From ba2432f8fe57181238cca97fd16742fa0fa98625 Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Mon, 16 Jun 2025 18:51:59 +0200
Subject: [PATCH] libutil: add asyncJoin, a Result-based joinPromises

we'll need this to asyncify withFramedSink and remove its thread pool.

Change-Id: I1a099392c094f8441482fde3b2d3843931420ffa
---
 lix/libutil/async-collect.hh | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/lix/libutil/async-collect.hh b/lix/libutil/async-collect.hh
index 3cf8a3e8a..76859a10f 100644
--- a/lix/libutil/async-collect.hh
+++ b/lix/libutil/async-collect.hh
@@ -136,4 +136,23 @@ kj::Promise<Result<void>> asyncSpread(Input && input, Fn fn)
     co_return result::success();
 }
 
+/**
+ * Run `promises` concurrently until they've all succeeded, or pass on
+ * the error returned by the first failing promise. This is similar to
+ * `kj::joinPromisesFailFast()`, but not limited to thrown exceptions.
+ */
+template<typename... Promises>
+kj::Promise<Result<void>> asyncJoin(Promises &&... promises)
+    requires(std::same_as<Promises, kj::Promise<Result<void>>> && ...)
+{
+    auto collect =
+        asyncCollect(kj::arr(std::pair{std::tuple{}, std::forward<Promises>(promises)}...));
+    while (auto r = co_await collect.next()) {
+        if (!r->second.has_value()) {
+            co_return std::move(r->second);
+        }
+    }
+
+    co_return result::success();
+}
 }