Merge branch 'ps/undecided-is-not-necessarily-sha1'

Before discovering the repository details, We used to assume SHA-1
as the "default" hash function, which has been corrected. Hopefully
this will smoke out codepaths that rely on such an unwarranted
assumptions.

* ps/undecided-is-not-necessarily-sha1:
  repository: stop setting SHA1 as the default object hash
  oss-fuzz/commit-graph: set up hash algorithm
  builtin/shortlog: don't set up revisions without repo
  builtin/diff: explicitly set hash algo when there is no repo
  builtin/bundle: abort "verify" early when there is no repository
  builtin/blame: don't access potentially unitialized `the_hash_algo`
  builtin/rev-parse: allow shortening to more than 40 hex characters
  remote-curl: fix parsing of detached SHA256 heads
  attr: fix BUG() when parsing attrs outside of repo
  attr: don't recompute default attribute source
  parse-options-cb: only abbreviate hashes when hash algo is known
  path: move `validate_headref()` to its only user
  path: harden validation of HEAD with non-standard hashes

attr.c

@@ -1205,15 +1205,16 @@ static void collect_some_attrs(struct index_state *istate,
 }
 
 static const char *default_attr_source_tree_object_name;
-static int ignore_bad_attr_tree;
 
 void set_git_attr_source(const char *tree_object_name)
 {
 	default_attr_source_tree_object_name = xstrdup(tree_object_name);
 }
 
-static void compute_default_attr_source(struct object_id *attr_source)
+static int compute_default_attr_source(struct object_id *attr_source)
 {
+	int ignore_bad_attr_tree = 0;
+
 	if (!default_attr_source_tree_object_name)
 		default_attr_source_tree_object_name = getenv(GIT_ATTR_SOURCE_ENVIRONMENT);
 
@@ -1222,22 +1223,34 @@ static void compute_default_attr_source(struct object_id *attr_source)
 		ignore_bad_attr_tree = 1;
 	}
 
-	if (!default_attr_source_tree_object_name || !is_null_oid(attr_source))
-		return;
+	if (!default_attr_source_tree_object_name)
+		return 0;
+
+	if (!startup_info->have_repository) {
+		if (!ignore_bad_attr_tree)
+			die(_("cannot use --attr-source or GIT_ATTR_SOURCE without repo"));
+		return 0;
+	}
 
 	if (repo_get_oid_treeish(the_repository,
 				 default_attr_source_tree_object_name,
-				 attr_source) && !ignore_bad_attr_tree)
-		die(_("bad --attr-source or GIT_ATTR_SOURCE"));
+				 attr_source)) {
+		if (!ignore_bad_attr_tree)
+			die(_("bad --attr-source or GIT_ATTR_SOURCE"));
+		return 0;
+	}
+
+	return 1;
 }
 
 static struct object_id *default_attr_source(void)
 {
 	static struct object_id attr_source;
+	static int has_attr_source = -1;
 
-	if (is_null_oid(&attr_source))
-		compute_default_attr_source(&attr_source);
-	if (is_null_oid(&attr_source))
+	if (has_attr_source < 0)
+		has_attr_source = compute_default_attr_source(&attr_source);
+	if (!has_attr_source)
 		return NULL;
 	return &attr_source;
 }

builtin/blame.c

@@ -915,7 +915,6 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	struct range_set ranges;
 	unsigned int range_i;
 	long anchor;
-	const int hexsz = the_hash_algo->hexsz;
 	long num_lines = 0;
 	const char *str_usage = cmd_is_annotate ? annotate_usage : blame_usage;
 	const char **opt_usage = cmd_is_annotate ? annotate_opt_usage : blame_opt_usage;
@@ -973,11 +972,11 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	} else if (show_progress < 0)
 		show_progress = isatty(2);
 
-	if (0 < abbrev && abbrev < hexsz)
+	if (0 < abbrev && abbrev < (int)the_hash_algo->hexsz)
 		/* one more abbrev length is needed for the boundary commit */
 		abbrev++;
 	else if (!abbrev)
-		abbrev = hexsz;
+		abbrev = the_hash_algo->hexsz;
 
 	if (revs_file && read_ancestry(revs_file))
 		die_errno("reading graft file '%s' failed", revs_file);

builtin/bundle.c

@@ -140,6 +140,11 @@ static int cmd_bundle_verify(int argc, const char **argv, const char *prefix) {
 			builtin_bundle_verify_usage, options, &bundle_file);
 	/* bundle internals use argv[1] as further parameters */
 
+	if (!startup_info->have_repository) {
+		ret = error(_("need a repository to verify a bundle"));
+		goto cleanup;
+	}
+
 	if ((bundle_fd = open_bundle(bundle_file, &header, &name)) < 0) {
 		ret = 1;
 		goto cleanup;

builtin/diff.c

@@ -465,6 +465,15 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			no_index = DIFF_NO_INDEX_IMPLICIT;
 	}
 
+	/*
+	 * When operating outside of a Git repository we need to have a hash
+	 * algorithm at hand so that we can generate the blob hashes. We
+	 * default to SHA1 here, but may eventually want to change this to be
+	 * configurable via a command line option.
+	 */
+	if (nongit)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	init_diff_ui_defaults();
 	git_config(git_diff_ui_config, NULL);
 	prefix = precompose_argv_prefix(argc, argv, prefix);

builtin/rev-parse.c

@@ -691,7 +691,6 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 	const char *name = NULL;
 	struct object_context unused;
 	struct strbuf buf = STRBUF_INIT;
-	const int hexsz = the_hash_algo->hexsz;
 	int seen_end_of_options = 0;
 	enum format_type format = FORMAT_DEFAULT;
 
@@ -867,8 +866,8 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				abbrev = strtoul(arg, NULL, 10);
 				if (abbrev < MINIMUM_ABBREV)
 					abbrev = MINIMUM_ABBREV;
-				else if (hexsz <= abbrev)
-					abbrev = hexsz;
+				else if ((int)the_hash_algo->hexsz <= abbrev)
+					abbrev = the_hash_algo->hexsz;
 				continue;
 			}
 			if (!strcmp(arg, "--sq")) {

builtin/shortlog.c

@@ -435,7 +435,7 @@ int cmd_shortlog(int argc, const char **argv, const char *prefix)
 		usage_with_options(shortlog_usage, options);
 	}
 
-	if (setup_revisions(argc, argv, &rev, NULL) != 1) {
+	if (!nongit && setup_revisions(argc, argv, &rev, NULL) != 1) {
 		error(_("unrecognized argument: %s"), argv[1]);
 		usage_with_options(shortlog_usage, options);
 	}

oss-fuzz/fuzz-commit-graph.c

@@ -19,6 +19,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 	 * touching the disk to keep the individual fuzz-test cases as fast as
 	 * possible.
 	 */
+	repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
 	the_repository->settings.commit_graph_generation_version = 2;
 	the_repository->settings.commit_graph_read_changed_paths = 1;
 	g = parse_commit_graph(&the_repository->settings, (void *)data, size);

parse-options-cb.c

@@ -7,6 +7,7 @@
 #include "environment.h"
 #include "gettext.h"
 #include "object-name.h"
+#include "setup.h"
 #include "string-list.h"
 #include "strvec.h"
 #include "oid-array.h"
@@ -29,7 +30,7 @@ int parse_opt_abbrev_cb(const struct option *opt, const char *arg, int unset)
 				     opt->long_name);
 		if (v && v < MINIMUM_ABBREV)
 			v = MINIMUM_ABBREV;
-		else if (v > the_hash_algo->hexsz)
+		else if (startup_info->have_repository && v > the_hash_algo->hexsz)
 			v = the_hash_algo->hexsz;
 	}
 	*(int *)(opt->value) = v;

path.c

@@ -5,7 +5,6 @@
 #include "abspath.h"
 #include "environment.h"
 #include "gettext.h"
-#include "hex.h"
 #include "repository.h"
 #include "strbuf.h"
 #include "string-list.h"
@@ -647,58 +646,6 @@ void strbuf_git_common_path(struct strbuf *sb,
 	va_end(args);
 }
 
-int validate_headref(const char *path)
-{
-	struct stat st;
-	char buffer[256];
-	const char *refname;
-	struct object_id oid;
-	int fd;
-	ssize_t len;
-
-	if (lstat(path, &st) < 0)
-		return -1;
-
-	/* Make sure it is a "refs/.." symlink */
-	if (S_ISLNK(st.st_mode)) {
-		len = readlink(path, buffer, sizeof(buffer)-1);
-		if (len >= 5 && !memcmp("refs/", buffer, 5))
-			return 0;
-		return -1;
-	}
-
-	/*
-	 * Anything else, just open it and try to see if it is a symbolic ref.
-	 */
-	fd = open(path, O_RDONLY);
-	if (fd < 0)
-		return -1;
-	len = read_in_full(fd, buffer, sizeof(buffer)-1);
-	close(fd);
-
-	if (len < 0)
-		return -1;
-	buffer[len] = '\0';
-
-	/*
-	 * Is it a symbolic ref?
-	 */
-	if (skip_prefix(buffer, "ref:", &refname)) {
-		while (isspace(*refname))
-			refname++;
-		if (starts_with(refname, "refs/"))
-			return 0;
-	}
-
-	/*
-	 * Is this a detached HEAD?
-	 */
-	if (!get_oid_hex(buffer, &oid))
-		return 0;
-
-	return -1;
-}
-
 static struct passwd *getpw_str(const char *username, size_t len)
 {
 	struct passwd *pw;

path.h

@@ -173,7 +173,6 @@ const char *git_path_fetch_head(struct repository *r);
 const char *git_path_shallow(struct repository *r);
 
 int ends_with_path_components(const char *path, const char *components);
-int validate_headref(const char *ref);
 
 int calc_shared_perm(int mode);
 int adjust_shared_perm(const char *path);

remote-curl.c

@@ -266,12 +266,23 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 	return list;
 }
 
+/*
+ * Try to detect the hash algorithm used by the remote repository when using
+ * the dumb HTTP transport. As dumb transports cannot tell us the object hash
+ * directly have to derive it from the advertised ref lengths.
+ */
 static const struct git_hash_algo *detect_hash_algo(struct discovery *heads)
 {
 	const char *p = memchr(heads->buf, '\t', heads->len);
 	int algo;
+
+	/*
+	 * In case the remote has no refs we have no way to reliably determine
+	 * the object hash used by that repository. In that case we simply fall
+	 * back to SHA1, which may or may not be correct.
+	 */
 	if (!p)
-		return the_hash_algo;
+		return &hash_algos[GIT_HASH_SHA1];
 
 	algo = hash_algo_by_length((p - heads->buf) / 2);
 	if (algo == GIT_HASH_UNKNOWN)
@@ -295,6 +306,12 @@ static struct ref *parse_info_refs(struct discovery *heads)
 		    "is this a git repository?",
 		    transport_anonymize_url(url.buf));
 
+	/*
+	 * Set the repository's hash algo to whatever we have just detected.
+	 * This ensures that we can correctly parse the remote references.
+	 */
+	repo_set_hash_algo(the_repository, hash_algo_by_ptr(options.hash_algo));
+
 	data = heads->buf;
 	start = NULL;
 	mid = data;

repository.c

@@ -26,26 +26,6 @@ void initialize_repository(struct repository *repo)
 	repo->parsed_objects = parsed_object_pool_new();
 	ALLOC_ARRAY(repo->index, 1);
 	index_state_init(repo->index, repo);
-
-	/*
-	 * Unfortunately, we need to keep this hack around for the time being:
-	 *
-	 *   - Not setting up the hash algorithm for `the_repository` leads to
-	 *     crashes because `the_hash_algo` is a macro that expands to
-	 *     `the_repository->hash_algo`. So if Git commands try to access
-	 *     `the_hash_algo` without a Git directory we crash.
-	 *
-	 *   - Setting up the hash algorithm to be SHA1 by default breaks other
-	 *     commands when running with SHA256.
-	 *
-	 * This is another point in case why having global state is a bad idea.
-	 * Eventually, we should remove this hack and stop setting the hash
-	 * algorithm in this function altogether. Instead, it should only ever
-	 * be set via our repository setup procedures. But that requires more
-	 * work.
-	 */
-	if (repo == the_repository)
-		repo_set_hash_algo(repo, GIT_HASH_SHA1);
 }
 
 static void expand_base_dir(char **out, const char *in,

setup.c

@@ -4,6 +4,7 @@
 #include "environment.h"
 #include "exec-cmd.h"
 #include "gettext.h"
+#include "hex.h"
 #include "object-name.h"
 #include "refs.h"
 #include "repository.h"
@@ -342,6 +343,58 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
 	return ret;
 }
 
+static int validate_headref(const char *path)
+{
+	struct stat st;
+	char buffer[256];
+	const char *refname;
+	struct object_id oid;
+	int fd;
+	ssize_t len;
+
+	if (lstat(path, &st) < 0)
+		return -1;
+
+	/* Make sure it is a "refs/.." symlink */
+	if (S_ISLNK(st.st_mode)) {
+		len = readlink(path, buffer, sizeof(buffer)-1);
+		if (len >= 5 && !memcmp("refs/", buffer, 5))
+			return 0;
+		return -1;
+	}
+
+	/*
+	 * Anything else, just open it and try to see if it is a symbolic ref.
+	 */
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return -1;
+	len = read_in_full(fd, buffer, sizeof(buffer)-1);
+	close(fd);
+
+	if (len < 0)
+		return -1;
+	buffer[len] = '\0';
+
+	/*
+	 * Is it a symbolic ref?
+	 */
+	if (skip_prefix(buffer, "ref:", &refname)) {
+		while (isspace(*refname))
+			refname++;
+		if (starts_with(refname, "refs/"))
+			return 0;
+	}
+
+	/*
+	 * Is this a detached HEAD?
+	 */
+	if (get_oid_hex_any(buffer, &oid) != GIT_HASH_UNKNOWN)
+		return 0;
+
+	return -1;
+}
+
 /*
  * Test if it looks like we're at a git directory.
  * We want to see:

t/t0003-attributes.sh

@@ -434,6 +434,21 @@ test_expect_success 'precedence of --attr-source, GIT_ATTR_SOURCE, then attr.tre
 	)
 '
 
+test_expect_success 'diff without repository with attr source' '
+	mkdir -p "$TRASH_DIRECTORY/outside/nongit" &&
+	(
+		cd "$TRASH_DIRECTORY/outside/nongit" &&
+		GIT_CEILING_DIRECTORIES="$TRASH_DIRECTORY/outside" &&
+		export GIT_CEILING_DIRECTORIES &&
+		touch file &&
+		cat >expect <<-EOF &&
+		fatal: cannot use --attr-source or GIT_ATTR_SOURCE without repo
+		EOF
+		test_must_fail env GIT_ATTR_SOURCE=HEAD git grep --no-index foo file 2>err &&
+		test_cmp expect err
+	)
+'
+
 test_expect_success 'bare repository: with --source' '
 	(
 		cd bare.git &&

t/t0040-parse-options.sh

@@ -176,6 +176,23 @@ test_expect_success 'long options' '
 	test_cmp expect output
 '
 
+test_expect_success 'abbreviate to something longer than SHA1 length' '
+	cat >expect <<-EOF &&
+	boolean: 0
+	integer: 0
+	magnitude: 0
+	timestamp: 0
+	string: (not set)
+	abbrev: 100
+	verbose: -1
+	quiet: 0
+	dry run: no
+	file: (not set)
+	EOF
+	test-tool parse-options --abbrev=100 >output &&
+	test_cmp expect output
+'
+
 test_expect_success 'missing required value' '
 	cat >expect <<-\EOF &&
 	error: switch `s'\'' requires a value

t/t1500-rev-parse.sh

@@ -304,4 +304,10 @@ test_expect_success 'rev-parse --bisect includes bad, excludes good' '
 	test_cmp expect actual
 '
 
+test_expect_success '--short= truncates to the actual hash length' '
+	git rev-parse HEAD >expect &&
+	git rev-parse --short=100 HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_done

t/t5550-http-fetch-dumb.sh

@@ -55,6 +55,21 @@ test_expect_success 'list refs from outside any repository' '
 	test_cmp expect actual
 '
 
+
+test_expect_success 'list detached HEAD from outside any repository' '
+	git clone --mirror "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" \
+		"$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" \
+		update-ref --no-deref HEAD refs/heads/main &&
+	git -C "$HTTPD_DOCUMENT_ROOT_PATH/repo-detached.git" update-server-info &&
+	cat >expect <<-EOF &&
+	$(git rev-parse main)	HEAD
+	$(git rev-parse main)	refs/heads/main
+	EOF
+	nongit git ls-remote "$HTTPD_URL/dumb/repo-detached.git" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'create password-protected repository' '
 	mkdir -p "$HTTPD_DOCUMENT_ROOT_PATH/auth/dumb/" &&
 	cp -Rf "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" \