Parse Rust toolchain file with a real script

.github/workflows/nightly.yaml

@@ -190,19 +190,12 @@ jobs:
           $VENV_PYTHON -m pip install --require-hashes -r requirements.txt
 
       - name: Set Artichoke Rust toolchain version
-        shell: python
+        shell: bash
         id: rust_toolchain
-        working-directory: artichoke
         run: |
-          import os
-          import tomllib
-
-          with open("rust-toolchain.toml", "rb") as f:
-              data = tomllib.load(f)
-          toolchain = data["toolchain"]["channel"]
-          print(f"Rust toolchain version: {toolchain}")
-          with open(os.environ["GITHUB_OUTPUT"], "a") as f:
-              print(f"version={toolchain}", file=f)
+          $VENV_PYTHON -m artichoke_nightly.rust_toolchain_version \
+            --file artichoke/rust-toolchain.toml \
+            --format github
 
       - name: Install Rust toolchain
         uses: artichoke/setup-rust/[email protected]

artichoke_nightly/github_actions.py

@@ -8,8 +8,11 @@ def set_output(*, name: str, value: str) -> None:
     """
     Set an output for a GitHub Actions job.
 
-    https://docs.github.com/en/actions/using-jobs/defining-outputs-for-jobs
-    https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
+    See the GitHub Actions documentation for [defining output for jobs] and
+    changes to [deprecate the set-output command].
+
+    [defining output for jobs]: https://docs.github.com/en/actions/using-jobs/defining-outputs-for-jobs
+    [deprecate the set-output command]: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
     """
 
     if github_output := os.getenv("GITHUB_OUTPUT"):
@@ -22,8 +25,18 @@ def log_group(group: str) -> Iterator[None]:
     """
     Create an expandable log group in GitHub Actions job logs.
 
-    https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#grouping-log-lines
+    Only prints log group markers when running in GitHub Actions CI. See the GitHub
+    Actions documentation for [grouping log lines].
+
+    Args:
+        group (str): The name of the log group.
+
+    [grouping log lines]: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#grouping-log-lines
     """
+    if os.getenv("CI") != "true" or os.getenv("GITHUB_ACTIONS") != "true":
+        # Do nothing if not running in GitHub Actions
+        yield
+        return
 
     print(f"::group::{group}")
     try:
@@ -33,7 +46,7 @@ def log_group(group: str) -> Iterator[None]:
 
 
 def emit_metadata() -> None:
-    if os.getenv("CI") != "true":
+    if os.getenv("CI") != "true" or os.getenv("GITHUB_ACTIONS") != "true":
         return
     with log_group("Workflow metadata"):
         if repository := os.getenv("GITHUB_REPOSITORY"):
@@ -55,6 +68,24 @@ def emit_metadata() -> None:
 
 
 def runner_tempdir() -> Path | None:
+    """
+    Get the temporary directory used by the GitHub Actions runner.
+
+    This function retrieves the path to the temporary directory used by the GitHub
+    Actions runner during job execution. The directory path is taken from the
+    `RUNNER_TEMP` environment variable, which is set by GitHub Actions. This
+    directory is used for storing temporary files generated during the job run.
+
+    Returns:
+        Optional[Path]: A Path object pointing to the runner's temporary directory if
+            the `RUNNER_TEMP` environment variable is set; otherwise, returns None.
+
+    Example:
+        >>> temp_dir = runner_tempdir()
+        >>> if temp_dir:
+        ...     print(f"Temporary directory: {temp_dir}")
+    """
+
     if temp := os.getenv("RUNNER_TEMP"):
         return Path(temp)
     return None

artichoke_nightly/rust_toolchain_version.py

@@ -0,0 +1,128 @@
+#!/usr/bin/env python3
+
+import argparse
+import sys
+import tomllib
+from dataclasses import dataclass, field
+from enum import StrEnum
+from pathlib import Path
+from typing import assert_never
+
+from .github_actions import emit_metadata, log_group, set_output
+
+
+class OutputFormat(StrEnum):
+    """Enum for output format options."""
+
+    PLAIN = "plain"
+    GITHUB = "github"
+
+
+@dataclass(frozen=True, kw_only=True)
+class Args:
+    """Dataclass to store command line arguments."""
+
+    file: Path = field(metadata={"help": "Path to the rust-toolchain.toml file."})
+    format: OutputFormat = field(
+        metadata={"help": "Output format: either 'plain' or 'github'."}
+    )
+
+
+def parse_args() -> Args:
+    """Parse command line arguments into an Args dataclass."""
+    parser = argparse.ArgumentParser(description="Set Rust toolchain version.")
+    parser.add_argument(
+        "-f",
+        "--file",
+        type=Path,
+        required=True,
+        help="Path to the rust-toolchain.toml file.",
+    )
+    parser.add_argument(
+        "--format",
+        type=OutputFormat,
+        choices=list(OutputFormat),
+        default=OutputFormat.PLAIN,
+        help="Output format: either 'plain' or 'github'.",
+    )
+    args = parser.parse_args()
+    return Args(file=args.file, format=args.format)
+
+
+def read_toolchain_version(file_path: Path) -> str:
+    """
+    Read the Rust toolchain version from the rust-toolchain.toml file.
+
+    Args:
+        file_path (Path): Path to the rust-toolchain.toml file.
+
+    Returns:
+        str: The Rust toolchain version specified in the TOML file.
+
+    Raises:
+        FileNotFoundError: If the file does not exist or cannot be accessed.
+        ValueError: If the TOML file is malformed.
+        TypeError: If the TOML file does not contain the expected structure.
+    """
+    try:
+        with file_path.open("rb") as f:
+            data = tomllib.load(f)
+    except tomllib.TOMLDecodeError as e:
+        raise ValueError(f"Failed to parse rust-toolchain.toml file: {e}") from e
+
+    # Validate the structure and type of the expected keys
+    toolchain = data.get("toolchain")
+    if toolchain is None:
+        raise TypeError("Malformed rust-toolchain.toml: 'toolchain' stanza is missing.")
+    if not isinstance(toolchain, dict):
+        raise TypeError(
+            "Malformed rust-toolchain.toml: 'toolchain' should be a dictionary."
+        )
+
+    channel = toolchain.get("channel")
+    if channel is None:
+        raise TypeError("Malformed rust-toolchain.toml: 'channel' field is missing.")
+    if not isinstance(channel, str):
+        raise TypeError("Malformed rust-toolchain.toml: 'channel' should be a string.")
+    if not channel:
+        raise ValueError("Malformed rust-toolchain.toml: 'channel' is empty.")
+
+    return channel
+
+
+def format_output(toolchain_version: str, output_format: OutputFormat) -> None:
+    """
+    Format the output based on the selected format.
+
+    Args:
+        toolchain_version (str): The Rust toolchain version.
+        output_format (OutputFormat): The desired output format, either 'plain' or
+            'github'.
+    """
+    match output_format:
+        case OutputFormat.PLAIN:
+            print(toolchain_version)
+        case OutputFormat.GITHUB:
+            set_output(name="version", value=toolchain_version)
+        case _:
+            assert_never(output_format)
+
+
+def main() -> int:
+    """Main function to set Rust toolchain version."""
+    args = parse_args()
+    emit_metadata()
+
+    with log_group("Setting Rust toolchain version"):
+        try:
+            toolchain_version = read_toolchain_version(args.file)
+            format_output(toolchain_version, args.format)
+        except (FileNotFoundError, OSError, ValueError, TypeError) as e:
+            print(e, file=sys.stderr)
+            return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())