git: implement upload-server-info. Fixes #731

This adds UpdateServerInfo along with a new go-git command to generate
info files to help git dumb http serve refs and their objects.
This also updates the docs to reflect this.

Docs: https://git-scm.com/docs/git-update-server-info
Fixes: https://github.com/go-git/go-git/issues/731

6 files changed, 332 insertions, 4 deletions

diff --git a/COMPATIBILITY.md b/COMPATIBILITY.md
index bbffea5..c1f280d 100644
--- a/COMPATIBILITY.md
+++ b/COMPATIBILITY.md
@@ -109,10 +109,10 @@ compatibility status with go-git.
 
 ## Server admin
 
-| Feature              | Sub-feature | Status | Notes | Examples |
-| -------------------- | ----------- | ------ | ----- | -------- |
-| `daemon`             |             | ❌     |       |          |
-| `update-server-info` |             | ❌     |       |          |
+| Feature              | Sub-feature | Status | Notes | Examples                                  |
+| -------------------- | ----------- | ------ | ----- | ----------------------------------------- |
+| `daemon`             |             | ❌     |       |                                           |
+| `update-server-info` |             | ✅     |       | [cli](./cli/go-git/update_server_info.go) |
 
 ## Advanced
 
diff --git a/cli/go-git/main.go b/cli/go-git/main.go
index 97b8c3e..0a5ad2c 100644
--- a/cli/go-git/main.go
+++ b/cli/go-git/main.go
@@ -22,6 +22,7 @@ func main() {
 	}
 
 	parser := flags.NewNamedParser(bin, flags.Default)
+	parser.AddCommand("update-server-info", "", "", &CmdUpdateServerInfo{})
 	parser.AddCommand("receive-pack", "", "", &CmdReceivePack{})
 	parser.AddCommand("upload-pack", "", "", &CmdUploadPack{})
 	parser.AddCommand("version", "Show the version information.", "", &CmdVersion{})
diff --git a/cli/go-git/update_server_info.go b/cli/go-git/update_server_info.go
new file mode 100644
index 0000000..a7f3e3e
--- /dev/null
+++ b/cli/go-git/update_server_info.go
@@ -0,0 +1,34 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/plumbing/serverinfo"
+	"github.com/go-git/go-git/v5/storage/filesystem"
+)
+
+// CmdUpdateServerInfo command updates the server info files in the repository.
+// This is used by git http transport (dumb) to generate a list of available
+// refs for the repository. See:
+// https://git-scm.com/docs/git-update-server-info
+type CmdUpdateServerInfo struct {
+	cmd
+}
+
+// Usage returns the usage of the command.
+func (CmdUpdateServerInfo) Usage() string {
+	return fmt.Sprintf("within a git repository run: %s", os.Args[0])
+}
+
+// Execute runs the command.
+func (c *CmdUpdateServerInfo) Execute(args []string) error {
+	r, err := git.PlainOpen(".")
+	if err != nil {
+		return err
+	}
+
+	fs := r.Storer.(*filesystem.Storage).Filesystem()
+	return serverinfo.UpdateServerInfo(r.Storer, fs)
+}
diff --git a/internal/reference/sort.go b/internal/reference/sort.go
new file mode 100644
index 0000000..726edbd
--- /dev/null
+++ b/internal/reference/sort.go
@@ -0,0 +1,14 @@
+package reference
+
+import (
+	"sort"
+
+	"github.com/go-git/go-git/v5/plumbing"
+)
+
+// Sort sorts the references by name to ensure a consistent order.
+func Sort(refs []*plumbing.Reference) {
+	sort.Slice(refs, func(i, j int) bool {
+		return refs[i].Name() < refs[j].Name()
+	})
+}
diff --git a/plumbing/serverinfo/serverinfo.go b/plumbing/serverinfo/serverinfo.go
new file mode 100644
index 0000000..d7ea7ef
--- /dev/null
+++ b/plumbing/serverinfo/serverinfo.go
@@ -0,0 +1,94 @@
+package serverinfo
+
+import (
+	"fmt"
+
+	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/internal/reference"
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/go-git/go-git/v5/plumbing/storer"
+	"github.com/go-git/go-git/v5/storage"
+)
+
+// UpdateServerInfo updates the server info files in the repository.
+//
+// It generates a list of available refs for the repository.
+// Used by git http transport (dumb), for more information refer to:
+// https://git-scm.com/book/id/v2/Git-Internals-Transfer-Protocols#_the_dumb_protocol
+func UpdateServerInfo(s storage.Storer, fs billy.Filesystem) error {
+	pos, ok := s.(storer.PackedObjectStorer)
+	if !ok {
+		return git.ErrPackedObjectsNotSupported
+	}
+
+	infoRefs, err := fs.Create("info/refs")
+	if err != nil {
+		return err
+	}
+
+	defer infoRefs.Close()
+
+	refsIter, err := s.IterReferences()
+	if err != nil {
+		return err
+	}
+
+	defer refsIter.Close()
+
+	var refs []*plumbing.Reference
+	if err := refsIter.ForEach(func(ref *plumbing.Reference) error {
+		refs = append(refs, ref)
+		return nil
+	}); err != nil {
+		return err
+	}
+
+	reference.Sort(refs)
+	for _, ref := range refs {
+		name := ref.Name()
+		hash := ref.Hash()
+		switch ref.Type() {
+		case plumbing.SymbolicReference:
+			if name == plumbing.HEAD {
+				continue
+			}
+			ref, err := s.Reference(ref.Target())
+			if err != nil {
+				return err
+			}
+
+			hash = ref.Hash()
+			fallthrough
+		case plumbing.HashReference:
+			fmt.Fprintf(infoRefs, "%s\t%s\n", hash, name)
+			if name.IsTag() {
+				tag, err := object.GetTag(s, hash)
+				if err == nil {
+					fmt.Fprintf(infoRefs, "%s\t%s^{}\n", tag.Target, name)
+				}
+			}
+		}
+	}
+
+	infoPacks, err := fs.Create("objects/info/packs")
+	if err != nil {
+		return err
+	}
+
+	defer infoPacks.Close()
+
+	packs, err := pos.ObjectPacks()
+	if err != nil {
+		return err
+	}
+
+	for _, p := range packs {
+		fmt.Fprintf(infoPacks, "P pack-%s.pack\n", p)
+	}
+
+	fmt.Fprintln(infoPacks)
+
+	return nil
+}
diff --git a/plumbing/serverinfo/serverinfo_test.go b/plumbing/serverinfo/serverinfo_test.go
new file mode 100644
index 0000000..0a52ea2
--- /dev/null
+++ b/plumbing/serverinfo/serverinfo_test.go
@@ -0,0 +1,185 @@
+package serverinfo
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-billy/v5/memfs"
+	fixtures "github.com/go-git/go-git-fixtures/v4"
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+	"github.com/go-git/go-git/v5/plumbing/storer"
+	"github.com/go-git/go-git/v5/storage"
+	"github.com/go-git/go-git/v5/storage/memory"
+	. "gopkg.in/check.v1"
+)
+
+type ServerInfoSuite struct{}
+
+var _ = Suite(&ServerInfoSuite{})
+
+func Test(t *testing.T) { TestingT(t) }
+
+func (s *ServerInfoSuite) TestUpdateServerInfoInit(c *C) {
+	fs := memfs.New()
+	st := memory.NewStorage()
+	r, err := git.Init(st, fs)
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+
+	err = UpdateServerInfo(st, fs)
+	c.Assert(err, IsNil)
+}
+
+func assertInfoRefs(c *C, st storage.Storer, fs billy.Filesystem) {
+	refsFile, err := fs.Open("info/refs")
+	c.Assert(err, IsNil)
+
+	defer refsFile.Close()
+	bts, err := io.ReadAll(refsFile)
+	c.Assert(err, IsNil)
+
+	localRefs := make(map[plumbing.ReferenceName]plumbing.Hash)
+	for _, line := range strings.Split(string(bts), "\n") {
+		if line == "" {
+			continue
+		}
+		parts := strings.Split(line, "\t")
+		c.Assert(parts, HasLen, 2)
+		hash := plumbing.NewHash(parts[0])
+		name := plumbing.ReferenceName(parts[1])
+		localRefs[name] = hash
+	}
+
+	refs, err := st.IterReferences()
+	c.Assert(err, IsNil)
+
+	err = refs.ForEach(func(ref *plumbing.Reference) error {
+		name := ref.Name()
+		hash := ref.Hash()
+		switch ref.Type() {
+		case plumbing.SymbolicReference:
+			if name == plumbing.HEAD {
+				return nil
+			}
+			ref, err := st.Reference(ref.Target())
+			c.Assert(err, IsNil)
+			hash = ref.Hash()
+			fallthrough
+		case plumbing.HashReference:
+			h, ok := localRefs[name]
+			c.Assert(ok, Equals, true)
+			c.Assert(h, Equals, hash)
+			if name.IsTag() {
+				tag, err := object.GetTag(st, hash)
+				if err == nil {
+					t, ok := localRefs[name+"^{}"]
+					c.Assert(ok, Equals, true)
+					c.Assert(t, Equals, tag.Target)
+				}
+			}
+		}
+		return nil
+	})
+
+	c.Assert(err, IsNil)
+}
+
+func assertObjectPacks(c *C, st storage.Storer, fs billy.Filesystem) {
+	infoPacks, err := fs.Open("objects/info/packs")
+	c.Assert(err, IsNil)
+
+	defer infoPacks.Close()
+	bts, err := io.ReadAll(infoPacks)
+	c.Assert(err, IsNil)
+
+	pos, ok := st.(storer.PackedObjectStorer)
+	c.Assert(ok, Equals, true)
+	localPacks := make(map[string]struct{})
+	packs, err := pos.ObjectPacks()
+	c.Assert(err, IsNil)
+
+	for _, line := range strings.Split(string(bts), "\n") {
+		if line == "" {
+			continue
+		}
+		parts := strings.Split(line, " ")
+		c.Assert(parts, HasLen, 2)
+		pack := strings.TrimPrefix(parts[1], "pack-")
+		pack = strings.TrimSuffix(pack, ".pack")
+		localPacks[pack] = struct{}{}
+	}
+
+	for _, p := range packs {
+		_, ok := localPacks[p.String()]
+		c.Assert(ok, Equals, true)
+	}
+}
+
+func (s *ServerInfoSuite) TestUpdateServerInfoTags(c *C) {
+	fs := memfs.New()
+	st := memory.NewStorage()
+	r, err := git.Clone(st, fs, &git.CloneOptions{
+		URL: fixtures.ByURL("https://github.com/git-fixtures/tags.git").One().URL,
+	})
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+
+	err = UpdateServerInfo(st, fs)
+	c.Assert(err, IsNil)
+
+	assertInfoRefs(c, st, fs)
+	assertObjectPacks(c, st, fs)
+}
+
+func (s *ServerInfoSuite) TestUpdateServerInfoBasic(c *C) {
+	fs := memfs.New()
+	st := memory.NewStorage()
+	r, err := git.Clone(st, fs, &git.CloneOptions{
+		URL: fixtures.Basic().One().URL,
+	})
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+
+	err = UpdateServerInfo(st, fs)
+	c.Assert(err, IsNil)
+
+	assertInfoRefs(c, st, fs)
+	assertObjectPacks(c, st, fs)
+}
+
+func (s *ServerInfoSuite) TestUpdateServerInfoBasicChange(c *C) {
+	fs := memfs.New()
+	st := memory.NewStorage()
+	r, err := git.Clone(st, fs, &git.CloneOptions{
+		URL: fixtures.Basic().One().URL,
+	})
+	c.Assert(err, IsNil)
+	c.Assert(r, NotNil)
+
+	err = UpdateServerInfo(st, fs)
+	c.Assert(err, IsNil)
+
+	assertInfoRefs(c, st, fs)
+	assertObjectPacks(c, st, fs)
+
+	head, err := r.Head()
+	c.Assert(err, IsNil)
+
+	ref := plumbing.NewHashReference("refs/heads/my-branch", head.Hash())
+	err = r.Storer.SetReference(ref)
+	c.Assert(err, IsNil)
+
+	_, err = r.CreateTag("test-tag", head.Hash(), &git.CreateTagOptions{
+		Message: "test-tag",
+	})
+	c.Assert(err, IsNil)
+
+	err = UpdateServerInfo(st, fs)
+
+	assertInfoRefs(c, st, fs)
+	assertObjectPacks(c, st, fs)
+}