1 files changed, 100 insertions, 0 deletions
diff --git a/plumbing/format/commitgraph/v2/chain.go b/plumbing/format/commitgraph/v2/chain.go
new file mode 100644
index 0000000..8da60d0
--- /dev/null
+++ b/plumbing/format/commitgraph/v2/chain.go
@@ -0,0 +1,100 @@
+package v2
+
+import (
+	"bufio"
+	"io"
+	"path"
+
+	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-git/v5/plumbing"
+)
+
+// OpenChainFile reads a commit chain file and returns a slice of the hashes within it
+//
+// Commit-Graph chains are described at https://git-scm.com/docs/commit-graph
+// and are new line separated list of graph file hashes, oldest to newest.
+//
+// This function simply reads the file and returns the hashes as a slice.
+func OpenChainFile(r io.Reader) ([]string, error) {
+	if r == nil {
+		return nil, io.ErrUnexpectedEOF
+	}
+	bufRd := bufio.NewReader(r)
+	chain := make([]string, 0, 8)
+	for {
+		line, err := bufRd.ReadSlice('\n')
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+			return nil, err
+		}
+
+		hashStr := string(line[:len(line)-1])
+		if !plumbing.IsHash(hashStr) {
+			return nil, ErrMalformedCommitGraphFile
+		}
+		chain = append(chain, hashStr)
+	}
+	return chain, nil
+}
+
+// OpenChainOrFileIndex expects a billy.Filesystem representing a .git directory.
+// It will first attempt to read a commit-graph index file, before trying to read a
+// commit-graph chain file and its index files. If neither are present, an error is returned.
+// Otherwise an Index will be returned.
+//
+// See: https://git-scm.com/docs/commit-graph
+func OpenChainOrFileIndex(fs billy.Filesystem) (Index, error) {
+	file, err := fs.Open(path.Join("objects", "info", "commit-graph"))
+	if err != nil {
+		// try to open a chain file
+		return OpenChainIndex(fs)
+	}
+
+	index, err := OpenFileIndex(file)
+	if err != nil {
+		// Ignore any file closing errors and return the error from OpenFileIndex instead
+		_ = file.Close()
+		return nil, err
+	}
+	return index, nil
+}
+
+// OpenChainIndex expects a billy.Filesystem representing a .git directory.
+// It will read a commit-graph chain file and return a coalesced index.
+// If the chain file or a graph in that chain is not present, an error is returned.
+//
+// See: https://git-scm.com/docs/commit-graph
+func OpenChainIndex(fs billy.Filesystem) (Index, error) {
+	chainFile, err := fs.Open(path.Join("objects", "info", "commit-graphs", "commit-graph-chain"))
+	if err != nil {
+		return nil, err
+	}
+
+	chain, err := OpenChainFile(chainFile)
+	_ = chainFile.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	var index Index
+	for _, hash := range chain {
+
+		file, err := fs.Open(path.Join("objects", "info", "commit-graphs", "graph-"+hash+".graph"))
+		if err != nil {
+			// Ignore all other file closing errors and return the error from opening the last file in the graph
+			_ = index.Close()
+			return nil, err
+		}
+
+		index, err = OpenFileIndexWithParent(file, index)
+		if err != nil {
+			// Ignore file closing errors and return the error from OpenFileIndex instead
+			_ = index.Close()
+			return nil, err
+		}
+	}
+
+	return index, nil
+}