From: Phillip Wood Date: Wed, 09 Jul 2025 14:12:43 GMT Subject: Re: [PATCH v3 0/4] daemon: explicitly allow EINTR during poll() Message-ID: In-Reply-To: On 26/06/2025 09:53, Carlo Marcelo Arenas Belón via GitGitGadget wrote: > This series addresses and ambiguity that is at least visible in OpenBSD, > where zombie proceses would only be cleared after a new connection is > received. > > The underlying problem is that when this code was originally introduced, > SA_RESTART was not widely implemented, and the signal() call usually > implemented SysV like semantics, at least until it started being > reimplemented by calling sigaction() internally. I'm all in favor of using sigaction() but I think the SA_RESTART parts of this series are an unnecessary complication that has the potential to hide bugs as we support platforms without SA_RESTART. Your EINTR patches have all been about efficiency rather than correctness so I think my initial assessment that the existing code handles EINTR correctly was probably true. As I said in my comments on patch 3 I think we can happily drop patches 1 and 4. Thanks Phillip > Changes since v2 > > * Add a new patch 2 that modifies windows' sigaction so there is no more > need for a fallback > * Hopefully no more silly mistakes and a variable that finally makes sense > > Changes since v1 > > * Almost all references to siginterrupt has been removed and a better named > variable used instead > * Changes had been abstracted to minimize ifdefs and their introduction > staged more naturally > > Carlo Marcelo Arenas Belón (4): > compat/posix.h: track SA_RESTART fallback > compat/mingw: allow sigaction(SIGCHLD) > daemon: use sigaction() to install child_handler() > daemon: explicitly allow EINTR during poll() > > Makefile | 5 +++++ > compat/mingw-posix.h | 2 +- > compat/mingw.c | 4 +++- > compat/posix.h | 8 ++++++++ > config.mak.uname | 7 ++++--- > configure.ac | 16 ++++++++++++++++ > daemon.c | 33 ++++++++++++++++++++++++++++----- > meson.build | 4 ++++ > 8 files changed, 69 insertions(+), 10 deletions(-) > > > base-commit: cb3b40381e1d5ee32dde96521ad7cfd68eb308a6 > Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2002%2Fcarenas%2Fsiginterrupt-v3 > Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2002/carenas/siginterrupt-v3 > Pull-Request: https://github.com/git/git/pull/2002 > > Range-diff vs v2: > > 1: e82b7425bbc ! 1: ae1ca6bb2b2 compat/posix.h: track SA_RESTART fallback > @@ Makefile: include shared.mak > # when attempting to read from an fopen'ed directory (or even to fopen > # it at all). > # > -+# Define USE_NON_POSIX_SIGNAL if don't have support for SA_RESTART or > -+# prefer to use ANSI C signal() over POSIX sigaction() > ++# Define NO_RESTARTABLE_SIGNALS if don't have support for SA_RESTART > +# > # Define OPEN_RETURNS_EINTR if your open() system call may return EINTR > # when a signal is received (as opposed to restarting). > @@ Makefile: ifdef FREAD_READS_DIRECTORIES > COMPAT_CFLAGS += -DFREAD_READS_DIRECTORIES > COMPAT_OBJS += compat/fopen.o > endif > -+ifdef USE_NON_POSIX_SIGNAL > -+ COMPAT_CFLAGS += -DUSE_NON_POSIX_SIGNAL > ++ifdef NO_RESTARTABLE_SIGNALS > ++ COMPAT_CFLAGS += -DNO_RESTARTABLE_SIGNALS > +endif > ifdef OPEN_RETURNS_EINTR > COMPAT_CFLAGS += -DOPEN_RETURNS_EINTR > @@ compat/posix.h: char *gitdirname(char *); > + * not on some systems (e.g. NonStop, QNX). > + */ > +#ifndef SA_RESTART > -+# define SA_RESTART 0 /* disabled for sigaction() */ > ++# define SA_RESTART 0 /* disabled for sigaction() */ > +#endif > + > typedef uintmax_t timestamp_t; > @@ config.mak.uname: ifeq ($(uname_S),Windows) > NO_STRTOUMAX = YesPlease > NO_MKDTEMP = YesPlease > NO_INTTYPES_H = YesPlease > -+ USE_NON_POSIX_SIGNAL = YesPlease > ++ NO_RESTARTABLE_SIGNALS = YesPlease > CSPRNG_METHOD = rtlgenrandom > # VS2015 with UCRT claims that snprintf and friends are C99 compliant, > # so we don't need this: > @@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL) > NO_MMAP = YesPlease > NO_POLL = YesPlease > NO_INTPTR_T = UnfortunatelyYes > -+ USE_NON_POSIX_SIGNAL = UnfortunatelyYes > ++ NO_RESTARTABLE_SIGNALS = UnfortunatelyYes > CSPRNG_METHOD = openssl > SANE_TOOL_PATH = /usr/coreutils/bin:/usr/local/bin > SHELL_PATH = /usr/coreutils/bin/bash > @@ config.mak.uname: ifeq ($(uname_S),MINGW) > NEEDS_LIBICONV = YesPlease > NO_STRTOUMAX = YesPlease > NO_MKDTEMP = YesPlease > -+ USE_NON_POSIX_SIGNAL = YesPlease > ++ NO_RESTARTABLE_SIGNALS = YesPlease > NO_SVN_TESTS = YesPlease > > # The builtin FSMonitor requires Named Pipes and Threads on Windows. > @@ config.mak.uname: ifeq ($(uname_S),QNX) > NO_PTHREADS = YesPlease > NO_STRCASESTR = YesPlease > NO_STRLCPY = YesPlease > -+ USE_NON_POSIX_SIGNAL = UnfortunatelyYes > ++ NO_RESTARTABLE_SIGNALS = UnfortunatelyYes > endif > > ## configure.ac ## > @@ configure.ac: fi > GIT_CONF_SUBST([ICONV_OMITS_BOM]) > fi > > -+# Define USE_NON_POSIX_SIGNAL if don't have support for SA_RESTART or > -+# prefer using ANSI C signal() over POSIX sigaction() > ++# Define NO_RESTARTABLE_SIGNALS if don't have support for SA_RESTART > + > +AC_CACHE_CHECK([whether SA_RESTART is supported], [ac_cv_siginterrupt], [ > + AC_COMPILE_IFELSE( > + [AC_LANG_PROGRAM([#include ], [[ > -+ #ifdef SA_RESTART > -+ #endif > -+ siginterrupt(SIGCHLD, 1) > -+ ]])],[ac_cv_siginterrupt=yes],[ > ++ #ifdef SA_RESTART > ++ restartable signals supported > ++ #endif > ++ ]])],[ > + ac_cv_siginterrupt=no > -+ USE_NON_POSIX_SIGNAL=UnfortunatelyYes > -+ ] > ++ NO_RESTARTABLE_SIGNALS=UnfortunatelyYes > ++ ], [ac_cv_siginterrupt=yes] > + ) > +]) > -+GIT_CONF_SUBST([USE_NON_POSIX_SIGNAL]) > ++GIT_CONF_SUBST([NO_RESTARTABLE_SIGNALS]) > + > ## Checks for typedefs, structures, and compiler characteristics. > AC_MSG_NOTICE([CHECKS for typedefs, structures, and compiler characteristics]) > @@ meson.build: else > endif > > +if compiler.get_define('SA_RESTART', prefix: '#include ') == '' > -+ libgit_c_args += '-DUSE_NON_POSIX_SIGNAL' > ++ libgit_c_args += '-DNO_RESTARTABLE_SIGNALS' > +endif > + > if not compiler.has_header('sys/select.h') > -: ----------- > 2: 3f63479119f compat/mingw: allow sigaction(SIGCHLD) > 2: 05d945aa1e5 ! 3: c66bda461f4 daemon: use sigaction() to install child_handler() > @@ Commit message > In a future change, the flags used for processing SIGCHLD will need to > be updated, which is only possible by using sigaction(). > > - Factor out the call to set the signal handler and use sigaction instead > - of signal for the systems that allow that, which has the added benefit > - of using BSD semantics reliably and therefore not needing the rearming > - call. > + Replace signal() with an equivalent invocation of sigaction(), which > + has the added benefit of using BSD semantics reliably and therefore > + not needing the rearming call in the signal handler. > > Signed-off-by: Carlo Marcelo Arenas Belón > > ## daemon.c ## > @@ daemon.c: static void handle(int incoming, struct sockaddr *addr, socklen_t addrlen) > - add_child(&cld, addr, addrlen); > - } > - > --static void child_handler(int signo UNUSED) > -+static void child_handler(int signo MAYBE_UNUSED) > + static void child_handler(int signo UNUSED) > { > /* > - * Otherwise empty handler because systemcalls will get interrupted > @@ daemon.c: static void handle(int incoming, struct sockaddr *addr, socklen_t addr > + * upon signal receipt. > */ > - signal(SIGCHLD, child_handler); > -+#ifdef USE_NON_POSIX_SIGNAL > -+ /* > -+ * SysV needs the handler to be rearmed, but this is known > -+ * to trigger infinite recursion crashes at least in AIX. > -+ */ > -+ signal(signo, child_handler); > -+#endif > } > > static int set_reuse_addr(int sockfd) > @@ daemon.c: static void socksetup(struct string_list *listen_addr, int listen_port, struct s > - } > - } > > -+#ifndef USE_NON_POSIX_SIGNAL > -+ > -+static void set_signal_handler(struct sigaction *psa) > -+{ > -+ sigemptyset(&psa->sa_mask); > -+ psa->sa_flags = SA_NOCLDSTOP | SA_RESTART; > -+ psa->sa_handler = child_handler; > -+ sigaction(SIGCHLD, psa, NULL); > -+} > -+ > -+#else > -+ > -+static void set_signal_handler(struct sigaction *psa UNUSED) > -+{ > -+ signal(SIGCHLD, child_handler); > -+} > -+ > static int service_loop(struct socketlist *socklist) > { > + struct sigaction sa; > @@ daemon.c: static int service_loop(struct socketlist *socklist) > } > > - signal(SIGCHLD, child_handler); > -+ set_signal_handler(&sa); > ++ sigemptyset(&sa.sa_mask); > ++ sa.sa_flags = SA_NOCLDSTOP | SA_RESTART; > ++ sa.sa_handler = child_handler; > ++ sigaction(SIGCHLD, &sa, NULL); > > for (;;) { > check_dead_children(); > 3: b737e0389df ! 4: 851d663be0b daemon: explicitly allow EINTR during poll() > @@ Commit message > Signed-off-by: Carlo Marcelo Arenas Belón > > ## daemon.c ## > -@@ daemon.c: static void set_signal_handler(struct sigaction *psa) > - sigaction(SIGCHLD, psa, NULL); > +@@ daemon.c: static void socksetup(struct string_list *listen_addr, int listen_port, struct s > + } > } > > ++#ifndef NO_RESTARTABLE_SIGNALS > ++ > +static void set_sa_restart(struct sigaction *psa, int enable) > +{ > + if (enable) > @@ daemon.c: static void set_signal_handler(struct sigaction *psa) > + sigaction(SIGCHLD, psa, NULL); > +} > + > - #else > - > - static void set_signal_handler(struct sigaction *psa UNUSED) > -@@ daemon.c: static void set_signal_handler(struct sigaction *psa UNUSED) > - signal(SIGCHLD, child_handler); > - } > - > ++#else > ++ > +static void set_sa_restart(struct sigaction *psa UNUSED, int enable UNUSED) > +{ > +} >