decode once, encode seven times, and one file at a time

A HEIC was being decoded once per rung. av-club is stateless per request, so
seven rungs asked for from the original are seven ImageMagick conversions of it
— measured on fofos at 8.5 s each, against 1.1 s for the resize and encode that
follow. Six of the seven produced a pixel-identical intermediate and threw it
away: about 67 seconds per photograph where 16 would do, paid again on every
future upload.

So when av-club says a format needs converting, convert it ONCE into the staging
directory and cut every rung from that. `needsConversion` is asked rather than
guessed, because which formats are expensive is av-club's fact and a list of
them kept here would go on paying after its build stopped needing it. JPEG and
PNG are untouched — writing a full-resolution intermediate to save six cheap
reads would cost more than it saved.

Concurrency drops to one. Two was chosen to match av-club and is the right
number on a machine with room; fofos has 1.87 GiB of RAM, and two concurrent
12-megapixel decodes left 520 MiB free with 648 MB already paged out. The work
is CPU-bound on a box already at 99%, so the second worker was not buying
throughput — it was buying a second resident bitmap.

And the sharp calls now name their output format properly. `output` takes
`{ format, options }`; passing `{ webp: { … } }` was silently ignored, so
everything built before this encoded at sharp's default quality rather than the
82 asked for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

api/derive.ts

@@ -202,6 +202,16 @@ export async function deriveOne(
   }
 }
 
+/**
+ * The intermediate a ladder is built from, when reading the original repeatedly would be the expensive
+ * part.
+ *
+ * Never compressed — `compressionLevel: 0`. It lives for seconds inside the staging directory and is
+ * then deleted, so every cycle spent shrinking it is spent twice for nothing: once packing, once
+ * unpacking, seven times over.
+ */
+const MASTER = '.master.png';
+
 async function buildImage(
   sourcePath: string,
   staging: string,
@@ -209,7 +219,7 @@ async function buildImage(
   source: { bytes: number; mtimeMs: number },
   email: string,
 ): Promise<Manifest> {
-  const meta = await avClub<{ width: number | null; height: number | null }>(
+  const meta = await avClub<{ width: number | null; height: number | null; needsConversion: boolean }>(
     `/v1/image?path=${encodeURIComponent(sourcePath)}`,
     email,
   );
@@ -217,6 +227,33 @@ async function buildImage(
   const height = meta.height ?? 0;
   if (!width || !height) throw new Error(`Could not read the dimensions of ${name}.`);
 
+  /*
+    DECODE ONCE.
+
+    av-club is stateless per request, so seven rungs asked for from the original are seven decodes of
+    it. For a JPEG that is fine — sharp reads one cheaply, and writing a full-resolution intermediate
+    to save six cheap reads would cost more than it saved. For a HEIC it is not: every request spawns
+    ImageMagick to produce a full-resolution PNG, measured on fofos at 8.5 s against 1.1 s for the
+    resize and encode that follow. Seven rungs spent about a minute making the same intermediate over
+    and over and throwing six of them away.
+
+    So when av-club says this format needs converting, it is converted ONCE, here, into the staging
+    directory nobody else can see, and every rung is cut from that. 67 seconds becomes 16.
+
+    `needsConversion` is asked rather than guessed. Which formats are expensive is av-club's fact and
+    changes when its build changes; a list of them kept here would go on paying for a conversion that
+    had stopped being necessary.
+  */
+  const master = meta.needsConversion ? join(staging, MASTER) : sourcePath;
+  if (meta.needsConversion) {
+    await post('/v1/sharp', email, {
+      inputPath: sourcePath,
+      outputPath: master,
+      pipeline: [{ rotate: {} }],
+      output: { format: 'png', options: { compressionLevel: 0 } },
+    });
+  }
+
   const variants: Variant[] = [];
 
   /*
@@ -226,14 +263,17 @@ async function buildImage(
     exists at all — the original renders nowhere. A lightbox, a zoom, or handing the client their own
     photograph back all need it.
   */
-  variants.push(await encodeWebp(sourcePath, join(staging, 'full.webp'), null, email, width, height));
+  variants.push(await encodeWebp(master, join(staging, 'full.webp'), null, email, width, height));
 
   // Strictly less than the original: a rung at or above it would duplicate `full.webp` byte for byte.
   for (const rung of LADDER) {
     if (rung >= width) continue;
-    variants.push(await encodeWebp(sourcePath, join(staging, `${rung}.webp`), rung, email, width, height));
+    variants.push(await encodeWebp(master, join(staging, `${rung}.webp`), rung, email, width, height));
   }
 
+  // Before the manifest is written, so a directory that has one never contains this.
+  if (meta.needsConversion) await rm(master, { force: true });
+
   return {
     kind: 'image',
     source: { name, bytes: source.bytes, mtimeMs: source.mtimeMs, width, height },
@@ -254,12 +294,14 @@ async function encodeWebp(
     inputPath: sourcePath,
     outputPath,
     // `rotate` with no angle applies the EXIF orientation, which phone photographs rely on entirely. A
-    // ladder built without it is a set of sideways pictures.
+    // ladder built without it is a set of sideways pictures. Kept on every rung including those cut
+    // from the master, where it is a no-op — a PNG carries no EXIF, and the master was written with the
+    // rotation already applied. One code path beats two that can disagree about which way up a photo is.
     pipeline: [
       { rotate: {} },
       ...(rung === null ? [] : [{ resize: { width: rung, fit: 'inside', withoutEnlargement: true } }]),
     ],
-    output: { webp: { quality: WEBP_QUALITY } },
+    output: { format: 'webp', options: { quality: WEBP_QUALITY } },
   });
 
   const width = rung ?? sourceWidth;
@@ -319,7 +361,7 @@ async function buildVideo(
   const poster = await post<{ path: string; bytes: number }>('/v1/sharp', email, {
     inputPath: stillJpg,
     outputPath: join(staging, 'poster.webp'),
-    output: { webp: { quality: WEBP_QUALITY } },
+    output: { format: 'webp', options: { quality: WEBP_QUALITY } },
   });
   await rm(stillJpg, { force: true });
   variants.push({ file: 'poster.webp', width: null, height: null, bytes: poster.bytes });