Why is PDO throwing an ErrorException rather than a PDOException? - php

Question

I have Sentry keeping track of uncaught exceptions in my PHP application, and I noticed a peculiar uncaught exception from PDO. The code looks like this:
/**
* #return boolean TRUE if the connection to the database worked; FALSE otherwise.
*/
public function verifyDatabase() {
try{
$this->pdo->query('SELECT 1');
return true;
}
catch (\PDOException $e) {
echo 'Lost connection to database: ' . $e->getMessage() . PHP_EOL;
return false;
}
}
This should catch errors like "MySQL server has gone away", and it indeed works on my development machine. However, Sentry recently recorded this error:
ErrorException
PDO::query(): MySQL server has gone away
According to sentry, this was thrown by the $this->pdo->query('SELECT 1'); statement above. Errors like this should have been caught by the try/catch. Why is PDO throwing an ErrorException rather than a PDOException?

Answer 1

I can't reproduce an ErrorException.
$pdo = new PDO('mysql:host=127.0.0.1;dbname=test', ..., ...);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
sleep(20); // during this sleep, I stop my MySQL Server instance.
$result = $pdo->query("SELECT 1");
Output:
Warning: PDO::query(): MySQL server has gone away
Warning: PDO::query(): Error reading result set's header
Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away
Stack trace:
#0 /Users/bkarwin/Documents/SO/pdo.php(8): PDO->query('SELECT 1')
This shows it throws a PDOException when the server has gone away, not an ErrorException.
Tested with MySQL 5.6.37 and PHP 7.1.23.
I wonder if the code you show in your question is actually the code that is deployed and throwing the exception to Sentry. Perhaps you have some code like:
catch (\PDOException $e) {
throw ErrorException($e->getMessage());
}
Either in your verifyDatabase() function, or else in the code that calls verifyDatabase(). In other words, what does your app do when verifyDatabase() returns false?

Answer 2

Okay, I think I've figured it out. It appears that this is related to a bug in which the PDO MySQL driver emits warnings even when they are supposed to be disabled (See also: this answer). I believe Sentry is then catching these as errors.
I was finally able to replicate, and solve this, by modifying Bill Karwin's test script:
// Initialize the Sentry reporting client
$ravenClient = new \Raven_Client(SENTRY_KEY);
$ravenClient->install();
echo 'Connecting...' . PHP_EOL;
$pdo = new PDO("mysql:host=$host;dbname=$dbname;charset=utf8mb4", $username, $password);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
echo 'Connected. Waiting...' . PHP_EOL;
sleep(20); // during this sleep, I stop my MySQL Server instance.
echo 'Querying...' . PHP_EOL;
try {
$result = $pdo->query("SELECT 1");
}
catch(\PDOException $e) {
echo 'Caught PDOException ' . $e->getMessage() . PHP_EOL;
}
This will print the following:
Connecting...
Connected. Waiting...
Querying...
PHP Warning: PDO::query(): MySQL server has gone away in /home/xxx/src/test.php on line 37
PHP Stack trace:
PHP 1. {main}() /home/xxx/src/test.php:0
PHP 2. PDO->query() /home/xxx/src/test.php:37
Caught PDOException SQLSTATE[HY000]: General error: 2006 MySQL server has gone away
Done.
And Sentry will record an ErrorException on the query() line.
I was able to solve this issue by implementing the "solution" posted by jferrer on the PHP bug report.
// Convert NOTICE, WARNING, ... in Exceptions
$convertErrorToException = function ($level, $message, $file, $line){
throw new ErrorException($message, 0, $level, $file, $line);
};
// The $previousErrorHandler will contain Sentry's handler
$previousErrorHandler = set_error_handler($convertErrorToException);
try {
$result = $pdo->query("SELECT 1");
}
catch(\PDOException $e) {
echo 'Caught PDOException ' . $e->getMessage() . PHP_EOL;
}
catch(\ErrorException $e) {
echo 'Caught ErrorException ' . $e->getMessage() . PHP_EOL;
}
// Restore Sentry as the default handler
set_error_handler($previousErrorHandler);
This results in just the ErrorException being thrown and caught:
Connecting...
Connected. Waiting...
Querying...
Caught ErrorException PDO::query(): MySQL server has gone away
Done.