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cache: Coalesce in-flight identical upstream fetches

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In-flight coalescing already dedups identical misses, but acceptance
still lacked a gated test (without a hold, waiters can become sequential
HITs after the first fill) and the Quick check metrics table omitted
cache_coalesced.

Add steamcache/coalesce_test.go: N concurrent identical GETs share one
upstream fill (leader MISS, waiters HIT-COALESCED) and a 5xx sibling.
Document cache_coalesced next to the other hit/miss fields.

Fixes #35
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2026-09-08 19:54:32 +00:00
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commit 59d1cc9923
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README.md
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@@ -81,6 +81,7 @@ curl -s -i http://localhost/lancache-heartbeat
| Field | Meaning |
| --- | --- |
| `cache_hits` / `cache_misses` / `hit_rate` | Whether later requests were served from cache |
| `cache_coalesced` | Waiters on an in-flight identical miss share one upstream fill (`X-LanCache-Status: HIT-COALESCED`) |
| `negative_cache_hits` | 404/410 served from a still-valid negative cache entry (also counted in `cache_hits`) |
| `range_cache` / `range_upstream` | Range GETs served as 206 from a cached object vs after a full upstream fetch |
| `memory_cache_hits` / `disk_cache_hits` | Which tier served the hits |
@@ -93,6 +94,8 @@ A first pass through new content is mostly misses (`hit_rate` near 0). Repeat th
Cache entries are keyed by depot object path (not the CDN `Host` header), so when Steam rotates CDN hostnames for the same depot path, hits still climb across the aliases.
Concurrent identical misses for the same key share one upstream GET: the leader is a `MISS` and waiters are `HIT-COALESCED` (`cache_coalesced`).
Steam clients lean on Range requests. When an object is already cached, a Range GET is served locally as 206 from that full object (`range_cache`). On a Range miss the cache still fetches and stores the full upstream body, then returns the requested byte range as 206 (`range_upstream`).
Definitive upstream 404/410 (gone depot objects) are stored as a short-TTL negative entry in the **same** cache, under the same depot-path key as a positive object. Repeating the request within `cache.negative_ttl` (default `5m`) is served as 404/410 without re-hitting upstream (`negative_cache_hits`). 5xx is not cached as negative. When the TTL expires the entry is deleted and the next request fetches again.