Make JS source-map symbolication race-safe and run it in parallel with native

Refactor the source-map worker to return position-keyed resolutions instead
of full table snapshots. The main thread applies the response against the
current shared state via applySourceMapSymbolicationResponse, allocating
sourceMapInfo rows, interning names, and deduping URLs against whatever
native symbolication has committed by then.

With per-funcIndex/per-frameIndex idempotency (skip rows already populated)
and JS funcs/frames being insulated from native symbolication, the worker
no longer needs to wait for native symbolication to finish. finalizeProfileView
now runs source-map fetch, native symbolication, and the source-map worker
all in parallel.

Author: canova

src/actions/receive-profile.ts

@@ -248,19 +248,19 @@ export function finalizeProfileView(
       }
     }
 
-    // Fetch source maps for all JS sources with a sourceMapURL, then apply JS
-    // symbolication. Fetching runs in parallel with native symbolication, but
-    // the worker is only dispatched after native symbolication has committed
-    // its Redux changes, so neither clobbers the other's funcTable updates.
-    // Requires WebChannel version 7+.
+    // Fetch source maps for all JS sources with a sourceMapURL, then run the
+    // source-map worker. Runs fully in parallel with native symbolication:
+    // native only touches funcs/frames belonging to library resources (JS
+    // funcs aren't in those sets), and the JS apply step reads current
+    // shared state at dispatch time so it composes with whatever native
+    // has committed by then. Requires WebChannel version 7+.
     let sourceMapSymbolicationPromise: Promise<void> | null = null;
     if (browserConnection !== null && browserConnection.supportsGetSourceMap) {
-      // Fetch source maps concurrently with native symbolication. Once both
-      // have completed, apply JS symbolication on top of the final state.
-      sourceMapSymbolicationPromise = Promise.all([
-        doResolveSourceMaps(profile, browserConnection, dispatch),
-        symbolicationPromise,
-      ]).then(([{ resolvedSourceMaps, compiledSources }]) =>
+      sourceMapSymbolicationPromise = doResolveSourceMaps(
+        profile,
+        browserConnection,
+        dispatch
+      ).then(({ resolvedSourceMaps, compiledSources }) =>
         dispatch(doSourceMapSymbolication(resolvedSourceMaps, compiledSources))
       );
     }

src/actions/source-map-symbolication.ts

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 import { getRawProfileSharedData } from 'firefox-profiler/selectors';
+import { applySourceMapSymbolicationResponse } from 'firefox-profiler/profile-logic/source-map-symbolication';
 
 import type {
   WorkerInput,
@@ -49,16 +50,29 @@ export function doSourceMapSymbolication(
     dispatch({ type: 'START_SOURCE_MAP_SYMBOLICATION' });
     const result = await _runSourceMapWorker(input);
     switch (result.type) {
-      case 'success':
+      case 'success': {
+        // Apply against the current shared state (not the snapshot the worker
+        // received), so concurrent worker runs compose instead of stomping
+        // each other's results.
+        const currentShared = getRawProfileSharedData(getState());
+        const applied = applySourceMapSymbolicationResponse(
+          currentShared,
+          result.response
+        );
+        if (applied === null) {
+          dispatch({ type: 'SOURCE_MAP_SYMBOLICATION_FAILED' });
+          break;
+        }
         dispatch({
           type: 'BULK_SOURCE_MAP_SYMBOLICATION',
-          newFuncTable: result.newFuncTable,
-          newFrameTable: result.newFrameTable,
-          newSourceLocationTable: result.newSourceLocationTable,
-          newSources: result.newSources,
-          newStringArray: result.newStringArray,
+          newFuncTable: applied.newFuncTable,
+          newFrameTable: applied.newFrameTable,
+          newSourceLocationTable: applied.newSourceLocationTable,
+          newSources: applied.newSources,
+          newStringArray: applied.newStringArray,
         });
         break;
+      }
       case 'error':
         console.warn('Source map worker error:', result.message);
         dispatch({ type: 'SOURCE_MAP_SYMBOLICATION_FAILED' });
@@ -79,26 +93,14 @@ export function doSourceMapSymbolication(
  *
  * Debugging tip: to step through symbolication from the page DevTools, paste
  * the snippet below over this function body. It runs the same core on the
- * main thread (blocking, debug only).
+ * main thread (blocking, debug only). The worker no longer mutates its
+ * input, so no defensive cloning is needed here.
  *
  *   const { runSourceMapSymbolicationCore } = await import(
  *     'firefox-profiler/profile-logic/source-map-symbolication'
  *   );
- *   const sources = {
- *     length: input.sources.length,
- *     id: input.sources.id.slice(),
- *     filename: input.sources.filename.slice(),
- *     startLine: input.sources.startLine.slice(),
- *     startColumn: input.sources.startColumn.slice(),
- *     sourceMapURL: input.sources.sourceMapURL.slice(),
- *     content: input.sources.content.slice(),
- *   };
- *   const stringArray = input.stringArray.slice();
  *   const wasmUrl = new URL('/mappings.wasm', window.location.href).href;
- *   return runSourceMapSymbolicationCore(
- *     { ...input, sources, stringArray },
- *     wasmUrl
- *   );
+ *   return runSourceMapSymbolicationCore(input, wasmUrl);
  */
 function _runSourceMapWorker(input: WorkerInput): Promise<WorkerOutput> {
   return new Promise((resolve) => {