From b19684fde92aafa67577429552ae6c39f59b145c Mon Sep 17 00:00:00 2001
From: Nicholas Molnar <65710+neekolas@users.noreply.github.com>
Date: Mon, 12 Oct 2026 17:53:58 -0700
Subject: [PATCH] Avoid duplicate browser migration archive copy

---
 crates/xmtp_legacy_migration/src/wasm.rs             |  5 ++++-
 .../test/platform/migration/archive-read-limit.mjs   | 12 +++++++++++-
 .../test/platform/migration/read-limit-worker.mjs    |  8 ++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/crates/xmtp_legacy_migration/src/wasm.rs b/crates/xmtp_legacy_migration/src/wasm.rs
index b904f7a29c..90f4071594 100644
--- a/crates/xmtp_legacy_migration/src/wasm.rs
+++ b/crates/xmtp_legacy_migration/src/wasm.rs
@@ -144,5 +144,8 @@ pub(crate) async fn read(path: &str) -> Result<Vec<u8>, MigrationError> {
     let bytes = read_output(path)
         .await
         .map_err(|value| MigrationError::Output(OutputError::Browser { value }))?;
-    Ok(js_sys::Uint8Array::new(&bytes).to_vec())
+    Ok(bytes
+        .dyn_into::<js_sys::Uint8Array>()
+        .map_err(output)?
+        .to_vec())
 }
diff --git a/sdks/browser/test/platform/migration/archive-read-limit.mjs b/sdks/browser/test/platform/migration/archive-read-limit.mjs
index 1a2238929c..1ce8ef65a7 100644
--- a/sdks/browser/test/platform/migration/archive-read-limit.mjs
+++ b/sdks/browser/test/platform/migration/archive-read-limit.mjs
@@ -5,6 +5,7 @@ import { createMigrationServer, withMigrationPage } from "./server.mjs";
 
 const limit = 100_000_000;
 const allocations = [];
+const copies = [];
 const server = await createMigrationServer(
   "crates/xmtp_legacy_migration/fixtures",
   ["stable"],
@@ -17,6 +18,10 @@ try {
         allocations.push(
           Number(event.text().slice("MIGRATION_ARCHIVE_READ ".length)),
         );
+      if (event.text().startsWith("MIGRATION_ARCHIVE_COPY "))
+        copies.push(
+          Number(event.text().slice("MIGRATION_ARCHIVE_COPY ".length)),
+        );
     });
     // verifies: MIG-008
     const result = await page.evaluate(async (limit) => {
@@ -127,6 +132,11 @@ try {
     assert.equal(result.retry, true);
     assert.equal(result.sourceUnchanged, true);
     assert.ok(allocations.includes(limit), "boundary read was not observed");
+    assert.equal(
+      copies.includes(limit),
+      false,
+      "archive read made a duplicate 100 MB byte-array copy",
+    );
     assert.equal(
       allocations.includes(limit + 1),
       false,
@@ -134,7 +144,7 @@ try {
     );
   });
   console.log(
-    "Browser archive read: inclusive 100 MB limit, rejection before allocation, preservation, and retry passed",
+    "Browser archive read: inclusive 100 MB limit, no duplicate copy, rejection before allocation, preservation, and retry passed",
   );
 } finally {
   await browser.close();
diff --git a/sdks/browser/test/platform/migration/read-limit-worker.mjs b/sdks/browser/test/platform/migration/read-limit-worker.mjs
index 302866129e..00a6485869 100644
--- a/sdks/browser/test/platform/migration/read-limit-worker.mjs
+++ b/sdks/browser/test/platform/migration/read-limit-worker.mjs
@@ -1,6 +1,14 @@
 // Observe the real OPFS archive read without replacing it.
 const pending = [];
 self.onmessage = (event) => pending.push(event);
+const NativeUint8Array = Uint8Array;
+globalThis.Uint8Array = new Proxy(NativeUint8Array, {
+  construct(target, args, newTarget) {
+    if (args[0] instanceof NativeUint8Array)
+      console.log("MIGRATION_ARCHIVE_COPY " + args[0].byteLength);
+    return Reflect.construct(target, args, newTarget);
+  },
+});
 const read = Blob.prototype.arrayBuffer;
 Blob.prototype.arrayBuffer = function (...args) {
   if (this instanceof File && this.name.startsWith("archive-"))