prevent get_backend from overflowing the stack

Prevent the `get_backend` methods from overflowing the stack/recurring
without bound.

Hoping this doesn't cause inference issues due to deeper call stacks.

Fixes #588

Author: nsajko

ext/LinearAlgebraExt.jl

@@ -3,7 +3,7 @@ module LinearAlgebraExt
 using KernelAbstractions: KernelAbstractions
 using LinearAlgebra: Tridiagonal, Diagonal
 
-KernelAbstractions.get_backend(A::Diagonal) = KernelAbstractions.get_backend(A.diag)
-KernelAbstractions.get_backend(A::Tridiagonal) = KernelAbstractions.get_backend(A.d)
+KernelAbstractions.get_backend(A::Diagonal) = KernelAbstractions.get_backend_recur(x -> x.diag, A)
+KernelAbstractions.get_backend(A::Tridiagonal) = KernelAbstractions.get_backend_recur(x -> x.d, A)
 
 end

src/KernelAbstractions.jl

@@ -510,8 +510,17 @@ Get a [`Backend`](@ref) instance suitable for array `A`.
 """
 function get_backend end
 
+function get_backend_recur(f::F, x) where {F}
+    t() = throw(ArgumentError("throwing to prevent a stack overflow, possibly a `get_backend` method is missing?"))
+    y = f(x)
+    if y isa typeof(x)
+        @noinline t()
+    end
+    get_backend(y)
+end
+
 # Should cover SubArray, ReshapedArray, ReinterpretArray, Hermitian, AbstractTriangular, etc.:
-get_backend(A::AbstractArray) = get_backend(parent(A))
+get_backend(A::AbstractArray) = get_backend_recur(parent, A)
 
 # Define:
 #   adapt_storage(::Backend, a::Array) = adapt(BackendArray, a)

test/test.jl

@@ -7,6 +7,9 @@ using Adapt
 
 identity(x) = x
 
+struct UnknownAbstractVector <: AbstractVector{Float32}  # issue #588
+end
+
 function unittest_testsuite(Backend, backend_str, backend_mod, BackendArrayT; skip_tests = Set{String}())
     @conditional_testset "partition" skip_tests begin
         backend = Backend()
@@ -80,6 +83,7 @@ function unittest_testsuite(Backend, backend_str, backend_mod, BackendArrayT; sk
         @test @inferred(KernelAbstractions.get_backend(view(A, 2:4, 1:3))) isa backendT
         @test @inferred(KernelAbstractions.get_backend(Diagonal(x))) isa backendT
         @test @inferred(KernelAbstractions.get_backend(Tridiagonal(A))) isa backendT
+        @test_throws ArgumentError KernelAbstractions.get_backend(UnknownAbstractVector())  # issue #588
     end
 
     @conditional_testset "sparse" skip_tests begin