std-aux: extract and add _nm_strerror_r() helper

We have nm_strerror_native_r(), which is the wrapper around strerror_r() that
we want to use in glib components (it also will ensure that the string is valid
UTF-8). However, it's not usable from non-glib components.

Move the part that abstracts strerror_r() out to libnm-std-aux as _nm_strerror_r().
The purpose is that non-glib componenent can use the thread-safe wrapper around
strerror_r().

src/libnm-glib-aux/nm-errno.c

@@ -6,6 +6,7 @@
 #include "libnm-glib-aux/nm-default-glib-i18n-lib.h"
 
 #include "nm-errno.h"
+#include "libnm-std-aux/nm-std-utils.h"
 
 /*****************************************************************************/
 
@@ -100,26 +101,10 @@ nm_strerror(int nmerr)
 const char *
 nm_strerror_native_r(int errsv, char *buf, gsize buf_size)
 {
-    char *buf2;
+    NM_AUTO_PROTECT_ERRNO(errsv2);
+    const char *buf2;
 
-    nm_assert(buf);
-    nm_assert(buf_size > 0);
-
-#if (!defined(__GLIBC__) && !defined(__UCLIBC__)) || ((_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE)
-    /* XSI-compliant */
-    {
-        int errno_saved = errno;
-
-        if (strerror_r(errsv, buf, buf_size) != 0) {
-            g_snprintf(buf, buf_size, "Unspecified errno %d", errsv);
-            errno = errno_saved;
-        }
-        buf2 = buf;
-    }
-#else
-    /* GNU-specific */
-    buf2 = strerror_r(errsv, buf, buf_size);
-#endif
+    buf2 = _nm_strerror_r(errsv, buf, buf_size);
 
     /* like g_strerror(), ensure that the error message is UTF-8. */
     if (!g_get_charset(NULL) && !g_utf8_validate(buf2, -1, NULL)) {

src/libnm-std-aux/nm-std-utils.c

@@ -92,3 +92,43 @@ out_huge:
     }
     return SIZE_MAX;
 }
+
+/*****************************************************************************/
+
+/**
+ * _nm_strerror_r:
+ * @errsv: the errno passed to strerror_r()
+ * @buf: the string buffer, must be non-null
+ * @buf_size: the size of the buffer, must be positive.
+ *
+ * A wrapper around strerror_r(). Does little else, aside clearing up the
+ * confusion about the different versions of the function.
+ *
+ * errno is preserved.
+ *
+ * Returns: the error string. This is either a static strong or @buf. It
+ *   is not guaranteed to be @buf.
+ */
+const char *
+_nm_strerror_r(int errsv, char *buf, size_t buf_size)
+{
+    NM_AUTO_PROTECT_ERRNO(errsv2);
+    char *buf2;
+
+    nm_assert(buf);
+    nm_assert(buf_size > 0);
+
+#if (!defined(__GLIBC__) && !defined(__UCLIBC__)) || ((_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE)
+    /* XSI-compliant */
+    if (strerror_r(errsv, buf, buf_size) != 0) {
+        snprintf(buf, buf_size, "Unspecified errno %d", errsv);
+    }
+    buf2 = buf;
+#else
+    /* GNU-specific */
+    buf2 = strerror_r(errsv, buf, buf_size);
+#endif
+
+    nm_assert(buf2);
+    return buf2;
+}

src/libnm-std-aux/nm-std-utils.h

@@ -35,4 +35,6 @@
 
 size_t nm_utils_get_next_realloc_size(bool true_realloc, size_t requested);
 
+const char *_nm_strerror_r(int errsv, char *buf, size_t buf_size);
+
 #endif /* __NM_STD_UTILS_H__ */