the family root's Makefile: under make -jN, every member's make warns jobserver unavailable: using -j1 and runs one job at a time #888

Goal: a person who types make -j4 bootstrap (or validate, siblings, park, resume) at a family root either gets the jobs they asked for in each member, or no warning that blames a rule they never wrote.

Filed by lane openRepoShape-5, from rv884's checklist candidate C1 in its review of #886 (#884), #886 issuecomment-6248806027. It is older than #884: rv884 measured it on 281df263 and on #886's head 82a84f88, under GNU Make 4.3.

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• Under the holder's make -j2 bootstrap, every member's make prints make[1]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule. and runs serially.
  • The holder's bootstrap: rule (templates/family-root/Makefile) runs $(PYTHON) scripts/bootstrap.py, with no + and no $(MAKE), so make does not pass the jobserver to it. run_make's Popen (templates/family-root/scripts/bootstrap.py) also closes every file descriptor past stderr by default.
  • The member's make still finds -j and --jobserver-auth in the MAKEFLAGS it inherits, cannot open the jobserver, and warns.
  • The warning tells the person to edit "the parent make rule". That rule is the holder's pinned Makefile, which the person does not own.
  • The same holds for validate (bootstrap.py --members-make validate) and for siblings.py's member makes (siblings, park, resume). That is read from the Makefile, not run.
  • Direction, one of:
    • mark the holder's member-running rules with + and pass the jobserver's descriptors through (pass_fds), so each member's make gets jobs;
    • or take -j and the jobserver out of the MAKEFLAGS a member's make inherits, so it runs serially with no warning;
    • or document that members run one job at a time.

Verify:

• Under make -j2 bootstrap at a holder with a member whose bootstrap target has two independent prerequisites, no jobserver unavailable line is printed. Its two jobs either run together or are documented as serial.
• tests/test_family_holder_bootstrap_texts.py, tests/test_family_bootstrap_gate_order.py and the trio pass.
• If the Makefile or bootstrap.py changes, each holder takes one upstream-changed row per pinned file (FAMILY_COPIED_VERBATIM).

Milestone: none set; lane 3's call. A warning and serial member runs under -j only. It is not a regression, and no verdict changes.

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