Bug#55436: buffer overflow in debug binary of dbug_buff in Field_new_decimal::store_value

There were some misunderstandings about parameters pertaining to buffer-size.

Patches fixes the reported off by one and
clarifies the documentation.

mysql-test/r/type_newdecimal.result:
  add test
mysql-test/t/type_newdecimal.test:
  add test
sql/field.cc:
  adjust buffer size by one to account for terminator.
sql/my_decimal.cc:
  adjust buffer size by one to account for terminator.
  clarify needs in comments.
sql/my_decimal.h:
  clarify buffer-size needs to prevent future off-by-one bugs.
strings/decimal.c:
  clarify buffer-size needs and parameters to prevent future off-by-one bugs

1 files changed, 2 insertions, 1 deletions

diff --git a/sql/my_decimal.h b/sql/my_decimal.h
index 21669e82c44..2c13142bb60 100644
--- a/sql/my_decimal.h
+++ b/sql/my_decimal.h
@@ -55,7 +55,7 @@ C_MODE_END
 
 /**
   maximum length of string representation (number of maximum decimal
-  digits + 1 position for sign + 1 position for decimal point)
+  digits + 1 position for sign + 1 position for decimal point, no terminator)
 */
 #define DECIMAL_MAX_STR_LENGTH (DECIMAL_MAX_POSSIBLE_PRECISION + 2)
 
@@ -212,6 +212,7 @@ inline uint32 my_decimal_precision_to_length(uint precision, uint8 scale,
 inline
 int my_decimal_string_length(const my_decimal *d)
 {
+  /* length of string representation including terminating '\0' */
   return decimal_string_size(d);
 }