MDEV-16288 ALTER TABLE…ALGORITHM=DEFAULT does not override alter_algorithm

- ALTER_ALGORITHM should be substituted when there is no mention of
algorithm in alter statement.
- Introduced algorithm(thd) in Alter_info. It returns the
user requested algorithm. If user doesn't specify algorithm explicitly then
it returns alter_algorithm variable.
- changed algorithm() to get_algorithm(thd) to return algorithm name for
displaying the error.
- set_requested_algorithm(algo_value) to avoid direct assignment on
requested_algorithm variable.
- Avoid direct access of requested_algorithm to encapsulate
requested_algorithm variable

mysql-test/suite/innodb/r/alter_algorithm,NOCOPY.rdiff

@@ -1,6 +1,6 @@
---- alter_algorithm.result
-+++ alter_algorithm.reject
-@@ -7,40 +7,32 @@
+--- alter_algorithm.result	2020-04-30 21:39:48.923115511 +0530
++++ alter_algorithm.reject	2020-04-30 21:52:10.785967739 +0530
+@@ -7,43 +7,35 @@
  INSERT INTO t1(f1, f2, f3) VALUES(1, 1, 1);
  SELECT @@alter_algorithm;
  @@alter_algorithm
@@ -47,10 +47,15 @@
 -affected rows: 1
 -info: Records: 1  Duplicates: 0  Warnings: 0
 +Got one of the listed errors
+ ALTER TABLE t1 FORCE, ALGORITHM=DEFAULT;
+-affected rows: 1
+-info: Records: 1  Duplicates: 0  Warnings: 0
++affected rows: 0
++info: Records: 0  Duplicates: 0  Warnings: 0
  DROP TABLE t1;
  affected rows: 0
  CREATE TABLE t1(f1 INT PRIMARY KEY, f2 INT NOT NULL,
-@@ -53,22 +45,22 @@
+@@ -56,22 +48,22 @@
  FOREIGN KEY fidx(f1) REFERENCES t1(f1))ENGINE=INNODB;
  INSERT INTO t1(f1, f2, f4, f5) VALUES(1, 2, 3, 4);
  ALTER TABLE t1 ADD INDEX idx1(f4), page_compressed=1;
@@ -83,7 +88,7 @@
  DROP TABLE t2, t1;
  affected rows: 0
  CREATE TABLE t1(f1 INT NOT NULL,
-@@ -81,27 +73,27 @@
+@@ -84,28 +76,27 @@
  INSERT INTO t1(f1, f2) VALUES(1, 1);
  # Add column at the end of the table
  ALTER TABLE t1 ADD COLUMN f4 char(100) default 'BIG WALL';
@@ -111,7 +116,9 @@
 +info: Records: 0  Duplicates: 0  Warnings: 0
  # Rename table
  ALTER TABLE t1 RENAME t3;
- affected rows: 0
+-affected rows: 1
+-info: Records: 1  Duplicates: 0  Warnings: 0
++affected rows: 0
  # Drop Virtual Column
  ALTER TABLE t3 DROP COLUMN vcol;
 -affected rows: 1
@@ -121,7 +128,7 @@
  # Column length varies
  ALTER TABLE t2 CHANGE f3 f3 VARCHAR(20);
  affected rows: 0
-@@ -109,12 +101,12 @@
+@@ -113,12 +104,12 @@
  SET foreign_key_checks = 0;
  affected rows: 0
  ALTER TABLE t3 ADD FOREIGN KEY fidx(f2) REFERENCES t2(f1);