Windows keeps the pinned directory locked while a handle inside it is open
(os.Root plus the pull temp file or the upload source), so renaming that
directory fails with a sharing violation. Every "pinned root/ancestor was
swapped" reproduction in the drive mirror tests relied on such a rename, so 13
tests failed on windows-latest. That, not a coverage shortfall, is why
Coverage (Windows) exited 1 before the gate ever ran.
Each reproduction now falls back to injecting the equivalent identity change
when the rename is refused. pinnedPullRoot.verify() and verifyParent() read
current identity only through pullPathStat / pullRootLstat, so pointing those
seams at another directory hits the same fail-closed branches. Unix still
performs the real move and loses no strength.
Assertions that need an actual replacement tree now branch on the helper's
return value. forcePinnedFallbackForTest makes the fallback path itself
regressible on any platform, and a dedicated test covers it.
Verified locally with the fallback forced on: all 13 tests pass and changed
code coverage stays at 100%.
chichuan
97b6022017