lib: Add str_truncate_utf8().

It works similar to str_truncate(), but it makes sure the truncated string
remains valid UTF-8.

src/lib/str.c

@@ -3,6 +3,7 @@
 #include "lib.h"
 #include "buffer.h"
 #include "printf-format-fix.h"
+#include "unichar.h"
 #include "str.h"
 
 #include <stdio.h>
@@ -146,3 +147,12 @@ void str_vprintfa(string_t *str, const char *fmt, va_list args)
 	/* drop the unused data, including terminating NUL */
 	buffer_set_used_size(str, pos + ret);
 }
+
+void str_truncate_utf8(string_t *str, size_t len)
+{
+	size_t size = str_len(str);
+
+	if (size <= len)
+		return;
+	str_truncate(str, uni_utf8_data_truncate(str_data(str), size, len));
+}

src/lib/str.h

@@ -78,4 +78,10 @@ static inline void str_truncate(string_t *str, size_t len)
 		buffer_set_used_size(str, len);
 }
 
+/* Truncate the string to specified length, but also make sure the truncation
+   doesn't happen in the middle of an UTF-8 character sequence. In that case,
+   the string will end up being up to a few bytes smaller than len. If it's
+   already smaller to begin with, do nothing. */
+void str_truncate_utf8(string_t *str, size_t len);
+
 #endif

src/lib/test-str.c

@@ -1,6 +1,7 @@
 /* Copyright (c) 2012-2018 Dovecot authors, see the included COPYING file */
 
 #include "test-lib.h"
+#include "unichar.h"
 #include "str.h"
 
 static void test_str_append(void)
@@ -127,6 +128,48 @@ static void test_str_truncate(void)
 	test_end();
 }
 
+static void test_str_truncate_utf8(void)
+{
+	string_t *str = t_str_new(8);
+	int i;
+
+	test_begin("str_truncate_utf8()");
+	str_append(str, "123456");
+	for (i = 100; i >= 6; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(str_len(str) == 6, i);
+	}
+	for (; i >= 0; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(str_len(str) == (unsigned int)i, i);
+	}
+
+	str_append(str, "\xE4\xB8\x80\xE4\xBa\x8C\xE4\xB8\x89"
+			"\xE5\x9b\x9b\xE4\xBa\x94\xE5\x85\xAD");
+	for (i = 100; i >= 18; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(str_len(str) == 18, i);
+	}
+	for (; i >= 0; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(str_len(str) % 3 == 0, i);
+		test_assert_idx((str_len(str) / 3) == ((unsigned int)i / 3), i);
+	}
+
+	str_append(str, "\xE4\xB8\x80""1""\xE4\xBa\x8C""2""\xE4\xB8\x89""3"
+			"\xE5\x9b\x9b""4""\xE4\xBa\x94""5""\xE5\x85\xAD""6");
+	for (i = 100; i >= 24; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(str_len(str) == 24, i);
+	}
+	for (; i >= 0; i--) {
+		str_truncate_utf8(str, i);
+		test_assert_idx(uni_utf8_data_is_valid(str_data(str),
+						       str_len(str)), i);
+	}
+	test_end();
+}
+
 void test_str(void)
 {
 	test_str_append();
@@ -135,4 +178,5 @@ void test_str(void)
 	test_str_delete();
 	test_str_append_max();
 	test_str_truncate();
+	test_str_truncate_utf8();
 }