Add a unit test for extract area out of bounds

[mhirsch]

This unit test checks to see that an assertion is thrown if extract is called for an area outside the image size.

This test doesn't pass right now. I'm not sure if this is even a good test to have -- but I'm submitting a pr in order to understand better. What's the recommended way to throw a proper exception from within native code in node? Obviously it isn't possible to know at the time execute is called in the js layer whether the area falls within the image bounds. So this will be caught in pipeline.cc, Execute(). There's this (baton->err).append(...); return Error(); pattern, but that results in the same type of error handling as an error that is thrown from vips, e.g.:

Unhandled rejection Error: extract_area: bad extract area

    at Error (native)

The test suite doesn't see that as a valid exception, so the unit test fails.

[lovell]

The test needs to include toBuffer( ... ) to invoke the pipeline.

[mhirsch]

With this test, there is an Unhandled rejection Error thrown -- you can see this on the node command line, but that doesn't register as the assert that the test suite is looking for. Are you aware if there's a way to catch this type of error and throw a proper assert?

[lovell]

Once you're in C++ land, any error becomes the first parameter of the invoked callback function, e.g.:

it('Bad image area', function(done) {
  sharp(fixtures.inputJpg)
    .extract({ left: 3000, top: 10, width: 10, height: 10 })
    .toBuffer(function(err) {
      assert(err instanceof Error);
      done();
    });
});

[mhirsch]

Ok! That's what I was looking for. Thanks.

[coveralls]

Coverage remained the same at 98.371% when pulling c4449dd on mhirsch:extract_bounds_unit_test into 8dd554b on lovell:master.

[lovell]

Thank you!