contrib/chg/Makefile

worker: rewrite error handling so os._exit covers all cases

Previously the worker error handling is like:

    pid = os.fork()   --+
    if pid == 0:        |
        ....            | problematic
        ....          --+
        try:          --+
            ....        | worker error handling
                      --+

If a signal arrives when Python is executing the "problematic" lines, an
external error handling (dispatch.py) will take over the control flow and
it's no longer guaranteed "os._exit" is called (see 86cd09bc13ba for why it
is necessary).

This patch rewrites the error handling so it covers all possible code paths
for a worker even during fork.

Note: "os.getpid() == parentpid" is used to test if the process is parent or
not intentionally, instead of checking "pid", because "pid = os.fork()" may
be not atomic - it's possible that that a signal hits the worker before the
assignment completes [1].  The newly added test replaces "os.fork" to
exercise that extreme case.

[1]: CPython compiles "pid = os.fork()" to 2 byte codes: "CALL_FUNCTION" and
"STORE_FAST", so it's probably not atomic:

    def f():
        pid = os.fork()

    dis.dis(f)
      2           0 LOAD_GLOBAL              0 (os)
                  3 LOAD_ATTR                1 (fork)
                  6 CALL_FUNCTION            0
                  9 STORE_FAST               0 (pid)
                 12 LOAD_CONST               0 (None)
                 15 RETURN_VALUE

HG = $(CURDIR)/../../hg

TARGET = chg
SRCS = chg.c hgclient.c procutil.c util.c
OBJS = $(SRCS:.c=.o)

CFLAGS ?= -O2 -Wall -Wextra -pedantic -g
CPPFLAGS ?= -D_FORTIFY_SOURCE=2
override CFLAGS += -std=gnu99
ifdef HGPATH
override CPPFLAGS += -DHGPATH=\"$(HGPATH)\"
endif

DESTDIR =
PREFIX = /usr/local
MANDIR = $(PREFIX)/share/man/man1

CHGSOCKDIR = /tmp/chg$(shell id -u)
CHGSOCKNAME = $(CHGSOCKDIR)/server

.PHONY: all
all: $(TARGET)

$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $(OBJS)

chg.o: hgclient.h procutil.h util.h
hgclient.o: hgclient.h procutil.h util.h
procutil.o: procutil.h util.h
util.o: util.h

.PHONY: install
install: $(TARGET)
	install -d $(DESTDIR)$(PREFIX)/bin
	install -m 755 $(TARGET) $(DESTDIR)$(PREFIX)/bin
	install -d $(DESTDIR)$(MANDIR)
	install -m 644 chg.1 $(DESTDIR)$(MANDIR)

.PHONY: serve
serve:
	[ -d $(CHGSOCKDIR) ] || ( umask 077; mkdir $(CHGSOCKDIR) )
	$(HG) serve --cwd / --cmdserver chgunix \
		--address $(CHGSOCKNAME) \
		--config cmdserver.log=/dev/stderr

.PHONY: clean
clean:
	$(RM) $(OBJS)

.PHONY: distclean
distclean:
	$(RM) $(OBJS) $(TARGET)