MDEV-20856 Bad values in metadata views for partitions on VARBINARY

The old code to print partition values was too complicated:
- it created new Items for character set conversion purposes.
- it mixed string conversion and partition error reporting
  in the same code blocks.

Simplifying the code as follows:

- Adding helper methods String::can_be_safely_convert_to() and
  String::append_introducer_and_hex().

- Adding DBUG_EXECUTE_IF("generate_partition_syntax_for_frm",  push_warning...)
  into generate_partition_syntax_for_frm(), to test the PARTITON
  clause written to FRM. Adding test partition_utf8-debug.test for this.

- Removing functions get_cs_converted_part_value_from_string() and
  get_cs_converted_string_value. Changing get_partition_column_description()
  to use Type_handler::partition_field_append_value() instead.
  This makes SHOW CREATE TABLE and SELECT FROM I_S.PARTITIONS
  use the same code path.

- Changing Type_handler::partition_field_append_value() not to
  call convert_charset_partition_constant(), to avoid creating a new Item
  for string conversion pursposes.
  Rewritting the code to use only String methods.

- Removing error reporting code (ER_PARTITION_FUNCTION_IS_NOT_ALLOWED)
  from Type_handler::partition_field_append_value().
  The error is correctly detected and reported on the caller level.
  So error reporting was redundant here.

Also:

- Moving methods Type_handler::partition_field_*() from sql_partition.cc
  to sql_type.cc. This fixes compilation problem with -DPLUGIN_PARTITION=NO,
  earlier introduced by the patch for MDEV-20831.

1 files changed, 17 insertions, 0 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index deff81f3333..8ced35657e8 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -840,6 +840,15 @@ public:
   bool copy_aligned(const char *s, size_t arg_length, size_t offset,
 		    CHARSET_INFO *cs);
   bool set_or_copy_aligned(const char *s, size_t arg_length, CHARSET_INFO *cs);
+  bool can_be_safely_converted_to(CHARSET_INFO *tocs) const
+  {
+    if (charset() == &my_charset_bin)
+      return Well_formed_prefix(tocs, ptr(), length()).length() == length();
+    String try_val;
+    uint try_conv_error= 0;
+    try_val.copy(ptr(), length(), charset(), tocs, &try_conv_error);
+    return try_conv_error == 0;
+  }
   bool copy(const char*s, size_t arg_length, CHARSET_INFO *csfrom,
 	    CHARSET_INFO *csto, uint *errors);
   bool copy(const String *str, CHARSET_INFO *tocs, uint *errors)
@@ -874,6 +883,14 @@ public:
   {
     return Binary_string::append_hex((const char*)src, srclen);
   }
+  bool append_introducer_and_hex(CHARSET_INFO *cs, const LEX_CSTRING &str)
+  {
+    return
+      append(STRING_WITH_LEN("_"))   ||
+      append(cs->csname)             ||
+      append(STRING_WITH_LEN(" 0x")) ||
+      append_hex(str.str, (uint32) str.length);
+  }
   bool append(IO_CACHE* file, uint32 arg_length)
   {
     return Binary_string::append(file, arg_length);