GitDriver: try to fetch default branch from remote

The initial clone determined what the default branch of the cache git repository was. Changing it on the remote didn't have any impact on the local data. However, cloning it on a different machine would then store a different default branch on that machine. This could lead to different results for the same command on different machines.

src/Composer/Repository/Vcs/GitDriver.php

@@ -13,6 +13,7 @@
 namespace Composer\Repository\Vcs;
 
 use Composer\Pcre\Preg;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Filesystem;
 use Composer\Util\Url;
@@ -93,6 +94,20 @@ public function getRootIdentifier(): string
         if (null === $this->rootIdentifier) {
             $this->rootIdentifier = 'master';
 
+            if (!(bool) Platform::getEnv('COMPOSER_DISABLE_NETWORK')) {
+                try {
+                    $this->process->execute('git remote show origin', $output, $this->repoDir);
+                    $lines = $this->process->splitLines($output);
+                    foreach ($lines as $line) {
+                        if (Preg::match('{^\s*HEAD branch:\s(.+)\s*$}', $line, $matches) > 0) {
+                            return $this->rootIdentifier = $matches[1];
+                        }
+                    }
+                } catch (\Exception $e) {
+                    $this->io->writeError('<error>Failed to fetch root identifier from remote: ' . $e->getMessage() . '</error>', true, IOInterface::DEBUG);
+                }
+            }
+
             // select currently checked out branch if master is not available
             $this->process->execute('git branch --no-color', $output, $this->repoDir);
             $branches = $this->process->splitLines($output);

tests/Composer/Test/Repository/Vcs/GitDriverTest.php

@@ -0,0 +1,93 @@
+<?php declare(strict_types=1);
+
+namespace Composer\Test\Repository\Vcs;
+
+use Composer\Config;
+use Composer\IO\IOInterface;
+use Composer\Repository\Vcs\GitDriver;
+use Composer\Test\TestCase;
+use Composer\Util\Filesystem;
+use Composer\Util\Platform;
+
+class GitDriverTest extends TestCase
+{
+    /** @var Config */
+    private $config;
+    /** @var string */
+    private $home;
+    /** @var false|string */
+    private $networkEnv;
+
+    public function setUp(): void
+    {
+        $this->home = self::getUniqueTmpDirectory();
+        $this->config = new Config();
+        $this->config->merge(array(
+            'config' => array(
+                'home' => $this->home,
+            ),
+        ));
+        $this->networkEnv = Platform::getEnv('COMPOSER_DISABLE_NETWORK');
+    }
+
+    protected function tearDown(): void
+    {
+        parent::tearDown();
+        $fs = new Filesystem;
+        $fs->removeDirectory($this->home);
+        if ($this->networkEnv === false) {
+            Platform::clearEnv('COMPOSER_DISABLE_NETWORK');
+        } else {
+            Platform::putEnv('COMPOSER_DISABLE_NETWORK', $this->networkEnv);
+        }
+    }
+
+    public function testGetRootIdentifierFromRemote(): void
+    {
+        $process = $this->getProcessExecutorMock();
+        $io = $this->getMockBuilder(IOInterface::class)->getMock();
+
+        $driver = new GitDriver(['url' => 'https://example.org/acme.git'], $io, $this->config, $this->getHttpDownloaderMock(), $process);
+
+        $process
+            ->expects([[
+                'cmd' => 'git remote show origin',
+                'stdout' => <<<GIT
+* remote origin
+  Fetch URL: https://example.org/acme.git
+  Push  URL: https://example.org/acme.git
+  HEAD branch: main
+  Remote branches:
+    1.10                       tracked
+    2.2                        tracked
+    main                       tracked
+GIT,
+            ]]);
+
+
+        $this->assertSame('main', $driver->getRootIdentifier());
+    }
+
+    public function testGetRootIdentifierFromLocalWithNetworkDisabled(): void
+    {
+        Platform::putEnv('COMPOSER_DISABLE_NETWORK', '1');
+
+        $process = $this->getProcessExecutorMock();
+        $io = $this->getMockBuilder(IOInterface::class)->getMock();
+
+        $driver = new GitDriver(['url' => 'https://example.org/acme.git'], $io, $this->config, $this->getHttpDownloaderMock(), $process);
+
+        $process
+            ->expects([[
+                'cmd' => 'git branch --no-color',
+                'stdout' => <<<GIT
+* main
+  2.2
+  1.10
+GIT,
+            ]]);
+
+
+        $this->assertSame('main', $driver->getRootIdentifier());
+    }
+}