Add manifest and package caching to dmf-get

[Copilot]

Implements caching to reduce network requests when downloading modules and manifests. Cache is stored in ~/.cache/dmod/dmf-get/ (customizable via DMOD_CACHE_DIR) with separate subdirectories for manifests and packages.

Changes

• Cache management: Added functions for directory creation, cache key generation (djb2 hash + sanitized URL suffix), file I/O, and cleanup
• Download integration: Modified DownloadWithCurl() to cache manifests; added DownloadWithCurlNoCache() helper; updated DownloadFile() to cache packages
• CLI: Added -c/--clear-cache flag
• Security hardening:
  • Replaced system() calls with safe C functions (mkdir(), unlink(), rmdir())
  • Added path traversal validation in RemoveDirectoryContents()
  • Increased buffer sizes (2048 bytes) with truncation checks
  • Used uint64_t with PRIx64 for portable hash function
  • Replaced fseek()/ftell() with fstat() for reliable file sizes

Usage

# Normal usage - automatic caching
dmf-get mymodule

# Clear cache
dmf-get --clear-cache

# Custom cache location
export DMOD_CACHE_DIR=/custom/path

On first download, manifests and packages are cached. Subsequent requests use cached files without network I/O. All existing tests pass.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)
• example.com
  • Triggering command: ../../build/bin/tools/dmf-get ../../build/bin/tools/dmf-get -m manifest.dmm -o output testmod -j ACCEPT (dns block)
  • Triggering command: ../../build/bin/tools/dmf-get ../../build/bin/tools/dmf-get -m manifest_vars.dmm -t arch/x86_64 -o output module1 o (dns block)
  • Triggering command: ../../build/bin/tools/dmf-get ../../build/bin/tools/dmf-get -d deps.dmd -m manifest.dmm -o output fig o etest-populate-s-I (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Dodać cache'owanie manifestów do dmf-get</issue_title>
<issue_description>dmf-get, aby ograniczyć liczbę zapytań o manifest, powinien sobie cache'ować manifesty i zipy w jakiejś znanej ścieżce i czyścić wszystko, jeśli użytkownik przekaże parametr --clear-cache lub -c. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Dodać cache'owanie manifestów do dmf-get #175

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