Share the link-preview fallback between the Meta adapters

Instagram and Threads both ended with the same twelve lines: read the Open
Graph tags, make og:video a video with og:image as its poster, otherwise make
og:image an image, and attach the referrer headers the CDN wants. Facebook
needs the same floor for the same reason, and three copies of a thing is where
it stops being a coincidence.

meta-media.ts already exists because these platforms are one product
underneath. The media schema it normalizes is Instagram's and Threads';
Facebook's is its own, but the headers and the link preview are shared with
both, so that is where this goes.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/platforms/instagram.ts

@@ -1,6 +1,12 @@
 import { ResolveError, oneSegment, type Media, type Post } from '../types.ts';
 import { readOpenGraph } from '../browser/capture.ts';
-import { findCaption, findMetaMedia, mediaFromMetaNode, metaHeaders } from './meta-media.ts';
+import {
+  findCaption,
+  findMetaMedia,
+  mediaFromMetaNode,
+  mediaFromOpenGraph,
+  metaHeaders,
+} from './meta-media.ts';
 import { extractJsonObjectAfterKey } from './scan.ts';
 import { hostMatcher } from './url.ts';
 import type { PlatformSpec, ResolveContext } from './types.ts';
@@ -202,22 +208,8 @@ function fromPayloads(texts: string[]): Scraped | undefined {
 /** Absolute floor: whatever the page offers a link preview. */
 async function fromOpenGraph(ctx: ResolveContext): Promise<Scraped> {
   const og = await readOpenGraph(ctx.page);
-  const fetchHeaders = metaHeaders(REFERER);
-  const media: Media[] = [];
-
-  if (og['og:video']) {
-    media.push({
-      kind: 'video',
-      url: og['og:video'],
-      fetchHeaders,
-      ...(og['og:image'] ? { poster: { url: og['og:image'], fetchHeaders } } : {}),
-    });
-  } else if (og['og:image']) {
-    media.push({ kind: 'image', url: og['og:image'], fetchHeaders });
-  }
-
   return {
-    media,
+    media: mediaFromOpenGraph(og, REFERER),
     ...(og['og:description'] ? { text: og['og:description'] } : {}),
   };
 }

src/platforms/meta-media.ts

@@ -1,8 +1,10 @@
 import type { Media } from '../types.ts';
 
 /**
- * Instagram and Threads are the same product underneath and serve the same
- * media schema, so both adapters normalize through here.
+ * Instagram, Threads and Facebook are the same product underneath. The first
+ * two serve the same media schema, so both normalize through here; Facebook's
+ * is its own, but the headers its CDN wants and the link preview it falls
+ * back to are shared with them.
  */
 
 type Candidate = { url?: string; width?: number; height?: number };
@@ -66,6 +68,27 @@ export function mediaFromMetaNode(node: MetaMediaNode, referer: string): Media[]
   return [];
 }
 
+/**
+ * The link preview as media: the floor on all three, and all that is left
+ * when a post is refused to a logged-out reader.
+ */
+export function mediaFromOpenGraph(og: Record<string, string>, referer: string): Media[] {
+  const fetchHeaders = metaHeaders(referer);
+
+  if (og['og:video']) {
+    return [{
+      kind: 'video',
+      url: og['og:video'],
+      fetchHeaders,
+      ...(og['og:image'] ? { poster: { url: og['og:image'], fetchHeaders } } : {}),
+    }];
+  }
+  if (og['og:image']) {
+    return [{ kind: 'image', url: og['og:image'], fetchHeaders }];
+  }
+  return [];
+}
+
 function nonEmptyArray(value: unknown): boolean {
   return Array.isArray(value) && value.length > 0;
 }

src/platforms/threads.ts

@@ -1,6 +1,12 @@
-import { ResolveError, type Media, type Post, type Segment } from '../types.ts';
+import { ResolveError, type Post, type Segment } from '../types.ts';
 import { readOpenGraph } from '../browser/capture.ts';
-import { findCaption, findMetaMedia, mediaFromMetaNode, metaHeaders, type MetaMediaNode } from './meta-media.ts';
+import {
+  findCaption,
+  findMetaMedia,
+  mediaFromMetaNode,
+  mediaFromOpenGraph,
+  type MetaMediaNode,
+} from './meta-media.ts';
 import { hostMatcher } from './url.ts';
 import type { PlatformSpec, ResolveContext } from './types.ts';
 
@@ -129,19 +135,7 @@ function segmentOf(post: ThreadsPost, isAnchor: boolean): Segment {
 /** The floor, when the structured payload is not there to be read. */
 async function fromOpenGraph(ctx: ResolveContext): Promise<Segment | undefined> {
   const og = await readOpenGraph(ctx.page);
-  const fetchHeaders = metaHeaders(REFERER);
-  const media: Media[] = [];
-
-  if (og['og:video']) {
-    media.push({
-      kind: 'video',
-      url: og['og:video'],
-      fetchHeaders,
-      ...(og['og:image'] ? { poster: { url: og['og:image'], fetchHeaders } } : {}),
-    });
-  } else if (og['og:image']) {
-    media.push({ kind: 'image', url: og['og:image'], fetchHeaders });
-  }
+  const media = mediaFromOpenGraph(og, REFERER);
 
   const text = og['og:description'];
   if (media.length === 0 && !text) return undefined;