hard error at backend creation with multi device (GPU + CPU) on OpenCL

[MichaelHirn]

using GPU + CPU for backend creation, results in the following non-recoverable error.

cargo test --test framework_opencl_specs -- --nocapture
     Running target/debug/framework_opencl_specs-f486119d3a03151b

running 3 tests
[Hardware { id: 140571595436224, name: Some("GeForce GT 750M\u{0}"), hardware_type: Some(GPU), compute_units: Some(2) }, Hardware { id: 140571595385432, name: Some("Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz\u{0}"), hardware_type: Some(CPU), compute_units: Some(8) }]
[0x7fd96002f0c0, 0x7fd960022a58]
[Hardware { id: 140571595436224, name: Some("GeForce GT 750M\u{0}"), hardware_type: Some(GPU), compute_units: Some(2) }, Hardware { id: 140571595385432, name: Some("Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz\u{0}"), hardware_type: Some(CPU), compute_units: Some(8) }]
Process didn't exit successfully: `/home/.../collenchyma/target/debug/framework_opencl_specs-f486119d3a03151b --nocapture` (signal: 11)

[alexandermorozov]

FYI, here is traceback under valgring on my system:

(skipped some probably unrelated warnings)

test backend_spec::native::it_can_use_ibackend_trait_object ... ok
==2715== Conditional jump or move depends on uninitialised value(s)
==2715==    at 0x1363D1: sync::mpsc::Receiver$LT$T$GT$::recv::h1839152013060402538 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x12450E: run_tests_console::h19f13b059fdf26c7JIb (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x121BC2: test_main::h02954d949b4d6ebeY2a (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x1280F7: test_main_static::h1c66d5b0032e74d3d5a (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x120CE6: __test::main::had5f854d465421a58da (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x18F174: sys_common::unwind::try::try_fn::h7821988306635677941 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x18C2EB: __rust_try (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x18EC0B: rt::lang_start::h582466266dfb2119IOz (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x120D29: main (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715== 
test backend_spec::native::it_can_create_default_backend ... ok
==2715== Warning: noted but unhandled ioctl 0x30000001 with no size/direction hints.
==2715==    This could cause spurious value errors to appear.
==2715==    See README_MISSING_SYSCALL_OR_IOCTL for guidance on writing a proper wrapper.
==2715== Warning: set address range perms: large range [0x200000000, 0x500000000) (noaccess)
==2715== Warning: set address range perms: large range [0x500000000, 0x800000000) (noaccess)
==2715== Warning: set address range perms: large range [0x805fba000, 0xa05fba000) (noaccess)
==2715== Warning: set address range perms: large range [0x805fba000, 0xa05fba000) (noaccess)
==2715== Warning: set address range perms: large range [0x900000000, 0xb00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xb00000000, 0xd00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xb00000000, 0xd00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xb00000000, 0xd00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xd00000000, 0xf00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xd00000000, 0xf00000000) (noaccess)
==2715== Warning: set address range perms: large range [0xd00000000, 0xf00000000) (noaccess)
==2715== Warning: set address range perms: large range [0x1000000000, 0x1200000000) (noaccess)
==2715== Thread 3 backend_spec::opencl::it_can_manually_create_backend:
==2715== Invalid read of size 8
==2715==    at 0xA93D805: ??? (in /usr/lib/x86_64-linux-gnu/nvidia/current/libnvidia-opencl.so.352.79)
==2715==    by 0x16BB6B: frameworks::opencl::api::context::API::ffi_create_context::h33fd890effe976e8tDc (context.rs:39)
==2715==    by 0x169C5F: frameworks::opencl::api::context::API::create_context::hc406bc8816d75d57aCc (context.rs:26)
==2715==    by 0x166695: frameworks::opencl::context::Context::new::ha2a8a5268f2467f2kjb (context.rs:27)
==2715==    by 0x166418: frameworks::opencl::OpenCL.IFramework::new_device::haef08581fd6fbe1e8te (mod.rs:96)
==2715==    by 0x120010: backend::Backend$LT$F$GT$::new::h3686179191400567503 (backend.rs:69)
==2715==    by 0x1206A4: backend_spec::opencl::it_can_manually_create_backend::hbc55225f33db6cfdVca (backend_specs.rs:55)
==2715==    by 0x13E796: boxed::F.FnBox$LT$A$GT$::call_box::h4965145969991418958 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x140E8B: sys_common::unwind::try::try_fn::h311507189721033412 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x18C2EB: __rust_try (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x189AAD: sys_common::unwind::inner_try::h6de88a54d8fb341cz2t (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x14120A: boxed::F.FnBox$LT$A$GT$::call_box::h9955032800241204795 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==  Address 0x408 is not stack'd, malloc'd or (recently) free'd
==2715== 
==2715== 
==2715== Process terminating with default action of signal 11 (SIGSEGV)
==2715==  Access not within mapped region at address 0x408
==2715==    at 0xA93D805: ??? (in /usr/lib/x86_64-linux-gnu/nvidia/current/libnvidia-opencl.so.352.79)
==2715==    by 0x16BB6B: frameworks::opencl::api::context::API::ffi_create_context::h33fd890effe976e8tDc (context.rs:39)
==2715==    by 0x169C5F: frameworks::opencl::api::context::API::create_context::hc406bc8816d75d57aCc (context.rs:26)
==2715==    by 0x166695: frameworks::opencl::context::Context::new::ha2a8a5268f2467f2kjb (context.rs:27)
==2715==    by 0x166418: frameworks::opencl::OpenCL.IFramework::new_device::haef08581fd6fbe1e8te (mod.rs:96)
==2715==    by 0x120010: backend::Backend$LT$F$GT$::new::h3686179191400567503 (backend.rs:69)
==2715==    by 0x1206A4: backend_spec::opencl::it_can_manually_create_backend::hbc55225f33db6cfdVca (backend_specs.rs:55)
==2715==    by 0x13E796: boxed::F.FnBox$LT$A$GT$::call_box::h4965145969991418958 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x140E8B: sys_common::unwind::try::try_fn::h311507189721033412 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x18C2EB: __rust_try (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x189AAD: sys_common::unwind::inner_try::h6de88a54d8fb341cz2t (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==    by 0x14120A: boxed::F.FnBox$LT$A$GT$::call_box::h9955032800241204795 (in /home/butler/projects/nn/collenchyma/target/debug/backend_specs-bf765cfe17a02bcb)
==2715==  If you believe this happened as a result of a stack
==2715==  overflow in your program's main thread (unlikely but
==2715==  possible), you can try to increase the size of the
==2715==  main thread stack using the --main-stacksize= flag.
==2715==  The main thread stack size used in this run was 8388608.