Refactor decryptLegacyDm to use ordered parallel race for decryptors.

Replaces Promise.any with raceOrdered utility to respect decryptor preference order while maintaining parallel execution.
Adds regression test for order preference.

Co-authored-by: PR0M3TH3AN <[email protected]>

js/dmDecryptor.js

@@ -467,6 +467,56 @@ async function decryptGiftWrap(event, decryptors, actorPubkey) {
   });
 }
 
+function raceOrdered(promises) {
+  if (!promises || promises.length === 0) {
+    return Promise.reject(new AggregateError([], "No promises to race"));
+  }
+
+  return new Promise((resolve, reject) => {
+    let pendingCount = promises.length;
+    const errors = new Array(promises.length);
+    const results = new Array(promises.length);
+    const states = new Array(promises.length).fill("pending");
+    let scanIndex = 0;
+
+    promises.forEach((p, i) => {
+      Promise.resolve(p).then(
+        (val) => {
+          states[i] = "fulfilled";
+          results[i] = val;
+
+          if (i === scanIndex) {
+            resolve(val);
+          }
+        },
+        (err) => {
+          states[i] = "rejected";
+          errors[i] = err;
+          pendingCount--;
+
+          if (i === scanIndex) {
+            while (scanIndex < promises.length) {
+              if (states[scanIndex] === "fulfilled") {
+                resolve(results[scanIndex]);
+                return;
+              }
+              if (states[scanIndex] === "rejected") {
+                scanIndex++;
+              } else {
+                return;
+              }
+            }
+          }
+
+          if (pendingCount === 0 && scanIndex >= promises.length) {
+            reject(new AggregateError(errors, "All promises rejected"));
+          }
+        },
+      );
+    });
+  });
+}
+
 async function decryptLegacyDm(event, decryptors, actorPubkey) {
   const ciphertext = typeof event?.content === "string" ? event.content : "";
   const senderPubkey = normalizeHex(event?.pubkey);
@@ -564,7 +614,7 @@ async function decryptLegacyDm(event, decryptors, actorPubkey) {
   // Race all decryption attempts to return the first successful result
   // This significantly reduces latency compared to sequential attempts
   try {
-    return await Promise.any(attempts);
+    return await raceOrdered(attempts);
   } catch (aggregateError) {
     const aggregatedErrors = aggregateError.errors || [];
     errors.push(...aggregatedErrors);

tests/dm-decryptor-order.test.mjs

@@ -0,0 +1,57 @@
+
+import assert from "node:assert/strict";
+import test from "node:test";
+import { decryptDM } from "../js/dmDecryptor.js";
+
+const SENDER_HEX = "a".repeat(64);
+const RECIPIENT_HEX = "b".repeat(64);
+
+test("decryptDM prefers higher priority decryptor even if slower", async () => {
+    // Mock event
+    const event = {
+        kind: 4,
+        content: "ciphertext",
+        pubkey: SENDER_HEX,
+        tags: [["p", RECIPIENT_HEX]]
+    };
+
+    const context = {
+        actorPubkey: RECIPIENT_HEX,
+        decryptors: [
+            {
+                scheme: "preferred",
+                priority: 0,
+                source: "slow-preferred",
+                decrypt: async (remote, cipher) => {
+                    // Normalize remote key might be done inside decryptDM
+                    if (remote === SENDER_HEX) {
+                        await new Promise(r => setTimeout(r, 100));
+                        return "preferred-plaintext";
+                    }
+                    throw new Error("wrong remote: " + remote);
+                }
+            },
+            {
+                scheme: "fallback",
+                priority: 10,
+                source: "fast-fallback",
+                decrypt: async (remote, cipher) => {
+                    if (remote === SENDER_HEX) {
+                         // Fast!
+                        return "fallback-plaintext";
+                    }
+                     throw new Error("wrong remote: " + remote);
+                }
+            }
+        ]
+    };
+
+    const result = await decryptDM(event, context);
+
+    assert.equal(result.ok, true, "Result should be ok. Errors: " + JSON.stringify(result.errors));
+
+    // If Promise.any is used, it will likely pick "fallback-plaintext" (fast).
+    // We WANT "preferred-plaintext".
+    assert.equal(result.plaintext, "preferred-plaintext",
+        `Expected preferred-plaintext but got ${result.plaintext}. Current implementation likely uses Promise.any which is sensitive to race conditions.`);
+});