fix(client/windows): settings persist when the app isn't installed on C: #62

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enricobuehler merged 1 commits from worktree-client-msix-persist into main 2026-08-05 20:53:40 +00:00

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Author	SHA1	Message	Date
enricobuehler	5ebe840320	fix(client/windows): settings persist when the app isn't installed on C: ... Reported from the field (2026-08-05): a fresh Windows 11 box with a data partition, "New apps will save to: D:", and the client installed there. It launches, finds hosts and streams — but no setting and no profile survives a restart. Reinstalling to C: fixes it completely. The reporter's read was "it's in read-only mode", and that is almost exactly right. The one clue that localises it: the client creates its mTLS identity with a plain `fs::write` on first run and hard-exits if that fails. Their app started, so ordinary file creation in the config directory works. Only the config stores were being lost — and those are the three files that go through `write_atomic`, which writes a sibling temp and renames it over the target. The rename is what breaks. The client ships as a full-trust MSIX package, so its `%APPDATA%` writes are redirected into the package container. When the package lives on a secondary drive, Windows keeps that redirected state on the package's own volume: `C:\Users\<u>\AppData\Local\Packages\<pfn>\` stays a real directory on C:, but its children (LocalCache, RoamingState, …) are junctions to `D:\WpSystem\<SID>\…`. Both sides of our rename still spell `C:\Users\…`, so nothing looks unusual, but they can resolve across that junction boundary — and `std::fs::rename` is `MoveFileExW` with `MOVEFILE_REPLACE_EXISTING` and *not* `MOVEFILE_COPY_ALLOWED`, so a cross-volume move fails outright rather than degrading to a copy. Creating files still works, which is why everything else about the install looks healthy. So the fix is not to make the rename work — it is to stop treating it as the only way to persist. `write_atomic` now falls back to writing the target in place when the atomic route fails. That is the same operation the identity files already use, and those demonstrably round-trip on the affected installs, so the fallback lands on a path we know resolves. It trades crash-atomicity for exactly the writes that would otherwise be lost, and nowhere else: temp+rename stays the normal route everywhere it works. Writing into a redirected location cannot desync from reading it — Microsoft documents one private-location-first resolution order for both, so whichever layer a write lands in is the layer the next read finds. The fallback verifies anyway, by reading the bytes straight back: a write that reports success and disappears is precisely the bug being fixed, so this path does not get to claim success on an `Ok(())` alone. It costs nothing normally — it only runs on an install that has already shown it does something unusual. Two things this uncovered on the way: The temp file was a single shared `<name>.json.tmp`, but these stores have five whole-file writers (WinUI shell, session, console UI, CLI, Decky). Two saving at once collide on it — on Windows the second write hits a sharing violation, and worse, one process can rename the other's half-written bytes over the target. The scratch path now carries the pid. And none of this was visible to anyone. Every save on this page is fire-and-forget by design (a failed settings write must never take a stream down), so ~15 call sites discard the error and the UI cheerfully shows the toggle you just moved. The reporter had no log file to send either, because "Open log folder" was handing out a phantom path — a separate bug, already fixed in f3c0ee47 but not in the 0.24.0 they were running. `store_health` records the last persistence failure centrally, and Settings shows an error bar naming the path when the store is refusing writes, so a client that cannot save says so instead of pretending. `update.rs` had hand-rolled the same temp+rename inline, so it neither cleaned up its temp on a failed rename nor picks up the fallback; it now goes through the one writer. The update floor silently never rising is how a declined update comes back forever. Deliberately NOT done: disabling MSIX AppData virtualization in the manifest (`desktop6:FileSystemWriteVirtualization`). It would stop the redirection at the source, but every existing packaged install's settings, profiles and pairings live inside the container today — turning it off points the client at an empty real `%APPDATA%` and silently resets all of them. That needs a migration, not a manifest flag. Also considered and not taken: resolving the destination directory with `GetFinalPathNameByHandleW` and creating the temp inside the resolved path, to keep atomicity. It does not reliably close this hole — when the target file exists only in the unvirtualized layer while its directory resolves to the private one, the rename still straddles the boundary — and it would rest on canonicalisation behaving through the redirection, which we have never verified on a packaged run. Verified on the RTX box (.173, Windows 11 26200), which is the platform that actually has these rename semantics: `cargo fmt --all --check`, the full `pf-client-core` lib suite (109 passed), and clippy `-D warnings --all-targets` on both `pf-client-core` and `punktfunk-client-windows` — all clean. Also green under linux/amd64 (116 passed). Three new tests: the pid-scoped scratch path, the fallback actually persisting and reading back when the atomic route is blocked, and a genuinely unwritable store surfacing its error instead of swallowing it. The mechanism above is established from documentation and third-party reports, not from a reproduction on a second-drive install — that box does not exist here. The fix does not depend on the diagnosis being exactly right: it repairs any install where the rename fails but a direct write succeeds.	2026-08-05 22:33:36 +02:00