Handle OpenCV exceptions instead of aborting

[lgov]

(This is intended as an umbrella issue to follow up my ongoing proposal on the dev mailing list & work on handling OpenCV exceptions)

Description

I am working on migrating a classification product currently using Caffe to
MXNet. Along the way I'm encountering some issues loading and augmenting
the images dataset.

Basically it seems my dataset contains some technically invalid images.
When loading them using mx.io.ImageRecordIter (from a Python script), they
get passed eventually to the OpenCV library which will throw a C++
exception. MXNet currently doesn't capture those, resulting in my script
aborting with a not very clear error message.

Error Message:

"
terminate called after throwing an instance of 'cv::Exception'

what(): OpenCV(3.4.3)
/home/lgo/dev/opencv-3.4.3/modules/imgproc/src/resize.cpp:4044: error:
(-215:Assertion failed) !ssize.empty() in function 'resize'

Aborted (core dumped)
"

Environment info (Required)

Linux (Ubuntu 18.04), OpenCV 3.4.3, mxmet master branch, Python 3.6.6

Build info (Required if built from source)

Compiler (gcc/clang/mingw/visual studio):
gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

Steps to reproduce

It's not clear exactly which image causes this particular exception. As the process aborts due to the unhandled exception, there is no easy way to capture good information about the process status - in this case the image name.

[lgov]

Draft proposal on how to handle OpenCV exceptions on the dev mailing list:

http://mail-archives.apache.org/mod_mbox/mxnet-dev/201811.mbox/%3cCANvU9seUjFbVYCk49T93R0BfDxbFq-cenirRRuZdCHYekOFD6Q@mail.gmail.com%3e

[lgov]

Related issues:
#9913 - TODO list for Exception Handling Support

#11912 - failed to catch imdecode exception
#10389 - Report clear errors when opencv::imdecode fails.
#9475 - OpenCV Error: Assertion failed
#12999 - throw mxnet exceptions when decoding invalid images.
#10393 - ImageRecordIOParser2 Assertion failed
#12280 - Reading/ignoring corrupt images with Gluon data loader (imdecode error cannot be captured)

[leleamol]

@mxnet-label-bot add [OpenCV, Exception Handling]

[eric-haibin-lin]

Do you know why this happens? @lgov

[lgov]

Do you know why this happens? @lgov

Late reply, I missed the notification. But what exactly do you mean?
An application using mxnet will abort whenever there is an Assert triggered in the 3rd party libraries it uses, or atleast in OpenCV. That can for instance happen when you pass it an incorrect image file (take a text file and load it as .jpg for instance).