tuklib_mbstr_nonprint: Preserve the value of errno

A typical use case is like this:

    printf("%s: %s\n", tuklib_mask_nonprint(filename), strerror(errno));

tuklib_mask_nonprint() may call mbrtowc() and malloc() which may modify
errno. If errno isn't preserved, the error message might be wrong if
a compiler decides to call tuklib_mask_nonprint() before strerror().

Fixes: 40e5733

src/common/tuklib_mbstr_nonprint.c

@@ -12,6 +12,7 @@
 #include "tuklib_mbstr_nonprint.h"
 #include <stdlib.h>
 #include <string.h>
+#include <errno.h>
 
 #ifdef HAVE_MBRTOWC
 #	include <wchar.h>
@@ -94,22 +95,29 @@ has_nonprint(const char *str, size_t len)
 extern bool
 tuklib_has_nonprint(const char *str)
 {
-	return has_nonprint(str, strlen(str));
+	const int saved_errno = errno;
+	const bool ret = has_nonprint(str, strlen(str));
+	errno = saved_errno;
+	return ret;
 }
 
 
 extern const char *
 tuklib_mask_nonprint_r(const char *str, char **mem)
 {
+	const int saved_errno = errno;
+
 	// Free the old string, if any.
 	free(*mem);
 	*mem = NULL;
 
 	// If the whole input string contains only printable characters,
 	// return the input string.
 	const size_t len = strlen(str);
-	if (!has_nonprint(str, len))
+	if (!has_nonprint(str, len)) {
+		errno = saved_errno;
 		return str;
+	}
 
 	// Allocate memory for the masked string. Since we use the single-byte
 	// character '?' to mask non-printable characters, it's possible that
@@ -119,8 +127,10 @@ tuklib_mask_nonprint_r(const char *str, char **mem)
 	// If allocation fails, return "???" because it should be safer than
 	// returning the unmasked string.
 	*mem = malloc(len + 1);
-	if (*mem == NULL)
+	if (*mem == NULL) {
+		errno = saved_errno;
 		return "???";
+	}
 
 	// Replace all non-printable characters with '?'.
 	char *dest = *mem;
@@ -139,6 +149,7 @@ tuklib_mask_nonprint_r(const char *str, char **mem)
 
 	*dest = '\0';
 
+	errno = saved_errno;
 	return *mem;
 }
 

src/common/tuklib_mbstr_nonprint.h

@@ -25,7 +25,8 @@ extern bool tuklib_has_nonprint(const char *str);
 /// \brief      Check if a string contains any non-printable characters
 ///
 /// \return     false if str contains only valid multibyte characters and
-///             iswprint(3) returns non-zero for all of them; true otherwise
+///             iswprint(3) returns non-zero for all of them; true otherwise.
+///             The value of errno is preserved.
 ///
 /// \note       In case mbrtowc(3) isn't available, single-byte character set
 ///             is assumed and isprint(3) is used instead of iswprint(3).
@@ -49,6 +50,7 @@ extern const char *tuklib_mask_nonprint_r(const char *str, char **mem);
 ///             allocated memory is also stored to *mem. A modified string
 ///             has the problematic characters replaced by '?'. If memory
 ///             allocation fails, "???" is returned and *mem is NULL.
+///             The value of errno is preserved.
 
 #define tuklib_mask_nonprint TUKLIB_SYMBOL(tuklib_mask_nonprint)
 extern const char *tuklib_mask_nonprint(const char *str);