20170918.ref.01.position.php
3ad727ba787cmaster

20170918.ref.01.position.php
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			<?php

			$table = new PhabricatorRepositoryRefPosition();
			$conn = $table->establishConnection('w');
			$key_name = 'key_position';

			try {
			queryfx(
			$conn,
			'ALTER TABLE %T DROP KEY %T',
			$table->getTableName(),
			$key_name);
			} catch (AphrontQueryException $ex) {
			// This key may or may not exist, depending on exactly when the install
			// ran previous migrations and adjustments. We're just dropping it if it
			// does exist.

			// We're doing this first (outside of the lock) because the MySQL
			// documentation says "if you ALTER TABLE a locked table, it may become
			// unlocked".
			}

			queryfx(
			$conn,
			'LOCK TABLES %T WRITE',
			$table->getTableName());

			$seen = array();
			foreach (new LiskMigrationIterator($table) as $position) {
			$cursor_id = $position->getCursorID();
			$hash = $position->getCommitIdentifier();

			// If this is the first copy of this row we've seen, mark it as seen and
			// move on.
			if (empty($seen[$cursor_id][$hash])) {
			$seen[$cursor_id][$hash] = true;
			continue;
			}

			// Otherwise, get rid of this row as it duplicates a row we saw previously.
			$position->delete();
			}

			queryfx(
			$conn,
			'ALTER TABLE %T ADD UNIQUE KEY %T (cursorID, commitIdentifier)',
			$table->getTableName(),
			$key_name);

			queryfx(
			$conn,
			'UNLOCK TABLES');