--no-wal mount option

[vbp1]

What this PR does

Adds a pbkfs mount --no-wal option that makes WAL under pg_wal/ effectively virtual: PostgreSQL sees a consistent PGDATA view, but WAL mutations are not materialized into pbk_diff. This reduces I/O and disk usage for recovery/analysis runs where WAL is disposable.

User-visible behavior

• New flag: pbkfs mount ... --no-wal (default: off).
• Applies only to files under pg_wal/.
• Unchanged WAL segments (never written/truncated/created/renamed during the mount):
  • Read directly from the backup chain.
  • Do not create diff copies.
• Changed or newly created WAL segments:
  • As soon as a WAL file is written/truncated/created/renamed/unlinked, it is treated as “changed”.
  • Reads return zero bytes within [0, logical_size) and EOF at/after logical_size.
  • Logical size grows/shrinks according to successful write/truncate operations, despite no data being persisted.
• WAL operations write, truncate, create/mknod, rename, unlink, fsync, fallocate are emulated in-memory and reported as successful, without touching pbk_diff or pbk_store.
• Diff directory becomes non-resumable after a --no-wal mount:
  • First --no-wal mount emits a warning.
  • Binding is persisted with no_wal_used=true.
  • Any later attempt to reuse that diff dir (with or without --no-wal) fails with a clear error.

Implementation overview

• CLI / Binding

  • Added no_wal to MountArgs, set PBKFS_NO_WAL consistently.
  • Persisted no_wal_used in binding and reject reuse when set.
  • Files: src/cli/mount.rs, src/binding/lock.rs.

• Overlay

  • OverlayInner.no_wal initialized from PBKFS_NO_WAL.
  • For WAL under no_wal, reads use a non-materializing path; legacy WAL copy-up fallback is disabled.
  • File: src/fs/overlay.rs.

• FUSE layer

  • Added no_wal flag and an in-memory wal_state map to OverlayFs.
  • Implemented WAL virtualization: write/truncate/create/rename/unlink/fsync/fallocate update only WAL state.
  • Reads return either real chain bytes (unchanged) or zero-filled bytes/ENOENT (changed/deleted).
  • readdir and stat_path/getattr synthesize WAL entries from WAL state.
  • File: src/fs/fuse.rs.

Tests

• Updated all test MountArgs initializations to include no_wal.
• Integration:
  • --no-wal + write to pg_wal/<seg> ⇒ reads return zeros, stat.size reflects logical growth, no WAL diff file is created.
  • --no-wal + unchanged WAL read ⇒ bytes come from backup chain without diff materialization.
  • File: tests/integration/mount_postgres_tests.rs.
• Contract:
  • Binding records no_wal_used on first --no-wal mount and reuse is rejected.
  • File: tests/contract/cli_contract_tests.rs.
• Unit:
  • Overlay reads PBKFS_NO_WAL into no_wal.
  • Binding no_wal_used round-trips and defaults to false.
  • Files: tests/unit/fs_overlay_tests.rs, tests/unit/binding_tests.rs.

Notes

• Default behavior is unchanged; virtualization is strictly opt-in.
• WAL data is intentionally never persisted in pbk_diff in this mode.