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
 
@@ -306,6 +307,15 @@ const THREAD_RNGs = MersenneTwister[]
 end
 function __init__()
     resize!(empty!(THREAD_RNGs), Threads.nthreads()) # ensures that we didn't save a bad object
+
+    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
 end
 
 

stdlib/Random/src/Random.jl

@@ -29,7 +29,7 @@ export rand!, randn!,
        shuffle, shuffle!,
        randperm, randperm!,
        randcycle, randcycle!,
-       AbstractRNG, MersenneTwister, RandomDevice
+       AbstractRNG, MersenneTwister, RandomDevice, RANDOM_DEVICE
 
 ## general definitions
 

stdlib/Random/test/runtests.jl

@@ -778,3 +778,7 @@ end
 @testset "RNGs broadcast as scalars: T" for T in (MersenneTwister, RandomDevice)
     @test length.(rand.(T(), 1:3)) == 1:3
 end
+
+@testset "RANDOM_DEVICE" begin
+    @test rand(Random.RANDOM_DEVICE) isa Float64
+end