normalize how git commits are interacted with, including changing VerifyComit -> VerifyCommits

---
type: change
message: |-
  normalize how git commits are interacted with, including changing VerifyComit -> VerifyCommits

  This commit attempts to normalize git commit interactions in order to reduce
  the amount of manual `GitRepo.CommitObject`, `GitRepo.TreeObject`,
  `Commit.UnmarshalText`, and `Commit.Interface` calls are done, by creating a
  single structure (`GitCommit`) which holds the output of those calls, and is
  only created by a single method (`GetGitCommit`), which is then used by a bunch
  of other methods to expand its functionality, including implementing a range
  request which can be used by verify and the pre-receive hook (though it's only
  used by the hook, currently).
change_hash: AMae4PL6+jrxhn2KEGHejstcdT37Gw/jjkl/UuovHcgd
credentials:
- type: pgp_signature
  pub_key_id: 95C46FA6A41148AC
  body: 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
  account: mediocregopher

cmd/dehub/cmd_commit.go

@@ -46,12 +46,12 @@ func cmdCommit(ctx context.Context, cmd *dcmd.Cmd) {
 			return fmt.Errorf("accrediting commit: %w", err)
 		}
 
-		hash, err := repo.Commit(commit, *accountID)
+		gitCommit, err := repo.Commit(commit, *accountID)
 		if err != nil {
 			return fmt.Errorf("committing to git: %w", err)
 		}
 
-		fmt.Printf("committed to HEAD as %s\n", hash)
+		fmt.Printf("committed to HEAD as %s\n", gitCommit.GitCommit.Hash)
 		return nil
 	}
 
@@ -110,28 +110,12 @@ func cmdCommit(ctx context.Context, cmd *dcmd.Cmd) {
 					return nil, errors.New("credential commit cannot have any files changed")
 				}
 
-				// TODO maybe nice to have a helper to do all of this
+				gitCommit, err := repo.GetGitRevision(plumbing.Revision(*rev))
-				h, err := repo.GitRepo.ResolveRevision(plumbing.Revision(*rev))
 				if err != nil {
-					return nil, fmt.Errorf("resolving revision: %w", err)
+					return nil, fmt.Errorf("resolving revision %q: %w", *rev, err)
 				}
 
-				commitObj, err := repo.GitRepo.CommitObject(*h)
+				credCommit, err := repo.NewCommitCredential(gitCommit.Interface.GetHash())
-				if err != nil {
-					return nil, fmt.Errorf("getting commit object %q: %w", h, err)
-				}
-
-				var commit dehub.Commit
-				if err := commit.UnmarshalText([]byte(commitObj.Message)); err != nil {
-					return nil, fmt.Errorf("unmarshaling commit message: %w", err)
-				}
-
-				commitInt, err := commit.Interface()
-				if err != nil {
-					return nil, fmt.Errorf("casting %#v to CommitInterface: %w", commit, err)
-				}
-
-				credCommit, err := repo.NewCommitCredential(commitInt.GetHash())
 				if err != nil {
 					return nil, fmt.Errorf("constructing credential commit: %w", err)
 				} else if err := accreditAndCommit(credCommit); err != nil {

cmd/dehub/cmd_hook.go

@@ -29,7 +29,7 @@ func cmdHook(ctx context.Context, cmd *dcmd.Cmd) {
 		for {
 			line, err := br.ReadString('\n')
 			if errors.Is(err, io.EOF) {
-				return nil, nil
+				break
 			} else if err != nil {
 				return nil, fmt.Errorf("error reading next line from stdin: %w", err)
 			}
@@ -40,64 +40,44 @@ func cmdHook(ctx context.Context, cmd *dcmd.Cmd) {
 				return nil, fmt.Errorf("malformed pre-receive hook stdin line %q", line)
 			}
 
+			startRev := plumbing.Revision(lineParts[0])
+			endRev := plumbing.Revision(lineParts[1])
 			branchName := plumbing.ReferenceName(lineParts[2])
 
-			// the zeroRevision gets sent on the very first push
+			if !branchName.IsBranch() {
-			const zeroRevision plumbing.Revision = "0000000000000000000000000000000000000000"
+				return nil, fmt.Errorf("reference %q is not a branch, can't push to it", branchName)
+			}
 
-			fromRev := plumbing.Revision(lineParts[0])
+			var startHash, endHash plumbing.Hash
-			var fromHash *plumbing.Hash
+			// startRev can be a zero hash if these are the first commits for a
-			if fromRev != zeroRevision {
+			// branch being pushed.
-				fromHash, err = repo.GitRepo.ResolveRevision(fromRev)
+			if startRev != plumbing.Revision(plumbing.ZeroHash.String()) {
+				h, err := repo.GitRepo.ResolveRevision(startRev)
 				if err != nil {
-					return nil, fmt.Errorf("unable to resolve revision %q: %w", fromRev, err)
+					return nil, fmt.Errorf("resolving revision %q: %w", startRev, err)
 				}
+				startHash = *h
 			}
-
+			{
-			toRev := plumbing.Revision(lineParts[1])
+				h, err := repo.GitRepo.ResolveRevision(endRev)
-			toHash, err := repo.GitRepo.ResolveRevision(toRev)
 				if err != nil {
-				return nil, fmt.Errorf("unable to resolve revision %q: %w", toRev, err)
+					return nil, fmt.Errorf("resolving revision %q: %w", endRev, err)
+				}
+				endHash = *h
 			}
 
-			toCommit, err := repo.GitRepo.CommitObject(*toHash)
+			gitCommits, err := repo.GetGitCommitRange(startHash, endHash)
 			if err != nil {
-				return nil, fmt.Errorf("unable to find commit %q: %w", *toHash, err)
+				return nil, fmt.Errorf("getting commits from %q to %q: %w",
+					startHash, endHash, err)
+
+			} else if err := repo.VerifyCommits(branchName, gitCommits); err != nil {
+				return nil, fmt.Errorf("verifying commits from %q to %q: %w",
+					startHash, endHash, err)
+			}
 		}
 
-			var hashesToCheck []plumbing.Hash
-			var found bool
-			for currCommit := toCommit; ; {
-				hashesToCheck = append(hashesToCheck, currCommit.Hash)
-				if currCommit.NumParents() == 0 {
-					break
-				} else if currCommit.NumParents() > 1 {
-					return nil, fmt.Errorf("commit %q has more than one parent: %+v",
-						currCommit.Hash, currCommit.ParentHashes)
-				}
-
-				parentCommit, err := currCommit.Parent(0)
-				if err != nil {
-					return nil, fmt.Errorf("unable to get parent of commit %q: %w", currCommit.Hash, err)
-				} else if fromHash != nil && parentCommit.Hash == *fromHash {
-					found = true
-					break
-				}
-				currCommit = parentCommit
-			}
-			if !found && fromHash != nil {
-				return nil, fmt.Errorf("unable to find commit %q as an ancestor of %q", *fromHash, *toHash)
-			}
-
-			for i := len(hashesToCheck) - 1; i >= 0; i-- {
-				hash := hashesToCheck[i]
-				fmt.Printf("Verifying change commit %q\n", hash)
-				if err := repo.VerifyCommit(branchName, hash); err != nil {
-					return nil, fmt.Errorf("could not verify change commit %q: %w", hash, err)
-				}
-			}
 		fmt.Println("All pushed commits have been verified, well done.")
 		return nil, nil
-		}
 	})
 }

cmd/dehub/cmd_verify.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"dehub"
 	"dehub/cmd/dehub/dcmd"
 	"fmt"
 
@@ -16,10 +17,11 @@ func cmdVerify(ctx context.Context, cmd *dcmd.Cmd) {
 	cmd.Run(func() (context.Context, error) {
 		repo := ctxRepo(ctx)
 
-		h, err := repo.GitRepo.ResolveRevision(plumbing.Revision(*rev))
+		gitCommit, err := repo.GetGitRevision(plumbing.Revision(*rev))
 		if err != nil {
-			return nil, fmt.Errorf("could not resolve revision %q: %w", *rev, err)
+			return nil, fmt.Errorf("resolving revision %q: %w", *rev, err)
 		}
+		gitCommitHash := gitCommit.GitCommit.Hash
 
 		var branchName plumbing.ReferenceName
 		if *branch == "" {
@@ -30,11 +32,12 @@ func cmdVerify(ctx context.Context, cmd *dcmd.Cmd) {
 			branchName = plumbing.NewBranchReferenceName(*branch)
 		}
 
-		if err := repo.VerifyCommit(branchName, *h); err != nil {
+		if err := repo.VerifyCommits(branchName, []dehub.GitCommit{gitCommit}); err != nil {
-			return nil, fmt.Errorf("could not verify commit at %q (%s): %w", *rev, *h, err)
+			return nil, fmt.Errorf("could not verify commit at %q (%s): %w",
+				*rev, gitCommitHash, err)
 		}
 
-		fmt.Printf("commit at %q (%s) is good to go!\n", *rev, *h)
+		fmt.Printf("commit at %q (%s) is good to go!\n", *rev, gitCommitHash)
 		return nil, nil
 	})
 }

cmd/dehub/tmp_file.go

@@ -20,7 +20,7 @@ func tmpFileMsg() (string, error) {
 		return "", fmt.Errorf("could not stat EDITOR %q: %w", editor, err)
 	}
 
-	tmpf, err := ioutil.TempFile("", "")
+	tmpf, err := ioutil.TempFile("", "dehub.*.txt")
 	if err != nil {
 		return "", fmt.Errorf("could not open temp file: %w", err)
 	}

commit.go

@@ -116,7 +116,7 @@ func (r *Repo) AccreditCommit(commit Commit, sigInt sigcred.SignifierInterface)
 		return commit, fmt.Errorf("could not cast commit %+v to interface: %w", commit, err)
 	}
 
-	headFS, err := r.HeadFS()
+	headFS, err := r.headFS()
 	if err != nil {
 		return commit, fmt.Errorf("could not grab snapshot of HEAD fs: %w", err)
 	}
@@ -132,22 +132,31 @@ func (r *Repo) AccreditCommit(commit Commit, sigInt sigcred.SignifierInterface)
 // Commit uses the given TextMarshaler to create a git commit object (with the
 // specified accountID as the author) and commits it to the current HEAD,
 // returning the hash of the commit.
-func (r *Repo) Commit(m encoding.TextMarshaler, accountID string) (plumbing.Hash, error) {
+func (r *Repo) Commit(m encoding.TextMarshaler, accountID string) (GitCommit, error) {
 	msgB, err := m.MarshalText()
 	if err != nil {
-		return plumbing.ZeroHash, fmt.Errorf("error marshaling %T to string: %v", m, err)
+		return GitCommit{}, fmt.Errorf("encoding %T to message string: %v", m, err)
 	}
 
 	w, err := r.GitRepo.Worktree()
 	if err != nil {
-		return plumbing.ZeroHash, fmt.Errorf("could not get git worktree: %w", err)
+		return GitCommit{}, fmt.Errorf("getting git worktree: %w", err)
 	}
-	return w.Commit(string(msgB), &git.CommitOptions{
+	h, err := w.Commit(string(msgB), &git.CommitOptions{
 		Author: &object.Signature{
 			Name: accountID,
 			When: time.Now(),
 		},
 	})
+	if err != nil {
+		return GitCommit{}, fmt.Errorf("committing to git worktree: %w", err)
+	}
+
+	gc, err := r.GetGitCommit(h)
+	if err != nil {
+		return GitCommit{}, fmt.Errorf("retrieving fresh commit %q back from git: %w", h, err)
+	}
+	return gc, nil
 }
 
 // HasStagedChanges returns true if there are file changes which have been
@@ -176,7 +185,6 @@ func (r *Repo) HasStagedChanges() (bool, error) {
 type verificationCtx struct {
 	commit                 *object.Commit
 	commitTree, parentTree *object.Tree
-	isRootCommit           bool
 }
 
 // non-gophers gonna hate on this method, but I say it's fine
@@ -188,9 +196,6 @@ func (r *Repo) verificationCtx(h plumbing.Hash) (vctx verificationCtx, err error
 		return vctx, fmt.Errorf("retrieving commit tree object %q: %w",
 			vctx.commit.TreeHash, err)
 
-	} else if vctx.isRootCommit = vctx.commit.NumParents() == 0; vctx.isRootCommit {
-		vctx.parentTree = new(object.Tree)
-
 	} else if parent, err := vctx.commit.Parent(0); err != nil {
 		return vctx, fmt.Errorf("retrieving commit parent: %w", err)
 
@@ -258,57 +263,84 @@ func (r *Repo) assertAccessControls(
 	return nil
 }
 
-// VerifyCommit verifies that the commit at the given hash, which is presumably
+// VerifyCommits verifies that the given commits, which are presumably on the
-// on the given branch, is gucci.
+// given branch, are gucci.
-func (r *Repo) VerifyCommit(branch plumbing.ReferenceName, h plumbing.Hash) error {
+func (r *Repo) VerifyCommits(branch plumbing.ReferenceName, gitCommits []GitCommit) error {
-	vctx, err := r.verificationCtx(h)
+
-	if err != nil {
+	for i, gitCommit := range gitCommits {
-		return err
+		// It's not a requirement that the given GitCommits are in ancestral
+		// order, but usually they are, so we can help verifyCommit not have to
+		// calculate the parentTree if the previous commit is the parent of this
+		// one.
+		var parentTree *object.Tree
+		if i > 0 && gitCommits[i-1].GitCommit.Hash == gitCommit.GitCommit.ParentHashes[0] {
+			parentTree = gitCommits[i-1].GitTree
+		}
+
+		if err := r.verifyCommit(branch, gitCommit, parentTree); err != nil {
+			return fmt.Errorf("verifying commit %q: %w",
+				gitCommit.GitCommit.Hash, err)
+		}
+	}
+	return nil
+}
+
+// if parentTree is nil then it will be inferred.
+func (r *Repo) verifyCommit(branch plumbing.ReferenceName, gitCommit GitCommit, parentTree *object.Tree) error {
+	isRoot := gitCommit.Root()
+
+	if parentTree == nil {
+		if isRoot {
+			parentTree = new(object.Tree)
+		} else if parentCommit, err := gitCommit.GitCommit.Parent(0); err != nil {
+			return fmt.Errorf("getting parent commit %q: %w",
+				gitCommit.GitCommit.ParentHashes[0], err)
+		} else if parentTree, err = r.GitRepo.TreeObject(parentCommit.TreeHash); err != nil {
+			return fmt.Errorf("getting parent tree object %q: %w",
+				parentCommit.TreeHash, err)
+		}
+	}
+
+	vctx := verificationCtx{
+		commit:     gitCommit.GitCommit,
+		commitTree: gitCommit.GitTree,
+		parentTree: parentTree,
 	}
 
 	var sigFS fs.FS
-	if vctx.isRootCommit {
+	if isRoot {
 		sigFS = fs.FromTree(vctx.commitTree)
 	} else {
 		sigFS = fs.FromTree(vctx.parentTree)
 	}
 
-	var commit Commit
-	if err := commit.UnmarshalText([]byte(vctx.commit.Message)); err != nil {
-		return err
-	}
-
 	cfg, err := r.loadConfig(sigFS)
 	if err != nil {
-		return fmt.Errorf("error loading config: %w", err)
+		return fmt.Errorf("loading config of parent %q: %w",
+			gitCommit.GitCommit.ParentHashes[0], err)
 	}
 
-	err = r.assertAccessControls(cfg.AccessControls, commit, vctx, branch)
+	err = r.assertAccessControls(cfg.AccessControls, gitCommit.Commit, vctx, branch)
 	if err != nil {
-		return fmt.Errorf("failed to satisfy all access controls: %w", err)
+		return fmt.Errorf("enforcing access controls: %w", err)
 	}
 
-	commitInt, err := commit.Interface()
+	changeHash := gitCommit.Interface.GetHash()
+	expectedChangeHash, err := gitCommit.Interface.Hash(vctx.parentTree, vctx.commitTree)
 	if err != nil {
-		return fmt.Errorf("could not cast commit %+v to interface: %w", commit, err)
+		return fmt.Errorf("calculating expected change hash: %w", err)
-	}
-
-	changeHash := commitInt.GetHash()
-	expectedChangeHash, err := commitInt.Hash(vctx.parentTree, vctx.commitTree)
-	if err != nil {
-		return fmt.Errorf("error calculating expected change hash: %w", err)
 	} else if !bytes.Equal(changeHash, expectedChangeHash) {
 		return fmt.Errorf("malformed change_hash in commit body, is %s but should be %s",
 			base64.StdEncoding.EncodeToString(expectedChangeHash),
 			base64.StdEncoding.EncodeToString(changeHash))
 	}
 
-	for _, cred := range commit.Credentials {
+	for _, cred := range gitCommit.Commit.Credentials {
 		sig, err := r.signifierForCredential(sigFS, cred)
 		if err != nil {
-			return fmt.Errorf("error finding signifier for credential %+v: %w", cred, err)
+			return fmt.Errorf("finding signifier for credential %+v: %w", cred, err)
 		} else if err := sig.Verify(sigFS, expectedChangeHash, cred); err != nil {
-			return fmt.Errorf("error verifying credential %+v: %w", cred, err)
+			return fmt.Errorf("verifying credential %+v: %w", cred, err)
 		}
 	}
 

commit_change.go

@@ -4,9 +4,9 @@ import (
 	"dehub/fs"
 	"dehub/yamlutil"
 	"errors"
+	"fmt"
 	"strings"
 
-	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
 )
 
@@ -22,11 +22,11 @@ var _ CommitInterface = CommitChange{}
 // encompassing the currently staged file changes. The Credentials of the
 // returned Commit will _not_ be filled in.
 func (r *Repo) NewCommitChange(msg string) (Commit, error) {
-	_, headTree, err := r.head()
+	headTree := new(object.Tree)
-	if errors.Is(err, plumbing.ErrReferenceNotFound) {
+	if head, err := r.GetGitHead(); err != nil && !errors.Is(err, ErrNoHead) {
-		headTree = &object.Tree{}
+		return Commit{}, fmt.Errorf("getting HEAD commit: %w", err)
-	} else if err != nil {
+	} else if err == nil {
-		return Commit{}, err
+		headTree = head.GitTree
 	}
 
 	_, stagedTree, err := fs.FromStagedChangesTree(r.GitRepo)

commit_change_test.go

@@ -76,24 +76,22 @@ func TestChangeCommitVerify(t *testing.T) {
 				h.stage(step.tree)
 				account := h.cfg.Accounts[0]
 
-				commit, hash := h.changeCommit(step.msg, account.ID, h.sig)
+				gitCommit := h.changeCommit(step.msg, account.ID, h.sig)
-				commitObj, err := h.repo.GitRepo.CommitObject(hash)
+				if step.msgHead == "" {
-				if err != nil {
-					t.Fatalf("failed to retrieve commit %v: %v", hash, err)
-				} else if step.msgHead == "" {
 					step.msgHead = strings.TrimSpace(step.msg) + "\n\n"
 				}
 
-				if !strings.HasPrefix(commitObj.Message, step.msgHead) {
+				if !strings.HasPrefix(gitCommit.GitCommit.Message, step.msgHead) {
-					t.Fatalf("commit message %q does not start with expected head %q", commitObj.Message, step.msgHead)
+					t.Fatalf("commit message %q does not start with expected head %q",
+						gitCommit.GitCommit.Message, step.msgHead)
 				}
 
 				var actualCommit Commit
-				if err := actualCommit.UnmarshalText([]byte(commitObj.Message)); err != nil {
+				if err := actualCommit.UnmarshalText([]byte(gitCommit.GitCommit.Message)); err != nil {
 					t.Fatalf("error unmarshaling commit body: %v", err)
-				} else if !reflect.DeepEqual(actualCommit, commit) {
+				} else if !reflect.DeepEqual(actualCommit, gitCommit.Commit) {
 					t.Fatalf("returned change commit:\n%s\ndoes not match actual one:\n%s",
-						spew.Sdump(commit), spew.Sdump(actualCommit))
+						spew.Sdump(gitCommit.Commit), spew.Sdump(actualCommit))
 				}
 			}
 		})

commit_credential_test.go

@@ -61,11 +61,11 @@ func TestCredentialCommitVerify(t *testing.T) {
 		},
 	}
 	h.stageCfg()
-	rootCommit, _ := h.changeCommit("initial commit", h.cfg.Accounts[0].ID, h.sig)
+	rootGitCommit := h.changeCommit("initial commit", h.cfg.Accounts[0].ID, h.sig)
 
 	// toot user wants to create a credential commit for the root commit, for
 	// whatever reason.
-	rootChangeHash := rootCommit.Change.ChangeHash
+	rootChangeHash := rootGitCommit.Commit.Change.ChangeHash
 	credCommit, err := h.repo.NewCommitCredential(rootChangeHash)
 	if err != nil {
 		t.Fatalf("creating credential commit for hash %x: %v", rootChangeHash, err)

commit_test.go

@@ -3,19 +3,17 @@ package dehub
 import (
 	"dehub/sigcred"
 	"testing"
-
-	"gopkg.in/src-d/go-git.v4/plumbing"
 )
 
 func TestConfigChange(t *testing.T) {
 	h := newHarness(t)
 
-	var hashes []plumbing.Hash
+	var gitCommits []GitCommit
 
 	// commit the initial staged changes, which merely include the config and
 	// public key
-	_, hash := h.changeCommit("commit configuration", h.cfg.Accounts[0].ID, h.sig)
+	gitCommit := h.changeCommit("commit configuration", h.cfg.Accounts[0].ID, h.sig)
-	hashes = append(hashes, hash)
+	gitCommits = append(gitCommits, gitCommit)
 
 	// create a new account and add it to the configuration. That commit should
 	// not be verifiable, though
@@ -38,17 +36,15 @@ func TestConfigChange(t *testing.T) {
 
 	// now add with the root user, this should work.
 	h.stageCfg()
-	_, hash = h.changeCommit("add toot user", h.cfg.Accounts[0].ID, h.sig)
+	gitCommit = h.changeCommit("add toot user", h.cfg.Accounts[0].ID, h.sig)
-	hashes = append(hashes, hash)
+	gitCommits = append(gitCommits, gitCommit)
 
 	// _now_ the toot user should be able to do things.
 	h.stage(map[string]string{"foo/bar": "what a cool file"})
-	_, hash = h.changeCommit("add a cool file", h.cfg.Accounts[1].ID, newSig)
+	gitCommit = h.changeCommit("add a cool file", h.cfg.Accounts[1].ID, newSig)
-	hashes = append(hashes, hash)
+	gitCommits = append(gitCommits, gitCommit)
 
-	for i, hash := range hashes {
+	if err := h.repo.VerifyCommits(MainRefName, gitCommits); err != nil {
-		if err := h.repo.VerifyCommit(MainRefName, hash); err != nil {
+		t.Fatal(err)
-			t.Fatalf("commit %d (%v) should have been verified but wasn't: %v", i, hash, err)
-		}
 	}
 }

config.go

@@ -44,7 +44,7 @@ func (r *Repo) loadConfig(fs fs.FS) (Config, error) {
 // LoadConfig loads the Config object from the HEAD of the repo, or directly
 // from the filesystem if there is no HEAD yet.
 func (r *Repo) LoadConfig() (Config, error) {
-	headFS, err := r.HeadFS()
+	headFS, err := r.headFS()
 	if err != nil {
 		return Config{}, fmt.Errorf("error retrieving repo HEAD: %w", err)
 	}

repo.go

@@ -95,7 +95,7 @@ func InitMemRepo() *Repo {
 func (r *Repo) init() error {
 	h := plumbing.NewSymbolicReference(plumbing.HEAD, MainRefName)
 	if err := r.GitRepo.Storer.SetReference(h); err != nil {
-		return fmt.Errorf("could not set HEAD to %q: %w", MainRefName, err)
+		return fmt.Errorf("setting HEAD reference to %q: %w", MainRefName, err)
 	}
 	return nil
 }
@@ -103,7 +103,7 @@ func (r *Repo) init() error {
 func (r *Repo) billyFilesystem() (billy.Filesystem, error) {
 	w, err := r.GitRepo.Worktree()
 	if err != nil {
-		return nil, fmt.Errorf("could not open git worktree: %w", err)
+		return nil, fmt.Errorf("opening git worktree: %w", err)
 	}
 	return w.Filesystem, nil
 }
@@ -114,7 +114,7 @@ func (r *Repo) CheckedOutBranch() (plumbing.ReferenceName, error) {
 	// newly initialized repo very well.
 	ogRef, err := r.GitRepo.Storer.Reference(plumbing.HEAD)
 	if err != nil {
-		return "", fmt.Errorf("couldn't de-reference HEAD (is it a bare repo?): %w", err)
+		return "", fmt.Errorf("de-referencing HEAD (is it a bare repo?): %w", err)
 	}
 
 	ref := ogRef
@@ -137,39 +137,137 @@ func (r *Repo) CheckedOutBranch() (plumbing.ReferenceName, error) {
 	return "", fmt.Errorf("could not de-reference HEAD to a branch: %w", err)
 }
 
-func (r *Repo) head() (*object.Commit, *object.Tree, error) {
+// headFS returns an FS based on the HEAD commit, or if there is no HEAD commit
-	head, err := r.GitRepo.Head()
-	if err != nil {
-		return nil, nil, fmt.Errorf("could not get repo HEAD: %w", err)
-	}
-
-	headHash := head.Hash()
-	headCommit, err := r.GitRepo.CommitObject(headHash)
-	if err != nil {
-		return nil, nil, fmt.Errorf("could not get commit at HEAD (%q): %w", headHash, err)
-	}
-
-	headTree, err := r.GitRepo.TreeObject(headCommit.TreeHash)
-	if err != nil {
-		return nil, nil, fmt.Errorf("could not get tree object at HEAD (commit:%q tree:%q): %w",
-			headHash, headCommit.TreeHash, err)
-	}
-
-	return headCommit, headTree, nil
-}
-
-// HeadFS returns an FS based on the HEAD commit, or if there is no HEAD commit
 // (it's an empty repo) an FS based on the raw filesystem.
-func (r *Repo) HeadFS() (fs.FS, error) {
+func (r *Repo) headFS() (fs.FS, error) {
-	_, headTree, err := r.head()
+	head, err := r.GetGitHead()
-	if errors.Is(err, plumbing.ErrReferenceNotFound) {
+	if errors.Is(err, ErrNoHead) {
 		bfs, err := r.billyFilesystem()
 		if err != nil {
-			return nil, fmt.Errorf("could not get underlying filesystem: %w", err)
+			return nil, fmt.Errorf("getting underlying filesystem: %w", err)
 		}
 		return fs.FromBillyFilesystem(bfs), nil
+
 	} else if err != nil {
 		return nil, fmt.Errorf("could not get HEAD tree: %w", err)
 	}
-	return fs.FromTree(headTree), nil
+
+	return fs.FromTree(head.GitTree), nil
+}
+
+// GitCommit wraps a single git commit object, and also contains various fields
+// which are parsed out of it. It is used as a convenience type, in place of
+// having to manually retrieve and parse specific information out of commit
+// objects.
+type GitCommit struct {
+	GitCommit *object.Commit
+
+	// Fields based on that Commit, which can't be directly gleaned from it.
+	GitTree   *object.Tree
+	Commit    Commit
+	Interface CommitInterface
+}
+
+// Root returns true if this commit is the root commit in its branch (i.e. it
+// has no parents)
+func (gc GitCommit) Root() bool {
+	return gc.GitCommit.NumParents() == 0
+}
+
+// GetGitCommit retrieves the commit at the given hash, and all of its sub-data
+// which can be pulled out of it.
+func (r *Repo) GetGitCommit(h plumbing.Hash) (gc GitCommit, err error) {
+	if gc.GitCommit, err = r.GitRepo.CommitObject(h); err != nil {
+		return gc, fmt.Errorf("getting git commit object: %w", err)
+	} else if gc.GitTree, err = r.GitRepo.TreeObject(gc.GitCommit.TreeHash); err != nil {
+		return gc, fmt.Errorf("getting git tree object %q: %w",
+			gc.GitCommit.TreeHash, err)
+	} else if gc.Commit.UnmarshalText([]byte(gc.GitCommit.Message)); err != nil {
+		return gc, fmt.Errorf("decoding commit message: %w", err)
+	} else if gc.Interface, err = gc.Commit.Interface(); err != nil {
+		return gc, fmt.Errorf("casting %+v to a CommitInterface: %w", gc.Commit, err)
+	}
+	return
+}
+
+// GetGitRevision resolves the revision and returns the GitCommit it references.
+func (r *Repo) GetGitRevision(rev plumbing.Revision) (GitCommit, error) {
+	// This returns a pointer for some reason, not sure why.
+	h, err := r.GitRepo.ResolveRevision(rev)
+	if err != nil {
+		return GitCommit{}, fmt.Errorf("resolving revision: %w", err)
+	}
+
+	gc, err := r.GetGitCommit(*h)
+	if err != nil {
+		return GitCommit{}, fmt.Errorf("getting commit %q: %w", *h, err)
+	}
+	return gc, nil
+}
+
+// ErrNoHead is returns from GetGitHead if there is no HEAD reference defined in
+// the repo. This can happen if the repo has no commits
+var ErrNoHead = errors.New("HEAD reference not found")
+
+// GetGitHead returns the GitCommit which is currently referenced by HEAD.
+// This method may return ErrNoHead if the repo has no commits.
+func (r *Repo) GetGitHead() (GitCommit, error) {
+	head, err := r.GitRepo.Head()
+	if errors.Is(err, plumbing.ErrReferenceNotFound) {
+		return GitCommit{}, ErrNoHead
+	} else if err != nil {
+		return GitCommit{}, fmt.Errorf("resolving HEAD: %w", err)
+	}
+	headHash := head.Hash()
+
+	gc, err := r.GetGitCommit(headHash)
+	if err != nil {
+		return GitCommit{}, fmt.Errorf("getting commit %q: %w", headHash, err)
+	}
+	return gc, nil
+}
+
+// GetGitCommitRange returns an ancestry of GitCommits, with the first being the
+// commit immediately following the given starting hash, and the last being the
+// given ending hash.
+//
+// If start is plumbing.ZeroHash then the root commit will be the starting one.
+func (r *Repo) GetGitCommitRange(start, end plumbing.Hash) ([]GitCommit, error) {
+	curr, err := r.GetGitCommit(end)
+	if err != nil {
+		return nil, fmt.Errorf("retrieving commit %q: %w", end, err)
+	}
+
+	var commits []GitCommit
+	var found bool
+	for {
+		commits = append(commits, curr)
+		numParents := curr.GitCommit.NumParents()
+		if numParents == 0 {
+			break
+		} else if numParents > 1 {
+			return nil, fmt.Errorf("commit %q has more than one parent: %+v",
+				curr.GitCommit.Hash, curr.GitCommit.ParentHashes)
+		}
+
+		parentHash := curr.GitCommit.ParentHashes[0]
+		parent, err := r.GetGitCommit(parentHash)
+		if err != nil {
+			return nil, fmt.Errorf("retrieving commit %q: %w", parentHash, err)
+		} else if start != plumbing.ZeroHash && parentHash == start {
+			found = true
+			break
+		}
+		curr = parent
+	}
+	if !found && start != plumbing.ZeroHash {
+		return nil, fmt.Errorf("unable to find commit %q as an ancestor of %q",
+			start, end)
+	}
+
+	// reverse the commits to be in the expected order
+	for l, r := 0, len(commits)-1; l < r; l, r = l+1, r-1 {
+		commits[l], commits[r] = commits[r], commits[l]
+	}
+	return commits, nil
 }

repo_test.go

@@ -122,7 +122,7 @@ func (h *harness) checkout(branch plumbing.ReferenceName) {
 		h.t.Fatal(err)
 	}
 
-	head, _, err := h.repo.head()
+	head, err := h.repo.GetGitHead()
 	if err != nil {
 		h.t.Fatal(err)
 	}
@@ -130,7 +130,7 @@ func (h *harness) checkout(branch plumbing.ReferenceName) {
 	_, err = h.repo.GitRepo.Branch(branch.Short())
 	if errors.Is(err, git.ErrBranchNotFound) {
 		err = w.Checkout(&git.CheckoutOptions{
-			Hash:   head.Hash,
+			Hash:   head.GitCommit.Hash,
 			Branch: branch,
 			Create: true,
 		})
@@ -166,9 +166,7 @@ func (h *harness) tryCommit(
 	shouldSucceed bool,
 	commit Commit,
 	accountID string, accountSig sigcred.SignifierInterface,
-) (
+) GitCommit {
-	Commit, plumbing.Hash,
-) {
 	if accountSig != nil {
 		var err error
 		if commit, err = h.repo.AccreditCommit(commit, accountSig); err != nil {
@@ -176,7 +174,7 @@ func (h *harness) tryCommit(
 		}
 	}
 
-	hash, err := h.repo.Commit(commit, accountID)
+	gitCommit, err := h.repo.Commit(commit, accountID)
 	if err != nil {
 		h.t.Fatalf("failed to commit ChangeCommit: %v", err)
 	}
@@ -186,35 +184,28 @@ func (h *harness) tryCommit(
 		h.t.Fatalf("determining checked out branch: %v", err)
 	}
 
-	err = h.repo.VerifyCommit(branch, hash)
+	err = h.repo.VerifyCommits(branch, []GitCommit{gitCommit})
 	if shouldSucceed && err != nil {
-		h.t.Fatalf("verifying commit %q: %v", hash, err)
+		h.t.Fatalf("verifying commit %q: %v", gitCommit.GitCommit.Hash, err)
 	} else if shouldSucceed {
-		return commit, hash
+		return gitCommit
 	} else if !shouldSucceed && err == nil {
-		h.t.Fatalf("verifying commit %q should have failed", hash)
+		h.t.Fatalf("verifying commit %q should have failed", gitCommit.GitCommit.Hash)
 	}
 
-	// commit verifying didn't succeed, reset it back. first get parent commit
+	if gitCommit.GitCommit.NumParents() == 0 {
-	// to reset to
+		h.t.Fatalf("unverifiable commit %q has no parents, but it should", gitCommit.GitCommit.NumParents())
-	commitObj, err := h.repo.GitRepo.CommitObject(hash)
-	if err != nil {
-		h.t.Fatalf("getting commit object of unverifiable hash %q: %v", hash, err)
-	} else if commitObj.NumParents() == 0 {
-		h.t.Fatalf("unverifiable commit %q has no parents, but it should", hash)
 	}
 
-	h.reset(commitObj.ParentHashes[0], git.HardReset)
+	h.reset(gitCommit.GitCommit.ParentHashes[0], git.HardReset)
-	return commit, hash
+	return gitCommit
 }
 
 func (h *harness) changeCommit(
 	msg string,
 	accountID string,
 	sig sigcred.SignifierInterface,
-) (
+) GitCommit {
-	Commit, plumbing.Hash,
-) {
 	commit, err := h.repo.NewCommitChange(msg)
 	if err != nil {
 		h.t.Fatalf("creating ChangeCommit: %v", err)
@@ -304,9 +295,15 @@ func TestThisRepoStillVerifies(t *testing.T) {
 		t.Fatalf("error opening repo: %v", err)
 	}
 
-	headCommit, _, err := repo.head()
+	headGitCommit, err := repo.GetGitHead()
 	if err != nil {
-		t.Fatalf("error getting repo head: %v", err)
+		t.Fatalf("getting repo head: %v", err)
+	}
+
+	allCommits, err := repo.GetGitCommitRange(plumbing.ZeroHash, headGitCommit.GitCommit.Hash)
+	if err != nil {
+		t.Fatalf("getting all commits (up to %q): %v",
+			headGitCommit.GitCommit.Hash, err)
 	}
 
 	checkedOutBranch, err := repo.CheckedOutBranch()
@@ -314,9 +311,7 @@ func TestThisRepoStillVerifies(t *testing.T) {
 		t.Fatalf("error determining checked out branch: %v", err)
 	}
 
-	for _, hash := range headCommit.ParentHashes {
+	if err := repo.VerifyCommits(checkedOutBranch, allCommits); err != nil {
-		if err := repo.VerifyCommit(checkedOutBranch, hash); err != nil {
+		t.Fatal(err)
-			t.Fatalf("error verifying commit %q of branch %q: %v", hash, checkedOutBranch, err)
-		}
 	}
 }