py/mpprint: Fix length calculation for strings with precision-modifier.

Two issues are tackled:

1. The calculation of the correct length to print is fixed to treat the
   precision as a maximum length instead as the exact length.
   This is done for both qstr (%q) and for regular str (%s).

2. Fix the incorrect use of mp_printf("%.*s") to mp_print_strn().

   Because of the fix of above issue, some testcases that would print
   an embedded null-byte (^@ in test-output) would now fail.
   The bug here is that "%s" was used to print null-bytes. Instead,
   mp_print_strn is used to make sure all bytes are outputted and the
   exact length is respected.

Test-cases are added for both %s and %q with a combination of precision
and padding specifiers.

1 files changed, 7 insertions, 6 deletions

diff --git a/py/mpprint.c b/py/mpprint.c
index 31cf73bb58..3218bd2f4d 100644
--- a/py/mpprint.c
+++ b/py/mpprint.c
@@ -484,10 +484,10 @@ int mp_vprintf(const mp_print_t *print, const char *fmt, va_list args) {
                 qstr qst = va_arg(args, qstr);
                 size_t len;
                 const char *str = (const char *)qstr_data(qst, &len);
-                if (prec < 0) {
-                    prec = len;
+                if (prec >= 0 && (size_t)prec < len) {
+                    len = prec;
                 }
-                chrs += mp_print_strn(print, str, prec, flags, fill, width);
+                chrs += mp_print_strn(print, str, len, flags, fill, width);
                 break;
             }
             case 's': {
@@ -499,10 +499,11 @@ int mp_vprintf(const mp_print_t *print, const char *fmt, va_list args) {
                     break;
                 }
                 #endif
-                if (prec < 0) {
-                    prec = strlen(str);
+                size_t len = strlen(str);
+                if (prec >= 0 && (size_t)prec < len) {
+                    len = prec;
                 }
-                chrs += mp_print_strn(print, str, prec, flags, fill, width);
+                chrs += mp_print_strn(print, str, len, flags, fill, width);
                 break;
             }
             case 'd': {