pidref: record pidfd inode number in PidRef struct

Besides internal comparisons, the inode number of pidfds
might be interesting directly to users, too. In the future
this field should also be exposed, so that it can serve as
a unique identifier of a process (but only for display,
as there's no method to map this back to a pid or pidfd).

src/basic/pidref.h

@@ -5,8 +5,10 @@
 
 /* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continuously. */
 typedef struct PidRef {
-        pid_t pid; /* always valid */
-        int fd;    /* only valid if pidfd are available in the kernel, and we manage to get an fd */
+        pid_t pid;      /* always valid */
+        int fd;         /* only valid if pidfd are available in the kernel, and we manage to get an fd */
+        uint64_t fd_id; /* the inode number of pidfd. only useful in kernel 6.9+ where pidfds live in
+                           their own pidfs and each process comes with a unique inode number */
 } PidRef;
 
 #define PIDREF_NULL (const PidRef) { .fd = -EBADF }
@@ -19,7 +21,8 @@ static inline bool pidref_is_set(const PidRef *pidref) {
         return pidref && pidref->pid > 0;
 }
 
-bool pidref_equal(const PidRef *a, const PidRef *b);
+int pidref_acquire_pidfd_id(PidRef *pidref);
+bool pidref_equal(PidRef *a, PidRef *b);
 
 /* This turns a pid_t into a PidRef structure, and acquires a pidfd for it, if possible. (As opposed to
  * PIDREF_MAKE_FROM_PID() above, which does not acquire a pidfd.) */