Return error instead of creating empty commits

BuildTree now returns an ErrEmptyCommit error, when there are no
changes to be committed. This can be opted-out via
CommitOptions.AllowEmptyCommits.

This is a breaking change which enables applications to detect when
empty commits are to be created.

Some instances in which this can occur is when the fs (e.g. `billy/osfs`)
make changes to the underlying files, causing a conflict between what
the previous Git worktree state was, and the current state. Changes to
the fs implementations are orthogonal to this, and will be dealt with
separately.

The new behaviour aligns with the Git CLI, in which empty commits
returns the error message: 'nothing to commit, working tree clean'.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/worktree_commit.go b/worktree_commit.go
index dc79569..e927721 100644
--- a/worktree_commit.go
+++ b/worktree_commit.go
@@ -2,6 +2,7 @@ package git
 
 import (
 	"bytes"
+	"errors"
 	"path"
 	"sort"
 	"strings"
@@ -16,6 +17,12 @@ import (
 	"github.com/go-git/go-billy/v5"
 )
 
+var (
+	// ErrEmptyCommit occurs when a commit is attempted using a clean
+	// working tree, with no changes to be committed.
+	ErrEmptyCommit = errors.New("cannot create empty commit: clean working tree")
+)
+
 // Commit stores the current contents of the index in a new commit along with
 // a log message from the user describing the changes.
 func (w *Worktree) Commit(msg string, opts *CommitOptions) (plumbing.Hash, error) {
@@ -39,7 +46,7 @@ func (w *Worktree) Commit(msg string, opts *CommitOptions) (plumbing.Hash, error
 		s:  w.r.Storer,
 	}
 
-	tree, err := h.BuildTree(idx)
+	tree, err := h.BuildTree(idx, opts)
 	if err != nil {
 		return plumbing.ZeroHash, err
 	}
@@ -145,7 +152,11 @@ type buildTreeHelper struct {
 
 // BuildTree builds the tree objects and push its to the storer, the hash
 // of the root tree is returned.
-func (h *buildTreeHelper) BuildTree(idx *index.Index) (plumbing.Hash, error) {
+func (h *buildTreeHelper) BuildTree(idx *index.Index, opts *CommitOptions) (plumbing.Hash, error) {
+	if len(idx.Entries) == 0 && (opts == nil || !opts.AllowEmptyCommits) {
+		return plumbing.ZeroHash, ErrEmptyCommit
+	}
+
 	const rootNode = ""
 	h.trees = map[string]*object.Tree{rootNode: {}}
 	h.entries = map[string]*object.TreeEntry{}