fix(context-bill): --exact refuses a walked file swapped for a FIFO instead of blocking on open

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

lib/context-bill.ts

@@ -891,13 +891,14 @@ export function exactSendSet(root: string): ExactSendSet {
  * following a final link, and only if it is still the same regular file at
  * the same realpath. A link retargeted, a file replaced, or a parent directory
  * swapped for a link after the walk is refused, so nothing the bill did not
- * walk can be sent.
+ * walk can be sent. The open does not block, so a file swapped for a FIFO is
+ * refused as not a regular file instead of hanging the run.
  */
 function readWalkedFile(real: string, dev: number, ino: number, file: string, root: string): string {
   const changed = () => new Error(`${file} changed after it was walked; nothing from ${root} was sent. Run again.`);
   let fd: number;
   try {
-    fd = fs.openSync(real, fs.constants.O_RDONLY | (fs.constants.O_NOFOLLOW ?? 0));
+    fd = fs.openSync(real, fs.constants.O_RDONLY | (fs.constants.O_NOFOLLOW ?? 0) | (fs.constants.O_NONBLOCK ?? 0));
   } catch {
     throw changed();
   }

test/context-bill.test.ts

@@ -16,6 +16,7 @@
  */
 import { describe, it, expect, beforeAll, afterAll } from "bun:test";
 import { canRevokeWrites } from "./helpers/fs-caps";
+import { bunIsolationArgs } from "../lib/bun-isolation";
 import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
@@ -1194,6 +1195,36 @@ describe("--exact (opt-in measurement; offline here via an injected fetch)", ()
     expect(sameFileMoved.err).toContain("changed after it was walked");
   });
 
+  it("refuses a walked file swapped for a FIFO instead of hanging", () => {
+    if (process.platform === "win32") return;
+    const tmp = fs.mkdtempSync(path.join(os.tmpdir(), "context-bill-fifo-"));
+    const skills = path.join(tmp, "skills");
+    fs.mkdirSync(path.join(skills, "qa"), { recursive: true });
+    fs.writeFileSync(path.join(skills, "qa", "SKILL.md"), "---\nname: qa\ndescription: d\n---\n# qa\n");
+    fs.writeFileSync(path.join(skills, "qa", "a.md"), "A");
+    const script = path.join(tmp, "probe.ts");
+    fs.writeFileSync(
+      script,
+      [
+        `import fs from "node:fs";`,
+        `import { spawnSync } from "node:child_process";`,
+        `import { exactSendSet, measureExactTokens } from ${JSON.stringify(path.join(ROOT, "lib", "context-bill.ts"))};`,
+        `const root = ${JSON.stringify(skills)};`,
+        `const sendSet = exactSendSet(root);`,
+        `fs.unlinkSync(${JSON.stringify(path.join(skills, "qa", "a.md"))});`,
+        `spawnSync("mkfifo", [${JSON.stringify(path.join(skills, "qa", "a.md"))}], { timeout: 5_000 });`,
+        `const fetchImpl = (async () => ({ ok: true, json: async () => ({ input_tokens: 9 }) })) as never;`,
+        `try { await measureExactTokens(root, { sendSet, model: "m", apiKey: "sk-test", fetchImpl, egressHome: ${JSON.stringify(tmp)} }); console.log("SENT"); }`,
+        `catch (error) { console.log("REFUSED " + (error as Error).message); }`,
+      ].join("\n"),
+    );
+    const result = Bun.spawnSync([process.execPath, ...bunIsolationArgs(), script], { timeout: 15_000 });
+    fs.rmSync(tmp, { recursive: true, force: true });
+    expect(result.exitCode).toBe(0);
+    expect(result.stdout.toString()).toContain("REFUSED");
+    expect(result.stdout.toString()).toContain("changed after it was walked");
+  });
+
   it("sends files in the root and the linked gstack checkout, and refuses and names anything else a link reaches", async () => {
     if (process.platform === "win32") return;
     const tmp = fs.mkdtempSync(path.join(os.tmpdir(), "context-bill-egress-"));