process-util: Add helpers to check for signal handler presence

Add new pidref_has_sigcgt() and pidref_has_sigblk() helpers. These read
the SigCgt and SigBlk bitmasks from /proc/<pid>/status to detect either
traditional signal handlers installed via sigaction(2), or blocked signals
that are typically handled via signalfd(2), such as through
sd_event_add_signal().

Both mechanisms represent valid ways to handle a signal without invoking
the default action. Share the parsing and pidref verification between the
two public helpers.

src/basic/process-util.c

@@ -21,6 +21,7 @@
 #include "alloc-util.h"
 #include "architecture.h"
 #include "argv-util.h"
+#include "bitfield.h"
 #include "capability-util.h"
 #include "cgroup-util.h"
 #include "dirent-util.h"
@@ -804,6 +805,53 @@ int get_process_umask(pid_t pid, mode_t *ret) {
         return parse_mode(m, ret);
 }
 
+static int pid_get_sigmask(pid_t pid, const char *field_name, uint64_t *ret) {
+        _cleanup_free_ char *field = NULL;
+        int r;
+
+        assert(pid >= 0);
+        assert(field_name);
+        assert(ret);
+
+        r = procfs_file_get_field(pid, "status", field_name, &field);
+        if (r == -ENOENT)
+                return -ESRCH;
+        if (r < 0)
+                return r;
+
+        return safe_atou64_full(field, 16, ret);
+}
+
+static int pidref_has_sigmask(const PidRef *pidref, int sig, const char *field_name) {
+        uint64_t mask;
+        int r;
+
+        if (!pidref_is_set(pidref))
+                return -ESRCH;
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+        if (!SIGNAL_VALID(sig))
+                return -EINVAL;
+
+        r = pid_get_sigmask(pidref->pid, field_name, &mask);
+        if (r < 0)
+                return r;
+
+        r = pidref_verify(pidref);
+        if (r < 0)
+                return r;
+
+        return BIT_SET(mask, sig - 1);
+}
+
+int pidref_has_sigcgt(const PidRef *pidref, int sig) {
+        return pidref_has_sigmask(pidref, sig, "SigCgt");
+}
+
+int pidref_has_sigblk(const PidRef *pidref, int sig) {
+        return pidref_has_sigmask(pidref, sig, "SigBlk");
+}
+
 /*
  * Return values:
  * < 0 : pidref_wait_for_terminate() failed to get the state of the

src/basic/process-util.h

@@ -53,6 +53,8 @@ int pidref_get_ppid_as_pidref(const PidRef *pidref, PidRef *ret);
 int pid_get_start_time(pid_t pid, usec_t *ret);
 int pidref_get_start_time(const PidRef *pid, usec_t *ret);
 int get_process_umask(pid_t pid, mode_t *ret);
+int pidref_has_sigcgt(const PidRef *pidref, int sig);
+int pidref_has_sigblk(const PidRef *pidref, int sig);
 
 static inline bool SIGINFO_CODE_IS_DEAD(int code) {
         return IN_SET(code, CLD_EXITED, CLD_KILLED, CLD_DUMPED);