@testset: with Xoshiro, restore Random.GLOBAL_SEED (#43188)

A `@testset` is supposed to restore the "global RNG state" as
it was before execution (so that they can be re-ordered easily, etc.)
Also, before a testset starts, the default RNG is re-seeded with the
"global seed" (to help reproduce test failures).

Before `Xoshiro` as the default RNG, the "global seed" was stored
within a `MersenneTwister` object. It was enough for a testset to
copy the default RNG at the start, and copy it back at the end.
But now the global seed is stored outside of the RNG, so it should
also be restored separately.

stdlib/Random/src/RNGs.jl

@@ -392,6 +392,7 @@ copy!(::_GLOBAL_RNG, src::Xoshiro) = copy!(default_rng(), src)
 copy(::_GLOBAL_RNG) = copy(default_rng())
 
 GLOBAL_SEED = 0
+set_global_seed!(seed) = global GLOBAL_SEED = seed
 
 function seed!(::_GLOBAL_RNG, seed=rand(RandomDevice(), UInt64, 4))
     global GLOBAL_SEED = seed

stdlib/Test/src/Test.jl

@@ -1340,6 +1340,7 @@ function testset_beginend_call(args, tests, source)
         # by wrapping the body in a function
         local RNG = default_rng()
         local oldrng = copy(RNG)
+        local oldseed = Random.GLOBAL_SEED
         try
             # RNG is re-seeded with its own seed to ease reproduce a failed test
             Random.seed!(Random.GLOBAL_SEED)
@@ -1353,6 +1354,7 @@ function testset_beginend_call(args, tests, source)
             record(ts, Error(:nontest_error, Expr(:tuple), err, Base.current_exceptions(), $(QuoteNode(source))))
         finally
             copy!(RNG, oldrng)
+            Random.set_global_seed!(oldseed)
             pop_testset()
             ret = finish(ts)
         end
@@ -1433,6 +1435,7 @@ function testset_forloop(args, testloop, source)
         local ts
         local RNG = default_rng()
         local oldrng = copy(RNG)
+        local oldseed = Random.GLOBAL_SEED
         Random.seed!(Random.GLOBAL_SEED)
         local tmprng = copy(RNG)
         try
@@ -1446,6 +1449,7 @@ function testset_forloop(args, testloop, source)
                 push!(arr, finish(ts))
             end
             copy!(RNG, oldrng)
+            Random.set_global_seed!(oldseed)
         end
         arr
     end

stdlib/Test/test/runtests.jl

@@ -923,6 +923,29 @@ end
     Random.seed!(seed)
     @test a == rand()
     @test b == rand()
+
+    # Even when seed!() is called within a testset A, subsequent testsets
+    # should start with the same "global RNG state" as what A started with,
+    # such that the test `refvalue == rand(Int)` below succeeds.
+    # Currently, this means that Random.GLOBAL_SEED has to be restored,
+    # in addition to the state of Random.default_rng().
+    GLOBAL_SEED_orig = Random.GLOBAL_SEED
+    local refvalue
+    @testset "GLOBAL_SEED is also preserved (setup)" begin
+        @test GLOBAL_SEED_orig == Random.GLOBAL_SEED
+        refvalue = rand(Int)
+        Random.seed!()
+        @test GLOBAL_SEED_orig != Random.GLOBAL_SEED
+    end
+    @test GLOBAL_SEED_orig == Random.GLOBAL_SEED
+    @testset "GLOBAL_SEED is also preserved (forloop)" for _=1:3
+        @test refvalue == rand(Int)
+        Random.seed!()
+    end
+    @test GLOBAL_SEED_orig == Random.GLOBAL_SEED
+    @testset "GLOBAL_SEED is also preserved (beginend)" begin
+        @test refvalue == rand(Int)
+    end
 end
 
 @testset "InterruptExceptions #21043" begin
@@ -1280,4 +1303,3 @@ let
         end
     end
 end
-