MDEV-30034 UNIQUE USING HASH accepts duplicate entries for tricky collations

- Adding a new argument "flag" to MY_COLLATION_HANDLER::strnncollsp_nchars()
  and a flag MY_STRNNCOLLSP_NCHARS_EMULATE_TRIMMED_TRAILING_SPACES.
  The flag defines if strnncollsp_nchars() should emulate trailing spaces
  which were possibly trimmed earlier (e.g. in InnoDB CHAR compression).
  This is important for NOPAD collations.

  For example, with this input:
   - str1= 'a '    (Latin letter a followed by one space)
   - str2= 'a  '   (Latin letter a followed by two spaces)
   - nchars= 3
  if the flag is given, strnncollsp_nchars() will virtually restore
  one trailing space to str1 up to nchars (3) characters and compare two
  strings as equal:
  - str1= 'a  '  (one extra trailing space emulated)
  - str2= 'a  '  (as is)

  If the flag is not given, strnncollsp_nchars() does not add trailing
  virtual spaces, so in case of a NOPAD collation, str1 will be compared
  as less than str2 because it is shorter.

- Field_string::cmp_prefix() now passes the new flag.
  Field_varstring::cmp_prefix() and Field_blob::cmp_prefix() do
  not pass the new flag.

- The branch in cmp_whole_field() in storage/innobase/rem/rem0cmp.cc
  (which handles the CHAR data type) now also passed the new flag.

- Fixing UCA collations to respect the new flag.
  Other collations are possibly also affected, however
  I had no success in making an SQL script demonstrating the problem.
  Other collations will be extended to respect this flags in a separate
  patch later.

- Changing the meaning of the last parameter of Field::cmp_prefix()
  from "number of bytes" (internal length)
  to "number of characters" (user visible length).

  The code calling cmp_prefix() from handler.cc was wrong.
  After this change, the call in handler.cc became correct.

  The code calling cmp_prefix() from key_rec_cmp() in key.cc
  was adjusted according to this change.

- Old strnncollsp_nchar() related tests in unittest/strings/strings-t.c
  now pass the new flag.
  A few new tests also were added, without the flag.

mysql-test/include/ctype_nopad_prefix_unique.inc

@@ -0,0 +1,85 @@
+--echo #
+--echo # MDEV-30034 UNIQUE USING HASH accepts duplicate entries for tricky collations
+--echo #
+
+# TEXT
+
+if (`SELECT UPPER(@@storage_engine) != 'MEMORY'`)
+{
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a TEXT COLLATE <COLLATION>,'
+  'UNIQUE(a(3)))',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+--error ER_DUP_ENTRY
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;
+
+
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a TEXT COLLATE <COLLATION>,'
+  'UNIQUE(a(3)) USING HASH)',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+--error ER_DUP_ENTRY
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;
+}
+
+
+# VARCHAR
+
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a VARCHAR(2000) COLLATE <COLLATION>,'
+  'UNIQUE(a(3)))',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+--error ER_DUP_ENTRY
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;
+
+
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a VARCHAR(2000) COLLATE <COLLATION>,'
+  'UNIQUE(a(3)) USING HASH)',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+--error ER_DUP_ENTRY
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;
+
+# CHAR
+
+# MyISAM is buggy on CHAR+BTREE+UNIQUE+PREFIX (see MDEV-30048), disable for now
+# Other engines work fine
+
+if (`SELECT UPPER(@@storage_engine) != 'MYISAM'`)
+{
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a CHAR(20) COLLATE <COLLATION>,'
+  'UNIQUE(a(3)))',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;
+}
+
+EXECUTE IMMEDIATE REPLACE(
+  'CREATE TABLE t1 ( '
+  ' a CHAR(20) COLLATE <COLLATION>,'
+  'UNIQUE(a(3)) USING HASH)',
+  '<COLLATION>', @@collation_connection);
+SHOW CREATE TABLE t1;
+INSERT INTO t1 VALUES ('ss ');
+INSERT INTO t1 VALUES (_utf8mb3 0xC39F20)/*SZ+SPACE*/;
+DROP TABLE t1;