[Random] Add s4 field to Xoshiro type (#51332)

This PR adds an optional field to the existing `Xoshiro` struct to be
able to faithfully copy the task-local RNG state.

Fixes #51255
Redo of #51271

Background context: #49110 added an additional state to the task-local
RNG. However, before this PR `copy(default_rng())` did not include this
extra state, causing subtle errors in `Test` where `copy(default_rng())`
is assumed to contain the full task-local RNG state.

stdlib/Random/src/Xoshiro.jl

@@ -48,28 +48,37 @@ mutable struct Xoshiro <: AbstractRNG
     s1::UInt64
     s2::UInt64
     s3::UInt64
+    s4::UInt64 # internal splitmix state
 
-    Xoshiro(s0::Integer, s1::Integer, s2::Integer, s3::Integer) = new(s0, s1, s2, s3)
+    Xoshiro(s0::Integer, s1::Integer, s2::Integer, s3::Integer, s4::Integer) = new(s0, s1, s2, s3, s4)
+    Xoshiro(s0::UInt64, s1::UInt64, s2::UInt64, s3::UInt64) = new(s0, s1, s2, s3, 1s0 + 3s1 + 5s2 + 7s3)
     Xoshiro(seed=nothing) = seed!(new(), seed)
 end
 
-function setstate!(x::Xoshiro, s0::UInt64, s1::UInt64, s2::UInt64, s3::UInt64)
+Xoshiro(s0::Integer, s1::Integer, s2::Integer, s3::Integer) = Xoshiro(UInt64(s0), UInt64(s1), UInt64(s2), UInt64(s3))
+
+function setstate!(
+    x::Xoshiro,
+    s0::UInt64, s1::UInt64, s2::UInt64, s3::UInt64, # xoshiro256 state
+    s4::UInt64, # internal splitmix state
+)
     x.s0 = s0
     x.s1 = s1
     x.s2 = s2
     x.s3 = s3
+    x.s4 = s4
     x
 end
 
-copy(rng::Xoshiro) = Xoshiro(rng.s0, rng.s1, rng.s2, rng.s3)
+copy(rng::Xoshiro) = Xoshiro(rng.s0, rng.s1, rng.s2, rng.s3, rng.s4)
 
 function copy!(dst::Xoshiro, src::Xoshiro)
-    dst.s0, dst.s1, dst.s2, dst.s3 = src.s0, src.s1, src.s2, src.s3
+    dst.s0, dst.s1, dst.s2, dst.s3, dst.s4 = src.s0, src.s1, src.s2, src.s3, src.s4
     dst
 end
 
 function ==(a::Xoshiro, b::Xoshiro)
-    a.s0 == b.s0 && a.s1 == b.s1 && a.s2 == b.s2 && a.s3 == b.s3
+    a.s0 == b.s0 && a.s1 == b.s1 && a.s2 == b.s2 && a.s3 == b.s3 && a.s4 == b.s4
 end
 
 rng_native_52(::Xoshiro) = UInt64
@@ -116,7 +125,7 @@ rng_native_52(::TaskLocalRNG) = UInt64
 function setstate!(
     x::TaskLocalRNG,
     s0::UInt64, s1::UInt64, s2::UInt64, s3::UInt64, # xoshiro256 state
-    s4::UInt64 = 1s0 + 3s1 + 5s2 + 7s3, # internal splitmix state
+    s4::UInt64, # internal splitmix state
 )
     t = current_task()
     t.rngState0 = s0
@@ -148,14 +157,20 @@ end
 function seed!(rng::Union{TaskLocalRNG,Xoshiro})
     # as we get good randomness from RandomDevice, we can skip hashing
     rd = RandomDevice()
-    setstate!(rng, rand(rd, UInt64), rand(rd, UInt64), rand(rd, UInt64), rand(rd, UInt64))
+    s0 = rand(rd, UInt64)
+    s1 = rand(rd, UInt64)
+    s2 = rand(rd, UInt64)
+    s3 = rand(rd, UInt64)
+    s4 = 1s0 + 3s1 + 5s2 + 7s3
+    setstate!(rng, s0, s1, s2, s3, s4)
 end
 
 function seed!(rng::Union{TaskLocalRNG,Xoshiro}, seed::Union{Vector{UInt32}, Vector{UInt64}})
     c = SHA.SHA2_256_CTX()
     SHA.update!(c, reinterpret(UInt8, seed))
     s0, s1, s2, s3 = reinterpret(UInt64, SHA.digest!(c))
-    setstate!(rng, s0, s1, s2, s3)
+    s4 = 1s0 + 3s1 + 5s2 + 7s3
+    setstate!(rng, s0, s1, s2, s3, s4)
 end
 
 seed!(rng::Union{TaskLocalRNG, Xoshiro}, seed::Integer) = seed!(rng, make_seed(seed))
@@ -178,24 +193,30 @@ end
 
 function copy(rng::TaskLocalRNG)
     t = current_task()
-    Xoshiro(t.rngState0, t.rngState1, t.rngState2, t.rngState3)
+    Xoshiro(t.rngState0, t.rngState1, t.rngState2, t.rngState3, t.rngState4)
 end
 
 function copy!(dst::TaskLocalRNG, src::Xoshiro)
     t = current_task()
-    setstate!(dst, src.s0, src.s1, src.s2, src.s3)
+    setstate!(dst, src.s0, src.s1, src.s2, src.s3, src.s4)
     return dst
 end
 
 function copy!(dst::Xoshiro, src::TaskLocalRNG)
     t = current_task()
-    setstate!(dst, t.rngState0, t.rngState1, t.rngState2, t.rngState3)
+    setstate!(dst, t.rngState0, t.rngState1, t.rngState2, t.rngState3, t.rngState4)
     return dst
 end
 
 function ==(a::Xoshiro, b::TaskLocalRNG)
     t = current_task()
-    a.s0 == t.rngState0 && a.s1 == t.rngState1 && a.s2 == t.rngState2 && a.s3 == t.rngState3
+    (
+        a.s0 == t.rngState0 &&
+        a.s1 == t.rngState1 &&
+        a.s2 == t.rngState2 &&
+        a.s3 == t.rngState3 &&
+        a.s4 == t.rngState4
+    )
 end
 
 ==(a::TaskLocalRNG, b::Xoshiro) = b == a