GitDriver: try to fetch default branch from remote

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 +1,95 @@
+<?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([
+            'config' => [
+                '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);
+
+        $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;
+
+        $process
+            ->expects([[
+                'cmd' => 'git remote show origin',
+                'stdout' => $stdout,
+            ]]);
+
+        $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);
+
+        $stdout = <<<GIT
+* main
+  2.2
+  1.10
+GIT;
+
+        $process
+            ->expects([[
+                'cmd' => 'git branch --no-color',
+                'stdout' => $stdout,
+            ]]);
+
+        $this->assertSame('main', $driver->getRootIdentifier());
+    }
+}