diff --git a/Makefile b/Makefile
index 1aad150b34f..4e3becb4980 100644
--- a/Makefile
+++ b/Makefile
@@ -831,6 +831,7 @@ LIB_OBJS += submodule-config.o
 LIB_OBJS += symlinks.o
 LIB_OBJS += tag.o
 LIB_OBJS += tempfile.o
+LIB_OBJS += tmp-objdir.o
 LIB_OBJS += trace.o
 LIB_OBJS += trailer.o
 LIB_OBJS += transport.o
diff --git a/tmp-objdir.c b/tmp-objdir.c
new file mode 100644
index 00000000000..9443868be1e
--- /dev/null
+++ b/tmp-objdir.c
@@ -0,0 +1,273 @@
+#include "cache.h"
+#include "tmp-objdir.h"
+#include "dir.h"
+#include "sigchain.h"
+#include "string-list.h"
+#include "strbuf.h"
+#include "argv-array.h"
+
+struct tmp_objdir {
+	struct strbuf path;
+	struct argv_array env;
+};
+
+/*
+ * Allow only one tmp_objdir at a time in a running process, which simplifies
+ * our signal/atexit cleanup routines.  It's doubtful callers will ever need
+ * more than one, and we can expand later if so.  You can have many such
+ * tmp_objdirs simultaneously in many processes, of course.
+ */
+static struct tmp_objdir *the_tmp_objdir;
+
+static void tmp_objdir_free(struct tmp_objdir *t)
+{
+	strbuf_release(&t->path);
+	argv_array_clear(&t->env);
+	free(t);
+}
+
+static int tmp_objdir_destroy_1(struct tmp_objdir *t, int on_signal)
+{
+	int err;
+
+	if (!t)
+		return 0;
+
+	if (t == the_tmp_objdir)
+		the_tmp_objdir = NULL;
+
+	/*
+	 * This may use malloc via strbuf_grow(), but we should
+	 * have pre-grown t->path sufficiently so that this
+	 * doesn't happen in practice.
+	 */
+	err = remove_dir_recursively(&t->path, 0);
+
+	/*
+	 * When we are cleaning up due to a signal, we won't bother
+	 * freeing memory; it may cause a deadlock if the signal
+	 * arrived while libc's allocator lock is held.
+	 */
+	if (!on_signal)
+		tmp_objdir_free(t);
+	return err;
+}
+
+int tmp_objdir_destroy(struct tmp_objdir *t)
+{
+	return tmp_objdir_destroy_1(t, 0);
+}
+
+static void remove_tmp_objdir(void)
+{
+	tmp_objdir_destroy(the_tmp_objdir);
+}
+
+static void remove_tmp_objdir_on_signal(int signo)
+{
+	tmp_objdir_destroy_1(the_tmp_objdir, 1);
+	sigchain_pop(signo);
+	raise(signo);
+}
+
+/*
+ * These env_* functions are for setting up the child environment; the
+ * "replace" variant overrides the value of any existing variable with that
+ * "key". The "append" variant puts our new value at the end of a list,
+ * separated by PATH_SEP (which is what separate values in
+ * GIT_ALTERNATE_OBJECT_DIRECTORIES).
+ */
+static void env_append(struct argv_array *env, const char *key, const char *val)
+{
+	const char *old = getenv(key);
+
+	if (!old)
+		argv_array_pushf(env, "%s=%s", key, val);
+	else
+		argv_array_pushf(env, "%s=%s%c%s", key, old, PATH_SEP, val);
+}
+
+static void env_replace(struct argv_array *env, const char *key, const char *val)
+{
+	argv_array_pushf(env, "%s=%s", key, val);
+}
+
+static int setup_tmp_objdir(const char *root)
+{
+	char *path;
+	int ret = 0;
+
+	path = xstrfmt("%s/pack", root);
+	ret = mkdir(path, 0777);
+	free(path);
+
+	return ret;
+}
+
+struct tmp_objdir *tmp_objdir_create(void)
+{
+	static int installed_handlers;
+	struct tmp_objdir *t;
+
+	if (the_tmp_objdir)
+		die("BUG: only one tmp_objdir can be used at a time");
+
+	t = xmalloc(sizeof(*t));
+	strbuf_init(&t->path, 0);
+	argv_array_init(&t->env);
+
+	strbuf_addf(&t->path, "%s/incoming-XXXXXX", get_object_directory());
+
+	/*
+	 * Grow the strbuf beyond any filename we expect to be placed in it.
+	 * If tmp_objdir_destroy() is called by a signal handler, then
+	 * we should be able to use the strbuf to remove files without
+	 * having to call malloc.
+	 */
+	strbuf_grow(&t->path, 1024);
+
+	if (!mkdtemp(t->path.buf)) {
+		/* free, not destroy, as we never touched the filesystem */
+		tmp_objdir_free(t);
+		return NULL;
+	}
+
+	the_tmp_objdir = t;
+	if (!installed_handlers) {
+		atexit(remove_tmp_objdir);
+		sigchain_push_common(remove_tmp_objdir_on_signal);
+		installed_handlers++;
+	}
+
+	if (setup_tmp_objdir(t->path.buf)) {
+		tmp_objdir_destroy(t);
+		return NULL;
+	}
+
+	env_append(&t->env, ALTERNATE_DB_ENVIRONMENT,
+		   absolute_path(get_object_directory()));
+	env_replace(&t->env, DB_ENVIRONMENT, absolute_path(t->path.buf));
+
+	return t;
+}
+
+/*
+ * Make sure we copy packfiles and their associated metafiles in the correct
+ * order. All of these ends_with checks are slightly expensive to do in
+ * the midst of a sorting routine, but in practice it shouldn't matter.
+ * We will have a relatively small number of packfiles to order, and loose
+ * objects exit early in the first line.
+ */
+static int pack_copy_priority(const char *name)
+{
+	if (!starts_with(name, "pack"))
+		return 0;
+	if (ends_with(name, ".keep"))
+		return 1;
+	if (ends_with(name, ".pack"))
+		return 2;
+	if (ends_with(name, ".idx"))
+		return 3;
+	return 4;
+}
+
+static int pack_copy_cmp(const char *a, const char *b)
+{
+	return pack_copy_priority(a) - pack_copy_priority(b);
+}
+
+static int read_dir_paths(struct string_list *out, const char *path)
+{
+	DIR *dh;
+	struct dirent *de;
+
+	dh = opendir(path);
+	if (!dh)
+		return -1;
+
+	while ((de = readdir(dh)))
+		if (!is_dot_or_dotdot(de->d_name))
+			string_list_append(out, de->d_name);
+
+	closedir(dh);
+	return 0;
+}
+
+static int migrate_paths(struct strbuf *src, struct strbuf *dst);
+
+static int migrate_one(struct strbuf *src, struct strbuf *dst)
+{
+	struct stat st;
+
+	if (stat(src->buf, &st) < 0)
+		return -1;
+	if (S_ISDIR(st.st_mode)) {
+		if (!mkdir(dst->buf, 0777)) {
+			if (adjust_shared_perm(dst->buf))
+				return -1;
+		} else if (errno != EEXIST)
+			return -1;
+		return migrate_paths(src, dst);
+	}
+	return finalize_object_file(src->buf, dst->buf);
+}
+
+static int migrate_paths(struct strbuf *src, struct strbuf *dst)
+{
+	size_t src_len = src->len, dst_len = dst->len;
+	struct string_list paths = STRING_LIST_INIT_DUP;
+	int i;
+	int ret = 0;
+
+	if (read_dir_paths(&paths, src->buf) < 0)
+		return -1;
+	paths.cmp = pack_copy_cmp;
+	string_list_sort(&paths);
+
+	for (i = 0; i < paths.nr; i++) {
+		const char *name = paths.items[i].string;
+
+		strbuf_addf(src, "/%s", name);
+		strbuf_addf(dst, "/%s", name);
+
+		ret |= migrate_one(src, dst);
+
+		strbuf_setlen(src, src_len);
+		strbuf_setlen(dst, dst_len);
+	}
+
+	string_list_clear(&paths, 0);
+	return ret;
+}
+
+int tmp_objdir_migrate(struct tmp_objdir *t)
+{
+	struct strbuf src = STRBUF_INIT, dst = STRBUF_INIT;
+	int ret;
+
+	if (!t)
+		return 0;
+
+	strbuf_addbuf(&src, &t->path);
+	strbuf_addstr(&dst, get_object_directory());
+
+	ret = migrate_paths(&src, &dst);
+
+	strbuf_release(&src);
+	strbuf_release(&dst);
+
+	tmp_objdir_destroy(t);
+	return ret;
+}
+
+const char **tmp_objdir_env(const struct tmp_objdir *t)
+{
+	if (!t)
+		return NULL;
+	return t->env.argv;
+}
+
+void tmp_objdir_add_as_alternate(const struct tmp_objdir *t)
+{
+	add_to_alternates_memory(t->path.buf);
+}
diff --git a/tmp-objdir.h b/tmp-objdir.h
new file mode 100644
index 00000000000..b1e45b4c75d
--- /dev/null
+++ b/tmp-objdir.h
@@ -0,0 +1,54 @@
+#ifndef TMP_OBJDIR_H
+#define TMP_OBJDIR_H
+
+/*
+ * This API allows you to create a temporary object directory, advertise it to
+ * sub-processes via GIT_OBJECT_DIRECTORY and GIT_ALTERNATE_OBJECT_DIRECTORIES,
+ * and then either migrate its object into the main object directory, or remove
+ * it. The library handles unexpected signal/exit death by cleaning up the
+ * temporary directory.
+ *
+ * Example:
+ *
+ *	struct tmp_objdir *t = tmp_objdir_create();
+ *	if (!run_command_v_opt_cd_env(cmd, 0, NULL, tmp_objdir_env(t)) &&
+ *	    !tmp_objdir_migrate(t))
+ *		printf("success!\n");
+ *	else
+ *		die("failed...tmp_objdir will clean up for us");
+ *
+ */
+
+struct tmp_objdir;
+
+/*
+ * Create a new temporary object directory; returns NULL on failure.
+ */
+struct tmp_objdir *tmp_objdir_create(void);
+
+/*
+ * Return a list of environment strings, suitable for use with
+ * child_process.env, that can be passed to child programs to make use of the
+ * temporary object directory.
+ */
+const char **tmp_objdir_env(const struct tmp_objdir *);
+
+/*
+ * Finalize a temporary object directory by migrating its objects into the main
+ * object database, removing the temporary directory, and freeing any
+ * associated resources.
+ */
+int tmp_objdir_migrate(struct tmp_objdir *);
+
+/*
+ * Destroy a temporary object directory, discarding any objects it contains.
+ */
+int tmp_objdir_destroy(struct tmp_objdir *);
+
+/*
+ * Add the temporary object directory as an alternate object store in the
+ * current process.
+ */
+void tmp_objdir_add_as_alternate(const struct tmp_objdir *);
+
+#endif /* TMP_OBJDIR_H */