libmount: add mnt_context_sprintf_errmsg()

Let's make it easy to create error message and return the message to
applications also when not generated by a syscall (kernel).

Signed-off-by: Karel Zak <kzak@redhat.com>

libmount/src/context.c

@@ -41,6 +41,7 @@
 #include "namespace.h"
 #include "match.h"
 
+#include <stdarg.h>
 #include <sys/wait.h>
 
 /**
@@ -2666,6 +2667,25 @@ int mnt_context_append_errmsg(struct libmnt_context *cxt, const char *msg)
 	return strappend(&cxt->errmsg, msg);
 }
 
+int mnt_context_sprintf_errmsg(struct libmnt_context *cxt, const char *msg, ...)
+{
+	int rc;
+	va_list ap;
+	char *p = NULL;
+
+	va_start(ap, msg);
+	rc = vasprintf(&p, msg, ap);
+	va_end(ap);
+
+	if (rc < 0 || !p)
+		return rc;
+
+	free(cxt->errmsg);
+	cxt->errmsg = p;
+
+	return 0;
+}
+
 /**
  * mnt_context_strerror
  * @cxt: context

libmount/src/context_mount.c

@@ -1433,6 +1433,11 @@ int mnt_context_get_mount_excode(
 	mnt_context_get_user_mflags(cxt, &uflags);	/* userspace flags */
 
 	if (!mnt_context_syscall_called(cxt)) {
+		if (buf && cxt->errmsg) {
+			xstrncpy(buf, cxt->errmsg, bufsz);
+			return MNT_EX_USAGE;
+		}
+
 		/*
 		 * libmount errors (extra library checks)
 		 */

libmount/src/mountP.h

@@ -628,6 +628,7 @@ extern int mnt_context_mount_setopt(struct libmnt_context *cxt, int c, char *arg
 
 extern int mnt_context_set_errmsg(struct libmnt_context *cxt, const char *msg);
 extern int mnt_context_append_errmsg(struct libmnt_context *cxt, const char *msg);
+extern int mnt_context_sprintf_errmsg(struct libmnt_context *cxt, const char *msg, ...);
 
 extern int mnt_context_propagation_only(struct libmnt_context *cxt)
 			__attribute__((nonnull));