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Reuse the segment handle across an iteration #8

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Reuse the segment handle across an iteration #8

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40 changes: 34 additions & 6 deletions include/microStore/FileStore.h
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Original file line number Diff line number Diff line change
Expand Up @@ -843,13 +843,37 @@ USTORE_LOG("[ustore] set_max_recs: %u\n", max_recs);

// Read the value from disk into current_.value. Called lazily by
// operator* / operator->. Marked const so it can mutate mutable members.
//
// The segment file is held open across the walk rather than reopened
// per record. Opening is not the cheap half: on LittleFS every open
// resolves the path afresh, and resolving a component replays that
// directory's metadata log, which grows with write activity until it
// is compacted. A store that is appended to constantly therefore
// charges more and more for each open while the read itself stays the
// same size. Measured on a 200-record path store on an ESP32-S3, one
// record cost 93-162 ms, nearly all of it under lfs_dir_find — the
// caller walking the table spent 32 s doing so and its task watchdog
// fired.
//
// Held for exactly the iteration's lifetime, which is all this claims:
// begin() flushes pending writes before iterating, so what is on disk
// is current, and mutating the store already invalidates the iterator.
// Every record is addressed by an explicit seek, so a handle shared
// with a copied iterator cannot be left on a stale position. It is not
// closed here on purpose — File holds a shared_ptr and FileImpl closes
// on destruction, so the last copy to go away closes it exactly once;
// closing it explicitly would shut the file under a copy (the one
// operator++(int) returns) that may still be walking.
void load_value() const {
char name[USTORE_MAX_FILENAME_LEN];
store_->segment_name(iv_.segment, name);

File f = store_->_filesystem.open(name, File::ModeRead);
if (!seg_file_ || seg_held_ != iv_.segment) {
char name[USTORE_MAX_FILENAME_LEN];
store_->segment_name(iv_.segment, name);
seg_file_ = store_->_filesystem.open(name, File::ModeRead);
seg_held_ = iv_.segment;
}
value_loaded_ = true;
if (!f) { current_.value.clear(); return; }
if (!seg_file_) { current_.value.clear(); return; }
File& f = seg_file_;

f.seek((long)iv_.offset, SeekModeSet);
RecordHeader hdr;
Expand All @@ -858,7 +882,6 @@ USTORE_LOG("[ustore] set_max_recs: %u\n", max_recs);
current_.value.resize(hdr.length);
if (hdr.length > 0)
f.read(current_.value.data(), hdr.length);
f.close();
}

BasicFileStore* store_;
Expand All @@ -867,6 +890,11 @@ USTORE_LOG("[ustore] set_max_recs: %u\n", max_recs);
IndexValue iv_;
mutable bool value_loaded_;
mutable Entry current_;
// The segment this iterator currently holds open, and which one it is.
// A store spans several segments, so a walk that crosses a boundary
// swaps the handle rather than keeping one per segment open.
mutable File seg_file_;
mutable uint32_t seg_held_ = 0;
};

/* -------- BEGIN / END -------- */
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