How Composer Prevents Duplicate Writes During Retries
Composer 2.x prevents duplicate writes and file corruption during retry cycles by utilizing a temporary-write-and-move pattern. The retry mechanism does not append to partial files; instead, it ensures that only fully validated archives are moved into the final destination.
Internal File Handling and Atomicity
To maintain idempotency, Composer follows these internal behaviors during the download phase:
- Temporary Staging: Packages are downloaded into a temporary file (typically within the system's temporary directory or the Composer cache). The retry logic is designed to overwrite these temporary files if a download is restarted, ensuring no data is appended to a corrupted partial write.
- Atomic Renaming: Composer does not extract files directly into the
vendor directory during the download stream. Only after a download is successfully completed is the file moved/renamed to its final location. This atomic operation ensures that the vendor directory never contains a "half-finished" package.
- Checksum Validation: Before extraction, Composer verifies the downloaded archive against the hash stored in the
composer.lock file. If a retry resulted in a corrupted file that somehow bypassed the transport layer's error detection, the checksum validation would fail, triggering another retry or a hard error.
Cleanup of Orphaned Writes
Orphaned partial writes are handled based on the nature of the failure:
- Soft Failures (Network Timeouts): The retry loop manages the temporary file handle, overwriting the partial data for the next attempt.
- Hard Failures (Process Crash): If the Composer process is killed, partial files may remain in the system temporary directory. These are treated as orphaned files and are subject to the operating system's standard temporary file cleanup policies rather than a specific Composer cleanup routine.
Verification Steps
To verify this behavior in your specific environment, you can use the following scoped commands:
# Run installation with maximum verbosity to trace file operations
composer install -vvv
While the process is running (or simulated via network throttling), inspect your system's temporary directory to observe the creation and overwriting of .tmp files. You can also manually verify the integrity of a cached package against the lock file:
# Example: Verify a cached archive hash (Linux/macOS)
sha384sum /path/to/composer/cache/archive.zip
Diagnostic Requirement
To provide a more specific recommendation regarding orphaned files, please specify the Operating System and Filesystem (e.g., Ubuntu/ext4, Windows/NTFS) being used, as temporary file locking and cleanup behaviors vary by OS.
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