Fix account_patch_reviews for mysql

Creation of the account_patch_reviews table failed on mysql with the
file_name column being set to varchar(4096), see [1].
mysql has a limit of 767 bytes max key length for InnoDb tables.
The max key length limit applies to each column in the (primary key)
index.

MySql uses up to 3 bytes per character when utf8 character set is used.
Therefore, the max length of the file_name column is 255 characgters
when using utf8 character set. This change sets the file_name column
length to 255 when using mysql.

The whole "CREATE TABLE ..." statement is extracted into the
MysqlAccountPatchReviewStore subclass. This gives more freedom
for further mysql specific tuning.

Change-Id: If35aff6c8dc515228c770729bfbf3a83e3dcc6bf

Documentation/pgm-MigrateAccountPatchReviewDb.txt

@@ -27,6 +27,12 @@ To migrate AccountPatchReviewDb:
 * Migrate data using this command
 * Start Gerrit
 
+[NOTE]
+When using MySQL, the file_name column length in the account_patch_reviews table will be shortened
+from the standard 4096 characters down to 255 characters. This is due to a
+link:https://dev.mysql.com/doc/refman/5.7/en/innodb-restrictions.html[MySQL limitation]
+on the max size of 767 bytes for each column in an index.
+
 == OPTIONS
 
 -d::

gerrit-server/src/main/java/com/google/gerrit/server/schema/JdbcAccountPatchReviewStore.java

@@ -181,7 +181,7 @@ public abstract class JdbcAccountPatchReviewStore
     }
   }
 
-  private static void doCreateTable(Statement stmt) throws SQLException {
+  protected void doCreateTable(Statement stmt) throws SQLException {
     stmt.executeUpdate(
         "CREATE TABLE IF NOT EXISTS account_patch_reviews ("
             + "account_id INTEGER DEFAULT 0 NOT NULL, "

gerrit-server/src/main/java/com/google/gerrit/server/schema/MysqlAccountPatchReviewStore.java

@@ -22,6 +22,7 @@ import com.google.gwtorm.server.OrmException;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import java.sql.SQLException;
+import java.sql.Statement;
 import org.eclipse.jgit.lib.Config;
 
 @Singleton
@@ -50,4 +51,17 @@ public class MysqlAccountPatchReviewStore extends JdbcAccountPatchReviewStore {
         return new OrmException(op + " failure on ACCOUNT_PATCH_REVIEWS", err);
     }
   }
+
+  @Override
+  protected void doCreateTable(Statement stmt) throws SQLException {
+    stmt.executeUpdate(
+        "CREATE TABLE IF NOT EXISTS account_patch_reviews ("
+            + "account_id INTEGER DEFAULT 0 NOT NULL, "
+            + "change_id INTEGER DEFAULT 0 NOT NULL, "
+            + "patch_set_id INTEGER DEFAULT 0 NOT NULL, "
+            + "file_name VARCHAR(255) DEFAULT '' NOT NULL, "
+            + "CONSTRAINT primary_key_account_patch_reviews "
+            + "PRIMARY KEY (change_id, patch_set_id, account_id, file_name)"
+            + ")");
+  }
 }