process-util: introduce namespace_get_leader helper

For a given PID and namespace type, this helper function gives the PID
of the leader of the namespace containing the given PID. Use this in
systemd-coredump instead of using the existing get_mount_namespace_leader.

This helper will be used again in a later commit.

src/basic/process-util.c

@@ -318,6 +318,33 @@ int container_get_leader(const char *machine, pid_t *pid) {
         return 0;
 }
 
+int namespace_get_leader(pid_t pid, NamespaceType type, pid_t *ret) {
+        int r;
+
+        assert(ret);
+
+        for (;;) {
+                pid_t ppid;
+
+                r = get_process_ppid(pid, &ppid);
+                if (r < 0)
+                        return r;
+
+                r = in_same_namespace(pid, ppid, type);
+                if (r < 0)
+                        return r;
+                if (r == 0) {
+                        /* If the parent and the child are not in the same
+                         * namespace, then the child is the leader we are
+                         * looking for. */
+                        *ret = pid;
+                        return 0;
+                }
+
+                pid = ppid;
+        }
+}
+
 int is_kernel_thread(pid_t pid) {
         _cleanup_free_ char *line = NULL;
         unsigned long long flags;

src/basic/process-util.h

@@ -14,6 +14,7 @@
 #include "alloc-util.h"
 #include "format-util.h"
 #include "macro.h"
+#include "namespace-util.h"
 #include "time-util.h"
 
 #define procfs_file_alloca(pid, field)                                  \
@@ -53,6 +54,8 @@ int get_process_umask(pid_t pid, mode_t *ret);
 
 int container_get_leader(const char *machine, pid_t *pid);
 
+int namespace_get_leader(pid_t pid, NamespaceType type, pid_t *ret);
+
 int wait_for_terminate(pid_t pid, siginfo_t *status);
 
 typedef enum WaitFlags {