From 0789d7796505019accea50696e7afe67df7b2a86 Mon Sep 17 00:00:00 2001
From: JamesWrigley <james@puiterwijk.org>
Date: Sun, 4 Aug 2024 01:02:34 +0200
Subject: [PATCH] Implement Base.isempty(::RemoteChannel)

The default method from Base has the nasty side-effect of take!()'ing from the
underlying channel.
---
 docs/src/_changelog.md   |  4 ++++
 src/remotecall.jl        |  3 +++
 test/distributed_exec.jl | 12 ++++++++++++
 3 files changed, 19 insertions(+)

diff --git a/docs/src/_changelog.md b/docs/src/_changelog.md
index 014863b..fe80fb4 100644
--- a/docs/src/_changelog.md
+++ b/docs/src/_changelog.md
@@ -9,6 +9,10 @@ This documents notable changes in DistributedNext.jl. The format is based on
 
 ## Unreleased
 
+### Fixed
+- Fixed behaviour of `isempty(::RemoteChannel)`, which previously had the
+  side-effect of taking an element from the channel ([#3]).
+
 ### Changed
 - Added a `project` argument to [`addprocs(::AbstractVector)`](@ref) to specify
   the project of a remote worker ([#2]).
diff --git a/src/remotecall.jl b/src/remotecall.jl
index 0b1143d..644ff04 100644
--- a/src/remotecall.jl
+++ b/src/remotecall.jl
@@ -768,6 +768,9 @@ close(rr::RemoteChannel) = call_on_owner(close_ref, rr)
 isopen_ref(rid) = isopen(lookup_ref(rid).c)
 isopen(rr::RemoteChannel) = call_on_owner(isopen_ref, rr)
 
+isempty_ref(rid) = isempty(lookup_ref(rid).c)
+Base.isempty(rr::RemoteChannel) = call_on_owner(isempty_ref, rr)
+
 getindex(r::RemoteChannel) = fetch(r)
 getindex(r::Future) = fetch(r)
 
diff --git a/test/distributed_exec.jl b/test/distributed_exec.jl
index a5aea3e..a5b833b 100644
--- a/test/distributed_exec.jl
+++ b/test/distributed_exec.jl
@@ -493,6 +493,18 @@ let ch = RemoteChannel(() -> Channel(1))
     @test 10 == test_iteration_collect(ch)
 end
 
+# Test isempty(::RemoteChannel). This should not modify the underlying
+# AbstractChannel, which Base's default implementation will do.
+let
+    chan = Channel(1)
+    push!(chan, 1)
+    remotechan = RemoteChannel(() -> chan)
+
+    @test !isempty(remotechan)
+    # Calling `isempty(remotechan)` shouldn't have modified `chan`
+    @test !isempty(chan)
+end
+
 # make sure exceptions propagate when waiting on Tasks
 @test_throws CompositeException (@sync (@async error("oops")))
 try