nss: block various signals while running NSS lookups

Let's make sure our poll() calls don't get interrupted where they shouldn't (SIGALRM, ...), but allow them to be
interrupted where they should (SIGINT, ...).

Fixes #1965

src/basic/nss-util.h

@@ -27,6 +27,8 @@
 #include <pwd.h>
 #include <resolv.h>
 
+#define NSS_SIGNALS_BLOCK SIGALRM,SIGVTALRM,SIGPIPE,SIGCHLD,SIGTSTP,SIGIO,SIGHUP,SIGUSR1,SIGUSR2,SIGPROF,SIGURG,SIGWINCH
+
 #define NSS_GETHOSTBYNAME_PROTOTYPES(module)            \
 enum nss_status _nss_##module##_gethostbyname4_r(       \
                 const char *name,                       \

src/basic/signal-util.h

@@ -41,3 +41,14 @@ int signal_from_string(const char *s) _pure_;
 int signal_from_string_try_harder(const char *s);
 
 void nop_signal_handler(int sig);
+
+static inline void block_signals_reset(sigset_t *ss) {
+        assert_se(sigprocmask(SIG_SETMASK, ss, NULL) >= 0);
+}
+
+#define BLOCK_SIGNALS(...)                                              \
+        _cleanup_(block_signals_reset) sigset_t _saved_sigset = ({      \
+                sigset_t t;                                             \
+                assert_se(sigprocmask_many(SIG_BLOCK, &t, __VA_ARGS__, -1) >= 0); \
+                t;                                                      \
+        })