builtin/stash: provide a way to export stashes to a ref

A common user problem is how to sync in-progress work to another
machine.  Users currently must use some sort of transfer of the working
tree, which poses security risks and also necessarily causes the index
to become dirty.  The experience is suboptimal and frustrating for
users.

A reasonable idea is to use the stash for this purpose, but the stash is
stored in the reflog, not in a ref, and as such it cannot be pushed or
pulled.  This also means that it cannot be saved into a bundle or
preserved elsewhere, which is a problem when using throwaway development
environments.

In addition, users often want to replicate stashes across machines, such
as when they must use multiple machines or when they use throwaway dev
environments, such as those based on the Devcontainer spec, where they
might otherwise lose various in-progress work.

Let's solve this problem by allowing the user to export the stash to a
ref (or, to just write it into the repository and print the hash, à la
git commit-tree).  Introduce git stash export, which writes a chain of
commits where the first parent is always a chain to the previous stash,
or to a single, empty commit (for the final item) and the second is the
stash commit normally written to the reflog.

Iterate over each stash from top to bottom, looking up the data for each
one, and then create the chain from the single empty commit back up in
reverse order.  Generate a predictable empty commit so our behavior is
reproducible.  Create a useful commit message, preserving the author and
committer information, to help users identify stash commits when viewing
them as normal commits.

If the user has specified specific stashes they'd like to export
instead, use those instead of iterating over all of the stashes.

As part of this, specifically request quiet behavior when looking up the
OID for a revision because we will eventually hit a revision that
doesn't exist and we don't want to die when that occurs.

When exporting stashes, be sure to verify that they look like valid
stashes and don't contain invalid data.  This will help avoid failures
on import or problems due to attempting to export invalid refs that are
not stashes.

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

Documentation/git-stash.adoc

@@ -23,6 +23,7 @@ SYNOPSIS
 'git stash' clear
 'git stash' create [<message>]
 'git stash' store [(-m | --message) <message>] [-q | --quiet] <commit>
+'git stash' export (--print | --to-ref <ref>) [<stash>...]
 
 DESCRIPTION
 -----------
@@ -154,6 +155,12 @@ store::
 	reflog.  This is intended to be useful for scripts.  It is
 	probably not the command you want to use; see "push" above.
 
+export ( --print | --to-ref <ref> ) [<stash>...]::
+
+	Export the specified stashes, or all of them if none are specified, to
+	a chain of commits which can be transferred using the normal fetch and
+	push mechanisms, then imported using the `import` subcommand.
+
 OPTIONS
 -------
 -a::
@@ -242,6 +249,19 @@ literally (including newlines and quotes).
 +
 Quiet, suppress feedback messages.
 
+--print::
+	This option is only valid for the `export` command.
++
+Create the chain of commits representing the exported stashes without
+storing it anywhere in the ref namespace and print the object ID to
+standard output.  This is designed for scripts.
+
+--to-ref::
+	This option is only valid for the `export` command.
++
+Create the chain of commits representing the exported stashes and store
+it to the specified ref.
+
 \--::
 	This option is only valid for `push` command.
 +
@@ -259,7 +279,7 @@ For more details, see the 'pathspec' entry in linkgit:gitglossary[7].
 
 <stash>::
 	This option is only valid for `apply`, `branch`, `drop`, `pop`,
-	`show` commands.
+	`show`, and `export` commands.
 +
 A reference of the form `stash@{<revision>}`. When no `<stash>` is
 given, the latest stash is assumed (that is, `stash@{0}`).

builtin/stash.c

@@ -28,7 +28,10 @@
 #include "log-tree.h"
 #include "diffcore.h"
 #include "reflog.h"
+#include "reflog-walk.h"
 #include "add-interactive.h"
+#include "oid-array.h"
+#include "commit.h"
 
 #define INCLUDE_ALL_FILES 2
 
@@ -56,6 +59,8 @@
 	   "          [-u | --include-untracked] [-a | --all] [<message>]")
 #define BUILTIN_STASH_CREATE_USAGE \
 	N_("git stash create [<message>]")
+#define BUILTIN_STASH_EXPORT_USAGE \
+	N_("git stash export (--print | --to-ref <ref>) [<stash>...]")
 #define BUILTIN_STASH_CLEAR_USAGE \
 	"git stash clear"
 
@@ -71,6 +76,7 @@ static const char * const git_stash_usage[] = {
 	BUILTIN_STASH_CLEAR_USAGE,
 	BUILTIN_STASH_CREATE_USAGE,
 	BUILTIN_STASH_STORE_USAGE,
+	BUILTIN_STASH_EXPORT_USAGE,
 	NULL
 };
 
@@ -124,6 +130,12 @@ static const char * const git_stash_save_usage[] = {
 	NULL
 };
 
+static const char * const git_stash_export_usage[] = {
+	BUILTIN_STASH_EXPORT_USAGE,
+	NULL
+};
+
+
 static const char ref_stash[] = "refs/stash";
 static struct strbuf stash_index_path = STRBUF_INIT;
 
@@ -160,6 +172,33 @@ static void free_stash_info(struct stash_info *info)
 	strbuf_release(&info->revision);
 }
 
+static int check_stash_topology(struct repository *r, struct commit *stash)
+{
+	struct commit *p1, *p2, *p3 = NULL;
+
+	/* stash must have two or three parents */
+	if (!stash->parents || !stash->parents->next ||
+			(stash->parents->next->next && stash->parents->next->next->next))
+		return -1;
+	p1 = stash->parents->item;
+	p2 = stash->parents->next->item;
+	if (stash->parents->next->next)
+		p3 = stash->parents->next->next->item;
+	if (repo_parse_commit(r, p1) || repo_parse_commit(r, p2) ||
+			(p3 && repo_parse_commit(r, p3)))
+		return -1;
+	/* p2 must have a single parent, p3 must have no parents */
+	if (!p2->parents || p2->parents->next || (p3 && p3->parents))
+		return -1;
+	if (repo_parse_commit(r, p2->parents->item))
+		return -1;
+	/* p2^1 must equal p1 */
+	if (!oideq(&p1->object.oid, &p2->parents->item->object.oid))
+		return -1;
+
+	return 0;
+}
+
 static void assert_stash_like(struct stash_info *info, const char *revision)
 {
 	if (get_oidf(&info->b_commit, "%s^1", revision) ||
@@ -1895,6 +1934,226 @@ static int save_stash(int argc, const char **argv, const char *prefix,
 	return ret;
 }
 
+static int write_commit_with_parents(struct repository *r,
+				     struct object_id *out,
+				     const struct object_id *oid,
+				     struct commit_list *parents)
+{
+	size_t author_len, committer_len;
+	struct commit *this;
+	const char *orig_author, *orig_committer;
+	char *author = NULL, *committer = NULL;
+	const char *buffer;
+	unsigned long bufsize;
+	const char *p;
+	struct strbuf msg = STRBUF_INIT;
+	int ret = 0;
+	struct ident_split id;
+
+	this = lookup_commit_reference(r, oid);
+	buffer = repo_get_commit_buffer(r, this, &bufsize);
+	orig_author = find_commit_header(buffer, "author", &author_len);
+	orig_committer = find_commit_header(buffer, "committer", &committer_len);
+
+	if (!orig_author || !orig_committer) {
+		ret = error(_("cannot parse commit %s"), oid_to_hex(oid));
+		goto out;
+	}
+
+	if (split_ident_line(&id, orig_author, author_len) < 0 ||
+	    split_ident_line(&id, orig_committer, committer_len) < 0) {
+		ret = error(_("invalid author or committer for %s"), oid_to_hex(oid));
+		goto out;
+	}
+
+	p = strstr(buffer, "\n\n");
+	strbuf_addstr(&msg, "git stash: ");
+
+	if (p)
+		strbuf_add(&msg, p + 2, bufsize - (p + 2 - buffer));
+	strbuf_complete_line(&msg);
+
+	author = xmemdupz(orig_author, author_len);
+	committer = xmemdupz(orig_committer, committer_len);
+
+	if (commit_tree_extended(msg.buf, msg.len,
+				 r->hash_algo->empty_tree, parents,
+				 out, author, committer,
+				 NULL, NULL)) {
+		ret = error(_("could not write commit"));
+		goto out;
+	}
+out:
+	strbuf_release(&msg);
+	repo_unuse_commit_buffer(r, this, buffer);
+	free(author);
+	free(committer);
+	return ret;
+}
+
+struct stash_entry_data {
+	struct repository *r;
+	struct commit_list **items;
+	size_t count;
+};
+
+static int collect_stash_entries(struct object_id *old_oid UNUSED,
+				 struct object_id *new_oid,
+				 const char *committer UNUSED,
+				 timestamp_t timestamp UNUSED,
+				 int tz UNUSED, const char *msg UNUSED,
+				 void *cb_data)
+{
+	struct stash_entry_data *data = cb_data;
+	struct commit *stash;
+
+	data->count++;
+	stash = lookup_commit_reference(data->r, new_oid);
+	if (!stash || check_stash_topology(data->r, stash)) {
+		return error(_("%s does not look like a stash commit"),
+			     oid_to_hex(new_oid));
+	}
+	data->items = commit_list_append(stash, data->items);
+	return 0;
+}
+
+static int do_export_stash(struct repository *r,
+			   const char *ref,
+			   int argc,
+			   const char **argv)
+{
+	struct object_id base;
+	struct object_context unused;
+	struct commit *prev;
+	struct commit_list *items = NULL, **iter = &items, *cur;
+	int res = 0;
+	int i;
+	struct strbuf revision = STRBUF_INIT;
+	const char *author, *committer;
+
+	/*
+	 * This is an arbitrary, fixed date, specifically the one used by git
+	 * format-patch.  The goal is merely to produce reproducible output.
+	 */
+	prepare_fallback_ident("git stash", "git@stash");
+	author = fmt_ident("git stash", "git@stash", WANT_BLANK_IDENT,
+			   "2001-09-17T00:00:00Z", 0);
+	committer = fmt_ident("git stash", "git@stash", WANT_BLANK_IDENT,
+			      "2001-09-17T00:00:00Z", 0);
+
+	/* First, we create a single empty commit. */
+	if (commit_tree_extended("", 0, r->hash_algo->empty_tree, NULL,
+				 &base, author, committer, NULL, NULL))
+		return error(_("unable to write base commit"));
+
+	prev = lookup_commit_reference(r, &base);
+
+	if (argc) {
+		/*
+		 * Find each specified stash, and load data into the array.
+		 */
+		for (i = 0; i < argc; i++) {
+			struct object_id oid;
+			struct commit *stash;
+
+			if (parse_stash_revision(&revision, argv[i], 1) ||
+			    get_oid_with_context(r, revision.buf,
+						 GET_OID_QUIETLY | GET_OID_GENTLY,
+						 &oid, &unused)) {
+				res = error(_("unable to find stash entry %s"), argv[i]);
+				goto out;
+			}
+
+			stash = lookup_commit_reference(r, &oid);
+			if (!stash || check_stash_topology(r, stash)) {
+				res = error(_("%s does not look like a stash commit"),
+						revision.buf);
+				goto out;
+			}
+			iter = commit_list_append(stash, iter);
+		}
+	} else {
+		/*
+		 * Walk the reflog, finding each stash entry, and load data into the
+		 * array.
+		 */
+		struct stash_entry_data cb_data = {
+			.r = r, .items = iter,
+		};
+		if (refs_for_each_reflog_ent_reverse(get_main_ref_store(r),
+						     "refs/stash",
+						     collect_stash_entries,
+						     &cb_data) && cb_data.count)
+			goto out;
+	}
+
+	/*
+	 * Now, create a set of commits identical to the regular stash commits,
+	 * but where their first parents form a chain to our original empty
+	 * base commit.
+	 */
+	items = reverse_commit_list(items);
+	for (cur = items; cur; cur = cur->next) {
+		struct commit_list *parents = NULL;
+		struct commit_list **next = &parents;
+		struct object_id out;
+		struct commit *stash = cur->item;
+
+		next = commit_list_append(prev, next);
+		next = commit_list_append(stash, next);
+		res = write_commit_with_parents(r, &out, &stash->object.oid, parents);
+		free_commit_list(parents);
+		if (res)
+			goto out;
+		prev = lookup_commit_reference(r, &out);
+	}
+	if (ref)
+		refs_update_ref(get_main_ref_store(r), NULL, ref,
+				&prev->object.oid, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
+	else
+		puts(oid_to_hex(&prev->object.oid));
+out:
+	strbuf_release(&revision);
+	free_commit_list(items);
+
+	return res;
+}
+
+enum export_action {
+	ACTION_NONE,
+	ACTION_PRINT,
+	ACTION_TO_REF,
+};
+
+static int export_stash(int argc,
+			const char **argv,
+			const char *prefix,
+			struct repository *repo)
+{
+	const char *ref = NULL;
+	enum export_action action = ACTION_NONE;
+	struct option options[] = {
+		OPT_CMDMODE(0, "print", &action,
+			    N_("print the object ID instead of writing it to a ref"),
+			    ACTION_PRINT),
+		OPT_STRING(0, "to-ref", &ref, "ref",
+			    N_("save the data to the given ref")),
+		OPT_END()
+	};
+
+	argc = parse_options(argc, argv, prefix, options,
+			     git_stash_export_usage,
+			     PARSE_OPT_KEEP_DASHDASH);
+
+	if (ref && action == ACTION_NONE)
+		action = ACTION_TO_REF;
+
+	if (action == ACTION_NONE || (ref && action == ACTION_PRINT))
+		return error(_("exactly one of --print and --to-ref is required"));
+
+	return do_export_stash(repo, ref, argc, argv);
+}
+
 int cmd_stash(int argc,
 	      const char **argv,
 	      const char *prefix,
@@ -1915,6 +2174,7 @@ int cmd_stash(int argc,
 		OPT_SUBCOMMAND("store", &fn, store_stash),
 		OPT_SUBCOMMAND("create", &fn, create_stash),
 		OPT_SUBCOMMAND("push", &fn, push_stash_unassumed),
+		OPT_SUBCOMMAND("export", &fn, export_stash),
 		OPT_SUBCOMMAND_F("save", &fn, save_stash, PARSE_OPT_NOCOMPLETE),
 		OPT_END()
 	};