web: do not send 500 on invalid If-Modified-Since header

[rgajrawala]

Fixing #3408

[mcg1969]

Thank you for doing this!

[bdarnell]

Looks good, but it could use a simple test in web_test.py.

[rgajrawala]

@bdarnell done!

[rgajrawala]

@bdarnell @mcg1969 Can we please merge and publish this if it's good? We're getting pinged for 500s every time our security scan runs and unfortunately we can't override StaticFileHandler since tornado is not a direct dependency.

[bdarnell]

Looks like the catch should be except Exception, since there no strong guarantees about what type of error you might see here.

I can merge this once the tests are passing but I'm not planning another release for a while (months) so you'll probably want to make your own custom build and put it in a private package index in the meantime.

[rgajrawala]

Updated per your suggestion.

I'm not planning another release for a while

Even for a patch? I'll try to look into private package indexes...

[bdarnell]

Even for a patch?

I can do patches for severe bugs, but this doesn't seem like it reaches that level since it concerns the handling of deliberately-invalid requests (and the failure is a spurious 500 response instead of doing work that shouldn't be done).

I'll try to look into private package indexes...

Some folks like to use tools like devpi for this but I prefer to keep it simple and use PIP_FIND_LINKS as in this ancient example: https://github.com/viewfinderco/viewfinder/blob/master/scripts/update-environment