Block signals during fpm master initialization

Fix PHP-FPM failure in the case of concurrent reload attempts.

Postpone signal delivery to the fpm master process till proper signal
handlers are set. Prevent the following case:

- Running master process receives `SIGUSR2` and performs `execvp()`.
- Another `SIGUSR2` is arrived before signal handlers are set.
- Master process dies.
- Requests to the HTTP server handled by PHP-fpm can not be served
  any more.

Block some signals using `sigprocmask()` before `execvp()` and early
in the `main()` function. Unblock signals as soon as proper
handlers are set.

Fixes bug #74083

Author: plmnikulin

sapi/fpm/fpm/fpm_main.c

@@ -86,6 +86,7 @@ int __riscosify_control = __RISCOSIFY_STRICT_UNIX_SPECS;
 #include "fpm.h"
 #include "fpm_request.h"
 #include "fpm_status.h"
+#include "fpm_signals.h"
 #include "fpm_conf.h"
 #include "fpm_php.h"
 #include "fpm_log.h"
@@ -1570,6 +1571,15 @@ int main(int argc, char *argv[])
 								closes it.  in apache|apxs mode apache
 								does that for us!  thies@thieso.net
 								20000419 */
+
+	/* Subset of signals from fpm_signals_init_main() to avoid unexpected death during early init
+		or during reload just after execvp() or fork */
+	int init_signal_array[] = { SIGUSR1, SIGUSR2, SIGCHLD };
+	if (0 > fpm_signals_init_mask(init_signal_array, sizeof(init_signal_array)/sizeof(init_signal_array[0])) ||
+	    0 > fpm_signals_block()) {
+		zlog(ZLOG_WARNING, "Could die in the case of too early reload signal");
+	}
+	zlog(ZLOG_DEBUG, "Blocked some signals");
 #endif
 
 #ifdef ZTS

sapi/fpm/fpm/fpm_process_ctl.c

@@ -77,6 +77,10 @@ static void fpm_pctl_exit() /* {{{ */
 
 static void fpm_pctl_exec() /* {{{ */
 {
+	zlog(ZLOG_DEBUG, "Blocking some signals before reexec");
+	if (0 > fpm_signals_block()) {
+		zlog(ZLOG_WARNING, "concurrent reloads may be unstable");
+	}
 
 	zlog(ZLOG_NOTICE, "reloading: execvp(\"%s\", {\"%s\""
 			"%s%s%s" "%s%s%s" "%s%s%s" "%s%s%s" "%s%s%s"

sapi/fpm/fpm/fpm_signals.c

@@ -19,6 +19,7 @@
 #include "zlog.h"
 
 static int sp[2];
+static sigset_t block_sigset;
 
 const char *fpm_signal_names[NSIG + 1] = {
 #ifdef SIGHUP
@@ -210,6 +211,11 @@ int fpm_signals_init_main() /* {{{ */
 		zlog(ZLOG_SYSERROR, "failed to init signals: sigaction()");
 		return -1;
 	}
+
+	zlog(ZLOG_DEBUG, "Unblocking all signals");
+	if (0 > fpm_signals_unblock()) {
+		return -1;
+	}
 	return 0;
 }
 /* }}} */
@@ -250,3 +256,50 @@ int fpm_signals_get_fd() /* {{{ */
 	return sp[0];
 }
 /* }}} */
+
+int fpm_signals_init_mask(int *signum_array, size_t size) /* {{{ */
+{
+	size_t i = 0;
+	if (0 > sigemptyset(&block_sigset)) {
+		zlog(ZLOG_SYSERROR, "failed to prepare signal block mask: sigemptyset()");
+		return -1;
+	}
+	for (i = 0; i < size; ++i) {
+		int sig_i = signum_array[i];
+		if (0 > sigaddset(&block_sigset, sig_i)) {
+			if (sig_i <= NSIG && fpm_signal_names[sig_i] != NULL) {
+				zlog(ZLOG_SYSERROR, "failed to prepare signal block mask: sigaddset(%s)",
+						fpm_signal_names[sig_i]);
+			} else {
+				zlog(ZLOG_SYSERROR, "failed to prepare signal block mask: sigaddset(%d)", sig_i);
+			}
+			return -1;
+		}
+	}
+	return 0;
+}
+/* }}} */
+
+int fpm_signals_block() /* {{{ */
+{
+	if (0 > sigprocmask(SIG_BLOCK, &block_sigset, NULL)) {
+		zlog(ZLOG_SYSERROR, "failed to block signals");
+		return -1;
+	}
+	return 0;
+}
+/* }}} */
+
+int fpm_signals_unblock() /* {{{ */
+{
+	/* Ensure that during reload after upgrade all signals are unblocked.
+		block_sigset could have different value before execve() */
+	sigset_t all_signals;
+	sigfillset(&all_signals);
+	if (0 > sigprocmask(SIG_UNBLOCK, &all_signals, NULL)) {
+		zlog(ZLOG_SYSERROR, "failed to unblock signals");
+		return -1;
+	}
+	return 0;
+}
+/* }}} */

sapi/fpm/fpm/fpm_signals.h

@@ -8,6 +8,9 @@
 int fpm_signals_init_main();
 int fpm_signals_init_child();
 int fpm_signals_get_fd();
+int fpm_signals_init_mask(int *signum_array, size_t size);
+int fpm_signals_block();
+int fpm_signals_unblock();
 
 extern const char *fpm_signal_names[NSIG + 1];