Stop parallel workers when serial tests are at the end

Co-authored-by: Claude <noreply@anthropic.com>

Author: giordano

src/ParallelTestRunner.jl

@@ -1082,8 +1082,31 @@ function runtests(mod::Module, args::ParsedArgs;
     #
 
     tests_to_start = Threads.Atomic{Int}(length(tests))
+    # After parallel-before-serial: stop extra workers so only one process is alive for
+    # serial tests, but keep one parallel worker so we do not add a third addworker (ID_COUNTER).
+    function drain_pool_leaving_one_worker!(pool, njobs)
+        alive = PTRWorker[]
+        for _ in 1:njobs
+            p = take!(pool)
+            if p !== nothing && Malt.isrunning(p)
+                push!(alive, p)
+            end
+        end
+        while length(alive) > 1
+            Malt.stop(pop!(alive))
+        end
+        kept = isempty(alive) ? nothing : alive[1]
+        if kept !== nothing
+            put!(pool, kept)
+        end
+        for _ in 1:(njobs - (kept === nothing ? 0 : 1))
+            put!(pool, nothing)
+        end
+    end
     try
-        for (phase_tests, sem, shared_worker) in test_phases
+        phases = test_phases
+        for i in 1:length(phases)
+            phase_tests, sem, shared_worker = phases[i]
             isempty(phase_tests) && continue
             # for serial phases, reserve one pool slot for the shared worker
             if !isnothing(shared_worker)
@@ -1197,6 +1220,14 @@ function runtests(mod::Module, args::ParsedArgs;
                 put!(worker_pool, shared_worker[])
                 shared_worker[] = nothing
             end
+            # parallel workers are not stopped while serial tests remain (tests_to_start > 0);
+            # drain before serial-after so only one worker is alive for the serial phase
+            if isnothing(shared_worker) && i < length(phases)
+                next_tests, _, next_sw = phases[i+1]
+                if !isempty(next_tests) && !isnothing(next_sw)
+                    drain_pool_leaving_one_worker!(worker_pool, jobs)
+                end
+            end
         end
     catch err
         if !(err isa InterruptException)

test/runtests.jl

@@ -840,47 +840,63 @@ end
     end
 
     @testset "serial tests run before parallel (default)" begin
+        serial_test_body = quote
+            children = _count_child_pids($(getpid()))
+            # Make sure serial tests run alone.
+            if children >= 0
+                @test children == 1
+            end
+        end
         testsuite = Dict(
-            "serial_a" => :(),
-            "serial_b" => :(),
+            "serial_1" => serial_test_body,
+            "serial_2" => serial_test_body,
+            "serial_3" => serial_test_body,
             "parallel_1" => :(),
             "parallel_2" => :(),
+            "parallel_3" => :(),
         )
         io = IOBuffer()
         jobs = 2
         old_id_counter = ParallelTestRunner.ID_COUNTER[]
-        runtests(ParallelTestRunner, ["--jobs=$(jobs)", "--verbose"];
-                 testsuite, stdout=io, stderr=io,
-                 serial=["serial_a", "serial_b"])
+        @show_if_error io runtests(ParallelTestRunner, ["--jobs=$(jobs)", "--verbose"];
+                                   testsuite, stdout=io, stderr=io,
+                                   init_code=:(include($(joinpath(@__DIR__, "utils.jl")))),
+                                   serial=["serial_1", "serial_2", "serial_3"])
         str = String(take!(io))
-        @test contains(str, "2 serial test(s) will run before")
+        @test contains(str, "Running 6 tests using 2 parallel jobs")
+        @test contains(str, "3 serial test(s) will run before")
         @test contains(str, "SUCCESS")
         @test ParallelTestRunner.ID_COUNTER[] == old_id_counter + jobs
     end
 
     @testset "serial tests run after parallel" begin
-        testsuite = Dict(
-            "serial_x" => quote
-                children = _count_child_pids($(getpid()))
-                # Make sure serial tests run alone.
-                if children >= 0
-                    @test children == 1
+        serial_test_body = quote
+            children = _count_child_pids($(getpid()))
+            # Make sure serial tests run alone.
+            if children >= 0
+                @test children == 1
                 end
-            end,
-            "parallel_y" => :(),
+        end
+        testsuite = Dict(
+            "serial_1" => serial_test_body,
+            "serial_2" => serial_test_body,
+            "serial_3" => serial_test_body,
+            "parallel_1" => :(),
+            "parallel_2" => :(),
+            "parallel_3" => :(),
         )
         io = IOBuffer()
         ioc = IOContext(io, :color => true)
         old_id_counter = ParallelTestRunner.ID_COUNTER[]
         @show_if_error io runtests(ParallelTestRunner, ["--jobs=2", "--verbose"];
                                    testsuite, stdout=ioc, stderr=ioc,
                                    init_code=:(include($(joinpath(@__DIR__, "utils.jl")))),
-                                   serial=["serial_x"], serial_position=:after)
+                                   serial=["serial_1", "serial_2", "serial_3"], serial_position=:after)
         str = String(take!(io))
-        @test contains(str, "Running 2 tests using 1 parallel jobs")
-        @test contains(str, "1 serial test(s) will run after")
+        @test contains(str, "Running 6 tests using 2 parallel jobs")
+        @test contains(str, "3 serial test(s) will run after")
         @test contains(str, "SUCCESS")
-        @test ParallelTestRunner.ID_COUNTER[] == old_id_counter + 1
+        @test ParallelTestRunner.ID_COUNTER[] == old_id_counter + 2
     end
 
     @testset "serial_position validation" begin