prefix import Prefix
+from pre_commit.util import cmd_output_b
+
+ENVIRONMENT_DIR = 'juliaenv'
+health_check = lang_base.basic_health_check
+get_default_version = lang_base.basic_get_default_version
+
+
+def run_hook(
+        prefix: Prefix,
+        entry: str,
+        args: Sequence[str],
+        file_args: Sequence[str],
+        *,
+        is_local: bool,
+        require_serial: bool,
+        color: bool,
+) -> tuple[int, bytes]:
+    # `entry` is a (hook-repo relative) file followed by (optional) args, e.g.
+    # `bin/id.jl` or `bin/hook.jl --arg1 --arg2` so we
+    # 1) shell parse it and join with args with hook_cmd
+    # 2) prepend the hooks prefix path to the first argument (the file)
+    # 3) prepend `julia` as the interpreter
+    cmd = lang_base.hook_cmd(entry, args)
+    cmd = ('julia', prefix.path(cmd[0]), *cmd[1:])
+    return lang_base.run_xargs(
+        cmd,
+        file_args,
+        require_serial=require_serial,
+        color=color,
+    )
+
+
+def get_env_patch(target_dir: str, version: str) -> PatchesT:
+    return (
+        # Single entry pointing to the hook env
+        ('JULIA_LOAD_PATH', target_dir),
+        # May be set, remove it to not interfer with LOAD_PATH
+        ('JULIA_PROJECT', ''),
+    )
+
+
+@contextlib.contextmanager
+def in_env(prefix: Prefix, version: str) -> Generator[None]:
+    envdir = lang_base.environment_dir(prefix, ENVIRONMENT_DIR, version)
+    with envcontext(get_env_patch(envdir, version)):
+        yield
+
+
+def install_environment(
+        prefix: Prefix,
+        version: str,
+        additional_dependencies: Sequence[str],
+) -> None:
+    envdir = lang_base.environment_dir(prefix, ENVIRONMENT_DIR, version)
+    with contextlib.ExitStack() as ctx:
+        ctx.enter_context(in_env(prefix, version))
+
+        # TODO: Support language_version with juliaup similar to rust via
+        # rustup
+        # if version != 'system':
+        #     ...
+
+        joined_deps = ''
+        if len(additional_dependencies) > 0:
+            # Pkg.REPLMode.pkgstr
+            joined_deps = ' '.join(additional_dependencies)
+
+        # Julia code to setup and instantiate the hook environment
+        # TODO: This would be easier to read and work with if it can be put in
+        # a .jl file instead.
+        julia_code = f"""
+        hook_env = "{envdir}"
+        mkdir(hook_env)
+        # Copy Project.toml to hook env
+        project_names = ("JuliaProject.toml", "Project.toml")
+        project_found = false
+        for project_name in project_names
+            isfile(project_name) || continue
+            cp(project_name, joinpath(hook_env, project_name))
+            global project_found = true
+            break
+        end
+        if !project_found
+            error("No (Julia)Project.toml found in hooks repository")
+        end
+
+        # Copy Manifest.toml to hook env (not mandatory)
+        manifest_names = ("JuliaManifest.toml", "Manifest.toml")
+        for manifest_name in manifest_names
+            isfile(manifest_name) || continue
+            cp(manifest_name, joinpath(hook_env, manifest_name))
+            break
+        end
+
+        # We prepend @stdlib here so that we can load the package manager even
+        # though `get_env_patch` limits `JULIA_LOAD_PATH` to just the hook env.
+        pushfirst!(LOAD_PATH, "@stdlib")
+        using Pkg
+        popfirst!(LOAD_PATH)
+
+        # Template in any additional_dependencies
+        joined_deps = "{joined_deps}"
+
+        # Instantiate the environment shipped with the hook repo. If we have
+        # additional dependencies we disable precompilation in this step to
+        # avoid double work.
+        precompile = isempty(joined_deps) ? "1" : "0"
+        withenv("JULIA_PKG_PRECOMPILE_AUTO" => precompile) do
+            Pkg.instantiate()
+        end
+
+        # Add additional_dependencies (with precompilation)
+        if !isempty(joined_deps)
+            withenv("JULIA_PKG_PRECOMPILE_AUTO" => "1") do
+                Pkg.REPLMode.pkgstr("add " * joined_deps)
+            end
+        end
+        """
+        cmd_output_b(
+            'julia', '-e', julia_code,
+            cwd=prefix.prefix_dir,
+        )