[LalitMaganti]

Summary

Records the files each process opens on macOS.

Third of seven steps towards #2. Stacked on #28.

Why

File events let a recording link each file to the step that wrote it and the steps that read it. The kernel reports each path the way the program passed it, which is often relative. Buildprof has to work out the full path itself.

Changes

• Working directories: Buildprof follows each process's working directory through chdir and fchdir. A new process starts with its parent's.
• Descriptors: Buildprof also follows the path behind each open file descriptor, through open, dup, dup2, fcntl and close-on-exec. This resolves paths given relative to a directory descriptor.
• Resolving: each reported path is resolved against the right directory. Opening / is the one open the kernel reports no path for, so it is handled separately.
• Flags: open flags are stored with Linux's numbers, so the UI reads them the same way on both platforms.
• --no-file-events: the flag now has an effect on macOS, and produces a smaller recording. Collection costs about the same either way, because the events that carry working directories are needed regardless.

A path that goes through a relative symbolic link loses its starting directory. #31 recovers most of those.

Testing

Unit tests cover translating open flags, a new process inheriting its parent's directory and descriptors, ignoring unrelated processes, and normalizing paths.