findings Q-2: a non-target sibling fetch failure no longer aborts the open

syncProjectToMemfs rethrew the first rejected fetchBytes whatever file it
was, so one missing/unreadable sibling (a .kicad_sch body gone, a Q-1
phantom row, a transient 5xx) took the whole board open down behind
"download failed (404)". Now only the TARGET's failure rejects; siblings
are logged, counted and reported through onStatus, and KiCad reports a
missing sheet itself. kicad-runner.test.ts pinned the old contract with a
non-target file — corrected to a target; new kicad-runner.findings-q.test
gates both branches.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D9KFksoViNPYYs1ygkcAoQ

web/standalone/src/wasm/kicad-runner.findings-q.test.ts

@@ -0,0 +1,95 @@
+import { describe, expect, it } from "vitest";
+import { driveProjectIntoTool, type DriveOptions } from "./kicad-runner";
+import { memfsFilePath } from "./constants";
+
+/**
+ * Findings Q-2 (docs/features/findings/groups/Q-…): a 404 on a NON-TARGET
+ * sibling must degrade, not abort the open. Today `syncProjectToMemfs`
+ * rethrows the first rejected fetch, so one missing `.kicad_sch` beside the
+ * board (or a Q-1 phantom row the listing invented) takes the whole editor
+ * boot down behind a "download failed (404)" that reads as corruption.
+ * The TARGET file failing must still reject — there is nothing to open.
+ */
+
+interface FakeFS {
+  files: Map<string, Uint8Array>;
+}
+
+function fakeWin(): { win: Parameters<typeof driveProjectIntoTool>[0]; fs: FakeFS } {
+  const fs: FakeFS = { files: new Map() };
+  const win = {
+    FS: {
+      writeFile: (path: string, data: Uint8Array) => {
+        fs.files.set(path, data);
+      },
+      mkdirTree: () => {},
+      readFile: () => new Uint8Array(),
+      analyzePath: () => ({ exists: false }),
+    },
+  } as unknown as Parameters<typeof driveProjectIntoTool>[0];
+  return { win, fs };
+}
+
+function opts(
+  files: string[],
+  fetchBytes: DriveOptions["fetchBytes"],
+  extra: Partial<DriveOptions> = {},
+): DriveOptions {
+  return {
+    tool: "pcbnew",
+    slug: "proj",
+    files: files.map((path) => ({ path })),
+    fetchBytes,
+    log: () => {},
+    onStatus: () => {},
+    ...extra,
+  };
+}
+
+describe("Q-2 · sibling fetch failures during MEMFS staging", () => {
+  it("a 404 on a non-target sibling is logged and skipped; every other file still stages", async () => {
+    const { win, fs } = fakeWin();
+    const files = ["board.kicad_pcb", "board.kicad_sch", "sub sheet.kicad_sch"];
+    const logs: string[] = [];
+    // No targetPath → the run stops after staging (no open flow), like the
+    // sibling tests in kicad-runner.test.ts. The failing file is NOT the
+    // target here either way.
+    await driveProjectIntoTool(
+      win,
+      opts(
+        files,
+        async (p) => {
+          if (p === "sub sheet.kicad_sch") {
+            throw new Error(`download failed (404): ${p}`);
+          }
+          return new TextEncoder().encode(`body:${p}`);
+        },
+        { log: (m) => logs.push(m) },
+      ),
+    );
+
+    expect(fs.files.has(memfsFilePath("proj", "board.kicad_pcb"))).toBe(true);
+    expect(fs.files.has(memfsFilePath("proj", "board.kicad_sch"))).toBe(true);
+    expect(fs.files.has(memfsFilePath("proj", "sub sheet.kicad_sch"))).toBe(false);
+    expect(logs.some((l) => l.includes("sub sheet.kicad_sch") && l.includes("404"))).toBe(
+      true,
+    );
+  });
+
+  it("the target file failing still rejects (nothing to open)", async () => {
+    const { win } = fakeWin();
+    await expect(
+      driveProjectIntoTool(
+        win,
+        opts(
+          ["board.kicad_pcb", "board.kicad_sch"],
+          async (p) => {
+            if (p === "board.kicad_pcb") throw new Error(`download failed (404): ${p}`);
+            return new TextEncoder().encode(`body:${p}`);
+          },
+          { targetPath: "board.kicad_pcb" },
+        ),
+      ),
+    ).rejects.toThrow("board.kicad_pcb");
+  });
+});

web/standalone/src/wasm/kicad-runner.test.ts

@@ -112,18 +112,18 @@ describe("MEMFS project staging", () => {
     expect(seen.at(-1)?.[1]).toBe(files.length);
   });
 
-  it("rejects when a fetch fails", async () => {
+  it("rejects when the TARGET's fetch fails (siblings: see kicad-runner.findings-q.test)", async () => {
     const { win } = fakeWin();
-    const files = ["ok1.kicad_sym", "bad.kicad_sym", "ok2.kicad_sym"];
+    const files = ["ok1.kicad_sym", "bad.kicad_pcb", "ok2.kicad_sym"];
 
     await expect(
-      driveProjectIntoTool(
-        win,
-        opts(files, async (p) => {
-          if (p === "bad.kicad_sym") throw new Error("fetch exploded");
+      driveProjectIntoTool(win, {
+        ...opts(files, async (p) => {
+          if (p === "bad.kicad_pcb") throw new Error("fetch exploded");
           return new Uint8Array([1]);
         }),
-      ),
+        targetPath: "bad.kicad_pcb",
+      }),
     ).rejects.toThrow("fetch exploded");
   });
 });

web/standalone/src/wasm/kicad-runner.ts

@@ -191,20 +191,41 @@ async function syncProjectToMemfs(win: ToolWindow, opts: DriveOptions): Promise<
     : Promise.resolve(opts.files));
 
   const queue = [...perFile];
+  const skipped: string[] = [];
   const worker = async (): Promise<void> => {
     for (let file = queue.shift(); file; file = queue.shift()) {
-      const bytes = await opts.fetchBytes(file.path);
+      let bytes: Uint8Array;
+      try {
+        bytes = await opts.fetchBytes(file.path);
+      } catch (err) {
+        // Findings Q-2: only the TARGET is load-bearing — without its bytes
+        // there is nothing to open, so that failure still rejects below. A
+        // sibling that cannot be fetched (a schematic whose body is gone, a
+        // stale listing row, a transient 5xx) must NOT abort the open of an
+        // unrelated board: log it, count it, and let KiCad report the missing
+        // sheet through its own dialog if it ever needs it.
+        if (file.path === opts.targetPath) throw err;
+        skipped.push(file.path);
+        opts.log(`[stage] skipped ${file.path}: ${String(err)}`);
+        opts.onFileProgress?.(++staged, opts.files.length);
+        continue;
+      }
       stageOne(file.path, bytes);
     }
   };
-  // One rejection fails the stage (same as the serial loop did), but let the
-  // in-flight siblings settle first so a failure can't leave a fetch writing
-  // into MEMFS after the caller has moved on.
+  // A target rejection fails the stage, but let the in-flight siblings settle
+  // first so a failure can't leave a fetch writing into MEMFS after the caller
+  // has moved on.
   const results = await Promise.allSettled(
     Array.from({ length: Math.min(STAGE_CONCURRENCY, queue.length) }, worker),
   );
   const failed = results.find((r) => r.status === "rejected");
   if (failed) throw (failed as PromiseRejectedResult).reason;
+  if (skipped.length) {
+    opts.onStatus(
+      `${skipped.length} project file(s) could not be loaded: ${skipped.join(", ")}`,
+    );
+  }
 }
 
 /**