NO_CLOSE_RANGE macro for old glibc/kernel builds

pgxn/neon_walredo/walredoproc.c

@@ -159,17 +159,21 @@ static XLogReaderState *reader_state;
  * If the Linux uAPI headers don't define the system call number,
  * fail the build deliberately rather than ifdef'ing it to ENOSYS.
  * We prefer a compile time over a runtime error for walredo.
+ *
+ * If, however, we need to build on old systems for development, e.g. Ubuntu 20
+ * with glibc 2.31, provide a NO_CLOSE_RANGE macro for suboptimal implementation
  */
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <errno.h>
 
+#ifdef __NR_close_range
 static int
 close_range_syscall(unsigned int start_fd, unsigned int count, unsigned int flags)
 {
     return syscall(__NR_close_range, start_fd, count, flags);
 }
-
+#endif
 
 static PgSeccompRule allowed_syscalls[] =
 {
@@ -213,10 +217,24 @@ enter_seccomp_mode(void)
 	 * it potentially leaked to us, _before_ we start processing potentially dangerous
 	 * wal records. See the comment in the Rust code that launches this process.
 	 */
+#ifdef __NR_close_range
 	if (close_range_syscall(3, ~0U, 0) != 0)
 		ereport(FATAL,
 				(errcode(ERRCODE_SYSTEM_ERROR),
 				 errmsg("seccomp: could not close files >= fd 3")));
+#else
+#ifdef NO_CLOSE_RANGE
+// close_range can return EINVAL (not our case, as start_fd and end_fd are hardcoded.
+// It doesn't return any other erorrs if CLOSE_RANGE_UNSHARE is set (it isn't in our case)
+// So we don't report any errors
+#warning "__NR_close_range unavailable, using suboptimal implementation"
+	int fd;
+    for (fd = 3; fd <= INT_MAX; ++fd)
+		close(fd);
+#else
+#error "__NR_close_range syscall not defined, NO_CLOSE_RANGE not defined either"
+#endif
+#endif
 
 #ifdef MALLOC_NO_MMAP
 	/* Ask glibc not to use mmap() */