pidref: use fd_inode_same to compare pidfds

src/basic/pidref.c

@@ -8,6 +8,33 @@
 #include "pidref.h"
 #include "process-util.h"
 #include "signal-util.h"
+#include "stat-util.h"
+
+bool pidref_equal(const PidRef *a, const PidRef *b) {
+        int r;
+
+        if (pidref_is_set(a)) {
+                if (!pidref_is_set(b))
+                        return false;
+
+                if (a->pid != b->pid)
+                        return false;
+
+                if (a->fd < 0 || b->fd < 0)
+                        return true;
+
+                /* pidfds live in their own pidfs and each process comes with a unique inode number since
+                 * kernel 6.8. We can safely do this on older kernels too though, as previously anonymous
+                 * inode was used and inode number was the same for all pidfds. */
+                r = fd_inode_same(a->fd, b->fd);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to check whether pidfds for pid " PID_FMT " are equal, assuming yes: %m",
+                                        a->pid);
+                return r != 0;
+        }
+
+        return !pidref_is_set(b);
+}
 
 int pidref_set_pid(PidRef *pidref, pid_t pid) {
         int fd;