Improve debug logging

[kleisauke]

When thumbnailing to 300x300 at our test environment, I now see:

[2017-08-30T19:59:41+00:00] debug: findLoad {"instance":null,"arguments":["##REDACTED##"]}
[2017-08-30T19:59:41+00:00] debug: findLoad {"result":["VipsForeignLoadJpegFile"]}
[2017-08-30T19:59:41+00:00] debug: newFromFile {"instance":null,"arguments":["##REDACTED##",{"access":"sequential"}]}
[2017-08-30T19:59:41+00:00] debug: newFromFile {"result":"{\"width\":1967,\"height\":2421,\"bands\":3,\"format\":\"uchar\",\"interpretation\":\"srgb\"}"}
[2017-08-30T19:59:41+00:00] debug: thumbnail {"instance":null,"arguments":["##REDACTED##",244,{"auto_rotate":false,"linear":false,"height":300,"size":"down"}]}
[2017-08-30T19:59:41+00:00] debug: thumbnail {"result":"{\"width\":244,\"height\":300,\"bands\":3,\"format\":\"uchar\",\"interpretation\":\"srgb\"}"}
[2017-08-30T19:59:41+00:00] debug: writeToBuffer {"instance":"{\"width\":244,\"height\":300,\"bands\":3,\"format\":\"uchar\",\"interpretation\":\"srgb\"}","arguments":[".jpg",{"strip":true,"Q":85,"interlace":false,"optimize_coding":true}]}
[2017-08-30T19:59:41+00:00] debug: writeToBuffer {"result":"##REDACTED##"}

Instead of:

[2017-08-30T18:39:21+00:00] debug: thumbnail {"instance":null,"arguments":["##REDACTED##",244,{"auto_rotate":false,"linear":false,"height":300,"size":"down"}]}
[2017-08-30T18:39:21+00:00] debug: thumbnail {"result":"{\"width\":244,\"height\":300,\"bands\":3,\"format\":\"uchar\",\"interpretation\":\"srgb\"}"}

When enabling the Vips\DebugLogger.

I didn't include the output of writeToFile (output: -1) and newInterpolator (output: GObject resource) because I think it's not needed for debugging.

If you want to hide the output/input of the *Buffer operations you can use this in the Vips\DebugLogger:

if (($message === 'findLoadBuffer' || $message === 'newFromBuffer') &&
    isset($context['arguments'][0])) {
    $context['arguments'][0] = '##REDACTED##';
}
if ($message === 'writeToBuffer' && isset($context['result'])) {
    $context['result'] = '##REDACTED##';
}

or to encode them with base64:

if (($message === 'findLoadBuffer' || $message === 'newFromBuffer') &&
    isset($context['arguments'][0])) {
    $context['arguments'][0] = base64_encode($context['arguments'][0]);
}
if ($message === 'writeToBuffer' && isset($context['result'])) {
    $context['result'] = base64_encode($context['result']);
}

(I didn't base64_encode them directly in the *Buffer operations because for performance reasons)

[jcupitt]

OK, looks good!

[jcupitt]

fwiw, I tried removing all the debug logging and it made no difference to the runtime of phpunit, so there's no performance impact, at least for the tests we do there.