Cache the decoded overlay in memory - #108
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- Add a process cache of decoded overlays, keyed by the overlay URL. - Clone the overlay under a lock, so one wand never serves two threads. - Bound the cache by a fixed count and expire an entry after DimsOverlayCacheMaxAge.
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What
The watermark operation keeps a process cache of decoded overlays, keyed by the overlay URL. A repeat request clones the cached overlay and skips the read and decode. The disk cache still holds the overlay bytes.
Why
Every watermark request read the cached overlay bytes and decoded them again with
MagickReadImage. The decode is the slow step, and it repeats for every request that uses the same overlay.The cache is safe for the worker threads. It clones the overlay under a lock, because one ImageMagick wand is not safe for two threads at once. It holds a fixed, small number of overlays, because a decoded overlay is large. It expires an entry after
DimsOverlayCacheMaxAge, the same age the disk cache uses. The allowlist check runs before the cache lookup, so a memory hit obeys the same rule as a disk hit.Verify
make -C test testpasses all 271 cases, including the watermark golden cases, which now hit the cache on the second identical overlay and match the golden bytes.make -C test sanitizereports no leak.make -C test valgrindreports no module allocation was definitely lost.Breaking
None. The output is identical and no directive is added.