* feat(mount): proactive flush of idle writable chunks
Add a background goroutine that periodically scans writable chunks
across all open file handles and seals those that are idle and
unlikely to receive further writes, submitting them for async upload.
A writable chunk is proactively flushed when it has been idle for
500ms AND meets one of: nearly full (>=90%), behind the sequential
write frontier by 2+ chunks, or stale for 5+ seconds. Flushing only
happens when the upload pipeline has spare capacity (< half of
concurrent writer slots in use).
This prevents partial chunks from accumulating until fsync/close,
which is particularly beneficial for bursty or small-file workloads
where chunks may never reach IsComplete().
Also fixes a latent bug in ActivityScore where MarkRead/MarkWrite
used value receivers, silently discarding all mutations.
* refactor(mount): reuse WriterPattern instead of duplicating sequential detection
Remove the isSequential atomic from UploadPipeline. The proactive
flusher now reads IsSequentialMode() from the existing WriterPattern
on PageWriter and passes it as a parameter to ProactiveFlush. This
avoids duplicating the sequential/random detection that WriterPattern
already maintains.
* fix(mount): address PR review feedback
- Make ActivityScore thread-safe using atomics (CAS loop for score
updates, atomic load/swap for timestamp). Previously MarkRead was
called under RLock while MarkWrite held a write lock, creating a
data race on the shared fields.
- Fix ProactiveFlush half-capacity guard: use multiplication
(uploaderCount*2 >= max) instead of floor division (max/2) which
misbehaves for odd or small concurrentWriterMax values.
* fix(mount): review fixes for proactive flush
- Fix TOCTOU race in lastWriteChunkIndex update: use CAS loop so
concurrent writers cannot regress the frontier.
- Remove unused UploaderCount() getter.
- Reuse the caller-provided tsNs instead of calling time.Now() again
in WriteDataAt for lastWriteTsNs, eliminating a redundant syscall
per write.
