The FN-5627 default-threshold fix changed the prerebase threshold default
from 0 (never fire on commit-count) to 1 (fire on any divergence). But
that only affected projects WITHOUT an explicit threshold. Projects with
user-set values like 'prerebaseDivergenceThreshold: 50' continued to
skip prerebase for small divergences (e.g., 4 commits behind), so the
squash built against stale base and update-ref refused non-FF \u2014
producing the same-SHA spurious-concurrent-advance signature that
stranded FN-5626/FN-5628/FN-5633.
Root distinction missed earlier:
- prerebaseDivergenceThreshold is for USER-VISIBLE SEVERITY REPORTING
(this branch is N commits behind, warn me).
- Engine correctness requires a SAFETY INVARIANT (any branch behind main
MUST be rebased before squash or update-ref will fail).
These are independent concerns. The safety invariant must not be gated
on the user's threshold.
decideAutoPrerebase() now returns fire=true with reason
'safety-fallback-any-divergence' whenever commitsBehind > 0, after the
hot-file and threshold checks. The threshold path still wins the reason
label when its condition trips, so user-visible severity reporting is
unchanged for non-pathological cases.
Full opt-out remains prerebaseAutoEnabled=false (skips the safety
fallback; user accepts behind-branch merges will fail).
prerebaseDivergenceThreshold=0 is no longer a complete opt-out from the
commit-count gate \u2014 it only suppresses the threshold-based reason label.
Tests (4 updated/new):
- safety-fallback-any-divergence reason added to AutoPrerebaseDecision
- 4 commits behind with threshold=50 fires via safety fallback
- prerebaseAutoEnabled=false respects full opt-out
- threshold trip still wins reason label
- commitsBehind=0 returns no-divergence (unchanged)
Engine suite: 6163 tests pass.
In-flight: FN-5626, FN-5628, FN-5633 manually SQL-reset to mergeRetries=0,
status=null, error=null, transientRecoveryCount=0 so the next merger tick
(after engine restart picks up this code) auto-prerebases via safety
fallback and lands the work. Future occurrences self-heal automatically.
Fusion-Task-Id: FN-5627
decideAutoPrerebase() previously defaulted prerebaseDivergenceThreshold
to 0, which meant the threshold path NEVER fired unless the user
explicitly set a positive value. Only hot-file matches could trigger
prerebase.
The result: tasks whose branch was started against an older main tip
(because other tasks landed concurrently) skipped prerebase, built their
squash commit against the stale base, and then failed at git update-ref
because the squash didn't descend from current main. The merger correctly
detected this as non-fast-forward and threw
IntegrationBranchConcurrentAdvanceError, but with both 'expected' and
'observed' SHAs set to current main tip \u2014 because observedCurrentSha was
captured from the pre-update rev-parse, not post-failure. This produced
the misleading 'expected X, observed X' same-SHA error signature that
stranded FN-5632 stuck at mergeRetries=3 after the FN-5627 merger fix
and engine restart.
New default: prerebaseDivergenceThreshold = 1. Any branch behind by at
least 1 commit auto-rebases before squash. Users who want the legacy
never-fire behavior can explicitly set prerebaseDivergenceThreshold = 0.
Threshold comparison also changed from > to >= so an explicit threshold
of N rebases at N+ commits behind instead of N+1+.
The self-healing classifier comment for spurious-concurrent-advance-same-sha
is updated to note the signature can come from either pre-FN-5627
misclassification OR the legitimate post-FN-5627 non-FF path; the
auto-recovery sweep is unchanged because both cases self-heal cleanly
once prerebase fires on the retry.
Tests (3 new):
- Default threshold (undefined) fires at 1 commit behind
- Explicit threshold = 0 stays as opt-out (never fire on commit-count)
- Default threshold doesn't fire when branch is up-to-date
Engine suite: 6160 tests pass.
In-flight: FN-5632 manually SQL-reset to mergeRetries=0 / status=null
once more so the next merger tick (after engine restart picks up this
code) auto-prerebases and lands the work. Future occurrences self-recover.
Fusion-Task-Id: FN-5627
