config: user: check for <aio.h>

And always zpool_read_label_slow() on non-conformant libcs

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13207
Closes #13254

config/user-aio.h.m4

@@ -0,0 +1,7 @@
+dnl #
+dnl # POSIX specifies <aio.h> as part of realtime extensions,
+dnl # and is missing from at least uClibc – force fallbacks there
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_USER_AIO_H], [
+	ZFS_AC_FIND_SYSTEM_LIBRARY(AIO_H, [], [aio.h], [], [rt], [lio_listio])
+])

config/user-libaio.m4

@@ -1,5 +1,5 @@
 dnl #
-dnl # Check for libaio - only used for libaiot test cases.
+dnl # Check for libaio - only used for mmap_libaio test cases.
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER_LIBAIO], [
 	ZFS_AC_FIND_SYSTEM_LIBRARY(LIBAIO, [], [libaio.h], [], [aio], [], [user_libaio=yes], [user_libaio=no])

config/user.m4

@@ -23,6 +23,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_LIBAIO
 	ZFS_AC_CONFIG_USER_LIBATOMIC
 	ZFS_AC_CONFIG_USER_LIBFETCH
+	ZFS_AC_CONFIG_USER_AIO_H
 	ZFS_AC_CONFIG_USER_CLOCK_GETTIME
 	ZFS_AC_CONFIG_USER_PAM
 	ZFS_AC_CONFIG_USER_RUNSTATEDIR

lib/libzutil/zutil_import.c

@@ -47,7 +47,9 @@
  * using our derived config, and record the results.
  */
 
+#ifdef HAVE_AIO_H
 #include <aio.h>
+#endif
 #include <ctype.h>
 #include <dirent.h>
 #include <errno.h>
@@ -982,6 +984,9 @@ zpool_read_label_slow(int fd, nvlist_t **config, int *num_labels)
 int
 zpool_read_label(int fd, nvlist_t **config, int *num_labels)
 {
+#ifndef HAVE_AIO_H
+	return (zpool_read_label_slow(fd, config, num_labels));
+#else
 	struct stat64 statbuf;
 	struct aiocb aiocbs[VDEV_LABELS];
 	struct aiocb *aiocbps[VDEV_LABELS];
@@ -1104,6 +1109,7 @@ zpool_read_label(int fd, nvlist_t **config, int *num_labels)
 	*config = expected_config;
 
 	return (0);
+#endif
 }
 
 /*