mysql_prepare_create_table() inconsistency

sql_field->key_length was 0 for blob fields when a field was
being added, but Field_blob::character_octet_length() on
subsequent ALTER TABLE's (when the Field object in the old table
already existed). This means mysql_prepare_create_table() couldn't
reliably detect if the keyseg was a prefix.

sql/sql_table.cc

@@ -4135,7 +4135,8 @@ mysql_prepare_create_table(THD *thd, HA_CREATE_INFO *create_info,
 	  key_info->flags|= HA_PACK_KEY;
       }
       /* Check if the key segment is partial, set the key flag accordingly */
-      if (key_part_length != sql_field->key_length)
+      if (key_part_length != sql_field->key_length &&
+          key_part_length != sql_field->type_handler()->max_octet_length())
         key_info->flags|= HA_KEY_HAS_PART_KEY_SEG;
 
       key_length+= key_part_length;
@@ -4482,7 +4483,7 @@ bool Column_definition::prepare_blob_field(THD *thd)
       set_handler(Type_handler::blob_type_handler((uint) length));
       pack_length= type_handler()->calc_pack_length(0);
     }
-    length= 0;
+    length= key_length= 0;
   }
   DBUG_RETURN(0);
 }

sql/sql_type.h

@@ -3228,6 +3228,7 @@ class Type_handler
   {
     return false;
   }
+  virtual uint max_octet_length() const { return 0; }
   /**
     Prepared statement long data:
     Check whether this parameter data type is compatible with long data.
@@ -5864,6 +5865,7 @@ class Type_handler_tiny_blob: public Type_handler_blob_common
                           const Record_addr &addr,
                           const Type_all_attributes &attr,
                           TABLE *table) const;
+  uint max_octet_length() const { return UINT_MAX8; }
 };
 
 
@@ -5879,6 +5881,7 @@ class Type_handler_medium_blob: public Type_handler_blob_common
                           const Record_addr &addr,
                           const Type_all_attributes &attr,
                           TABLE *table) const;
+  uint max_octet_length() const { return UINT_MAX24; }
 };
 
 
@@ -5896,6 +5899,7 @@ class Type_handler_long_blob: public Type_handler_blob_common
                           const Record_addr &addr,
                           const Type_all_attributes &attr,
                           TABLE *table) const;
+  uint max_octet_length() const { return UINT_MAX32; }
 };
 
 
@@ -5911,6 +5915,7 @@ class Type_handler_blob: public Type_handler_blob_common
                           const Record_addr &addr,
                           const Type_all_attributes &attr,
                           TABLE *table) const;
+  uint max_octet_length() const { return UINT_MAX16; }
 };