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');