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[PATCH 11/12] builtin/show-ref: add new mode to check for reference existence

From
Patrick Steinhardt <ps@pks.im>
Date
Oct 24, 2023, 13:11 UTC
Message-ID
<2f876e61dd36a8887a1286bb8db9fb6577c55c9b.1698152926.git.ps@pks.im>
In-Reply-To
<cover.1698152926.git.ps@pks.im>

While we have multiple ways to show the value of a given reference, we do not have any way to check whether a reference exists at all. While commands like git-rev-parse(1) or git-show-ref(1) can be used to check for reference existence in case the reference resolves to something sane, neither of them can be used to check for existence in some other scenarios where the reference does not resolve cleanly:

    - References which have an invalid name cannot be resolved.
    - References to nonexistent objects cannot be resolved.
    - Dangling symrefs can be resolved via git-symbolic-ref(1), but this
      requires the caller to special case existence checks depending on
      whteher or not a reference is symbolic or direct.

Furthermore, git-rev-list(1) and other commands do not let the caller distinguish easily between an actually missing reference and a generic error.

Taken together, this gseems like sufficient motivation to introduce a separate plumbing command to explicitly check for the existence of a reference without trying to resolve its contents.

This new command comes in the form of `git show-ref --exists`. This new mode will exit successfully when the reference exists, with a specific error code of 2 when it does not exist, or with 1 when there has been a generic error.

Note that the only way to properly implement this command is by using the internal `refs_read_raw_ref()` function. While the public function `refs_resolve_ref_unsafe()` can be made to behave in the same way by passing various flags, it does not provide any way to obtain the errno with which the reference backend failed when reading the reference. As such, it becomes impossible for us to distinguish generic errors from the explicit case where the reference wasn't found.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 Documentation/git-show-ref.txt | 11 ++++++
 builtin/show-ref.c             | 47 ++++++++++++++++++++++--
 t/t1403-show-ref.sh            | 67 +++++++++++++++++++++++++++++++++-
 3 files changed, 120 insertions(+), 5 deletions(-)
diff --git a/Documentation/git-show-ref.txt b/Documentation/git-show-ref.txt
index ab23e0b62e1..a7e9374bc2b 100644
--- a/Documentation/git-show-ref.txt
+++ b/Documentation/git-show-ref.txt
@@ -15,6 +15,7 @@ SYNOPSIS
 	     [-s | --hash[=<n>]] [--abbrev[=<n>]]
 	     [--] [<ref>...]
 'git show-ref' --exclude-existing[=<pattern>]
+'git show-ref' --exists <ref>
 
 DESCRIPTION
 -----------
@@ -30,6 +31,10 @@ The `--exclude-existing` form is a filter that does the inverse. It reads
 refs from stdin, one ref per line, and shows those that don't exist in
 the local repository.
 
+The `--exists` form can be used to check for the existence of a single
+references. This form does not verify whether the reference resolves to an
+actual object.
+
 Use of this utility is encouraged in favor of directly accessing files under
 the `.git` directory.
 
@@ -65,6 +70,12 @@ OPTIONS
 	Aside from returning an error code of 1, it will also print an error
 	message if `--quiet` was not specified.
 
+--exists::
+
+	Check whether the given reference exists. Returns an error code of 0 if
+	it does, 2 if it is missing, and 128 in case looking up the reference
+	failed with an error other than the reference being missing.
+
 --abbrev[=<n>]::
 
 	Abbreviate the object name.  When using `--hash`, you do
diff --git a/builtin/show-ref.c b/builtin/show-ref.c
index d0a32d07404..617e754bbed 100644
--- a/builtin/show-ref.c
+++ b/builtin/show-ref.c
@@ -2,7 +2,7 @@
 #include "config.h"
 #include "gettext.h"
 #include "hex.h"
-#include "refs.h"
+#include "refs/refs-internal.h"
 #include "object-name.h"
 #include "object-store-ll.h"
 #include "object.h"
@@ -18,6 +18,7 @@ static const char * const show_ref_usage[] = {
 	   "             [-s | --hash[=<n>]] [--abbrev[=<n>]]\n"
 	   "             [--] [<ref>...]"),
 	N_("git show-ref --exclude-existing[=<pattern>]"),
+	N_("git show-ref --exists <ref>"),
 	NULL
 };
 
@@ -214,6 +215,41 @@ static int cmd_show_ref__patterns(const struct patterns_options *opts,
 	return 0;
 }
 
+static int cmd_show_ref__exists(const char **refs)
+{
+	struct strbuf unused_referent = STRBUF_INIT;
+	struct object_id unused_oid;
+	unsigned int unused_type;
+	int failure_errno = 0;
+	const char *ref;
+	int ret = 1;
+
+	if (!refs || !*refs)
+		die("--exists requires a reference");
+	ref = *refs++;
+	if (*refs)
+		die("--exists requires exactly one reference");
+
+	if (refs_read_raw_ref(get_main_ref_store(the_repository), ref,
+			      &unused_oid, &unused_referent, &unused_type,
+			      &failure_errno)) {
+		if (failure_errno == ENOENT) {
+			error(_("reference does not exist"));
+			ret = 2;
+		} else {
+			error(_("failed to look up reference: %s"), strerror(failure_errno));
+		}
+
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	strbuf_release(&unused_referent);
+	return ret;
+}
+
 static int hash_callback(const struct option *opt, const char *arg, int unset)
 {
 	struct show_one_options *opts = opt->value;
@@ -243,10 +279,11 @@ int cmd_show_ref(int argc, const char **argv, const char *prefix)
 	struct exclude_existing_options exclude_existing_opts = {0};
 	struct patterns_options patterns_opts = {0};
 	struct show_one_options show_one_opts = {0};
-	int verify = 0;
+	int verify = 0, exists = 0;
 	const struct option show_ref_options[] = {
 		OPT_BOOL(0, "tags", &patterns_opts.tags_only, N_("only show tags (can be combined with heads)")),
 		OPT_BOOL(0, "heads", &patterns_opts.heads_only, N_("only show heads (can be combined with tags)")),
+		OPT_BOOL(0, "exists", &exists, N_("check for reference existence without resolving")),
 		OPT_BOOL(0, "verify", &verify, N_("stricter reference checking, "
 			    "requires exact ref path")),
 		OPT_HIDDEN_BOOL('h', NULL, &patterns_opts.show_head,
@@ -272,13 +309,15 @@ int cmd_show_ref(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, show_ref_options,
 			     show_ref_usage, 0);
 
-	if ((!!exclude_existing_opts.enabled + !!verify) > 1)
-		die(_("only one of --exclude-existing or --verify can be given"));
+	if ((!!exclude_existing_opts.enabled + !!verify + !!exists) > 1)
+		die(_("only one of --exclude-existing, --exists or --verify can be given"));
 
 	if (exclude_existing_opts.enabled)
 		return cmd_show_ref__exclude_existing(&exclude_existing_opts);
 	else if (verify)
 		return cmd_show_ref__verify(&show_one_opts, argv);
+	else if (exists)
+		return cmd_show_ref__exists(argv);
 	else
 		return cmd_show_ref__patterns(&patterns_opts, &show_one_opts, argv);
 }
diff --git a/t/t1403-show-ref.sh b/t/t1403-show-ref.sh
index 3a312c8b27c..17eba350ce5 100755
--- a/t/t1403-show-ref.sh
+++ b/t/t1403-show-ref.sh
@@ -197,8 +197,73 @@ test_expect_success 'show-ref --verify with dangling ref' '
 '
 
 test_expect_success 'show-ref sub-modes are mutually exclusive' '
+	cat >expect <<-EOF &&
+	fatal: only one of --exclude-existing, --exists or --verify can be given
+	EOF
+
 	test_must_fail git show-ref --verify --exclude-existing 2>err &&
-	grep "only one of --exclude-existing or --verify can be given" err
+	test_cmp expect err &&
+
+	test_must_fail git show-ref --verify --exists 2>err &&
+	test_cmp expect err &&
+
+	test_must_fail git show-ref --exclude-existing --exists 2>err &&
+	test_cmp expect err
+'
+
+test_expect_success '--exists with existing reference' '
+	git show-ref --exists refs/heads/$GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+'
+
+test_expect_success '--exists with missing reference' '
+	test_expect_code 2 git show-ref --exists refs/heads/does-not-exist
+'
+
+test_expect_success '--exists does not use DWIM' '
+	test_expect_code 2 git show-ref --exists $GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME 2>err &&
+	grep "reference does not exist" err
+'
+
+test_expect_success '--exists with HEAD' '
+	git show-ref --exists HEAD
+'
+
+test_expect_success '--exists with bad reference name' '
+	test_when_finished "git update-ref -d refs/heads/bad...name" &&
+	new_oid=$(git rev-parse HEAD) &&
+	test-tool ref-store main update-ref msg refs/heads/bad...name $new_oid $ZERO_OID REF_SKIP_REFNAME_VERIFICATION &&
+	git show-ref --exists refs/heads/bad...name
+'
+
+test_expect_success '--exists with arbitrary symref' '
+	test_when_finished "git symbolic-ref -d refs/symref" &&
+	git symbolic-ref refs/symref refs/heads/$GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME &&
+	git show-ref --exists refs/symref
+'
+
+test_expect_success '--exists with dangling symref' '
+	test_when_finished "git symbolic-ref -d refs/heads/dangling" &&
+	git symbolic-ref refs/heads/dangling refs/heads/does-not-exist &&
+	git show-ref --exists refs/heads/dangling
+'
+
+test_expect_success '--exists with nonexistent object ID' '
+	test-tool ref-store main update-ref msg refs/heads/missing-oid $(test_oid 001) $ZERO_OID REF_SKIP_OID_VERIFICATION &&
+	git show-ref --exists refs/heads/missing-oid
+'
+
+test_expect_success '--exists with non-commit object' '
+	tree_oid=$(git rev-parse HEAD^{tree}) &&
+	test-tool ref-store main update-ref msg refs/heads/tree ${tree_oid} $ZERO_OID REF_SKIP_OID_VERIFICATION &&
+	git show-ref --exists refs/heads/tree
+'
+
+test_expect_success '--exists with directory fails with generic error' '
+	cat >expect <<-EOF &&
+	error: failed to look up reference: Is a directory
+	EOF
+	test_expect_code 1 git show-ref --exists refs/heads 2>err &&
+	test_cmp expect err
 '
 
 test_done

-- 
2.42.0
Previous: Patrick SteinhardtNext: Eric Sunshine
Message 19 of 66 in “show-ref: introduce mode to check for ref existence”
  1. 00/12 show-ref: introduce mode to check for ref existencePatrick Steinhardt, Oct 24, 2023
  2. 01/12 builtin/show-ref: convert pattern to a local variablePatrick Steinhardt, Oct 24, 2023
  3. 02/12 builtin/show-ref: split up different subcommandsPatrick Steinhardt, Oct 24, 2023
  4. Eric SunshineOct 24, 2023
  5. 03/12 builtin/show-ref: fix leaking string bufferPatrick Steinhardt, Oct 24, 2023
  6. 04/12 builtin/show-ref: fix dead code when passing patternsPatrick Steinhardt, Oct 24, 2023
  7. Eric SunshineOct 24, 2023
  8. 05/12 builtin/show-ref: refactor `--exclude-existing` optionsPatrick Steinhardt, Oct 24, 2023
  9. Eric SunshineOct 24, 2023
  10. Patrick SteinhardtOct 25, 2023
  11. 06/12 builtin/show-ref: stop using global variable to count matchesPatrick Steinhardt, Oct 24, 2023
  12. 07/12 builtin/show-ref: stop using global vars for `show_one()`Patrick Steinhardt, Oct 24, 2023
  13. 08/12 builtin/show-ref: refactor options for patterns subcommandPatrick Steinhardt, Oct 24, 2023
  14. 09/12 builtin/show-ref: ensure mutual exclusiveness of subcommandsPatrick Steinhardt, Oct 24, 2023
  15. Eric SunshineOct 24, 2023
  16. 10/12 builtin/show-ref: explicitly spell out different modes in synopsisPatrick Steinhardt, Oct 24, 2023
  17. Eric SunshineOct 24, 2023
  18. Patrick SteinhardtOct 25, 2023
  19. 11/12 builtin/show-ref: add new mode to check for reference existencePatrick Steinhardt, Oct 24, 2023
  20. Eric SunshineOct 24, 2023
  21. Patrick SteinhardtOct 25, 2023
  22. 12/12 t: use git-show-ref(1) to check for ref existencePatrick Steinhardt, Oct 24, 2023
  23. Junio C HamanoOct 24, 2023
  24. Han-Wen NienhuysOct 25, 2023
  25. Phillip WoodOct 25, 2023
  26. Patrick SteinhardtOct 26, 2023
  27. Phillip WoodOct 27, 2023
  28. Patrick SteinhardtOct 26, 2023
  29. 00/12 show-ref: introduce mode to check for ref existencePatrick Steinhardt, Oct 26, 2023
  30. 01/12 builtin/show-ref: convert pattern to a local variablePatrick Steinhardt, Oct 26, 2023
  31. 02/12 builtin/show-ref: split up different subcommandsPatrick Steinhardt, Oct 26, 2023
  32. 03/12 builtin/show-ref: fix leaking string bufferPatrick Steinhardt, Oct 26, 2023
  33. Taylor BlauOct 30, 2023
  34. 04/12 builtin/show-ref: fix dead code when passing patternsPatrick Steinhardt, Oct 26, 2023
  35. Taylor BlauOct 30, 2023
  36. 05/12 builtin/show-ref: refactor `--exclude-existing` optionsPatrick Steinhardt, Oct 26, 2023
  37. Taylor BlauOct 30, 2023
  38. Patrick SteinhardtOct 31, 2023
  39. Taylor BlauOct 30, 2023
  40. Patrick SteinhardtOct 31, 2023
  41. 06/12 builtin/show-ref: stop using global variable to count matchesPatrick Steinhardt, Oct 26, 2023
  42. Taylor BlauOct 30, 2023
  43. 07/12 builtin/show-ref: stop using global vars for `show_one()`Patrick Steinhardt, Oct 26, 2023
  44. 08/12 builtin/show-ref: refactor options for patterns subcommandPatrick Steinhardt, Oct 26, 2023
  45. 09/12 builtin/show-ref: ensure mutual exclusiveness of subcommandsPatrick Steinhardt, Oct 26, 2023
  46. Taylor BlauOct 30, 2023
  47. Patrick SteinhardtOct 31, 2023
  48. 10/12 builtin/show-ref: explicitly spell out different modes in synopsisPatrick Steinhardt, Oct 26, 2023
  49. 11/12 builtin/show-ref: add new mode to check for reference existencePatrick Steinhardt, Oct 26, 2023
  50. 12/12 t: use git-show-ref(1) to check for ref existencePatrick Steinhardt, Oct 26, 2023
  51. Taylor BlauOct 30, 2023
  52. Junio C HamanoOct 31, 2023
  53. 00/12 builtin/show-ref: introduce mode to check for ref existencePatrick Steinhardt, Oct 31, 2023
  54. 01/12 builtin/show-ref: convert pattern to a local variablePatrick Steinhardt, Oct 31, 2023
  55. 02/12 builtin/show-ref: split up different subcommandsPatrick Steinhardt, Oct 31, 2023
  56. 03/12 builtin/show-ref: fix leaking string bufferPatrick Steinhardt, Oct 31, 2023
  57. 04/12 builtin/show-ref: fix dead code when passing patternsPatrick Steinhardt, Oct 31, 2023
  58. 05/12 builtin/show-ref: refactor `--exclude-existing` optionsPatrick Steinhardt, Oct 31, 2023
  59. 06/12 builtin/show-ref: stop using global variable to count matchesPatrick Steinhardt, Oct 31, 2023
  60. 07/12 builtin/show-ref: stop using global vars for `show_one()`Patrick Steinhardt, Oct 31, 2023
  61. 08/12 builtin/show-ref: refactor options for patterns subcommandPatrick Steinhardt, Oct 31, 2023
  62. 09/12 builtin/show-ref: ensure mutual exclusiveness of subcommandsPatrick Steinhardt, Oct 31, 2023
  63. 10/12 builtin/show-ref: explicitly spell out different modes in synopsisPatrick Steinhardt, Oct 31, 2023
  64. 11/12 builtin/show-ref: add new mode to check for reference existencePatrick Steinhardt, Oct 31, 2023
  65. 12/12 t: use git-show-ref(1) to check for ref existencePatrick Steinhardt, Oct 31, 2023
  66. Taylor BlauOct 31, 2023

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