ruby-git · jcouball · Mar 4, 2023 · Mar 4, 2023
diff --git a/README.md b/README.md
@@ -197,6 +197,8 @@ g.show('v2.8', 'README.md')
 Git.ls_remote('https://github.com/ruby-git/ruby-git.git') # returns a hash containing the available references of the repo.
 Git.ls_remote('/path/to/local/repo')
 Git.ls_remote() # same as Git.ls_remote('.')
+
+Git.default_branch('https://github.com/ruby-git/ruby-git') #=> 'master'
 ```
 
 And here are the operations that will need to write to your git repository.

diff --git a/lib/git.rb b/lib/git.rb
@@ -175,6 +175,46 @@ def self.clone(repository_url, directory = nil, options = {})
     Base.clone(repository_url, directory, options)
   end
 
+  # Returns the name of the default branch of the given repository
+  #
+  # @example with a URI string
+  #   Git.default_branch('https://github.com/ruby-git/ruby-git') # => 'master'
+  #   Git.default_branch('https://github.com/rspec/rspec-core') # => 'main'
+  #
+  # @example with a URI object
+  #   repository_uri = URI('https://github.com/ruby-git/ruby-git')
+  #   Git.default_branch(repository_uri) # => 'master'
+  #
+  # @example with a local repository
+  #   Git.default_branch('.') # => 'master'
+  #
+  # @example with a local repository Pathname
+  #   repository_path = Pathname('.')
+  #   Git.default_branch(repository_path) # => 'master'
+  #
+  # @example with the logging option
+  #   logger = Logger.new(STDOUT, level: Logger::INFO)
+  #   Git.default_branch('.', log: logger) # => 'master'
+  #   I, [2022-04-13T16:01:33.221596 #18415]  INFO -- : git '-c' 'core.quotePath=true' '-c' 'color.ui=false' ls-remote '--symref' '--' '.' 'HEAD'  2>&1
+  #
+  # @param repository [URI, Pathname, String] The (possibly remote) repository to get the default branch name for
+  #
+  #   See [GIT URLS](https://git-scm.com/docs/git-clone#_git_urls_a_id_urls_a)
+  #   for more information.
+  #
+  # @param [Hash] options The options for this command (see list of valid
+  #   options below)
+  #
+  # @option options [Logger] :log A logger to use for Git operations.  Git
+  #   commands are logged at the `:info` level.  Additional logging is done
+  #   at the `:debug` level.
+  #
+  # @return [String] the name of the default branch
+  #
+  def self.default_branch(repository, options = {})
+    Base.repository_default_branch(repository, options)
+  end
+
   # Export the current HEAD (or a branch, if <tt>options[:branch]</tt>
   # is specified) into the +name+ directory, then remove all traces of git from the
   # directory.

diff --git a/lib/git/base.rb b/lib/git/base.rb
@@ -24,6 +24,11 @@ def self.clone(repository_url, directory, options = {})
       new(new_options)
     end
 
+    # (see Git.default_branch)
+    def self.repository_default_branch(repository, options = {})
+      Git::Lib.new(nil, options[:log]).repository_default_branch(repository)
+    end
+
     # Returns (and initialize if needed) a Git::Config instance
     #
     # @return [Git::Config] the current config instance.

diff --git a/lib/git/lib.rb b/lib/git/lib.rb
@@ -124,6 +124,24 @@ def return_base_opts_from_clone(clone_dir, opts)
       base_opts
     end
 
+    # Returns the name of the default branch of the given repository
+    #
+    # @param repository [URI, Pathname, String] The (possibly remote) repository to clone from
+    #
+    # @return [String] the name of the default branch
+    #
+    def repository_default_branch(repository)
+      output = command('ls-remote', '--symref', '--', repository, 'HEAD')
+
+      match_data = output.match(%r{^ref: refs/remotes/origin/(?<default_branch>[^\t]+)\trefs/remotes/origin/HEAD$})
+      return match_data[:default_branch] if match_data
+
+      match_data = output.match(%r{^ref: refs/heads/(?<default_branch>[^\t]+)\tHEAD$})
+      return match_data[:default_branch] if match_data
+
+      raise 'Unable to determine the default branch'
+    end
+
     ## READ COMMANDS ##
 
     #

diff --git a/tests/units/test_git_default_branch.rb b/tests/units/test_git_default_branch.rb
@@ -0,0 +1,41 @@
+#!/usr/bin/env ruby
+
+require File.dirname(__FILE__) + '/../test_helper'
+
+require 'logger'
+require 'stringio'
+
+# Tests for Git::Lib#repository_default_branch
+#
+class TestLibRepositoryDefaultBranch < Test::Unit::TestCase
+  def test_default_branch
+    repository = 'new_repo'
+    in_temp_dir do
+      create_local_repository(repository, initial_branch: 'main')
+      assert_equal('main', Git.default_branch(repository))
+    end
+  end
+
+  def test_default_branch_with_logging
+    repository = 'new_repo'
+    in_temp_dir do
+      create_local_repository(repository, initial_branch: 'main')
+      log_device = StringIO.new
+      logger = Logger.new(log_device, level: Logger::INFO)
+      Git.default_branch(repository, log: logger)
+      assert_match(/git.*ls-remote/, log_device.string)
+    end
+  end
+
+  private
+
+  def create_local_repository(subdirectory, initial_branch: 'main')
+    git = Git.init(subdirectory, initial_branch: initial_branch)
+
+    FileUtils.cd(subdirectory) do
+      File.write('README.md', '# This is a README')
+      git.add('README.md')
+      git.commit('Initial commit')
+    end
+  end
+end
diff --git a/tests/units/test_lib_repository_default_branch.rb b/tests/units/test_lib_repository_default_branch.rb
@@ -0,0 +1,96 @@
+#!/usr/bin/env ruby
+
+require File.dirname(__FILE__) + '/../test_helper'
+
+# Tests for Git::Lib#repository_default_branch
+#
+class TestLibRepositoryDefaultBranch < Test::Unit::TestCase
+  def setup
+    clone_working_repo
+    @git = Git.open(@wdir)
+
+    @lib = Git.open(@wdir).lib
+  end
+
+  # This is the one real test that actually calls git.  The rest of the tests
+  # mock Git::Lib#command to return specific responses.
+  #
+  def test_local_repository
+    in_temp_dir do
+      git = Git.init('new_repo', initial_branch: 'main')
+      FileUtils.cd('new_repo') do
+        File.write('README.md', '# This is a README')
+        git.add('README.md')
+        git.commit('Initial commit')
+      end
+      FileUtils.touch('new_repo/README.md')
+
+      assert_equal('main', @lib.repository_default_branch('new_repo'))
+    end
+  end
+
+  def mock_command(lib, repository, response)
+    test_case = self
+    lib.define_singleton_method(:command) do |cmd, *opts, &_block|
+      test_case.assert_equal('ls-remote', cmd)
+      test_case.assert_equal(['--symref', '--', repository, 'HEAD'], opts.flatten)
+      response
+    end
+  end
+
+  def test_remote_repository
+    repository = 'https://github.com/ruby-git/ruby-git'
+    mock_command(@lib, repository, <<~RESPONSE)
+      ref: refs/heads/default_branch\tHEAD
+      292087efabc8423c3cf616d78fac5311d58e7425\tHEAD
+    RESPONSE
+    assert_equal('default_branch', @lib.repository_default_branch(repository))
+  end
+
+  def test_local_repository_with_origin
+    repository = 'https://github.com/ruby-git/ruby-git'
+    mock_command(@lib, repository, <<~RESPONSE)
+      ref: refs/heads/master\tHEAD
+      292087efabc8423c3cf616d78fac5311d58e7425\tHEAD
+      ref: refs/remotes/origin/default_branch\trefs/remotes/origin/HEAD
+      292087efabc8423c3cf616d78fac5311d58e7425\trefs/remotes/origin/HEAD
+    RESPONSE
+    assert_equal('default_branch', @lib.repository_default_branch(repository))
+  end
+
+  def test_local_repository_without_remotes
+    repository = '.'
+    mock_command(@lib, repository, <<~RESPONSE)
+      ref: refs/heads/default_branch\tHEAD
+      d7b79c31113c42c7aa3fe915186c1d6bcd3fbd39\tHEAD
+    RESPONSE
+    assert_equal('default_branch', @lib.repository_default_branch(repository))
+  end
+
+  def test_repository_with_no_commits
+    # Local or remote, the result is the same
+    repository = '.'
+    mock_command(@lib, repository, '')
+    assert_raise_with_message(RuntimeError, 'Unable to determine the default branch') do
+      @lib.repository_default_branch(repository)
+    end
+  end
+
+  def test_repository_not_found
+    # Local or remote, the result is the same
+    repository = 'does_not_exist'
+    assert_raise(Git::FailedError) do
+      @lib.repository_default_branch(repository)
+    end
+  end
+
+  def test_not_a_repository
+    in_temp_dir do
+      repository = 'exists_but_not_a_repository'
+      FileUtils.mkdir repository
+      assert_raise(Git::FailedError) do
+        @lib.repository_default_branch(repository)
+      end
+    end
+  end
+end