workaround to support pidfd_open (backported) while waitpid is missing

src/include/override/sys/pidfd.h

@@ -8,7 +8,8 @@
 /* Defined since glibc-2.36.
  * Supported since kernel v5.3 (7615d9e1780e26e0178c93c55b73309a5dc093d7). */
 int pidfd_open_shim(pid_t pid, unsigned flags);
-#define pidfd_open pidfd_open_shim
+int pidfd_open_checked(pid_t pid, unsigned flags);
+#define pidfd_open pidfd_open_checked
 
 /* Defined since glibc-2.36.
  * Supported since kernel v5.1 (3eb39f47934f9d5a3027fe00d906a45fe3a15fad). */

src/libc/pidfd.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <sys/pidfd.h>
+#include <sys/wait.h>
 
 #include "libc-shim.h"
 
@@ -8,6 +9,32 @@ DEFINE_SYSCALL_SHIM(pidfd_open, int,
                     pid_t, pid,
                     unsigned, flags)
 
+/* ANCIENT-MOD: vendor kernels < 5.4 (e.g. Android 4.19) may backport pidfd_open() without
+ * waitid(P_PIDFD); report pidfds as unsupported there so the plain-PID fallbacks are used. */
+int pidfd_open_checked(pid_t pid, unsigned flags) {
+        static int usable = -1;
+
+        if (usable < 0) {
+                int fd = pidfd_open_shim(getpid(), 0);
+                if (fd < 0) {
+                        if (errno == ENOSYS)
+                                usable = 0;
+                        return fd;
+                }
+
+                siginfo_t si = {};
+                usable = waitid(P_PIDFD, fd, &si, WEXITED|WNOHANG) >= 0 || errno != EINVAL;
+                close(fd);
+        }
+
+        if (!usable) {
+                errno = ENOSYS;
+                return -1;
+        }
+
+        return pidfd_open_shim(pid, flags);
+}
+
 DEFINE_SYSCALL_SHIM(pidfd_send_signal, int,
                     int, fd,
                     int, sig,