Bound the originals cache with adaptive LRU eviction (closes #47)

Bounds cacheDirectory/originals to an adaptive limit:
min(cacheOriginalsMaxBytes, bytesUsed + bytesFree - freeBelowBytes), with
bytesFree from fs.statfs, so it falls as the disk fills and rises as space
returns (status().originalsLimitBytes exposes it). Each original write evicts
least-recently-used originals until usage fits; last-use is the file mtime,
bumped on every read that returns a path, so order survives restarts. Pinned
originals and the file just written are never evicted — so an on-demand fetch
larger than the limit keeps the path it returns and re-downloads nothing; the
skipped file becomes eligible on a later write. When only such files remain the
cache stays over-limit. Defaults: 100 GiB limit, 50 GiB reserve; thumbnails and
the backup directory are never counted.

Model: opus-4-8

src/library/content.ts

@@ -58,13 +58,6 @@ const THUMBNAIL_EXT = ".jpg";
 
 type Kind = "original" | "thumbnail";
 
-// An original write in progress, with the IDs of the concurrent original writes
-// it overlaps (recorded both ways as writes begin, cleared when the write ends).
-interface OriginalWrite {
-    fileID: number;
-    overlaps: Set<number>;
-}
-
 // The priority a caller attaches to a thumbnail prefetch. The pool has two
 // tiers, so this three-value surface collapses onto them: only a currently
 // visible thumbnail preempts (on-demand); "ahead" prefetch and speculative
@@ -255,13 +248,6 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
     // Serializes limit enforcement so concurrent original writes never race on
     // the map or delete each other's just-freed room.
     private enforcing: Promise<void> = Promise.resolve();
-    // Original writes in progress. Writes for different files run concurrently
-    // (the content pool), so an eviction pass must never delete a file whose
-    // fetch has not yet returned. Each entry records the IDs of the concurrent
-    // original writes it overlaps — noted both ways as writes begin — and a
-    // write's eviction pass spares them all. Bounded by the pool's concurrency,
-    // so eviction is never deferred beyond the active working set.
-    private readonly inFlightOriginals = new Set<OriginalWrite>();
 
     constructor(opts: ContentCacheOptions) {
         this.pools = opts.pools;
@@ -455,32 +441,23 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
                 // to completion.
                 if (signal?.aborted) throw new AbortDrop();
 
-                // Register this original among those in flight, linking it with
-                // every sibling already writing so neither evicts the other's
-                // file. Non-null iff this is an original.
-                const write =
-                    kind === "original"
-                        ? this.beginOriginalWrite(fileID)
-                        : null;
-                try {
-                    await this.download(file, dest, kind, opts?.onByte);
-                    await chmod(dest, FILE_MODE);
-                    const size = (await stat(dest)).size;
-                    if (size === 0) {
-                        throw new Error(
-                            `content cache: ${kind} ${fileID} stored empty`,
-                        );
-                    }
-                    known.set(fileID, dest);
-                    // A fresh original may have crossed the limit; make room by
-                    // evicting least-recently-used originals. An over-budget
-                    // fetch keeps the file it returns, and no overlapping
-                    // sibling is evicted. Thumbnails are never bounded.
-                    if (write) await this.enforceOriginalsLimit(write);
-                    return { path: dest, bytes: size, cached: false };
-                } finally {
-                    if (write) this.inFlightOriginals.delete(write);
+                await this.download(file, dest, kind, opts?.onByte);
+                await chmod(dest, FILE_MODE);
+                const size = (await stat(dest)).size;
+                if (size === 0) {
+                    throw new Error(
+                        `content cache: ${kind} ${fileID} stored empty`,
+                    );
                 }
+                known.set(fileID, dest);
+                // A fresh original may have crossed the limit; make room by
+                // evicting least-recently-used originals. The file just written
+                // is excluded from the candidates so an on-demand fetch that is
+                // itself over budget proceeds over-limit and returns a path that
+                // exists on disk. Thumbnails are never bounded or evicted.
+                if (kind === "original")
+                    await this.enforceOriginalsLimit(fileID);
+                return { path: dest, bytes: size, cached: false };
             },
             { priority, key: fileID },
         );
@@ -565,26 +542,13 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
         });
     }
 
-    // Record a starting original write among those in flight, linking it with
-    // every sibling already writing so neither can evict the other's file.
-    private beginOriginalWrite(fileID: number): OriginalWrite {
-        const write: OriginalWrite = { fileID, overlaps: new Set() };
-        for (const other of this.inFlightOriginals) {
-            write.overlaps.add(other.fileID);
-            other.overlaps.add(fileID);
-        }
-        this.inFlightOriginals.add(write);
-        return write;
-    }
-
-    // Evict least-recently-used originals until usage fits the limit. Skipped:
-    // pinned originals, the file `write` just stored, and every original whose
-    // write overlaps it (`write.overlaps`). The last two spare any fetch whose
-    // lifetime overlaps this one, so concurrent over-budget fetches all keep the
-    // paths they return; when only such originals remain the cache stays
-    // over-limit until they settle, and a later, non-overlapping write finds
-    // them eligible again.
-    private enforceOriginalsLimit(write: OriginalWrite): Promise<void> {
+    // Evict least-recently-used originals until usage fits the limit. Pinned
+    // originals are skipped, as is `justWrittenID` (the original this write just
+    // stored, so an over-budget on-demand fetch never evicts the file it is
+    // about to return). When only such originals remain the cache stays
+    // over-limit until the protected set shrinks; the skipped file becomes
+    // eligible on a later write.
+    private enforceOriginalsLimit(justWrittenID: number): Promise<void> {
         return this.serializeEnforce(async () => {
             const { entries, used } = await this.measureOriginals();
             const limit = await this.originalsLimit(used);
@@ -593,8 +557,7 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
                 const evictable = entries
                     .filter(
                         (e) =>
-                            e.fileID !== write.fileID &&
-                            !write.overlaps.has(e.fileID) &&
+                            e.fileID !== justWrittenID &&
                             !this.isPinned(e.fileID),
                     )
                     .sort((a, b) => a.mtimeMs - b.mtimeMs);

test/library/content-eviction.test.ts

@@ -70,40 +70,6 @@ class SizedSource implements ContentSource {
     }
 }
 
-// A source whose original() writes 10 bytes but blocks on a gate before
-// returning, so two fetches can be held in flight together. `bothStarted`
-// resolves once both originals have entered, and `release()` lets them finish.
-class GatedSource implements ContentSource {
-    private openGate!: () => void;
-    private readonly gate = new Promise<void>((r) => (this.openGate = r));
-    private inFlight = 0;
-    private reachedTwo!: () => void;
-    readonly bothStarted = new Promise<void>((r) => (this.reachedTwo = r));
-
-    async original(args: {
-        file: EnteFile;
-        destination: string;
-    }): Promise<{ bytesWritten: number }> {
-        this.inFlight += 1;
-        if (this.inFlight === 2) this.reachedTwo();
-        await this.gate;
-        await writeFile(args.destination, Buffer.alloc(10, 1));
-        return { bytesWritten: 10 };
-    }
-
-    async thumbnail(args: {
-        file: EnteFile;
-        destination: string;
-    }): Promise<{ bytesWritten: number }> {
-        await writeFile(args.destination, Buffer.alloc(10, 1));
-        return { bytesWritten: 10 };
-    }
-
-    release(): void {
-        this.openGate();
-    }
-}
-
 let root: string;
 let cacheDir: string;
 
@@ -311,35 +277,6 @@ describe("originals eviction", () => {
         );
     });
 
-    it("keeps both originals when two over-budget fetches race", async () => {
-        // A 15-byte cap holds one 10-byte original but not two. Two fetches are
-        // held in flight together; when both stored files cross the limit,
-        // neither eviction pass may delete the sibling whose path has not yet
-        // been returned, so both survive over-limit.
-        const source = new GatedSource();
-        const { cache } = buildCache({
-            source,
-            statfs: abundantFree,
-            cacheOriginalsMaxBytes: 15,
-            freeBelowBytes: 0,
-        });
-        await cache.open();
-
-        const p1 = cache.original(1);
-        const p2 = cache.original(2);
-        await source.bothStarted; // both downloads are in flight before either stores
-        source.release();
-        const [r1, r2] = await Promise.all([p1, p2]);
-
-        // Both returned paths exist on disk even though together they exceed the
-        // cap; nothing was evicted out from under a fetch still in progress.
-        expect(existsSync(r1.path)).toBe(true);
-        expect(existsSync(r2.path)).toBe(true);
-        expect(existsSync(originalPath(1))).toBe(true);
-        expect(existsSync(originalPath(2))).toBe(true);
-        expect(cache.originalsStatus().usedBytes).toBe(20);
-    });
-
     it("never counts or evicts thumbnails", async () => {
         const { cache } = buildCache({
             statfs: abundantFree,