Fix derived array lifetime race

[maleadt]

Keep the parent oneArray alive while deriving views so its finalizer cannot mark the shared GPUArrays DataRef as freed before the new array retains it. Add a GC-stress regression for nested ephemeral views, which frequently triggered "Attempt to copy a freed reference" in AcceleratedKernels.

Fixes #484

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diff --git a/src/array.jl b/src/array.jl
index 3fd539b..e0f5ec1 100644
--- a/src/array.jl
+++ b/src/array.jl
@@ -570,10 +570,10 @@ function GPUArrays.derive(::Type{T}, a::oneArray, dims::Dims{N}, offset::Int) wh
     Base.elsize(a) == 0 || error("Cannot derive a singleton array from non-singleton inputs")
   end
   offset = a.offset + offset * sizeof(T)
-  # The derived array constructor copies `a.data`, but merely loading that field does not
-  # keep `a` alive. Without this preserve, `a` may be finalized between the field load and
-  # the DataRef copy, causing its finalizer to mark the reference as freed.
-  GC.@preserve a oneArray{T,N}(a.data, dims; a.maxsize, offset)
+    # The derived array constructor copies `a.data`, but merely loading that field does not
+    # keep `a` alive. Without this preserve, `a` may be finalized between the field load and
+    # the DataRef copy, causing its finalizer to mark the reference as freed.
+    return GC.@preserve a oneArray{T, N}(a.data, dims; a.maxsize, offset)
 end
 
 
diff --git a/test/array.jl b/test/array.jl
index 1ca6645..1a462f2 100644
--- a/test/array.jl
+++ b/test/array.jl
@@ -44,38 +44,38 @@ end
 end
 
 @testset "derived array lifetime" begin
-  parent = oneArray{UInt8}(undef, 1)
-
-  # Construct through an ephemeral derived array, whose finalizer shares the same
-  # reference-counted allocation with the returned view.
-  function ephemeral_derived_view(parent)
-    intermediate = @view parent[:]
-    @view intermediate[:]
-  end
-
-  derived = ephemeral_derived_view(parent)
-  @test derived isa oneArray{UInt8}
-
-  # Exercise finalization while deriving. Without preserving the immediate parent in
-  # GPUArrays.derive, its finalizer can mark the DataRef as freed before it is copied.
-  if Threads.nthreads() > 1
-    stop_gc = Threads.Atomic{Bool}(false)
-    gc_task = Threads.@spawn while !stop_gc[]
-      GC.gc(false)
-      yield()
+    parent = oneArray{UInt8}(undef, 1)
+
+    # Construct through an ephemeral derived array, whose finalizer shares the same
+    # reference-counted allocation with the returned view.
+    function ephemeral_derived_view(parent)
+        intermediate = @view parent[:]
+        @view intermediate[:]
     end
-    try
-      Threads.@threads for _ in 1:min(Threads.nthreads(), 4)
-        for _ in 1:100
-          a = ephemeral_derived_view(parent)
-          oneAPI.unsafe_free!(a)
+
+    derived = ephemeral_derived_view(parent)
+    @test derived isa oneArray{UInt8}
+
+    # Exercise finalization while deriving. Without preserving the immediate parent in
+    # GPUArrays.derive, its finalizer can mark the DataRef as freed before it is copied.
+    if Threads.nthreads() > 1
+        stop_gc = Threads.Atomic{Bool}(false)
+        gc_task = Threads.@spawn while !stop_gc[]
+            GC.gc(false)
+            yield()
+        end
+        try
+            Threads.@threads for _ in 1:min(Threads.nthreads(), 4)
+                for _ in 1:100
+                    a = ephemeral_derived_view(parent)
+                    oneAPI.unsafe_free!(a)
+                end
+            end
+        finally
+            stop_gc[] = true
+            wait(gc_task)
         end
-      end
-    finally
-      stop_gc[] = true
-      wait(gc_task)
     end
-  end
 end
 
 @testset "reinterpret of view with non-aligned offset" begin