Goal

Move ccache out of core compilation into a swappable extension. After this PR a parallel cache backend (sccache, distcc-style, distributed cache) can ship as a sibling extension without touching core.

A concrete near-term motivation: a future extensions/sccache.sh would let CI workflows running on GitHub-hosted runners route their compile-cache straight into the native GitHub Actions cache (SCCACHE_GHA_ENABLED=on, via mozilla-actions/sccache-action) — 10 GB free per repository, low-latency on the runner's local region. The same extension framework also covers self-hosted runners (S3/R2/MinIO bucket, WebDAV endpoint, Redis-on-disk, ...) without core changes — backend choice is purely an env-var concern per workflow.

What changes (vs armbian/build:main)

New files

• extensions/ccache.sh — implements compile_prepare_vars, compile_wrapper_pre/_post, and {kernel,uboot,atf,crust}_make_config hooks.
• lib/functions/compilation/compile-wrapper.sh — do_with_compile_wrapper + compile_wrapper_pre/_post hooks.

Removed

• lib/functions/compilation/ccache.sh (logic moved into the extension).

Modified

• lib/functions/compilation/atf.sh — adds atf_make_config hook; replaces hardcoded ccache with ${CCACHE} (latent bug: USE_CCACHE=no was still dragging ccache into ATF).
• lib/functions/compilation/crust.sh — adds crust_make_config hook; drops dead binutils_flags_crust="" declaration (preexisting SC2034 since 2023).
• lib/functions/compilation/{kernel,uboot}.sh — compile phase wrapped in do_with_compile_wrapper.
• lib/functions/configuration/compilation-config.sh — dispatches compile_prepare_vars instead of inlining configure-ccache; resets stale CCACHE between artifacts; warns if USE_CCACHE is set in userpatches/lib.config (sourced too late for enable_extension).
• lib/functions/configuration/main-config.sh — auto-enables the ccache extension when USE_CCACHE=yes, PRIVATE_CCACHE=yes, or ENABLE_EXTENSIONS=ccache-remote, before initialize_extension_manager.
• lib/functions/general/extensions.sh — adds extension_list_normalized helper (used by the auto-enable shim and per-extension mutex checks).
• lib/library-functions.sh — regenerated.

User-visible behavior

Unchanged. USE_CCACHE=yes (default for kernel/u-boot) and USE_CCACHE=no behave exactly as before; ENABLE_EXTENSIONS=ccache-remote implicitly pulls in ccache, matching the pre-refactor coupling.

Test plan

Built on a fresh DigitalOcean cloud builder (AMD EPYC 8c, Ubuntu 24.04), BOARD=helios64 BRANCH=edge:

Logs: paste.armbian.com/{enaretezuc,noxayoyoni,vonuyajido}.

Known design choices

• USE_CCACHE=yes in userpatches/lib.config does not auto-enable the
  ccache extension. lib.config is sourced after
  initialize_extension_manager, too late to enable extensions; the
  framework limitation is documented in main-config.sh (too late to define hook functions or add extensions in lib.config). The shim
  emits a warning at compile-vars phase nudging users to migrate to
  ENABLE_EXTENSIONS=ccache (the auto-enable code path).
• Auto-enable triggers on three signals: USE_CCACHE=yes,
  PRIVATE_CCACHE=yes, or ENABLE_EXTENSIONS=ccache-remote (the only
  extension in-tree that sets USE_CCACHE=yes from its
  extension_prepare_config). Any future extension that also sets
  USE_CCACHE from its prepare hook will need to add itself to the
  same trigger list, since the shim runs before
  initialize_extension_manager.

Summary by CodeRabbit

• New Features

  • Extensible compile-wrapper that runs pre/post build hooks and guarantees post-hook execution on interruption.
  • New ccache compile-cache backend with shared/private modes, configurable cache dir, optional pre/post-build cache reporting, and umask preservation across clean envs.
  • Automatic activation of the compile-cache backend when requested by config.

• Refactor

  • Compilation cache handling moved to an extension-based system and integrated with build steps (ATF/Crust/kernel/uboot) while preserving legacy post-build reporting.