RAM-backed small-file cache with auto-flush; mount without PUT-Range; cache_threshold option; robust unmount

[carli2]

Summary

• Adds a per-handle in-RAM cache with full PUT on close/fsync and inactivity auto-flush.
• Allows read–write mounts even when the server lacks partial write support.
• Adds cache_threshold options to bypass RAM cache for large files (when ranged I/O is supported).
• Improves unmount on signals and process exit.

Motivation

• Many WebDAV servers don’t support partial writes, making efficient random I/O hard. This change enables reliable read–write behavior via an in-memory buffer that uploads on close, while
  still using ranged I/O where available and beneficial.

Key Changes

• RAM cache handle for reads/writes, PUT on close/fsync, 10s inactivity auto-flush:
  • memhandle.go:1
  • fuse.go:805 (Open), 520 (Create), 572 (ftruncate path for RAM), 281 (Remove local-only), 176 (Rename fast path)
• Size-based decision: use RAM cache for small files, ranged I/O for large files:
  • fuse.go:829
  • main.go:202 (default 64 MiB)
  • mountoptions.go:9 (CacheThreshold), 87 (cache_threshold), 94 (cache_threshold_mb)
• Allow mounting and operating without PUT-Range; warn and fallback to RAM cache:
  • main.go:286 (dav.Mount), 202–206, 286–305 (mount + capability messaging)
• Robust unmount on signals and exit (graceful + lazy detach fallback):
  • main.go:336–359

New/Updated Options

• cache_threshold: Size in bytes above which files prefer non‑cached ranged I/O (when available). Default 67108864 (64 MiB).
• cache_threshold_mb: Same as above, but in MiB.
• Behavior notes:
  • If the server lacks partial writes, all files use the in‑RAM cache regardless of size.
  • RAM cache auto‑flushes after 10s of inactivity per handle, and on fsync/close.

Docs

• README updated with RAM cache mode, auto-flush, options, and behavior:
  • README.md:1
• DAV requirements expanded with fallback semantics and references:
  • DAV-requirements.md:1

Behavior Details

• Open/Create: Loads file into RAM buffer for small files or when no range support; large files use ranged I/O when supported:
  • fuse.go:805, 520, 829
• Truncate: Resizes in-memory buffer when using RAM handles; otherwise uses PUT/PUT-Range appropriately:
  • fuse.go:572
• Rename/Delete: Optimized local operations for files not yet uploaded or with open RAM handles:
  • fuse.go:176, 281
• Unmount: On SIGINT/SIGTERM/SIGHUP, attempts graceful unmount, closes FUSE, falls back to lazy unmount on Linux:
  • main.go:336–359

Backward Compatibility

• No breaking changes to existing mounts.
• Defaults preserve previous ranged-I/O behavior where supported, with a clear default threshold and safe fallback when not.

Testing Notes

• Scenarios exercised:
  • Mount against SabreDAV/Apache with partial update support and without.
  • Small file random writes; verify upload on close and after ~10s inactivity.
  • Large file I/O above threshold uses ranged operations.
  • Create with O_EXCL semantics; rename/delete before upload completes.
  • Signal handling triggers unmount and process exit.
• Example:
  • mount -t webdavfs -o username=...,password=...,cache_threshold_mb=64 https://host/dav /mnt
  • Write small file, keep open >10s to see auto-flush; write large file and observe ranged I/O logs (with -T webdav,httpreq).

Notes for Reviewers

• The inactivity timer is per open handle and aims to balance durability vs. overhead.
• Conditional headers are used on full PUTs to avoid overwriting unexpected states where possible.
• Default 64 MiB threshold is conservative; can be tuned per environment.

Checklist

• Feature: RAM cache, auto-flush, and thresholded I/O
• Mounting without PUT-Range support
• Updated docs: README and DAV requirements
• Manual tests across supported scenarios
• Optional: add integration tests against mock DAV endpoints

[carli2]

This makes webdavfs mount against Servers using the PHP sabre/dav library and improves performance on small file handling