pidfd-util: add helper for getting our own pidfdid

let's start caching our own pidfd id, since it's somewhat involved to
get but a much better identifier for things than the classic PID is.

src/basic/pidfd-util.c

@@ -9,6 +9,7 @@
 #include "macro.h"
 #include "memory-util.h"
 #include "missing_magic.h"
+#include "missing_threads.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "pidfd-util.h"
@@ -243,3 +244,32 @@ int pidfd_get_inode_id(int fd, uint64_t *ret) {
                 *ret = (uint64_t) st.st_ino;
         return 0;
 }
+
+int pidfd_get_inode_id_self_cached(uint64_t *ret) {
+        static thread_local uint64_t cached = 0;
+        static thread_local pid_t initialized = 0; /* < 0: cached error; == 0: invalid; > 0: valid and pid that was current */
+        int r;
+
+        assert(ret);
+
+        if (initialized == getpid_cached()) {
+                *ret = cached;
+                return 0;
+        }
+        if (initialized < 0)
+                return initialized;
+
+        _cleanup_close_ int fd = pidfd_open(getpid_cached(), 0);
+        if (fd < 0)
+                return -errno;
+
+        r = pidfd_get_inode_id(fd, &cached);
+        if (ERRNO_IS_NEG_NOT_SUPPORTED(r))
+                return (initialized = -EOPNOTSUPP);
+        if (r < 0)
+                return r;
+
+        *ret = cached;
+        initialized = getpid_cached();
+        return 0;
+}

src/basic/pidfd-util.h

@@ -17,3 +17,5 @@ int pidfd_get_uid(int fd, uid_t *ret);
 int pidfd_get_cgroupid(int fd, uint64_t *ret);
 
 int pidfd_get_inode_id(int fd, uint64_t *ret);
+
+int pidfd_get_inode_id_self_cached(uint64_t *ret);