Each time we run a loader, save the loader information to the cache. …

…Using a big JSON file for now, but we might want to use a sqlite db instead to solve the race conditions.

The loader information will be used to complete the build manifest.

src/loader.ts

@@ -1,13 +1,14 @@
 import {createHash} from "node:crypto";
 import type {FSWatcher, WatchListener, WriteStream} from "node:fs";
 import {createReadStream, existsSync, statSync, watch} from "node:fs";
-import {open, readFile, rename, unlink} from "node:fs/promises";
+import type {FileHandle} from "node:fs/promises";
+import {open, readFile, rename, unlink, writeFile} from "node:fs/promises";
 import {dirname, extname, join} from "node:path/posix";
 import {createGunzip} from "node:zlib";
 import {spawn} from "cross-spawn";
 import JSZip from "jszip";
 import {extract} from "tar-stream";
-import {enoent} from "./error.js";
+import {enoent, isEnoent} from "./error.js";
 import {maybeStat, prepareOutput, visitFiles} from "./files.js";
 import {FileWatchers} from "./fileWatchers.js";
 import {formatByteSize} from "./format.js";
@@ -463,12 +464,57 @@ abstract class AbstractLoader implements Loader {
         effects.logger.log(`${red("error")} ${faint(`in ${formatElapsed(start)}:`)} ${red(error.message)}`);
       }
     );
+    command.then(() => saveLoadInfo(this));
     return command;
   }
 
   abstract exec(output: WriteStream, options?: LoadOptions, effects?: LoadEffects): Promise<void>;
 }
 
+/**
+ * Saves the loaders information for the *targetPath* page, module or file, to
+ * the cache. The information will be added to the build manifest.
+ */
+async function saveLoadInfo({
+  root,
+  targetPath,
+  path: loader,
+  params
+}: {
+  root: string;
+  targetPath: string;
+  loader: string;
+  params?: {[key: string]: string};
+}) {
+  let buildInfo: {[path: string]: {loader: string; params?: {[key: string]: string}}} = {};
+  const cachePath = join(root, ".observablehq", "cache", "_loaders.json");
+  const lock = `${cachePath}.lock`;
+  let handle: FileHandle | null = null;
+  let retry = 10;
+  await prepareOutput(cachePath);
+  do {
+    if (--retry === 0) throw new Error(`Could not acquire ${lock}`);
+    try {
+      handle = await open(lock, "wx");
+    } catch (error) {
+      if (error?.["code"] !== "EEXIST") throw error;
+      await new Promise((resolve) => setTimeout(resolve, 100 * (1 + Math.random())));
+    }
+  } while (!handle);
+  try {
+    buildInfo = JSON.parse(await readFile(cachePath, "utf-8"));
+  } catch (error) {
+    if (!isEnoent(error)) throw error;
+  } finally {
+    buildInfo[targetPath] = {loader, params};
+    try {
+      await writeFile(cachePath, JSON.stringify(buildInfo), "utf-8");
+    } finally {
+      await unlink(lock);
+    }
+  }
+}
+
 interface CommandLoaderOptions extends LoaderOptions {
   command: string;
   args: string[];