Duplicate entries in MySQL after performing a check (PHP) - php

Question

Lately, I have been getting some strange duplicate entries in my MySQL database. Entries in this table are inserted during a PUT request to a PHP page. The table contains 3 fields with no external references:
manga_id (primary key; auto increment) - bigint(20);
name - varchar(255);
manga_cid - varchar(255).
The PHP code is the following:
class MangaHandler {
private function getMangaFromName($name) {
$id = $this->generateId($name);
$mangas = new Query("SELECT * FROM tbl_manga WHERE manga_cid = '" . $this->conn->escapeString($id) . "'", $this->conn);
if(!$mangas || $mangas->hasError()) {
logError("getMangaFromName($name): " . $this->conn->getError());
return null;
}
if($mangas->moveNext()) {
return $mangas->getRow();
}
return null;
}
private function addManga($name) {
$manga_row = null;
$error = false;
$cid = $this->generateId($name);
$sql = sprintf("INSERT INTO tbl_manga(name, manga_cid) VALUES ('%s', '%s')", $this->conn->escapeString($name), $this->conn->escapeString($cid));
if(!$this->conn->execute($sql))
$error = true;
// some more code ...
if($error) {
logError("addManga($name): " . $this->conn->getError());
}
return $manga_row;
}
public function addMangaSourceAndFollow($name, $url, $source_id, $user_id, $stick_source = false, $stick_lang = 'English') {
// validate url
$manga = $this->getMangaFromUrl($url, $source_id);
if(!$manga) {
$manga = $this->getMangaFromName($name);
if(!$manga) $manga = $this->addManga($name);
// ...
}
// ...
return true;
}
}
class MangaRestService extends CommonRestService
{
public function performPut($url, $arguments, $accept, $raw) {
header('Content-type: application/json');
header("Cache-Control: no-cache, must-revalidate");
$json = json_decode($raw, true);
// some input validation and auth
$ms = new MangaHandler();
try {
$ret = $ms->addMangaSourceAndFollow(null, $json['url'], $source['source_id'], $user['user_id'], $enforce == 1);
// throw exception if some ret is invalid
// return proper json response
} catch(Exception $e) {
$conn->rollback();
logError("MangaRestService.performPut($url, [" . implode("; ", $arguments) . "], $accept, $raw): " . $e->getMessage());
echo RestResponse::getSomeErrorResponse()->toJSON();
}
}
}
$serv = new MangaRestService();
// performs some checks and invokes the appropriate GET, POST, PUT or DELETE method
// in this case it's the performPut method above
$serv->handleRawRequest();
The manga name gets filtered (only alphanumeric chars, underscore and some more characters are allowed) and becomes the manga_cid (which should be unique in the table).
The code basically checks if a manga with a specific manga_cid exists. Only if it doesn't, a new entry is inserted. I have tested the code and it works, however, after deploying the app., I have been getting entries with duplicate manga_cid (sometimes more than 2). This is a rare occurrence, as most entries remain unique. Also, I have no errors in my logs.
Could it be that for some reason, multiple HTTP PUT requests are being executed concurrently and, since there is no synchronization, INSERT gets called multiple times?
I find it very unlikely, specially because the app only lets you press the button that performs this request once before it disappears.
I have tried using MySQL transactions but it didn't solve the problem. I know that setting the field as unique will probably allow me to avoid these duplicate entries, however I would have to perform some heavy maintenance on the database in order to remove the duplicate entries first. Although this table has a simple structure, the manga_id is referenced in several other tables.
Also, I am curious as to why this is happening :-)
Just in case, here's the Query class:
class Query extends QueryBase
{
function Query($query, &$conn)
{
$this->recordset = array();
$this->has_error=0;
$regs = mysqli_query($conn->getConnection(), $query);
if(!$regs)
{
$this->has_error=1;
return;
}
$index = 0;
$this->current_index=-1;
while(($row = mysqli_fetch_array($regs, MYSQL_ASSOC)))
{
$this->recordset[$index]=$row;
$index++;
}
mysqli_free_result($regs);
}
public function moveNext()
{
if($this->current_index<(sizeof($this->recordset)-1))
{
$this->current_index++;
return 1;
}
else
return 0;
}
public function moveBack()
{
if($this->current_index>=1)
{
$this->current_index--;
return 1;
}
else
return 0;
}
public function recordCount()
{
return sizeof($this->recordset);
}
public function get($field)
{
return $this->recordset[$this->current_index][$field];
}
public function getRow()
{
return $this->recordset[$this->current_index];
}
public function hasError()
{
return $this->has_error;
}
}
Thank you for your help.

Answer 1

It sounds as though you are executing that block of code more than once, can you post the rest of the code? The code that deals with the 'put'?

Answer 2

As a general rule, if the process for checking for the existence of a value and insertion of a value are separate, there can be double entries due to someone accidentally clicking the submit button more than once (for example - or other action? image click...)
It's often a good practice to embed a duplicate submission check so that double submissions are avoided.

Answer 3

I'm not sure if this will fix it for you, but it would be better to return "false;" instead of returning "null" upon failure from your getManageFromName() method.
Another thing to check would your hasErrors() method; it might be returning errors for some reason sometimes?
You might be better off to write it like this:
private function getMangaFromName($name) {
$id = $this->generateId($name);
$mangas = new Query("SELECT * FROM tbl_manga WHERE manga_cid = '" . $this->conn->escapeString($id) . "'", $this->conn);
if($mangas->moveNext()) {
return $mangas->getRow();
} else {
if(!$mangas || $mangas->hasError()) {
logError("getMangaFromName($name): " . $this->conn->getError());
}
return null;
}
}
// Then check it like this:
$manga = $this->getMangaFromName($name);
if ( empty($manga) ) {
$manga = $this->addManga($name);
}