bug symfony#50437 [Filesystem] Follow symlinks when dumping files (ausi)

This PR was squashed before being merged into the 5.4 branch.

Discussion
----------

[Filesystem] Follow symlinks when dumping files

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Currently, if `Filesystem::dumpFile()` is called on a symlink, the link gets replaced with a new file.

This is unexpected I think because both `Filesystem::appendToFile()` as well as `file_put_contents()` follow the symlink instead.

This PR changes `dumpFile()` to behave the same way.

Commits
-------

4aa8b46 [Filesystem] Follow symlinks when dumping files

Author: nicolas-grekas

src/Symfony/Component/Filesystem/Filesystem.php

@@ -669,6 +669,12 @@ public function dumpFile(string $filename, $content)
 
         $dir = \dirname($filename);
 
+        if (is_link($filename) && $linkTarget = $this->readlink($filename)) {
+            $this->dumpFile(Path::makeAbsolute($linkTarget, $dir), $content);
+
+            return;
+        }
+
         if (!is_dir($dir)) {
             $this->mkdir($dir);
         }

src/Symfony/Component/Filesystem/Tests/FilesystemTest.php

@@ -1091,14 +1091,16 @@ public function testReadBrokenLink()
     {
         $this->markAsSkippedIfSymlinkIsMissing();
 
-        if ('\\' === \DIRECTORY_SEPARATOR) {
-            $this->markTestSkipped('Windows does not support creating "broken" symlinks');
+        if ('\\' === \DIRECTORY_SEPARATOR && \PHP_VERSION_ID < 70400) {
+            $this->markTestSkipped('Windows does not support reading "broken" symlinks in PHP < 7.4.0');
         }
 
         $file = $this->workspace.'/file';
         $link = $this->workspace.'/link';
 
+        touch($file);
         $this->filesystem->symlink($file, $link);
+        $this->filesystem->remove($file);
 
         $this->assertEquals($file, $this->filesystem->readlink($link));
         $this->assertNull($this->filesystem->readlink($link, true));
@@ -1605,6 +1607,38 @@ public function testDumpFileOverwritesAnExistingFile()
         $this->assertStringEqualsFile($filename, 'bar');
     }
 
+    public function testDumpFileFollowsSymlink()
+    {
+        $filename = $this->workspace.\DIRECTORY_SEPARATOR.'foo.txt';
+        file_put_contents($filename, 'FOO BAR');
+        $linknameA = $this->workspace.\DIRECTORY_SEPARATOR.'bar.txt';
+        $linknameB = $this->workspace.\DIRECTORY_SEPARATOR.'baz.txt';
+        $this->filesystem->symlink($filename, $linknameA);
+        $this->filesystem->symlink($linknameA, $linknameB);
+
+        $this->filesystem->dumpFile($linknameB, 'bar');
+
+        $this->assertFileExists($filename);
+        $this->assertFileExists($linknameA);
+        $this->assertFileExists($linknameB);
+        $this->assertStringEqualsFile($filename, 'bar');
+        $this->assertStringEqualsFile($linknameA, 'bar');
+        $this->assertStringEqualsFile($linknameB, 'bar');
+
+        // Windows does not support reading "broken" symlinks in PHP < 7.4.0
+        if ('\\' === \DIRECTORY_SEPARATOR && \PHP_VERSION_ID < 70400) {
+            return;
+        }
+
+        $this->filesystem->remove($filename);
+        $this->filesystem->dumpFile($linknameB, 'baz');
+
+        $this->assertFileExists($filename);
+        $this->assertStringEqualsFile($filename, 'baz');
+        $this->assertStringEqualsFile($linknameA, 'baz');
+        $this->assertStringEqualsFile($linknameB, 'baz');
+    }
+
     public function testDumpFileWithFileScheme()
     {
         $scheme = 'file://';
@@ -1678,6 +1712,35 @@ public function testAppendToFileWithResource()
         }
     }
 
+    public function testAppendToFileFollowsSymlink()
+    {
+        $filename = $this->workspace.\DIRECTORY_SEPARATOR.'foo.txt';
+        file_put_contents($filename, 'foo');
+        $linknameA = $this->workspace.\DIRECTORY_SEPARATOR.'bar.txt';
+        $linknameB = $this->workspace.\DIRECTORY_SEPARATOR.'baz.txt';
+        $this->filesystem->symlink($filename, $linknameA);
+        $this->filesystem->symlink($linknameA, $linknameB);
+
+        $this->filesystem->appendToFile($linknameA, 'bar');
+        $this->filesystem->appendToFile($linknameB, 'baz');
+
+        $this->assertFileExists($filename);
+        $this->assertFileExists($linknameA);
+        $this->assertFileExists($linknameB);
+        $this->assertStringEqualsFile($filename, 'foobarbaz');
+        $this->assertStringEqualsFile($linknameA, 'foobarbaz');
+        $this->assertStringEqualsFile($linknameB, 'foobarbaz');
+
+        $this->filesystem->remove($filename);
+        $this->filesystem->appendToFile($linknameB, 'foo');
+        $this->filesystem->appendToFile($linknameA, 'bar');
+
+        $this->assertFileExists($filename);
+        $this->assertStringEqualsFile($filename, 'foobar');
+        $this->assertStringEqualsFile($linknameA, 'foobar');
+        $this->assertStringEqualsFile($linknameB, 'foobar');
+    }
+
     public function testAppendToFileWithScheme()
     {
         $scheme = 'file://';