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[DNM] macos: record the process tree with the kernel trace facility - #34
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macOS has no unprivileged way to follow a process tree. Configuring the kernel trace facility needs root, but the kernel keeps granting access to the process that owns a session, so recording is privileged only while it starts: configure the session, give up root for good, then run the build, read events and write the trace as the person who ran it. This records the process tree: which process started which, what program each ran, and how each exited. The kernel also reports an exit when a process replaces its image, so an exit is held briefly and cancelled by anything further from that pid. Command lines, file events and compiler traces follow.
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Prototype for #2, step 1 of 6, on top of #33. Do not merge; the design is still TBD.
Records the process tree on macOS by reading the kernel trace facility. Starting a trace session needs root, but access is granted to the owning process and survives dropping privileges, so buildprof is privileged only while it starts: it configures the session, gives up root for good, then runs the build, reads events and writes the trace as you. The build does not run as root and keeps your environment.
This step records which process started which, what program each ran, and how each exited. Command lines, file events and compiler traces follow in the rest of the stack.
Verified on macOS 26: records a 23-process build and propagates its exit status.