From 08c3a4dc3d909a6b227e132a3af7073a910d9b08 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julius=20H=C3=A4rtl?= <jus@bitgrid.net>
Date: Fri, 15 Dec 2023 21:52:31 +0100
Subject: [PATCH] feat: First attempt to track dirty tables after writes and
 switch back to replicas if reads go to other tables
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Signed-off-by: Julius Härtl <jus@bitgrid.net>
---
 lib/private/DB/Connection.php | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/lib/private/DB/Connection.php b/lib/private/DB/Connection.php
index d3cc93cdbe009..cae50da1455f2 100644
--- a/lib/private/DB/Connection.php
+++ b/lib/private/DB/Connection.php
@@ -81,6 +81,8 @@ class Connection extends PrimaryReadReplicaConnection {
 
 	protected ?float $transactionActiveSince = null;
 
+	protected $tableDirtyWrites = [];
+
 	/**
 	 * Initializes a new instance of the Connection class.
 	 *
@@ -257,6 +259,18 @@ public function prepare($sql, $limit = null, $offset = null): Statement {
 	 * @throws \Doctrine\DBAL\Exception
 	 */
 	public function executeQuery(string $sql, array $params = [], $types = [], QueryCacheProfile $qcp = null): Result {
+		$tables = $this->getQueriedTables($sql);
+		if (count(array_intersect($this->tableDirtyWrites, $tables)) === 0 && !$this->isTransactionActive()) {
+			// No tables read that could have been written already in the same request and no transaction active
+			// so we can switch back to the replica for reading as long as no writes happen that switch back to the primary
+			$this->ensureConnectedToReplica();
+			$this->logger->debug('no dirty table reads: ' . $sql, ['tables' => $this->tableDirtyWrites, 'reads' => $tables]);
+		} else {
+			// Read to a table that was previously written to
+			// While this might not necessarily mean that we did a read after write it is an indication for a code path to check
+			$this->logger->debug('dirty table reads: ' . $sql, ['tables' => $this->tableDirtyWrites, 'reads' => $tables, 'exception' => new \Exception()]);
+		}
+
 		$sql = $this->replaceTablePrefix($sql);
 		$sql = $this->adapter->fixupStatement($sql);
 		$this->queriesExecuted++;
@@ -264,6 +278,16 @@ public function executeQuery(string $sql, array $params = [], $types = [], Query
 		return parent::executeQuery($sql, $params, $types, $qcp);
 	}
 
+	/**
+	 * Helper function to get the list of tables affected by a given query
+	 * used to track dirty tables that received a write with the current request
+	 */
+	private function getQueriedTables(string $sql): array {
+		$re = '/(\*PREFIX\*[A-z0-9_-]+)/mi';
+		preg_match_all($re, $sql, $matches);
+		return array_map([$this, 'replaceTablePrefix'], $matches[0] ?? []);
+	}
+
 	/**
 	 * @throws Exception
 	 */
@@ -290,6 +314,9 @@ public function executeUpdate(string $sql, array $params = [], array $types = []
 	 * @throws \Doctrine\DBAL\Exception
 	 */
 	public function executeStatement($sql, array $params = [], array $types = []): int {
+		$tables = $this->getQueriedTables($sql);
+		$this->tableDirtyWrites = array_unique(array_merge($this->tableDirtyWrites, $tables));
+		$this->logger->debug('dirty table writes: ' . $sql, ['tables' => $this->tableDirtyWrites]);
 		$sql = $this->replaceTablePrefix($sql);
 		$sql = $this->adapter->fixupStatement($sql);
 		$this->queriesExecuted++;