fix(secrets): read the age identity once per process so <(...) keys survive --all (#36)

A process substitution (`CAST_AGE_KEY_FILE_<ENV>=<(pm read ...)`) is a
read-once pipe, but `diff --all` / `apply --all` call decryptSecrets once
per project. The first project drained the pipe; every later project
re-read the key file, handed age an empty identity, and failed — the
fleet loop then misreported the project as unreachable (diff) or aborted
the fleet (apply). Latent today because only one registered project has
a prod store; real the moment a second one gains one.

Cache the key bytes by key path, module-level, so the identity is read
exactly once per process. Exposure is unchanged: the key already
transits this process's memory on every call.

The regression test uses a FIFO, which really drains — unlike the
existing /proc/self/fd test, whose regular file re-opens at offset 0 on
every read. A second writer serves emptiness after the first decrypt so
a regression fails loudly (age: no secret keys found) instead of
blocking the suite on a writerless FIFO open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/secrets.ts

@@ -9,12 +9,30 @@ import { join } from "node:path";
 // holding the fd — this one. A freshly spawned age has no such fd and fails
 // with ENOENT. Not `-i /dev/stdin` either: node closes the pipe before age
 // re-opens it by path (ENXIO); `-` makes age read the inherited fd directly.
+//
+// And it is read exactly ONCE per process: a process substitution is a
+// read-once pipe, but `diff --all` / `apply --all` call decryptSecrets once
+// per project. The first read drains the pipe; a re-read would hand age an
+// empty identity and the second project's decrypt would fail. Caching by key
+// path changes nothing about exposure — the key already transits this
+// process's memory on every call.
+const identities = new Map<string, Buffer>();
+
+function readIdentity(keyFile: string): Buffer {
+  let identity = identities.get(keyFile);
+  if (identity === undefined) {
+    identity = readFileSync(keyFile);
+    identities.set(keyFile, identity);
+  }
+  return identity;
+}
+
 export function decryptSecrets(
   file: string,
   keyFile: string,
 ): Record<string, string> {
   const out = execFileSync("age", ["-d", "-i", "-", file], {
-    input: readFileSync(keyFile),
+    input: readIdentity(keyFile),
     encoding: "utf8",
   });
   const secrets: Record<string, string> = {};

test/secrets.test.ts

@@ -1,4 +1,4 @@
-import { execFileSync } from "node:child_process";
+import { execFileSync, spawn } from "node:child_process";
 import {
   closeSync,
   mkdirSync,
@@ -62,6 +62,38 @@ describe("secretsFileFor", () => {
   });
 });
 
+// Pins the read-once shape of `<(pm read …)` that the fd-path test above
+// cannot: a regular file behind /proc/self/fd re-opens at offset 0 on every
+// read, but a pipe drains. `diff --all` / `apply --all` decrypt once per
+// project, so the identity must be read once per process and reused.
+describe("decryptSecrets identity caching", () => {
+  it("a read-once pipe key survives two decrypts — the --all loop shape", () => {
+    const { keyFile, enc } = ageFixture();
+    const dir = mkdtempSync(join(tmpdir(), "infra-fifo-"));
+    const fifo = join(dir, "key.fifo");
+    execFileSync("mkfifo", [fifo]);
+    // One writer, one serving of the key: exactly what a process substitution
+    // delivers. It pairs with the first decrypt's open and exits.
+    const once = spawn("sh", ["-c", `cat "${keyFile}" > "${fifo}"`], {
+      stdio: "ignore",
+    });
+    const expected = { MAILGUN_KEY: "mk-123", OPENROUTER_KEY: "or-456" };
+    expect(decryptSecrets(enc, fifo)).toEqual(expected);
+    // The pipe is now drained. A second writer serves nothing, so if the
+    // per-process cache ever regresses, the re-read hands age an empty
+    // identity and fails loudly instead of blocking the suite on a
+    // writerless FIFO open. With the cache, nobody opens the FIFO again and
+    // the writer is still blocked in open() when we kill it.
+    const drained = spawn("sh", ["-c", `: > "${fifo}"`], { stdio: "ignore" });
+    try {
+      expect(decryptSecrets(enc, fifo)).toEqual(expected);
+    } finally {
+      once.kill("SIGKILL");
+      drained.kill("SIGKILL");
+    }
+  });
+});
+
 describe("keyFileFor", () => {
   it("an env with no injected var and no standing key refuses, naming both ways in", () => {
     Reflect.deleteProperty(process.env, "CAST_AGE_KEY_FILE_PROD");