xtc/docs/RECOVERY.md

Persistence and failure recovery

The project format is v12. Readers migrate v1–v11, validate bounded input and atomically install a candidate only after complete success. Existing tests cover exact migration/roundtrip, truncations, malformed references, counts, IDs, nonfinite values and failed GUI/backend installation. Native processors are session-only; saving their graph returns UNSUPPORTED rather than writing process addresses or an unreadable project. See PROJECT_FORMAT.md.

Project Save writes a mode-0600 same-directory NAME.tmp.XXXXXX, flushes and fsyncs it, closes and renames it over the destination. Before rename, handled failures remove the temporary and retain the destination. POSIX rename gives complete old-or-new destination contents during process failure. The containing directory is fsynced after rename. If that sync fails, Save reports I/O failure but the complete new file may already be present. Preserve and validate it before retrying; filesystem/hardware durability still varies. Symlinks are replaced rather than followed at rename; original file permissions are not kept.

Recording similarly writes/finalizes a same-directory temporary, then renames it. Stop must synchronize callbacks before draining/finalizing. Ring overflow returns an explicit error and may finalize a partial take for manual recovery; I/O failure retains the previous destination. A killed recorder does not finish its WAV header. Source WAV files remain external; references inside the project folder are saved relative to it. Save As does not collect them. Back up the project and assets together.

After a crash

1. Preserve the destination and any .tmp.* files before attempting repair.
2. Run xtc-check --validate project.xtc for metadata validation without a GUI, display or audio server. Run xtc-check --inspect project.xtc to list resolved media/module paths and plugin identities: status 2 means missing/unreadable references, 1 means invalid metadata/I/O, 0 means files are available. Neither command verifies file contents, plugin initialization or realtime preparation.
3. A valid destination can be opened normally. If only a temporary contains useful data, copy it to a separate candidate and validate that copy before replacing anything. Partial project files are rejected; there is no salvage parser.
4. An unfinished WAV temporary may have a zero/incorrect data length and needs a separate recovery tool or manual header repair on a copy. xtc does not guess its intended length or automatically rename orphan temporaries.
5. Missing assets preserve silent clips. Use Relink selected audio, then Play to revalidate. For server loss/format changes, Stop/Reconnect and explicit routing are required. Panic/F9 clears held notes. Failed close retains ownership for retry.

There is no autosave, crash journal, automatic orphan recovery or persisted Undo history. Existing finalized files are protected against the tested process-level failures; unsaved edits/in-flight takes can still be lost.

make test-recovery uses temporary directories and child processes only. It forces an EFBIG-style write failure with a child-local file-size limit, kills eight project writers while temporaries are visible, and kills a recorder after draining but before finalization. Complete old/new destinations and explicit orphan temporaries are verified. It does not cut power, terminate the shared audio server, simulate a failing filesystem, or qualify disk/device endurance.

Retained recording prefixes

Recording now defaults to no-clobber publication; the GUI requires an unused path. Failed nonempty temporary recordings survive destruction. Query the recorder recovery path before destruction, or use the GUI's reported path. xtc-check --recover-recording TEMP NEW.wav reconstructs a canonical mono float32 WAV from complete on-disk frames, ignoring a trailing partial frame. It never changes the source or overwrites an existing destination. This supersedes the earlier lack of a WAV recovery tool described above. See AUDIO_FILES.md for format/ownership/size limits and make test-recording.

Plugin load and replacement failures

GUI Open reports an unavailable/incompatible plugin's label and absolute module path and retains the current document. Preserve the saved file; restore a compatible module at its original path and retry Open. xtc-check --validate checks metadata without executing plugins, so success does not certify that a module exists or initializes. Metadata-only C load/save preserves unavailable plugin records. There is no automatic missing-plugin substitution or GUI relink for an unopened document.

For an already loaded project, Replace selected plugin preserves wires and node ID only for the same audio layout. New defaults replace controls/automation; Undo restores the original configuration. Failed discovery/compile/instantiate retains the original graph, controls and history position. Cancel in the generic parameter editor discards its temporary values. Reset defaults clears automation only when applied. No host recovery can contain a crash or hang in trusted native plugin code; LADSPA has no runtime error-return channel.

Save diagnostics

make test-project-diagnostics deterministically injects flush, file-sync, close, rename and directory-sync errors. Pre-rename failures retain the previous bytes and clean temporary files. Post-rename sync errors leave a complete new file and return failure; there is no rollback after replacement. Load diagnostics identify the expected record and syntax line, or line zero for whole-model/I/O stages. Unknown versions report the supported range (1–12); unknown records/fields are rejected without partially changing the current project.

Modern CLAP/VST2/VST3 module identity is preserved in v11 metadata even when the binary is unavailable. Restore the compatible saved binary path and retry Open. CLAP restart and VST2 I/O-change requests fail/silence subsequent blocks; stop and rebuild/reload rather than attempting callback-time reconfiguration. Modern instances restart at default configuration; no edited opaque state is exposed or saved by the initial adapters. Trusted plugin crashes cannot be contained.

Instrument support

MIDI tracks can bind CLAP, VST2 and VST3 instruments. In the Track menu, select Load instrument plugin for the selected sequencer track; Use built-in instrument removes the binding. Assignment also selects the track for live MIDI. Live MIDI to selected instrument changes that target; Live MIDI to built-in sine restores the legacy live source. MIDI clip notes always go to their own track's instrument. Independent instances feed track gain/pan, mute/solo, bus routing and automation. Bindings/live target save in v12.

Supply the actual trusted Linux binary (including the inner .so of a VST3 bundle). Supported instruments have zero audio inputs, one mono output, float32 processing, zero latency and one note/event input. Default configuration only: no custom editor, opaque state/preset restoration, modern parameter editing/automation, stereo/multichannel instruments or MIDI output. No arbitrary third-party instrument is qualified by the original ABI fixture tests.

CLAP accepts MIDI or native notes; VST2 uses MIDI events; VST3 uses IEventList note events. Live CC reaches CLAP MIDI/VST2; native CLAP/VST3 have no controller mapping (including sustain). Live MIDI goes to exactly one selected target. Seeks/loops retrigger notes crossing the destination with fresh envelopes; plugin oscillator/opaque DSP state cannot be reconstructed. Panic delivers all-off (VST3 held-key note-offs); plugin release tails may remain. Processing uses preallocated 256-frame chunks and 256 incoming events per instance/chunk; overflow or processing failure silences the block and schedules reset events. The existing 32 simultaneous scheduled-note and 4096 stored-note bounds remain. Do not unload/replace modules during playback; candidate plans are prepared off the callback and old instances are reclaimed on the control thread. Missing references remain inspectable without loading; failed GUI preparation retains the current project. Save a backup for applications limited to v11 or earlier.