tests: add hexquote_strndup function to libtests

* tests/tests.h (hexquote_strndup): New prototype.
* tests/hexquote_strndup.c: New file.
* tests/Makefile.am (libtests_a_SOURCES): Add it.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 667d863..da74adc 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -44,6 +44,7 @@
 	error_msg.c \
 	get_page_size.c \
 	hexdump_strdup.c \
+	hexquote_strndup.c \
 	inode_of_sockfd.c \
 	tail_alloc.c \
 	tests.h \
diff --git a/tests/hexquote_strndup.c b/tests/hexquote_strndup.c
new file mode 100644
index 0000000..3ad3970
--- /dev/null
+++ b/tests/hexquote_strndup.c
@@ -0,0 +1,58 @@
+/*
+ * Make a hexquoted copy of a string
+ *
+ * Copyright (c) 2016 Dmitry V. Levin <ldv@altlinux.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "tests.h"
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+const char *
+hexquote_strndup(const char *src, const size_t src_len)
+{
+	const size_t dst_size = 4 * src_len + 1;
+	assert(dst_size > src_len);
+
+	char *dst = malloc(dst_size);
+	if (!dst)
+		perror_msg_and_fail("malloc(%zu)", dst_size);
+
+	char *p = dst;
+	size_t i;
+	for (i = 0; i < src_len; ++i) {
+		unsigned int c = ((const unsigned char *) src)[i];
+		*(p++) = '\\';
+		*(p++) = 'x';
+		*(p++) = "0123456789abcdef"[c >> 4];
+		*(p++) = "0123456789abcdef"[c & 0xf];
+	}
+	*p = '\0';
+
+	return dst;
+}
diff --git a/tests/tests.h b/tests/tests.h
index 37db2eb..826f8b2 100644
--- a/tests/tests.h
+++ b/tests/tests.h
@@ -69,6 +69,9 @@
 /* Make a hexdump copy of C string */
 const char *hexdump_strdup(const char *);
 
+/* Make a hexquoted copy of a string */
+const char *hexquote_strndup(const char *, size_t);
+
 /* Return inode number of socket descriptor. */
 unsigned long inode_of_sockfd(int);