I noticed this while investigating the tog issue reported by xs on IRC.

Opening a bunch of files in /tmp for pack temp files and opening the
repository just to validate the work tree's base-commit ID is overkill.
Simply parse the ID directly. If the commit object no longer exists we
will run into another error sooner or later. The commit is already
protected by the work tree reference. If it is missing then something
has badly gone wrong.

ok?

M  lib/worktree_open.c  |  29+  22-

1 file changed, 29 insertions(+), 22 deletions(-)

commit - 06dcca949ee0cb85e6246bbcb2b6415c74a2b450
commit + 120020c7f39fb55bbcf56e62da533a41f05acde1
blob - 6ae0f96f5546d6f7eaac0b1b2d32dd6249c51d52
blob + 017608a6f88968416720497e95e0aab838d924b0
--- lib/worktree_open.c
+++ lib/worktree_open.c
@@ -41,6 +41,7 @@
 
 #include "got_lib_worktree.h"
 #include "got_lib_gotconfig.h"
+#include "got_lib_hash.h"
 
 #ifndef nitems
 #define nitems(_a)	(sizeof((_a)) / sizeof((_a)[0]))
@@ -124,14 +125,19 @@ open_worktree(struct got_worktree **worktree, const ch
 	char *uuidstr = NULL;
 	char *path_lock = NULL;
 	char *base_commit_id_str = NULL;
+	struct got_object_id *base_commit_id = NULL;
 	int version, fd = -1;
 	const char *errstr;
-	struct got_repository *repo = NULL;
-	int *pack_fds = NULL;
 	uint32_t uuid_status;
+	enum got_hash_algorithm algo;
+	int have_algo;
 
 	*worktree = NULL;
 
+	base_commit_id = calloc(1, sizeof(*base_commit_id));
+	if (base_commit_id == NULL)
+		return got_error_from_errno("calloc");
+
 	if (asprintf(&path_meta, "%s/%s", path, meta_dir) == -1) {
 		err = got_error_from_errno("asprintf");
 		path_meta = NULL;
@@ -204,18 +210,29 @@ open_worktree(struct got_worktree **worktree, const ch
 		goto done;
 	}
 
-	err = got_repo_pack_fds_open(&pack_fds);
-	if (err)
-		goto done;
+	switch (strlen(base_commit_id_str)) {
+	case SHA1_DIGEST_STRING_LENGTH - 1:
+		algo = GOT_HASH_SHA1;
+		have_algo = 1;
+		break;
+	case SHA256_DIGEST_STRING_LENGTH - 1:
+		algo = GOT_HASH_SHA256;
+		have_algo = 1;
+		break;
+	default:
+		have_algo = 0;
+		break;
+	}
 
-	err = got_repo_open(&repo, (*worktree)->repo_path, NULL, pack_fds);
-	if (err)
+	if (!have_algo ||
+	    !got_parse_object_id(base_commit_id, base_commit_id_str, algo)) {
+		err = got_error_fmt(GOT_ERR_BAD_OBJ_ID_STR,
+		    "%s/%s", meta_dir, GOT_WORKTREE_BASE_COMMIT);
 		goto done;
+	}
 
-	err = got_object_resolve_id_str(&(*worktree)->base_commit_id, repo,
-	    base_commit_id_str);
-	if (err)
-		goto done;
+	(*worktree)->base_commit_id = base_commit_id;
+	base_commit_id = NULL;
 
 	err = read_meta_file(&(*worktree)->head_ref_name, path_meta,
 	    GOT_WORKTREE_HEAD_REF);
@@ -241,20 +258,10 @@ open_worktree(struct got_worktree **worktree, const ch
 		goto done;
 	}
 done:
-	if (repo) {
-		const struct got_error *close_err = got_repo_close(repo);
-		if (err == NULL)
-			err = close_err;
-	}
-	if (pack_fds) {
-		const struct got_error *pack_err =
-		    got_repo_pack_fds_close(pack_fds);
-		if (err == NULL)
-			err = pack_err;
-	}
 	free(path_meta);
 	free(path_lock);
 	free(base_commit_id_str);
+	free(base_commit_id);
 	free(uuidstr);
 	free(formatstr);
 	if (err) {