Random: add a global RANDOM_DEVICE generator

Creating a RandomDevice object is cheap but it opens a file
on unix systems, so one can't call `rand(RandomDevice())`
repeatedly too quicly (e.g. in a loop). It can therefore be
convenient to have a readily available generator.

stdlib/Random/src/RNGs.jl

@@ -67,6 +67,7 @@ RandomDevice
 RandomDevice(::Nothing) = RandomDevice()
 seed!(rng::RandomDevice) = rng
 
+const RANDOM_DEVICE = RandomDevice()
 
 ## MersenneTwister
 

stdlib/Random/src/Random.jl

@@ -23,7 +23,7 @@ export rand!, randn!,
        shuffle, shuffle!,
        randperm, randperm!,
        randcycle, randcycle!,
-       AbstractRNG, MersenneTwister, RandomDevice
+       AbstractRNG, MersenneTwister, RandomDevice, RANDOM_DEVICE
 
 ## general definitions
 
@@ -264,6 +264,14 @@ rand(                ::Type{X}, d::Integer, dims::Integer...) where {X} = rand(X
 function __init__()
     try
         seed!()
+        if !Sys.iswindows()
+            # open /dev/urandom "in-place" (in RANDOM_DEVICE.file)
+            RANDOM_DEVICE.file.handle = pointer(RANDOM_DEVICE.file.ios)
+            systemerror("opening file /dev/urandom",
+                ccall(:ios_file, Ptr{Cvoid},
+                      (Ptr{UInt8}, Cstring, Cint, Cint, Cint, Cint),
+                      RANDOM_DEVICE.file.ios, "/dev/urandom", 1, 0, 0, 0) == C_NULL)
+        end
     catch ex
         Base.showerror_nostdio(ex,
             "WARNING: Error during initialization of module Random")

stdlib/Random/test/runtests.jl

@@ -694,3 +694,7 @@ end
     a = rand(Int, 1)
     @test shuffle(a) == a
 end
+
+@testset "RANDOM_DEVICE" begin
+    @test rand(Random.RANDOM_DEVICE) isa Float64
+end