bug symfony#57261 [Yaml] Throw on duplicate key even when value is NU…

…LL (olsavmic)

This PR was squashed before being merged into the 7.2 branch.

Discussion
----------

[Yaml] Throw on duplicate key even when value is NULL

| Q             | A
| ------------- | ---
| Branch?       | 5.4 (since v3.0)
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | Fix symfony#57262
| License       | MIT

Duplicate keys are not valid by definition in YAML. The current implementation contains a bug that allows a key to be defined multiple times when the value is not set.

```yaml
services:
    Foo:
    Bar:
    Foo:
```

Extends symfony/yaml@8094454
to throw when a key is set twice in YAML without a value.

It may be technically a breaking change (as it suddenly makes some yaml-like files invalid), even though I'd classify it as a bugfix (as by definition, such files were not valid).

If we classify it as a bug, we should probably backport the fix to the oldest maintained version.

Commits
-------

f9df19b [Yaml] Throw on duplicate key even when value is NULL

src/Symfony/Component/Yaml/CHANGELOG.md

@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+7.2
+---
+
+ * duplicate mapping keys throw a `ParseException` even when such key has a NULL value
+
 7.1
 ---
 

src/Symfony/Component/Yaml/Parser.php

@@ -299,7 +299,7 @@ private function doParse(string $value, int $flags): mixed
                     if (!$this->isNextLineIndented() && !$this->isNextLineUnIndentedCollection()) {
                         // Spec: Keys MUST be unique; first one wins.
                         // But overwriting is allowed when a merge node is used in current block.
-                        if ($allowOverwrite || !isset($data[$key])) {
+                        if ($allowOverwrite || !\array_key_exists($key, $data)) {
                             if (null !== $subTag) {
                                 $data[$key] = new TaggedValue($subTag, '');
                             } else {
@@ -320,7 +320,7 @@ private function doParse(string $value, int $flags): mixed
                             }
 
                             $data += $value;
-                        } elseif ($allowOverwrite || !isset($data[$key])) {
+                        } elseif ($allowOverwrite || !\array_key_exists($key, $data)) {
                             // Spec: Keys MUST be unique; first one wins.
                             // But overwriting is allowed when a merge node is used in current block.
                             if (null !== $subTag) {
@@ -336,7 +336,7 @@ private function doParse(string $value, int $flags): mixed
                     $value = $this->parseValue(rtrim($values['value']), $flags, $context);
                     // Spec: Keys MUST be unique; first one wins.
                     // But overwriting is allowed when a merge node is used in current block.
-                    if ($allowOverwrite || !isset($data[$key])) {
+                    if ($allowOverwrite || !\array_key_exists($key, $data)) {
                         $data[$key] = $value;
                     } else {
                         throw new ParseException(sprintf('Duplicate key "%s" detected.', $key), $this->getRealCurrentLineNb() + 1, $this->currentLine);

src/Symfony/Component/Yaml/Tests/ParserTest.php

@@ -1026,6 +1026,14 @@ public static function getParseExceptionOnDuplicateData()
 EOD;
         $tests[] = [$yaml, 'child_sequence', 6];
 
+        $yaml = <<<EOD
+parent:
+  child:
+  child2:
+  child:
+EOD;
+        $tests[] = [$yaml, 'child', 4];
+
         return $tests;
     }