thodg/libgit2

diff --git a/src/cherrypick.c b/src/cherrypick.c
index 3754b1f..ebc9fcd 100644
--- a/src/cherrypick.c
+++ b/src/cherrypick.c
@@ -172,9 +172,8 @@ int git_cherrypick(
 	char commit_oidstr[GIT_OID_HEXSZ + 1];
 	const char *commit_msg, *commit_summary;
 	git_buf their_label = GIT_BUF_INIT;
-	git_index *index = NULL, *index_new = NULL;
+	git_index *index = NULL;
 	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
-	bool write_index = false;
 	int error = 0;
 
 	assert(repo && commit);
@@ -194,28 +193,16 @@ int git_cherrypick(
 
 	if ((error = write_merge_msg(repo, commit_msg)) < 0 ||
 		(error = git_buf_printf(&their_label, "%.7s... %s", commit_oidstr, commit_summary)) < 0 ||
-		(error = cherrypick_normalize_opts(repo, &opts, given_opts, git_buf_cstr(&their_label))) < 0)
-		goto on_error;
-
-	write_index = (opts.checkout_opts.checkout_strategy & GIT_CHECKOUT_DONT_WRITE_INDEX) == 0;
-
-	if (write_index) {
-		/* Never let checkout update the index, we'll update it ourselves. */
-		opts.checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
-
-		if ((error = git_repository_index(&index, repo)) < 0 ||
-			(error = git_indexwriter_init(&indexwriter, index)) < 0)
-			goto on_error;
-	}
-
-	if ((error = write_cherrypick_head(repo, commit_oidstr)) < 0 ||
+		(error = cherrypick_normalize_opts(repo, &opts, given_opts, git_buf_cstr(&their_label))) < 0 ||
+		(error = git_indexwriter_init_for_operation(&indexwriter, repo, &opts.checkout_opts.checkout_strategy)) < 0 ||
+		(error = write_cherrypick_head(repo, commit_oidstr)) < 0 ||
 		(error = git_repository_head(&our_ref, repo)) < 0 ||
 		(error = git_reference_peel((git_object **)&our_commit, our_ref, GIT_OBJ_COMMIT)) < 0 ||
-		(error = git_cherrypick_commit(&index_new, repo, commit, our_commit, opts.mainline, &opts.merge_opts)) < 0 ||
-		(error = git_merge__check_result(repo, index_new)) < 0 ||
-		(error = git_merge__append_conflicts_to_merge_msg(repo, index_new)) < 0 ||
-		(error = git_checkout_index(repo, index_new, &opts.checkout_opts)) < 0 ||
-		(write_index && (error = git_indexwriter_commit(&indexwriter)) < 0))
+		(error = git_cherrypick_commit(&index, repo, commit, our_commit, opts.mainline, &opts.merge_opts)) < 0 ||
+		(error = git_merge__check_result(repo, index)) < 0 ||
+		(error = git_merge__append_conflicts_to_merge_msg(repo, index)) < 0 ||
+		(error = git_checkout_index(repo, index, &opts.checkout_opts)) < 0 ||
+		(error = git_indexwriter_commit(&indexwriter)) < 0)
 		goto on_error;
 
 	goto done;
@@ -226,7 +213,6 @@ on_error:
 done:
 	git_indexwriter_cleanup(&indexwriter);
 	git_index_free(index);
-	git_index_free(index_new);
 	git_commit_free(our_commit);
 	git_reference_free(our_ref);
 	git_buf_free(&their_label);
diff --git a/src/index.c b/src/index.c
index 58575fe..6c01b40 100644
--- a/src/index.c
+++ b/src/index.c
@@ -2669,6 +2669,8 @@ int git_indexwriter_init(
 {
 	int error;
 
+	GIT_REFCOUNT_INC(index);
+
 	writer->index = index;
 
 	if (!index->index_file_path)
@@ -2677,12 +2679,33 @@ int git_indexwriter_init(
 
 	if ((error = git_filebuf_open(
 		&writer->file, index->index_file_path, GIT_FILEBUF_HASH_CONTENTS, GIT_INDEX_FILE_MODE)) < 0) {
+
 		if (error == GIT_ELOCKED)
 			giterr_set(GITERR_INDEX, "The index is locked. This might be due to a concurrent or crashed process");
 
 		return error;
 	}
 
+	writer->should_write = 1;
+
+	return 0;
+}
+
+int git_indexwriter_init_for_operation(
+	git_indexwriter *writer,
+	git_repository *repo,
+	unsigned int *checkout_strategy)
+{
+	git_index *index;
+	int error;
+
+	if ((error = git_repository_index__weakptr(&index, repo)) < 0 ||
+		(error = git_indexwriter_init(writer, index)) < 0)
+		return error;
+
+	writer->should_write = (*checkout_strategy & GIT_CHECKOUT_DONT_WRITE_INDEX) == 0;
+	*checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
+
 	return 0;
 }
 
@@ -2690,6 +2713,9 @@ int git_indexwriter_commit(git_indexwriter *writer)
 {
 	int error;
 
+	if (!writer->should_write)
+		return 0;
+
 	if (index_sort_if_needed(writer->index, true) < 0)
 		return -1;
 
@@ -2711,10 +2737,16 @@ int git_indexwriter_commit(git_indexwriter *writer)
 
 	writer->index->on_disk = 1;
 
+	git_index_free(writer->index);
+	writer->index = NULL;
+
 	return 0;
 }
 
 void git_indexwriter_cleanup(git_indexwriter *writer)
 {
 	git_filebuf_cleanup(&writer->file);
+
+	git_index_free(writer->index);
+	writer->index = NULL;
 }
diff --git a/src/index.h b/src/index.h
index d151f66..6d2904f 100644
--- a/src/index.h
+++ b/src/index.h
@@ -97,6 +97,7 @@ extern int git_index_snapshot_find(
 typedef struct {
 	git_index *index;
 	git_filebuf file;
+	unsigned int should_write:1;
 } git_indexwriter;
 
 #define GIT_INDEXWRITER_INIT { NULL, GIT_FILEBUF_INIT }
@@ -104,6 +105,16 @@ typedef struct {
 /* Lock the index for eventual writing. */
 extern int git_indexwriter_init(git_indexwriter *writer, git_index *index);
 
+/* Lock the index for eventual writing by a repository operation: a merge,
+ * revert, cherry-pick or a rebase.  Note that the given checkout strategy
+ * will be updated for the operation's use so that checkout will not write
+ * the index.
+ */
+extern int git_indexwriter_init_for_operation(
+	git_indexwriter *writer,
+	git_repository *repo,
+	unsigned int *checkout_strategy);
+
 /* Write the index and unlock it. */
 extern int git_indexwriter_commit(git_indexwriter *writer);
 
diff --git a/src/merge.c b/src/merge.c
index 8201c0c..a92e6ae 100644
--- a/src/merge.c
+++ b/src/merge.c
@@ -2651,9 +2651,8 @@ int git_merge(
 	git_checkout_options checkout_opts;
 	git_annotated_commit *ancestor_head = NULL, *our_head = NULL;
 	git_tree *ancestor_tree = NULL, *our_tree = NULL, **their_trees = NULL;
-	git_index *index = NULL, *index_new = NULL;
+	git_index *index = NULL;
 	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
-	bool write_index = false;
 	size_t i;
 	int error = 0;
 
@@ -2667,24 +2666,12 @@ int git_merge(
 	their_trees = git__calloc(their_heads_len, sizeof(git_tree *));
 	GITERR_CHECK_ALLOC(their_trees);
 
-	if ((error = merge_heads(&ancestor_head, &our_head, repo, their_heads, their_heads_len)) < 0)
-		goto on_error;
-
-	if ((error = merge_normalize_checkout_opts(repo, &checkout_opts, given_checkout_opts,
-		ancestor_head, our_head, their_heads_len, their_heads)) < 0)
+	if ((error = merge_heads(&ancestor_head, &our_head, repo, their_heads, their_heads_len)) < 0 ||
+		(error = merge_normalize_checkout_opts(repo, &checkout_opts, given_checkout_opts,
+			ancestor_head, our_head, their_heads_len, their_heads)) < 0 ||
+		(error = git_indexwriter_init_for_operation(&indexwriter, repo, &checkout_opts.checkout_strategy)) < 0)
 		goto on_error;
 
-	write_index = (checkout_opts.checkout_strategy & GIT_CHECKOUT_DONT_WRITE_INDEX) == 0;
-
-	if (write_index) {
-		/* Never let checkout update the index, we'll update it ourselves. */
-		checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
-
-		if ((error = git_repository_index(&index, repo)) < 0 ||
-			(error = git_indexwriter_init(&indexwriter, index)) < 0)
-			goto on_error;
-	}
-
 	/* Write the merge files to the repository. */
 	if ((error = git_merge__setup(repo, our_head, their_heads, their_heads_len)) < 0)
 		goto on_error;
@@ -2703,11 +2690,11 @@ int git_merge(
 
 	/* TODO: recursive, octopus, etc... */
 
-	if ((error = git_merge_trees(&index_new, repo, ancestor_tree, our_tree, their_trees[0], merge_opts)) < 0 ||
-		(error = git_merge__check_result(repo, index_new)) < 0 ||
-		(error = git_merge__append_conflicts_to_merge_msg(repo, index_new)) < 0 ||
-		(error = git_checkout_index(repo, index_new, &checkout_opts)) < 0 ||
-		(write_index && (error = git_indexwriter_commit(&indexwriter)) < 0))
+	if ((error = git_merge_trees(&index, repo, ancestor_tree, our_tree, their_trees[0], merge_opts)) < 0 ||
+		(error = git_merge__check_result(repo, index)) < 0 ||
+		(error = git_merge__append_conflicts_to_merge_msg(repo, index)) < 0 ||
+		(error = git_checkout_index(repo, index, &checkout_opts)) < 0 ||
+		(error = git_indexwriter_commit(&indexwriter)) < 0)
 		goto on_error;
 
 	goto done;
@@ -2719,7 +2706,6 @@ done:
 	git_indexwriter_cleanup(&indexwriter);
 
 	git_index_free(index);
-	git_index_free(index_new);
 
 	git_tree_free(ancestor_tree);
 	git_tree_free(our_tree);
diff --git a/src/rebase.c b/src/rebase.c
index 9e6c4c1..b24830b 100644
--- a/src/rebase.c
+++ b/src/rebase.c
@@ -726,9 +726,8 @@ static int rebase_next_merge(
 	git_checkout_options checkout_opts = {0};
 	git_commit *current_commit = NULL, *parent_commit = NULL;
 	git_tree *current_tree = NULL, *head_tree = NULL, *parent_tree = NULL;
-	git_index *index = NULL, *index_new = NULL;
+	git_index *index = NULL;
 	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
-	bool write_index = false;
 	git_rebase_operation *operation;
 	char current_idstr[GIT_OID_HEXSZ];
 	unsigned int parent_count;
@@ -758,27 +757,15 @@ static int rebase_next_merge(
 
 	git_oid_fmt(current_idstr, &operation->id);
 
-	write_index = (checkout_opts.checkout_strategy & GIT_CHECKOUT_DONT_WRITE_INDEX) == 0;
-
-	if (write_index) {
-		/* Never let checkout update the index, we'll update it ourselves. */
-		checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
-
-		if ((error = git_repository_index(&index, rebase->repo)) < 0 ||
-			(error = git_indexwriter_init(&indexwriter, index)) < 0)
-			goto done;
-	}
-
-	if ((error = rebase_setupfile(rebase, MSGNUM_FILE, -1, "%d\n", rebase->current+1)) < 0 ||
-		(error = rebase_setupfile(rebase, CURRENT_FILE, -1, "%.*s\n", GIT_OID_HEXSZ, current_idstr)) < 0)
-		goto done;
-
 	normalize_checkout_opts(rebase, current_commit, &checkout_opts, given_checkout_opts);
 
-	if ((error = git_merge_trees(&index, rebase->repo, parent_tree, head_tree, current_tree, NULL)) < 0 ||
-		(error = git_merge__check_result(rebase->repo, index_new)) < 0 ||
-		(error = git_checkout_index(rebase->repo, index_new, &checkout_opts)) < 0 ||
-		(write_index && (error = git_indexwriter_commit(&indexwriter)) < 0))
+	if ((error = git_indexwriter_init_for_operation(&indexwriter, rebase->repo, &checkout_opts.checkout_strategy)) < 0 ||
+		(error = rebase_setupfile(rebase, MSGNUM_FILE, -1, "%d\n", rebase->current+1)) < 0 ||
+		(error = rebase_setupfile(rebase, CURRENT_FILE, -1, "%.*s\n", GIT_OID_HEXSZ, current_idstr)) < 0 ||
+		(error = git_merge_trees(&index, rebase->repo, parent_tree, head_tree, current_tree, NULL)) < 0 ||
+		(error = git_merge__check_result(rebase->repo, index)) < 0 ||
+		(error = git_checkout_index(rebase->repo, index, &checkout_opts)) < 0 ||
+		(error = git_indexwriter_commit(&indexwriter)) < 0)
 		goto done;
 
 	*out = operation;
@@ -786,7 +773,6 @@ static int rebase_next_merge(
 done:
 	git_indexwriter_cleanup(&indexwriter);
 	git_index_free(index);
-	git_index_free(index_new);
 	git_tree_free(current_tree);
 	git_tree_free(head_tree);
 	git_tree_free(parent_tree);
diff --git a/src/revert.c b/src/revert.c
index f5f1a5d..f8a7f43 100644
--- a/src/revert.c
+++ b/src/revert.c
@@ -175,9 +175,8 @@ int git_revert(
 	char commit_oidstr[GIT_OID_HEXSZ + 1];
 	const char *commit_msg;
 	git_buf their_label = GIT_BUF_INIT;
-	git_index *index = NULL, *index_new = NULL;
+	git_index *index = NULL;
 	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
-	bool write_index = false;
 	int error;
 
 	assert(repo && commit);
@@ -196,29 +195,17 @@ int git_revert(
 	}
 
 	if ((error = git_buf_printf(&their_label, "parent of %.7s... %s", commit_oidstr, commit_msg)) < 0 ||
-		(error = revert_normalize_opts(repo, &opts, given_opts, git_buf_cstr(&their_label))) < 0)
-		goto on_error;
-
-	write_index = (opts.checkout_opts.checkout_strategy & GIT_CHECKOUT_DONT_WRITE_INDEX) == 0;
-
-	if (write_index) {
-		/* Never let checkout update the index, we'll update it ourselves. */
-		opts.checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
-
-		if ((error = git_repository_index(&index, repo)) < 0 ||
-			(error = git_indexwriter_init(&indexwriter, index)) < 0)
-			goto on_error;
-	}
-
-	if ((error = write_revert_head(repo, commit_oidstr)) < 0 ||
+		(error = revert_normalize_opts(repo, &opts, given_opts, git_buf_cstr(&their_label))) < 0 ||
+		(error = git_indexwriter_init_for_operation(&indexwriter, repo, &opts.checkout_opts.checkout_strategy)) < 0 ||
+		(error = write_revert_head(repo, commit_oidstr)) < 0 ||
 		(error = write_merge_msg(repo, commit_oidstr, commit_msg)) < 0 ||
 		(error = git_repository_head(&our_ref, repo)) < 0 ||
 		(error = git_reference_peel((git_object **)&our_commit, our_ref, GIT_OBJ_COMMIT)) < 0 ||
-		(error = git_revert_commit(&index_new, repo, commit, our_commit, opts.mainline, &opts.merge_opts)) < 0 ||
-		(error = git_merge__check_result(repo, index_new)) < 0 ||
-		(error = git_merge__append_conflicts_to_merge_msg(repo, index_new)) < 0 ||
-		(error = git_checkout_index(repo, index_new, &opts.checkout_opts)) < 0 ||
-		(write_index && (error = git_indexwriter_commit(&indexwriter)) < 0))
+		(error = git_revert_commit(&index, repo, commit, our_commit, opts.mainline, &opts.merge_opts)) < 0 ||
+		(error = git_merge__check_result(repo, index)) < 0 ||
+		(error = git_merge__append_conflicts_to_merge_msg(repo, index)) < 0 ||
+		(error = git_checkout_index(repo, index, &opts.checkout_opts)) < 0 ||
+		(error = git_indexwriter_commit(&indexwriter)) < 0)
 		goto on_error;
 
 	goto done;
@@ -229,7 +216,6 @@ on_error:
 done:
 	git_indexwriter_cleanup(&indexwriter);
 	git_index_free(index);
-	git_index_free(index_new);
 	git_commit_free(our_commit);
 	git_reference_free(our_ref);
 	git_buf_free(&their_label);