fix(models): hold the destination claim lock for foreground downloads too

[mattmillerai]

What

A foreground comfy model download now takes the destination's claim lock, the same one a background submit uses, and holds it in-process for the whole transfer. The finally writes the terminal status first and then releases the lock.

Before this, the foreground path wrote a state record and arbitrated with _enforce_claim, which writes, re-scans and hopes the competitor's record is already visible. Two foreground runs, or a foreground run and a --background submit, could each miss the other's record and both transfer. With --downloader aria2 the two transfers wrote into the same destination file. The background-vs-background case was already closed by #856. This PR closes the remaining pairs.

How

• _acquire_dest_claim(..., hold=True): the background arbitration (held lock → refuse; free lock whose payload names a background submit still in its spawn gap → refuse; anything else → stamp it) now has a mode that returns the won lock instead of releasing it. _hold_foreground_claim calls it right after _claim_foreground writes the record. The loser withdraws its record (_withdraw_record) and raises model_download_in_flight, the same refusal used before.

• Release order: the lock is released only after _persist_record writes the terminal status. A competitor that grabs the lock immediately judges our id by our record, so it needs to see completed/failed, not downloading.

• SIGKILL: the kernel drops the lock, so the next submit takes the destination immediately, with no grace wait and no sweep. The download-cancel foreground refusal ("press Ctrl-C") is unchanged. Its record-demotion path is now belt-and-braces, and a comment there says so.

• _in_spawn_gap ignores foreground records that carry a pid. A foreground run holds the lock from before it stamps the payload until after its terminal write, so if a lockable claim names one, that claim is stale. Without this, a foreground run whose terminal write failed could block its destination for up to STARTUP_GRACE_S. A pidless record that reads foreground can only be a background submit whose kind hit the tolerant fallback, so it keeps its spawn-gap protection.

• dest.exists() is re-checked once the lock is won, in both modes. A competitor that finished its whole transfer during our record write, auth probe and lock wait leaves a free lock and a stale payload. The re-check withdraws our record and refuses with model_file_exists instead of downloading over the finished file.

• download-cancel leaves a successor's partials alone. The lock drops with a dead run, so the next submit may already be streaming into the same .part (or, with aria2, into dest). Both sweep paths skip cleanup while another download holds the claim lock.

• Hold-mode edge cases: a failed payload write blanks the payload rather than leaving it naming the stale holder, and an unexpected exit while taking the lock (Ctrl-C included) withdraws the just-written record.

• _enforce_claim is now a degradation path only. Every arbitration pair is decided by the lock (plus the spawn-gap payload check):

  pair	decided by
  fg / fg	held lock (2nd run refused)
  fg holding → bg submit	held lock (_refuse_held_claim, kind: foreground)
  bg worker holding → fg	held lock
  bg in spawn gap → fg	payload names a young active record (_spawn_gap_holder)
  bg / bg	unchanged from fix(models): hold an exclusive OS lock on the destination claim for the transfer #856

  So I removed the pre-claim re-scan from _submit_background_download and the post-write re-scan from the foreground path. I did not delete _enforce_claim/_claim_order: point 2 of the ticket requires the OSError degradation to fall back to _enforce_claim. It now runs only when the claims dir is unavailable, the filesystem can't lock, or (background only) the payload write fails. Its docstring says this.

• Updated the stale comments and docstrings: pre-flight scan, _claim_order, _active_download_for exclude_id, and the background submit comment, which still said "a foreground transfer writes no claim file". Also updated the model_download_claim_contested description in error_codes.py, which said --background only. Added a CHANGELOG Unreleased → Fixed entry.

Tests

In tests/comfy_cli/command/test_model_download_background.py:

• Rewritten against the lock, not removed: test_a_claim_that_landed_first_wins_the_claim_lock (was …_post_write_recheck), test_a_live_foreground_holder_refuses_a_background_submit (real holder process), test_a_rival_holding_the_lock_takes_the_destination, test_a_rival_record_without_the_lock_does_not_take_the_destination, test_identical_timestamps_produce_exactly_one_winner (now crossed over id order × rival locked/unlocked; the lock decides, not _claim_order), test_a_claim_we_cannot_unlink_is_made_inert_instead. test_a_claim_left_by_a_killed_worker_is_taken_over now goes through the download command, because the persisted demotion it asserts came from the removed re-scan and is still done by the command's pre-flight scan.

• New:

  • four real foreground processes racing one destination: exactly one transfers, three get model_download_in_flight, and the winner holds until every loser has exited
  • SIGKILL a real foreground run mid-transfer, then a submit to the same destination succeeds immediately (POSIX)
  • a background worker holding the lock (real process) refuses a foreground run
  • a foreground run refuses a mid-transfer --background submit, end to end
  • a background submit in its spawn gap refuses a foreground run
  • the lock is held during the transfer and released after the terminal write (success / failure / Ctrl-C)
  • an unusable claims dir still downloads, and the degraded re-scan still refuses
  • an unlockable filesystem (ENOLCK) still downloads
  • foreground records are never treated as being in a spawn gap

  The record-visibility tests blind the pre-flight scan (_active_download_for → None), so they don't depend on when a record becomes visible.

• Negative control: with this PR's tests and main's models.py, 12 of the new or rewritten tests fail. All pass with the change.

Verified

• ruff check . and ruff format --check . are clean.
• pytest tests/comfy_cli/command/test_model_download_background.py: 270 passed, 1 skipped at the current head. The review round's 7 new tests all fail against the previous head's models.py.
• Review round: tests/comfy_cli/command/models/, the other suites that import download_state/models, tests/comfy_cli/output and test_aria2_download.py: 830 passed. The full serial pytest was not re-run this round because it exceeded the local time budget, so CI is authoritative for the rest.
• Full pytest (serial, local Linux host): 9368 passed, 38 skipped, 24 failed. The failures are outside the code this PR touches. I only captured the summary tail, which showed test_project_command.py::test_status_pretty_mode_mentions_root and test_file_utils.py::test_atomic_write_{text,bytes}_new_file_uses_umask_default (umask/environment-dependent). I did not confirm the full list of 24 is pre-existing on main, so treat CI as the source of truth there. Every model-download suite passes.
• Doc-drift sweep: Doc-drift sweep: 9 references checked, 2 updated. The sweep listed 395 lines across 60 identifiers, nearly all generic words (success, failure, reset, failed) unrelated to this change. I judged by hand the 9 claim/foreground references in download_state.py, schemas/download_status.json, error_codes.py and CHANGELOG.md, and updated the model_download_claim_contested description plus a new CHANGELOG entry.

Judgment calls / deviations

• Bounded wait, not strictly non-blocking. The ticket says "non-blocking". The foreground path reuses the background submit's SUBMIT_CLAIM_LOCK_TIMEOUT_S (1 s) retry window. Non-owners take the same lock briefly (prune's claim sweep, another submit reading the payload), and a foreground run that lands on one of those holds must not be refused. A real owner holds the lock for the whole transfer, so the only cost is that the refusal comes up to 1 s later.
• Payload write failure while holding is not a degradation in foreground mode. The held lock decides ownership. The payload only names the holder in a competitor's refusal; if it's unresolvable, the refusal becomes model_download_claim_contested.
• No lock when _claim_foreground returns None (record unwritable, or unverifiable pid). This keeps the documented None-degradation as it was.
• This supersedes the approach in the still-open, now-conflicting fix(models): hold the destination claim lock on the foreground download path too #855, which predates the held lock and adds an O_EXCL claim to the foreground path. That PR should probably be closed in favour of this one. I haven't touched it.

Residual

• aria2 after a SIGKILL. The lock drops with the CLI, but the external aria2 daemon may still be writing dest. This predates this PR (reconcile's dead-pid demotion had the same effect). The comment and the CHANGELOG now scope "frees the destination at once" accordingly.
• Mixed-version / partially-degraded arbitration. The background path no longer runs a record re-scan. A foreground run from an older comfy-cli (one that writes a record but takes no lock), or one whose lock attempt fails with a transient OSError while the background side's succeeds on the same claims dir, is caught only by the background side's pre-flight scan. If its record lands after that scan, both can transfer. The degraded side still runs _enforce_claim, so this is the same single-sided check-then-act window that existed before the lock, now limited to these degenerate cases. Closing it would mean keeping a pre-claim re-scan on every background submit forever. The ticket asked to remove it once the lock decides every pair, and that's what this PR does.
• Cross-workspace collisions remain open (unchanged; documented in _active_download_for). Claims live under get_workspace(), so runs from different workspaces aimed at one file via --relative-path ../.. or an absolute path can't see each other.
• Network filesystems: flock over NFS/SMB is best-effort, as documented in download_state.py. This PR doesn't change that.
• Unexercised artifacts: the plan comes from an investigation and a parent epic on an internal tracker. Neither was readable from this environment, so this PR is based on reading the code on main (post-fix(models): hold an exclusive OS lock on the destination claim for the transfer #856) plus the tests above. The SIGKILL test is POSIX-only (skipped on Windows), and the race and SIGKILL tests were run on Linux only. macOS and Windows coverage comes from CI.

Provenance

• Authored by: agent-work loop
• Verified:
  • ruff check + format clean at the current head
  • test_model_download_background.py: 270 passed / 1 skipped; the 7 review-round tests fail against the previous models.py
  • model, download_state, output and aria2 suites: 830 passed
  • full serial pytest: last run on the first head (24 failures in unrelated suites, not confirmed pre-existing); not re-run this round, so CI is authoritative
  • four-real-process foreground race and real-process SIGKILL test pass on Linux
• Deviations:
  • 1 s bounded lock wait instead of strictly non-blocking
  • _enforce_claim/_claim_order kept as the degradation path (ticket point 2 requires the fallback)
  • beyond the plan: _in_spawn_gap foreground exclusion (pid-gated), post-lock dest.exists() re-check, download-cancel successor guard, CHANGELOG entry and error_codes.py wording fix

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📝 Walkthrough

Foreground downloads now hold a destination claim lock through transfer and terminal-record persistence. Background submissions use destination locks for arbitration, with advisory enforcement retained when locking is unavailable. Cancellation avoids removing partial output when another download holds the destination claim.

Changes: Destination claim locking

Layer / File(s)	Summary
Destination claim arbitration comfy_cli/command/models/models.py, tests/comfy_cli/command/test_model_download_background.py	Destination locks arbitrate claims instead of record ordering. Foreground claim acquisition can retain the lock; advisory scans remain for fallback enforcement. Tests cover lock ownership, race outcomes, stale-claim takeover, and lock failures.
Foreground and background transfer lifecycle comfy_cli/command/models/models.py, comfy_cli/error_codes.py, CHANGELOG.md, tests/comfy_cli/command/test_model_download_background.py	Foreground downloads hold the lock through transfer and terminal-record persistence. Background submission uses lock arbitration and spawn-gap checks. Tests cover cross-kind contention, terminal states, concurrent processes, and SIGKILL. The changelog and error description include foreground downloads.
Cancellation and partial-file cleanup comfy_cli/command/models/models.py, comfy_cli/download_state.py, tests/comfy_cli/command/test_model_download_background.py	Cancellation checks whether a successor holds the destination claim before removing partial output. Tests verify that successor files remain untouched.

Sequence Diagram(s)

sequenceDiagram
  participant models.download
  participant _hold_foreground_claim
  participant ClaimLock
  participant Transfer
  participant ClaimRecord
  models.download->>_hold_foreground_claim: request destination claim
  _hold_foreground_claim->>ClaimLock: acquire destination lock
  ClaimLock-->>_hold_foreground_claim: return held lock
  models.download->>Transfer: perform transfer while lock is held
  models.download->>ClaimRecord: persist terminal status
  models.download->>ClaimLock: release lock

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Priority: ⬇️ Low

Merge Risk: 🟡 Moderate · up to 21346

A background download can still overlap with an older or lockless client and silently overwrite the same destination. Resolve or explicitly accept this arbitration gap before merging.

Pre-merge checks: 2

Passed checks: 2 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Triggered by @mattmillerai.

Found 8 finding(s).

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @comfy_cli/command/models/models.py:
- Around line 1522-1541: Update the docstring for _dest_held_by_another to
clarify that an unreadable claim payload (None) counts as held by another, even
when the cancelled download may be the holder. Preserve the existing comparison
and behavior.

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