From: Phillip Wood Date: Wed, 25 Jun 2025 08:39:25 GMT Subject: Re: [PATCH v2 0/3] daemon: explicitly allow EINTR during poll() Message-ID: <907a79d1-da2e-4c8e-963f-05c6e313643f@gmail.com> In-Reply-To: On 25/06/2025 08:35, 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. There is still a race where a child that exits after it has been checked in check_dead_children() but before we call poll() will not be collected until a new connection is received or a child exits while we're polling. If we used the self-pipe trick described on the select(2) man page [1] we would avoid that race and would not need to mess with SA_RESTART and so would not need to introduce USE_NON_POSIX_SIGNAL. Best Wishes Phillip [1] https://www.man7.org/linux/man-pages/man2/select.2.html > 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. > > Changes since v1 > > * Almost all references to siginterrupt has been removed and a better named > variable used instead > * Changes had been anstracted to minimize ifdefs and their introduction > staged more naturally > > Carlo Marcelo Arenas Belón (3): > compat/posix.h: track SA_RESTART fallback > daemon: use sigaction() to install child_handler() > daemon: explicitly allow EINTR during poll() > > Makefile | 6 +++++ > compat/mingw-posix.h | 1 - > compat/posix.h | 8 +++++++ > config.mak.uname | 7 +++--- > configure.ac | 17 +++++++++++++++ > daemon.c | 52 +++++++++++++++++++++++++++++++++++++++----- > meson.build | 4 ++++ > 7 files changed, 85 insertions(+), 10 deletions(-) > > > base-commit: cb3b40381e1d5ee32dde96521ad7cfd68eb308a6 > Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2002%2Fcarenas%2Fsiginterrupt-v2 > Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2002/carenas/siginterrupt-v2 > Pull-Request: https://github.com/git/git/pull/2002 > > Range-diff vs v1: > > 1: 2b5a58e53ac ! 1: e82b7425bbc compat/posix.h: track SA_RESTART fallback > @@ Metadata > ## Commit message ## > compat/posix.h: track SA_RESTART fallback > > - Systems without SA_RESTART where using custom CFLAGS instead of > - the standard header file. > + Systems without SA_RESTART are using custom CFLAGS or headers > + instead of the standard header file. > > - Consolidate that, so it will be easier to use in a future commit. > + Correct that, and invent a Makefile variable to track the > + exceptions which will become handy in the next commits. > > Signed-off-by: Carlo Marcelo Arenas Belón > > + ## Makefile ## > +@@ 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 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 > ++endif > + ifdef OPEN_RETURNS_EINTR > + COMPAT_CFLAGS += -DOPEN_RETURNS_EINTR > + endif > + > + ## compat/mingw-posix.h ## > +@@ compat/mingw-posix.h: struct sigaction { > + sig_handler_t sa_handler; > + unsigned sa_flags; > + }; > +-#define SA_RESTART 0 > + > + struct itimerval { > + struct timeval it_value, it_interval; > + > ## compat/posix.h ## > @@ compat/posix.h: char *gitdirname(char *); > #define NAME_MAX 255 > #endif > > -+/* On most systems would have given us this, but > ++/* > ++ * On most systems would have given us this, but > + * 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; > @@ compat/posix.h: char *gitdirname(char *); > #define parse_timestamp strtoumax > > ## config.mak.uname ## > +@@ config.mak.uname: ifeq ($(uname_S),Windows) > + NO_STRTOUMAX = YesPlease > + NO_MKDTEMP = YesPlease > + NO_INTTYPES_H = YesPlease > ++ USE_NON_POSIX_SIGNAL = YesPlease > + CSPRNG_METHOD = rtlgenrandom > + # VS2015 with UCRT claims that snprintf and friends are C99 compliant, > + # so we don't need this: > @@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL) > FREAD_READS_DIRECTORIES = UnfortunatelyYes > > @@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL) > # Apparently needed in compat/fnmatch/fnmatch.c. > COMPAT_CFLAGS += -DHAVE_STRING_H=1 > NO_ST_BLOCKS_IN_STRUCT_STAT = YesPlease > +@@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL) > + NO_MMAP = YesPlease > + NO_POLL = YesPlease > + NO_INTPTR_T = UnfortunatelyYes > ++ USE_NON_POSIX_SIGNAL = UnfortunatelyYes > + CSPRNG_METHOD = openssl > + SANE_TOOL_PATH = /usr/coreutils/bin:/usr/local/bin > + SHELL_PATH = /usr/coreutils/bin/bash > +@@ config.mak.uname: ifeq ($(uname_S),MINGW) > + NEEDS_LIBICONV = YesPlease > + NO_STRTOUMAX = YesPlease > + NO_MKDTEMP = YesPlease > ++ USE_NON_POSIX_SIGNAL = YesPlease > + NO_SVN_TESTS = YesPlease > + > + # The builtin FSMonitor requires Named Pipes and Threads on Windows. > @@ config.mak.uname: ifeq ($(uname_S),MINGW) > endif > endif > @@ config.mak.uname: ifeq ($(uname_S),MINGW) > EXPAT_NEEDS_XMLPARSE_H = YesPlease > HAVE_STRINGS_H = YesPlease > NEEDS_SOCKET = YesPlease > +@@ config.mak.uname: ifeq ($(uname_S),QNX) > + NO_PTHREADS = YesPlease > + NO_STRCASESTR = YesPlease > + NO_STRLCPY = YesPlease > ++ USE_NON_POSIX_SIGNAL = 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() > ++ > ++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],[ > ++ ac_cv_siginterrupt=no > ++ USE_NON_POSIX_SIGNAL=UnfortunatelyYes > ++ ] > ++ ) > ++]) > ++GIT_CONF_SUBST([USE_NON_POSIX_SIGNAL]) > ++ > + ## Checks for typedefs, structures, and compiler characteristics. > + AC_MSG_NOTICE([CHECKS for typedefs, structures, and compiler characteristics]) > + # > + > + ## meson.build ## > +@@ meson.build: else > + build_options_config.set('NO_EXPAT', '1') > + endif > + > ++if compiler.get_define('SA_RESTART', prefix: '#include ') == '' > ++ libgit_c_args += '-DUSE_NON_POSIX_SIGNAL' > ++endif > ++ > + if not compiler.has_header('sys/select.h') > + libgit_c_args += '-DNO_SYS_SELECT_H' > + endif > 2: 2e8c4643a60 ! 2: 05d945aa1e5 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(). > > - Replace the call, which hs the added benefit of using BSD semantics > - reliably and therefore not needing the rearming call. > + 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. > > Signed-off-by: Carlo Marcelo Arenas Belón > > ## daemon.c ## > -@@ daemon.c: static void child_handler(int signo UNUSED) > +@@ 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) > + { > /* > - * Otherwise empty handler because systemcalls will get interrupted > - * upon signal receipt > +- * Otherwise empty handler because systemcalls will get interrupted > +- * upon signal receipt > - * SysV needs the handler to be rearmed > ++ * Otherwise empty handler because systemcalls should get interrupted > ++ * 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 pollfd *pfd; > + struct sigaction sa; > + struct pollfd *pfd; > > CALLOC_ARRAY(pfd, socklist->nr); > - > @@ daemon.c: static int service_loop(struct socketlist *socklist) > pfd[i].events = POLLIN; > } > > - signal(SIGCHLD, child_handler); > -+ sigemptyset(&sa.sa_mask); > -+ sa.sa_flags = SA_NOCLDSTOP | SA_RESTART; > -+ sa.sa_handler = child_handler; > -+ sigaction(SIGCHLD, &sa, NULL); > ++ set_signal_handler(&sa); > > for (;;) { > check_dead_children(); > 3: a450bdb0066 ! 3: b737e0389df daemon: explicitly allow EINTR during poll() > @@ Commit message > might not return with -1 and set errno to EINTR when a signal is > received. > > - Since the logic to reap zombie childs relies om those interruptions > + Since the logic to reap zombie childs relies on those interruptions > make sure to explicitly disable SA_RESTART around this function. > > - Add a Makefile flag for portability to systems that don't have the > - functionality to change those flags or where it is not needed. > - > Signed-off-by: Carlo Marcelo Arenas Belón > > - ## Makefile ## > -@@ Makefile: include shared.mak > - # Define NO_PREAD if you have a problem with pread() system call (e.g. > - # cygwin1.dll before v1.5.22). > - # > -+# Define NO_SIGINTERRUPT if you don't have siginterrupt() or SA_RESTART > -+# or if your signal(SIGCHLD) implementation doesn't set SA_RESTART. > -+# > - # Define NO_SETITIMER if you don't have setitimer() > - # > - # Define NO_STRUCT_ITIMERVAL if you don't have struct itimerval > -@@ Makefile: ifdef NO_PREAD > - COMPAT_CFLAGS += -DNO_PREAD > - COMPAT_OBJS += compat/pread.o > - endif > -+ifdef NO_SIGINTERRUPT > -+ COMPAT_CFLAGS += -DNO_SIGINTERRUPT > -+endif > - ifdef NO_FAST_WORKING_DIRECTORY > - BASIC_CFLAGS += -DNO_FAST_WORKING_DIRECTORY > - endif > - > - ## config.mak.uname ## > -@@ config.mak.uname: ifeq ($(uname_S),Windows) > - NO_STRTOUMAX = YesPlease > - NO_MKDTEMP = YesPlease > - NO_INTTYPES_H = YesPlease > -+ NO_SIGINTERRUPT = YesPlease > - CSPRNG_METHOD = rtlgenrandom > - # VS2015 with UCRT claims that snprintf and friends are C99 compliant, > - # so we don't need this: > -@@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL) > - NO_PREAD = YesPlease > - NO_MMAP = YesPlease > - NO_POLL = YesPlease > -+ NO_SIGINTERRUPT = UnfortunatelyYes > - NO_INTPTR_T = UnfortunatelyYes > - CSPRNG_METHOD = openssl > - SANE_TOOL_PATH = /usr/coreutils/bin:/usr/local/bin > -@@ config.mak.uname: ifeq ($(uname_S),MINGW) > - NEEDS_LIBICONV = YesPlease > - NO_STRTOUMAX = YesPlease > - NO_MKDTEMP = YesPlease > -+ NO_SIGINTERRUPT = YesPlease > - NO_SVN_TESTS = YesPlease > - > - # The builtin FSMonitor requires Named Pipes and Threads on Windows. > -@@ config.mak.uname: ifeq ($(uname_S),QNX) > - NO_PTHREADS = YesPlease > - NO_STRCASESTR = YesPlease > - NO_STRLCPY = YesPlease > -+ NO_SIGINTERRUPT = UnfortunatelyYes > - endif > - > - ## configure.ac ## > -@@ configure.ac: GIT_CHECK_FUNC(getdelim, > - [HAVE_GETDELIM=]) > - GIT_CONF_SUBST([HAVE_GETDELIM]) > - # > -+# Define NO_SIGINTERRUPT if you don't have siginterrupt. > -+GIT_CHECK_FUNC(siginterrupt, > -+[NO_SIGINTERRUPT=], > -+[NO_SIGINTERRUPT=YesPlease]) > -+GIT_CONF_SUBST([NO_SIGINTERRUPT]) > - # > - # Define NO_MMAP if you want to avoid mmap. > - # > - > ## daemon.c ## > -@@ daemon.c: static void handle(int incoming, struct sockaddr *addr, socklen_t addrlen) > - add_child(&cld, addr, addrlen); > +@@ daemon.c: static void set_signal_handler(struct sigaction *psa) > + sigaction(SIGCHLD, psa, NULL); > } > > --static void child_handler(int signo UNUSED) > -+static void child_handler(int signo) > - { > - /* > -- * Otherwise empty handler because systemcalls will get interrupted > -- * upon signal receipt > -+ * Empty handler because systemcalls should get interrupted > -+ * upon signal receipt. > - */ > -+#ifdef NO_SIGINTERRUPT > -+ /* SysV needs the handler to be rearmed */ > -+ signal(signo, child_handler); > -+#endif > ++static void set_sa_restart(struct sigaction *psa, int enable) > ++{ > ++ if (enable) > ++ psa->sa_flags |= SA_RESTART; > ++ else > ++ psa->sa_flags &= ~SA_RESTART; > ++ 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); > } > > - static int set_reuse_addr(int sockfd) > -@@ daemon.c: static void socksetup(struct string_list *listen_addr, int listen_port, struct s > - > ++static void set_sa_restart(struct sigaction *psa UNUSED, int enable UNUSED) > ++{ > ++} > ++ > ++#endif > ++ > static int service_loop(struct socketlist *socklist) > { > -- struct pollfd *pfd; > -+#ifndef NO_SIGINTERRUPT > struct sigaction sa; > -+#endif > -+ struct pollfd *pfd; > - > - CALLOC_ARRAY(pfd, socklist->nr); > - > @@ daemon.c: static int service_loop(struct socketlist *socklist) > - pfd[i].events = POLLIN; > - } > - > -+#ifdef NO_SIGINTERRUPT > -+ signal(SIGCHLD, child_handler); > -+#else > - sigemptyset(&sa.sa_mask); > - sa.sa_flags = SA_NOCLDSTOP | SA_RESTART; > - sa.sa_handler = child_handler; > - sigaction(SIGCHLD, &sa, NULL); > -+#endif > - > for (;;) { > check_dead_children(); > > -+#ifndef NO_SIGINTERRUPT > -+ sa.sa_flags &= ~SA_RESTART; > -+ sigaction(SIGCHLD, &sa, NULL); > -+#endif > ++ set_sa_restart(&sa, 0); > if (poll(pfd, socklist->nr, -1) < 0) { > if (errno != EINTR) { > logerror("Poll failed, resuming: %s", > @@ daemon.c: static int service_loop(struct socketlist *socklist) > } > continue; > } > -+#ifndef NO_SIGINTERRUPT > -+ sa.sa_flags |= SA_RESTART; > -+ sigaction(SIGCHLD, &sa, NULL); > -+#endif > ++ set_sa_restart(&sa, 1); > > for (size_t i = 0; i < socklist->nr; i++) { > if (pfd[i].revents & POLLIN) { > - > - ## meson.build ## > -@@ meson.build: checkfuncs = { > - 'setenv' : ['setenv.c'], > - 'mkdtemp' : ['mkdtemp.c'], > - 'initgroups' : [], > -+ 'siginterrupt' : [], > - 'strtoumax' : ['strtoumax.c', 'strtoimax.c'], > - 'pread' : ['pread.c'], > - } >