fix(samd): fetch the core from Adafruit's bundle so submodules come with it

[zackees]

Closes #1400.

The break

samd-core was fetched from GitHub's auto-generated source archive, which omits submodules by design. Tag 1.7.16 declares two under libraries/: Adafruit_TinyUSB_Arduino and Adafruit_ZeroDMA.

Until #1401 this was latent — FastLED's SAMD builds compile sketches that include neither library, so they were green for months. #1401's check fires on the package rather than on use, so every SAMD build started failing before a compiler ran:

build error: package error: samd-core unpacked without its submodule contents.
  - libraries/Adafruit_TinyUSB_Arduino
  - libraries/Adafruit_ZeroDMA

Downstream that took out metro_m4, samd21, samd21_zero, samd51j and samd51p in FastLED the moment it pinned 2.5.22 (FastLED/FastLED#4153).

The open question in #1400, answered

#1400 could not say whether Adafruit's board-index bundle actually carries the submodule contents, since Adafruit publishes no release asset for 1.7.16. It does:

	files
libraries/Adafruit_TinyUSB_Arduino/	368 (incl. Adafruit_TinyUSB.h, tusb.h)
libraries/Adafruit_ZeroDMA/	25

So this is a real fix, not a way to quiet the check. The bundle also carries no .gitmodules, which #1401 already treats as clean — it is a prepared bundle, not a git archive.

What changed

• URL moves to https://adafruit.github.io/arduino-board-index/boards/adafruit-samd-1.7.16.tar.bz2.
• The SHA-256 from package_adafruit_index.json is now pinned and verified (the old URL passed None). The bundle is served from a GitHub Pages site rather than an immutable release asset, so this matters.
• Doc comments and the find_core_root test updated for the adafruit-samd-1.7.16/ top-level name.

Two things needed no change, and are worth stating because they are the parts that could have bitten:

• find_core_root scans for any subdirectory containing cores/, so the top-level rename is handled generically.
• .tar.bz2 was already routed to extract_tar_bz2 by extractor::extract, which dispatches on the filename download_file_with_progress derives from the URL. This is the only .tar.bz2 package in the tree, so I checked rather than assumed.

Verification

• fbuild build tests/platform/samd21 -e samd21 — build succeeded in 3.0s, flash 11420 bytes / ram 3828 bytes, unpacked to adafruit-samd-1.7.16/cores/arduino/ with the submodule check passing.
• Downloaded bundle's SHA-256 matches the pin exactly.
• bash test -p fbuild-library — 253 passed, 0 failed.
• soldr cargo clippy -p fbuild-library --all-targets -- -D warnings — clean.

Regression guard

Two tests pin the invariant so a future edit cannot quietly reintroduce #1400: one rejects a /archive/refs/ URL form, one requires the checksum to stay pinned.

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Summary by CodeRabbit

• Improvements
  • The SAMD core is now downloaded from Adafruit’s official Arduino board index bundle rather than a GitHub source archive.
  • Downloads use a pinned SHA-256 checksum to verify package integrity before use.
  • Updated package layout handling ensures the bundled SAMD core is detected correctly.
  • Additional validation helps ensure the expected download source and package version remain consistent.

[coderabbitai]

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📝 Walkthrough

Walkthrough

The SAMD core now downloads Adafruit’s prepared adafruit-samd-1.7.16.tar.bz2 bundle instead of a GitHub source archive. All constructors pass a pinned SHA-256 checksum, and documentation and tests reflect the bundle’s extraction layout.

Changes

SAMD bundle integration

Layer / File(s)	Summary
Bundle URL and checksum contract crates/fbuild-library/src/library/samd_core.rs	SAMD_CORE_URL now references Adafruit’s bundle. SAMD_CORE_SHA256 defines its pinned checksum. Documentation describes the new source and root directory.
Checksum wiring and bundle layout validation crates/fbuild-library/src/library/samd_core.rs	All three constructors pass the checksum to PackageBase. Tests validate the URL, checksum, and adafruit-samd-1.7.16 extraction directory.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 69c12

This updates SAMD core downloads to a checksum-verified Adafruit bundle containing required submodules, with matching layout coverage. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the primary change: fetching the SAMD core from Adafruit’s bundle so required submodules are included.
Linked Issues check	✅ Passed	The changes address issue #1400 by replacing the GitHub source archive with Adafruit’s bundled archive, pinning and verifying its checksum, and updating tests and documentation for the bundled layout.…
Out of Scope Changes check	✅ Passed	The changes are limited to SAMD core fetching, checksum verification, related documentation, tests, and fixture updates. No unrelated code changes are identified.
Docstring Coverage	✅ Passed	Docstring coverage is 88.89% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 1 files.

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[zackees]

Audit: the same trap is still armed on two other cores

While verifying this fix I checked .gitmodules at the pinned ref of every core fbuild fetches from a GitHub archive. Two are still exposed, and #1401's check will fail them the same way the moment they unpack on a cold cache:

core	ref	declared submodules	checksum	status
silabs-core	SiliconLabs/arduino@2.2.0	extra/core-api	None	exposed
ch32v-core	openwch/arduino_core_ch32@d767162	libraries/Adafruit_TinyUSB_Arduino	None	exposed

ch32v is not mentioned in #1400 — it was missed in the original sweep. silabs was flagged there as unverified; it is now confirmed to declare a submodule.

Confirmed clean (no .gitmodules at the pinned ref), so nothing to do:

arduino/ArduinoCore-API@1.5.2, arduino/ArduinoCore-avr@1.8.6, arduino/ArduinoCore-mbed@4.5.0, SpenceKonde/ATTinyCore@v1.5.2, stm32duino/Arduino_Core_STM32@2.9.0, FastLED/framework-arduino-lpc8xx@9e8be02.

esp8266 is already on its release asset from #1398, so it is fine.

Neither exposed core is in FastLED's CI matrix, so neither is red today — same latent shape SAMD had before 2.5.22. I left both out of this PR because each needs its own artifact hunt and verification, exactly as #1400 argued for SAMD. Worth tracking separately.

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Actionable comments posted: 1

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Inline comments:
In `@crates/fbuild-library/src/library/samd_core.rs`:
- Around line 263-265: Update the tests around
test_core_url_is_not_a_github_source_archive to assert that SAMD_CORE_URL
exactly matches the expected Adafruit bundle URL, and update the checksum test
to compare directly against the expected SHA-256 value rather than only
validating its format. Preserve the existing test structure while enforcing both
exact constants.

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[zackees]

Blocked on #1419, not on anything in this branch.

Check (ubuntu-latest) fails in native_release_workflow_uses_current_cross_toolchains, which asserts the exact soldr version template_native_build.yml pins. #1416 moved that pin 0.9.6 → 0.9.12 without updating the test, so main has been red since e4a871a and every open PR inherits it. #1419 follows the pin; I'll rebase this branch once it lands.