support analysis on-the-fly (interposition)

[dkrupp]

from cfe-dev:"
One important use case that scan-build/ccc-analyzer supports but that the LD_PRELOAD + compilation database approach doesn’t is running the analyzer as the code is being built rather than replaying the build commands later. This is important build systems that move or modify build system intermediates. I think it would be good to continue to have a way to support such projects. "

should support this feature through the CC/CXX environment variables and through the ld_logger.

[Xazax-hun]

scan-build-py supports this feature, probably we should use that.

[bruntib]

Are we going to join logging and analysis phase ever? I don't think it would be relevant in the future.

[Xazax-hun]

I think the main advantage of interposition is that it supports build systems where generated code is cleaned up at the and of the build process. We cannot handle such systems using the current model. We can say that such scenarios are not and never will be supported but it should be stated somewhere in the documentation.

[whisperity]

Cheap man's solution:

• we could provide a custom POSIX/GNU rm entrypoint by prepending the PATH of the running build, which only logs the files-to-be-deleted and do the actual remove in post-processing
• if the build system / compiler isn't using rm but rather the unlink() syscall, LD_PRELOAD might be able to give a custom behaviour for unlink too.

@Xazax-hun Do you have an overview/sketch idea how scan-build(-py?) does this?