From 43abd0aebb12bb5c2f2d99327de75bf93d4be7d0 Mon Sep 17 00:00:00 2001
From: Tim Besard <tim.besard@gmail.com>
Date: Tue, 15 Sep 2026 10:04:45 +0200
Subject: [PATCH] Keep the libuv event loop alive during builds

Julia 1.12 can lose the wakeup of the thread that services the libuv
event loop: when a worker thread arms IO (Pkg downloads artifacts from a
Threads.@spawn task) while thread 0 is parked, nobody runs uv_run again
and the download stalls forever with data waiting in the socket. On
Yggdrasil this shows up as jobs stuck at "Installing N artifacts" until
the 5-hour timeout.

Arm a periodic Timer for the lifetime of build_tarballs when running
with more than one thread. An active handle keeps the loop alive, so
thread 0 blocks inside uv_run rather than parking and registers new
watchers on its next iteration. Remove once the runtime bug is fixed.

Works around JuliaLang/julia#63173.
---
 src/AutoBuild.jl | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/AutoBuild.jl b/src/AutoBuild.jl
index 75b75326..280a1ff0 100644
--- a/src/AutoBuild.jl
+++ b/src/AutoBuild.jl
@@ -137,6 +137,21 @@ const BUILD_HELP = (
     """
 )
 
+# Julia 1.12 can lose the wakeup of the thread that services the libuv event
+# loop when a worker thread arms IO (e.g. Pkg downloading artifacts in a
+# `Threads.@spawn` task) while that thread is parked. The download then stalls
+# forever with data waiting in the socket. Keeping a libuv handle active means
+# the event loop never becomes idle, so thread 0 blocks inside `uv_run` instead
+# of parking and picks up new watchers on its next iteration.
+# See https://github.com/JuliaLang/julia/issues/63173.
+const EVENT_LOOP_KEEPALIVE = Ref{Union{Nothing,Timer}}(nothing)
+function keep_event_loop_alive()
+    if EVENT_LOOP_KEEPALIVE[] === nothing && Threads.nthreads() > 1
+        EVENT_LOOP_KEEPALIVE[] = Timer(_ -> nothing, 1; interval=1)
+    end
+    return nothing
+end
+
 """
     build_tarballs(ARGS, src_name, src_version, sources, script, platforms,
                    products, dependencies; kwargs...)
@@ -210,6 +225,8 @@ function build_tarballs(ARGS, src_name, src_version, sources, script,
         return nothing
     end
 
+    keep_event_loop_alive()
+
     if compression_format != "gzip" && minimum_compat(julia_compat) < v"1.6"
         error("Compression formats different from gzip are supported only from Julia v1.6, increase the Julia compat if you want to use a non-default format")
     end