Make threads test more robust + fix bug parsing JULIA_NUM_THREADS (#4241)

Co-authored-by: Ian Butterworth <[email protected]>

Author: gbaraldi

src/Operations.jl

@@ -2014,6 +2014,9 @@ end
 
 function get_threads_spec()
     if haskey(ENV, "JULIA_NUM_THREADS")
+        if isempty(ENV["JULIA_NUM_THREADS"])
+            throw(ArgumentError("JULIA_NUM_THREADS is set to an empty string. It is not clear what Pkg.test should set for `-t` on the test worker."))
+        end
         # if set, prefer JULIA_NUM_THREADS because this is passed to the test worker via --threads
         # which takes precedence in the worker
         ENV["JULIA_NUM_THREADS"]

test/new.jl

@@ -2128,6 +2128,7 @@ end
                         withenv(
                             "EXPECTED_NUM_THREADS_DEFAULT" => "$default_nthreads_default",
                             "EXPECTED_NUM_THREADS_INTERACTIVE" => "$default_nthreads_interactive",
+                            "JULIA_NUM_THREADS" => nothing,
                         ) do
                             Pkg.test("TestThreads")
                         end
@@ -2154,6 +2155,7 @@ end
                         withenv(
                             "EXPECTED_NUM_THREADS_DEFAULT" => "$other_nthreads_default",
                             "EXPECTED_NUM_THREADS_INTERACTIVE" => "$default_nthreads_interactive",
+                            "JULIA_NUM_THREADS" => nothing,
                         ) do
                             Pkg.test("TestThreads"; julia_args=`--threads=$other_nthreads_default`)
                         end
@@ -2162,6 +2164,7 @@ end
                         withenv(
                             "EXPECTED_NUM_THREADS_DEFAULT" => "$other_nthreads_default",
                             "EXPECTED_NUM_THREADS_INTERACTIVE" => "$other_nthreads_interactive",
+                            "JULIA_NUM_THREADS" => nothing,
                         ) do
                             Pkg.test("TestThreads"; julia_args=`--threads=$other_nthreads_default,$other_nthreads_interactive`)
                         end