Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

Description

Infer kernels in IE backend works asynchronously and implemented via RequestPool which has a set of InferRequest's and concurrent bounded queue to track free requests.

Before run the task RequestPool pop first free request and if there are no free requests it blocks. (*)
When execution is finished (all frames are processed or at least started processing) need to wait for all requests are free.
There is some trick: try to pop all requests from queue, if specific request is busy it blocks until it's free. When it finishes guarantee all requests are free, but the queue of free requests is empty.

cv::GArray<cv::Rect> rr;
cv::GMat in;
cv::GArray<cv::GMat> age, gender;
std::tie(age, gender) = cv::gapi::infer<AgeGender>(rr, in);
cv::GComputation comp(cv::GIn(in, rr), cv::GOut(age, gender));

// Start RequestPool
cc = comp.compile(/* args */);

// Run first time
cc(cv::gin(/* inputs */), cv::gout(/* outputs */));
// RequestPool emptied the queue of free requests

// Second run cause hanging because the queue of free request is empty, see (*)
cc(cv::gin(/* inputs */), cv::gout(/* outputs */));

Build configuration

force_builders=Custom,Custom Win,Custom Mac
build_gapi_standalone:Linux x64=ade-0.1.1f
build_gapi_standalone:Win64=ade-0.1.1f
build_gapi_standalone:Mac=ade-0.1.1f
build_gapi_standalone:Linux x64 Debug=ade-0.1.1f

Xbuild_image:Custom=centos:7
Xbuildworker:Custom=linux-1
build_gapi_standalone:Custom=ade-0.1.1f

build_image:Custom=ubuntu-openvino-2021.3.0:20.04
build_image:Custom Win=openvino-2021.3.0
build_image:Custom Mac=openvino-2021.3.0

test_modules:Custom=gapi,python2,python3,java
test_modules:Custom Win=gapi,python2,python3,java
test_modules:Custom Mac=gapi,python2,python3,java

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*