Implement comment commits

---
type: change
message: |-
  Implement comment commits

  This ended up requiring a little refactoring here and there, as well as a fix
  for typeobj after running into a bug. Also made the commit message unmarshaling
  a bit more durable, after running into a case where the error message that gets
  produced for an invalid commit is not very helpful.

  Currently comment commits are not very well tested. The tests for commit objects
  in general need to be rethought completely, so they better test the hashes as
  well as message head parsing.
change_hash: AO7Cnfcgbuusch969HHVuo8OJCKh+uuiof+qhnmiD5xo
credentials:
- type: pgp_signature
  pub_key_id: 95C46FA6A41148AC
  body: 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
  account: mediocregopher

ROADMAP.md

@@ -6,7 +6,6 @@ set, only a sequence of milestones and the requirements to hit them.
 
 ## Milestone: Be able to add other developers to the project
 
-* Thread commits (comments)
 * Coalesce command
 * Fast-forward perms on branches
 * Authorship in commit messages

cmd/dehub/cmd_commit.go

@@ -126,5 +126,29 @@ func cmdCommit(ctx context.Context, cmd *dcmd.Cmd) {
 		},
 	)
 
+	cmd.SubCmd("comment", "Commit a comment to a branch",
+		func(ctx context.Context, cmd *dcmd.Cmd) {
+			flag := cmd.FlagSet()
+			msg := flag.String("msg", "", "Comment message")
+			cmd.Run(func() (context.Context, error) {
+				if *msg == "" {
+					var err error
+					if *msg, err = tmpFileMsg(); err != nil {
+						return nil, fmt.Errorf("collecting comment message from user: %w", err)
+
+					} else if *msg == "" {
+						return nil, errors.New("empty comment message, not doing anything")
+					}
+				}
+
+				commit, err := repo.NewCommitComment(*msg)
+				if err != nil {
+					return nil, fmt.Errorf("constructing comment commit: %w", err)
+				}
+				return nil, accreditAndCommit(commit)
+			})
+		},
+	)
+
 	cmd.Run(body)
 }

cmd/dehub/tmp_file.go

@@ -29,7 +29,7 @@ func tmpFileMsg() (string, error) {
 
 	tmpBody := bytes.NewBufferString(`
 
-# Please enter the commit message for your changes. Lines starting
+# Please enter the commit message for your commit. Lines starting
 # with '#' will be ignored, and an empty message aborts the commit.`)
 
 	_, err = io.Copy(tmpf, tmpBody)

commit.go

@@ -10,7 +10,9 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
+	"reflect"
 	"sort"
+	"strings"
 	"time"
 
 	"gopkg.in/src-d/go-git.v4"
@@ -59,11 +61,23 @@ func (cc CommitCommon) credAccountIDs() []string {
 	return s
 }
 
+func abbrevCommitMessage(msg string) string {
+	i := strings.Index(msg, "\n")
+	if i > 0 {
+		msg = msg[:i]
+	}
+	if len(msg) > 80 {
+		msg = msg[:80] + "..."
+	}
+	return msg
+}
+
 // Commit represents a single Commit which is being added to a branch. Only one
 // field should be set on a Commit, unless otherwise noted.
 type Commit struct {
 	Change     *CommitChange     `type:"change,default"`
 	Credential *CommitCredential `type:"credential"`
+	Comment    *CommitComment    `type:"comment"`
 
 	Common CommitCommon `yaml:",inline"`
 }
@@ -133,7 +147,12 @@ func (c *Commit) UnmarshalText(msg []byte) error {
 
 	if err := yaml.Unmarshal(msgBody, c); err != nil {
 		return fmt.Errorf("could not unmarshal Commit message from yaml: %w", err)
+
+	} else if reflect.DeepEqual(*c, Commit{}) {
+		// a basic check, but worthwhile
+		return errors.New("commit message is malformed, could not unmarshal yaml object")
 	}
+
 	return nil
 }
 

commit_change.go

@@ -5,7 +5,6 @@ import (
 	"dehub/yamlutil"
 	"errors"
 	"fmt"
-	"strings"
 
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
 )
@@ -46,11 +45,7 @@ func (r *Repo) NewCommitChange(msg string) (Commit, error) {
 
 // MessageHead implements the method for the CommitInterface interface.
 func (cc CommitChange) MessageHead(CommitCommon) (string, error) {
-	i := strings.Index(cc.Message, "\n")
-	if i > 0 {
-		return cc.Message[:i], nil
-	}
-	return cc.Message, nil
+	return abbrevCommitMessage(cc.Message), nil
 }
 
 // Hash implements the method for the CommitInterface interface.

commit_comment.go

@@ -0,0 +1,46 @@
+package dehub
+
+import (
+	"dehub/yamlutil"
+	"fmt"
+	"strings"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/object"
+)
+
+// CommitComment describes the structure of a comment commit message.
+type CommitComment struct {
+	Message     string        `yaml:"message"`
+	MessageHash yamlutil.Blob `yaml:"message_hash"`
+}
+
+var _ CommitInterface = CommitComment{}
+
+// NewCommitComment constructs and returns a COmmit populated with a
+// CommitComment encompassing the given message. The Credentials of the returned
+// Commit will _not_ be filled in.
+func (r *Repo) NewCommitComment(msg string) (Commit, error) {
+	cc := CommitComment{Message: msg}
+	var err error
+	if cc.MessageHash, err = cc.Hash(nil, nil); err != nil {
+		return Commit{}, fmt.Errorf("calculating comment hash: %w", err)
+	}
+	return Commit{Comment: &cc}, nil
+}
+
+// MessageHead implements the method for the CommitInterface interface.
+func (cc CommitComment) MessageHead(common CommitCommon) (string, error) {
+	msgAbbrev := abbrevCommitMessage(cc.Message)
+	credAccounts := strings.Join(common.credAccountIDs(), ", ")
+	return fmt.Sprintf("Comment by %s: %s", credAccounts, msgAbbrev), nil
+}
+
+// Hash implements the method for the CommitInterface.
+func (cc CommitComment) Hash(_, _ *object.Tree) ([]byte, error) {
+	return genCommentHash(nil, cc.Message), nil
+}
+
+// GetHash implements the method for the CommitInterface.
+func (cc CommitComment) GetHash() []byte {
+	return cc.MessageHash
+}

hash.go

@@ -31,6 +31,12 @@ func newHashHelper(h hash.Hash) *hashHelper {
 	return s
 }
 
+func (s *hashHelper) sum(prefix []byte) []byte {
+	out := make([]byte, len(prefix), len(prefix)+s.Hash.Size())
+	copy(out, prefix)
+	return s.Hash.Sum(out)
+}
+
 func (s *hashHelper) writeUint(i uint64) {
 	n := binary.PutUvarint(s.varintBuf, i)
 	if _, err := s.Write(s.varintBuf[:n]); err != nil {
@@ -64,12 +70,22 @@ func (s *hashHelper) writeTreeDiff(from, to *object.Tree) {
 
 }
 
-var changeHashVersion = []byte{0}
+var (
+	changeHashVersion  = []byte{0}
+	commentHashVersion = []byte{0}
+)
 
 // if h is nil it then defaultHashHelperAlgo will be used
 func genChangeHash(h hash.Hash, msg string, from, to *object.Tree) []byte {
 	s := newHashHelper(h)
 	s.writeStr(msg)
 	s.writeTreeDiff(from, to)
-	return s.Sum(changeHashVersion)
+	return s.sum(changeHashVersion)
+}
+
+// if h is nil it then defaultHashHelperAlgo will be used
+func genCommentHash(h hash.Hash, comment string) []byte {
+	s := newHashHelper(h)
+	s.writeStr(comment)
+	return s.sum(commentHashVersion)
 }

typeobj/typeobj.go

@@ -124,7 +124,7 @@ func element(val reflect.Value) (reflect.Value, string, []int, error) {
 		}
 	}
 
-	if fieldVal.IsZero() {
+	if !fieldVal.IsValid() {
 		return reflect.Value{}, "", nil, errors.New(`no non-zero fields tagged with "type"`)
 	}
 	return fieldVal, typeTag, nonTypeFields, nil