Support `.git/commondir` repository layout

Git creates `.git/commondir` when there are custom worktrees (see "git worktree add" related commands).

`.git/commondir` in such case contains a link to another dot-git repository tree, which could contain some folders like:
 - objects;
 - config;
 - refs;
 - etc.

In this PR a new dotgit.RepositoryFilesystem struct is defined, which is billy.Filesystem interface compatible object-wrapper, that can handle commondir and dispatch all operations to the correct file path.

`git.PlainOpen` remain unchanged, but `git.PlainOpenWithOptions` has a new option: `PlainOpenOptions.EnableDotGitCommonDir=true|false` (which is false by default). When `EnableDotGitCommonDir=true` repository-open procedure will read `.git/commondir` (if it exists) and then create dotgit.RepositoryFilesystem object initialized with 2 filesystems. This object then passed into storage and then into dotgit.DotGit as `billy.Filesystem` interface. This object will catch all filesystem operations and dispatch to the correct repository-filesystem (dot-git or common-dot-git) according to the rules described in the doc: https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt. EnableDotGitCommonDir option will only work with the filesystem-backed storage.

Also worktree_test.go has been adopted from an older, already existing existing PR: https://github.com/src-d/go-git/pull/1098. This PR needs new fixtures added in the following PR: https://github.com/go-git/go-git-fixtures/pull/1.

1 files changed, 124 insertions, 0 deletions

diff --git a/storage/filesystem/dotgit/repository_filesystem_test.go b/storage/filesystem/dotgit/repository_filesystem_test.go
new file mode 100644
index 0000000..880ec0d
--- /dev/null
+++ b/storage/filesystem/dotgit/repository_filesystem_test.go
@@ -0,0 +1,124 @@
+package dotgit
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+
+	"github.com/go-git/go-billy/v5/osfs"
+
+	. "gopkg.in/check.v1"
+)
+
+func (s *SuiteDotGit) TestRepositoryFilesystem(c *C) {
+	dir, err := ioutil.TempDir("", "repository_filesystem")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	fs := osfs.New(dir)
+
+	err = fs.MkdirAll("dotGit", 0777)
+	c.Assert(err, IsNil)
+	dotGitFs, err := fs.Chroot("dotGit")
+	c.Assert(err, IsNil)
+
+	err = fs.MkdirAll("commonDotGit", 0777)
+	c.Assert(err, IsNil)
+	commonDotGitFs, err := fs.Chroot("commonDotGit")
+	c.Assert(err, IsNil)
+
+	repositoryFs := NewRepositoryFilesystem(dotGitFs, commonDotGitFs)
+	c.Assert(repositoryFs.Root(), Equals, dotGitFs.Root())
+
+	somedir, err := repositoryFs.Chroot("somedir")
+	c.Assert(err, IsNil)
+	c.Assert(somedir.Root(), Equals, repositoryFs.Join(dotGitFs.Root(), "somedir"))
+
+	_, err = repositoryFs.Create("somefile")
+	c.Assert(err, IsNil)
+
+	_, err = repositoryFs.Stat("somefile")
+	c.Assert(err, IsNil)
+
+	file, err := repositoryFs.Open("somefile")
+	c.Assert(err, IsNil)
+	err = file.Close()
+	c.Assert(err, IsNil)
+
+	file, err = repositoryFs.OpenFile("somefile", os.O_RDONLY, 0666)
+	c.Assert(err, IsNil)
+	err = file.Close()
+	c.Assert(err, IsNil)
+
+	file, err = repositoryFs.Create("somefile2")
+	c.Assert(err, IsNil)
+	err = file.Close()
+	c.Assert(err, IsNil)
+	_, err = repositoryFs.Stat("somefile2")
+	c.Assert(err, IsNil)
+	err = repositoryFs.Rename("somefile2", "newfile")
+	c.Assert(err, IsNil)
+
+	tempDir, err := repositoryFs.TempFile("tmp", "myprefix")
+	c.Assert(err, IsNil)
+	c.Assert(repositoryFs.Join(repositoryFs.Root(), "tmp", tempDir.Name()), Equals, repositoryFs.Join(dotGitFs.Root(), "tmp", tempDir.Name()))
+
+	err = repositoryFs.Symlink("newfile", "somelink")
+	c.Assert(err, IsNil)
+
+	_, err = repositoryFs.Lstat("somelink")
+	c.Assert(err, IsNil)
+
+	link, err := repositoryFs.Readlink("somelink")
+	c.Assert(err, IsNil)
+	c.Assert(link, Equals, "newfile")
+
+	err = repositoryFs.Remove("somelink")
+	c.Assert(err, IsNil)
+
+	_, err = repositoryFs.Stat("somelink")
+	c.Assert(os.IsNotExist(err), Equals, true)
+
+	dirs := []string{objectsPath, refsPath, packedRefsPath, configPath, branchesPath, hooksPath, infoPath, remotesPath, logsPath, shallowPath, worktreesPath}
+	for _, dir := range dirs {
+		err := repositoryFs.MkdirAll(dir, 0777)
+		c.Assert(err, IsNil)
+		_, err = commonDotGitFs.Stat(dir)
+		c.Assert(err, IsNil)
+		_, err = dotGitFs.Stat(dir)
+		c.Assert(os.IsNotExist(err), Equals, true)
+	}
+
+	exceptionsPaths := []string{repositoryFs.Join(logsPath, "HEAD"), repositoryFs.Join(refsPath, "bisect"), repositoryFs.Join(refsPath, "rewritten"), repositoryFs.Join(refsPath, "worktree")}
+	for _, path := range exceptionsPaths {
+		_, err := repositoryFs.Create(path)
+		c.Assert(err, IsNil)
+		_, err = commonDotGitFs.Stat(path)
+		c.Assert(os.IsNotExist(err), Equals, true)
+		_, err = dotGitFs.Stat(path)
+		c.Assert(err, IsNil)
+	}
+
+	err = repositoryFs.MkdirAll("refs/heads", 0777)
+	c.Assert(err, IsNil)
+	_, err = commonDotGitFs.Stat("refs/heads")
+	c.Assert(err, IsNil)
+	_, err = dotGitFs.Stat("refs/heads")
+	c.Assert(os.IsNotExist(err), Equals, true)
+
+	err = repositoryFs.MkdirAll("objects/pack", 0777)
+	c.Assert(err, IsNil)
+	_, err = commonDotGitFs.Stat("objects/pack")
+	c.Assert(err, IsNil)
+	_, err = dotGitFs.Stat("objects/pack")
+	c.Assert(os.IsNotExist(err), Equals, true)
+
+	err = repositoryFs.MkdirAll("a/b/c", 0777)
+	c.Assert(err, IsNil)
+	_, err = commonDotGitFs.Stat("a/b/c")
+	c.Assert(os.IsNotExist(err), Equals, true)
+	_, err = dotGitFs.Stat("a/b/c")
+	c.Assert(err, IsNil)
+}