compat: replace qsort_r() where missing

This should sort properly on Windows and any other platform without
qsort_r(). It does _not_ fix any potential issues on any platforms with
an incompatible qsort_r() prototype (such as FreeBSD < 13).

compat/qsort_r.c

@@ -0,0 +1,56 @@
+/*****************************************************************************
+ * qsort_r.c: future POSIX qsort_r() replacement
+ *****************************************************************************
+ * Copyright © 2018 Rémi Denis-Courmont
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <stdlib.h>
+
+static _Thread_local struct
+{
+    int (*compar)(const void *, const void *, void *);
+    void *arg;
+} state;
+
+static int compar_wrapper(const void *a, const void *b)
+{
+    return state.compar(a, b, state.arg);
+}
+
+/* Follow the upcoming POSIX prototype, coming from GNU/libc.
+ * Note that this differs from the BSD prototype. */
+
+void qsort_r(void *base, size_t nmemb, size_t size,
+             int (*compar)(const void *, const void *, void *),
+             void *arg)
+{
+    int (*saved_compar)(const void *, const void *, void *) = state.compar;
+    void *saved_arg = state.arg;
+
+    state.compar = compar;
+    state.arg = arg;
+
+    qsort(base, nmemb, size, compar_wrapper);
+
+    /* Restore state for nested reentrant calls */
+    state.compar = saved_compar;
+    state.arg = saved_arg;
+}

configure.ac

@@ -581,7 +581,7 @@ need_libc=false
 
 dnl Check for usual libc functions
 AC_CHECK_FUNCS([accept4 daemon fcntl flock fstatvfs fork getenv getpwuid_r isatty memalign mkostemp mmap open_memstream newlocale openat pipe2 pread posix_fadvise posix_madvise posix_memalign setlocale stricmp strnicmp strptime uselocale])
-AC_REPLACE_FUNCS([aligned_alloc atof atoll dirfd fdopendir flockfile fsync getdelim getpid lfind lldiv memrchr nrand48 poll recvmsg rewind sendmsg setenv strcasecmp strcasestr strdup strlcpy strndup strnlen strnstr strsep strtof strtok_r strtoll swab tdestroy tfind timegm timespec_get strverscmp pathconf])
+AC_REPLACE_FUNCS([aligned_alloc atof atoll dirfd fdopendir flockfile fsync getdelim getpid lfind lldiv memrchr nrand48 poll qsort_r recvmsg rewind sendmsg setenv strcasecmp strcasestr strdup strlcpy strndup strnlen strnstr strsep strtof strtok_r strtoll swab tdestroy tfind timegm timespec_get strverscmp pathconf])
 AC_REPLACE_FUNCS([gettimeofday])
 AC_CHECK_FUNC(fdatasync,,
   [AC_DEFINE(fdatasync, fsync, [Alias fdatasync() to fsync() if missing.])

include/vlc_fixups.h

@@ -90,6 +90,7 @@ typedef struct
 
 #if !defined (HAVE_ALIGNED_ALLOC) || \
     !defined (HAVE_MEMRCHR) || \
+    !defined (HAVE_QSORT_R) || \
     !defined (HAVE_STRLCPY) || \
     !defined (HAVE_STRNDUP) || \
     !defined (HAVE_STRNLEN) || \
@@ -223,6 +224,11 @@ long long atoll (const char *);
 lldiv_t lldiv (long long, long long);
 #endif
 
+#ifndef HAVE_QSORT_R
+void (qsort_r)(void *, size_t, size_t,
+               int (*)(const void *, const void *, void *), void *);
+#endif
+
 #ifndef HAVE_STRTOF
 #ifndef __ANDROID__
 float strtof (const char *, char **);