fetch: print left-hand side when fetching HEAD:foo

`store_updated_refs()` parses the remote reference for two purposes:

    - It gets used as a note when writing FETCH_HEAD.

    - It is passed through to `display_ref_update()` to display
      updated references in the following format:

      ```
       * branch               master          -> master
      ```

In most cases, the parsed remote reference is the prettified reference
name and can thus be used for both cases. But if the remote reference is
HEAD, the parsed remote reference becomes empty. This is intended when
we write the FETCH_HEAD, where we skip writing the note in that case.
But when displaying the updated references this leads to inconsistent
output where the left-hand side of reference updates is missing in some
cases:

```
$ git fetch origin HEAD HEAD:explicit-head :implicit-head main
From https://github.com/git/git
 * branch                  HEAD       -> FETCH_HEAD
 * [new ref]                          -> explicit-head
 * [new ref]                          -> implicit-head
 * branch                  main       -> FETCH_HEAD
```

This behaviour has existed ever since the table-based output has been
introduced for git-fetch(1) via 165f390250 (git-fetch: more terse fetch
output, 2007-11-03) and was never explicitly documented either in the
commit message or in any of our tests. So while it may not be a bug per
se, it feels like a weird inconsistency and not like it was a concious
design decision.

The logic of how we compute the remote reference name that we ultimately
pass to `display_ref_update()` is not easy to follow. There are three
different cases here:

    - When the remote reference name is "HEAD" we set the remote
      reference name to the empty string. This is the case that causes
      the left-hand side to go missing, where we would indeed want to
      print "HEAD" instead of the empty string. This is what
      `prettify_refname()` would return.

    - When the remote reference name has a well-known prefix then we
      strip this prefix. This matches what `prettify_refname()` does.

    - Otherwise, we keep the fully qualified reference name. This also
      matches what `prettify_refname()` does.

As the return value of `prettify_refname()` would do the correct thing
for us in all three cases, we can thus fix the inconsistency by passing
through the full remote reference name to `display_ref_update()`, which
learns to call `prettify_refname()`. At the same time, this also
simplifies the code a bit.

Note that this patch also changes formatting of the block that computes
the "kind" (which is the category like "branch" or "tag") and "what"
(which is the prettified reference name like "master" or "v1.0")
variables. This is done on purpose so that it is part of the diff,
hopefully making the change easier to comprehend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 66 insertions, 18 deletions

diff --git a/builtin/fetch.c b/builtin/fetch.c
index 71959b2479..2b8d15fb9e 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -920,12 +920,14 @@ static void display_ref_update(struct display_state *display_state, char code,
 	}
 
 	width = (summary_width + strlen(summary) - gettext_width(summary));
+	remote = prettify_refname(remote);
+	local = prettify_refname(local);
 
 	strbuf_addf(&display_state->buf, " %c %-*s ", code, width, summary);
 	if (!display_state->compact_format)
-		print_remote_to_local(display_state, remote, prettify_refname(local));
+		print_remote_to_local(display_state, remote, local);
 	else
-		print_compact(display_state, remote, prettify_refname(local));
+		print_compact(display_state, remote, local);
 	if (error)
 		strbuf_addf(&display_state->buf, "  (%s)", error);
 	strbuf_addch(&display_state->buf, '\n');
@@ -936,7 +938,7 @@ static void display_ref_update(struct display_state *display_state, char code,
 static int update_local_ref(struct ref *ref,
 			    struct ref_transaction *transaction,
 			    struct display_state *display_state,
-			    const char *remote, const struct ref *remote_ref,
+			    const struct ref *remote_ref,
 			    int summary_width)
 {
 	struct commit *current = NULL, *updated;
@@ -948,7 +950,7 @@ static int update_local_ref(struct ref *ref,
 	if (oideq(&ref->old_oid, &ref->new_oid)) {
 		if (verbosity > 0)
 			display_ref_update(display_state, '=', _("[up to date]"), NULL,
-					   remote, ref->name, summary_width);
+					   remote_ref->name, ref->name, summary_width);
 		return 0;
 	}
 
@@ -961,7 +963,7 @@ static int update_local_ref(struct ref *ref,
 		 */
 		display_ref_update(display_state, '!', _("[rejected]"),
 				   _("can't fetch into checked-out branch"),
-				   remote, ref->name, summary_width);
+				   remote_ref->name, ref->name, summary_width);
 		return 1;
 	}
 
@@ -972,12 +974,12 @@ static int update_local_ref(struct ref *ref,
 			r = s_update_ref("updating tag", ref, transaction, 0);
 			display_ref_update(display_state, r ? '!' : 't', _("[tag update]"),
 					   r ? _("unable to update local ref") : NULL,
-					   remote, ref->name, summary_width);
+					   remote_ref->name, ref->name, summary_width);
 			return r;
 		} else {
 			display_ref_update(display_state, '!', _("[rejected]"),
 					   _("would clobber existing tag"),
-					   remote, ref->name, summary_width);
+					   remote_ref->name, ref->name, summary_width);
 			return 1;
 		}
 	}
@@ -1010,7 +1012,7 @@ static int update_local_ref(struct ref *ref,
 		r = s_update_ref(msg, ref, transaction, 0);
 		display_ref_update(display_state, r ? '!' : '*', what,
 				   r ? _("unable to update local ref") : NULL,
-				   remote, ref->name, summary_width);
+				   remote_ref->name, ref->name, summary_width);
 		return r;
 	}
 
@@ -1033,7 +1035,7 @@ static int update_local_ref(struct ref *ref,
 		r = s_update_ref("fast-forward", ref, transaction, 1);
 		display_ref_update(display_state, r ? '!' : ' ', quickref.buf,
 				   r ? _("unable to update local ref") : NULL,
-				   remote, ref->name, summary_width);
+				   remote_ref->name, ref->name, summary_width);
 		strbuf_release(&quickref);
 		return r;
 	} else if (force || ref->force) {
@@ -1045,12 +1047,12 @@ static int update_local_ref(struct ref *ref,
 		r = s_update_ref("forced-update", ref, transaction, 1);
 		display_ref_update(display_state, r ? '!' : '+', quickref.buf,
 				   r ? _("unable to update local ref") : _("forced update"),
-				   remote, ref->name, summary_width);
+				   remote_ref->name, ref->name, summary_width);
 		strbuf_release(&quickref);
 		return r;
 	} else {
 		display_ref_update(display_state, '!', _("[rejected]"), _("non-fast-forward"),
-				   remote, ref->name, summary_width);
+				   remote_ref->name, ref->name, summary_width);
 		return 1;
 	}
 }
@@ -1255,14 +1257,13 @@ static int store_updated_refs(struct display_state *display_state,
 			if (!strcmp(rm->name, "HEAD")) {
 				kind = "";
 				what = "";
-			}
-			else if (skip_prefix(rm->name, "refs/heads/", &what))
+			} else if (skip_prefix(rm->name, "refs/heads/", &what)) {
 				kind = "branch";
-			else if (skip_prefix(rm->name, "refs/tags/", &what))
+			} else if (skip_prefix(rm->name, "refs/tags/", &what)) {
 				kind = "tag";
-			else if (skip_prefix(rm->name, "refs/remotes/", &what))
+			} else if (skip_prefix(rm->name, "refs/remotes/", &what)) {
 				kind = "remote-tracking branch";
-			else {
+			} else {
 				kind = "";
 				what = rm->name;
 			}
@@ -1280,7 +1281,7 @@ static int store_updated_refs(struct display_state *display_state,
 					  display_state->url_len);
 
 			if (ref) {
-				rc |= update_local_ref(ref, transaction, display_state, what,
+				rc |= update_local_ref(ref, transaction, display_state,
 						       rm, summary_width);
 				free(ref);
 			} else if (write_fetch_head || dry_run) {
@@ -1291,7 +1292,7 @@ static int store_updated_refs(struct display_state *display_state,
 				 */
 				display_ref_update(display_state, '*',
 						   *kind ? kind : "branch", NULL,
-						   *what ? what : "HEAD",
+						   rm->name,
 						   "FETCH_HEAD", summary_width);
 			}
 		}
diff --git a/t/t5574-fetch-output.sh b/t/t5574-fetch-output.sh
index 8a344e6790..9890f6f381 100755
--- a/t/t5574-fetch-output.sh
+++ b/t/t5574-fetch-output.sh
@@ -61,6 +61,53 @@ test_expect_success 'fetch compact output' '
 	test_cmp expect actual
 '
 
+test_expect_success 'fetch output with HEAD' '
+	test_when_finished "rm -rf head" &&
+	git clone . head &&
+
+	git -C head fetch --dry-run origin HEAD >actual.out 2>actual.err &&
+	cat >expect <<-EOF &&
+	From $(test-tool path-utils real_path .)/.
+	 * branch            HEAD       -> FETCH_HEAD
+	EOF
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err &&
+
+	git -C head fetch origin HEAD >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err &&
+
+	git -C head fetch --dry-run origin HEAD:foo >actual.out 2>actual.err &&
+	cat >expect <<-EOF &&
+	From $(test-tool path-utils real_path .)/.
+	 * [new ref]         HEAD       -> foo
+	EOF
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err &&
+
+	git -C head fetch origin HEAD:foo >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err
+'
+
+test_expect_success 'fetch output with object ID' '
+	test_when_finished "rm -rf object-id" &&
+	git clone . object-id &&
+	commit=$(git rev-parse HEAD) &&
+
+	git -C object-id fetch --dry-run origin $commit:object-id >actual.out 2>actual.err &&
+	cat >expect <<-EOF &&
+	From $(test-tool path-utils real_path .)/.
+	 * [new ref]         $commit -> object-id
+	EOF
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err &&
+
+	git -C object-id fetch origin $commit:object-id >actual.out 2>actual.err &&
+	test_must_be_empty actual.out &&
+	test_cmp expect actual.err
+'
+
 test_expect_success '--no-show-forced-updates' '
 	mkdir forced-updates &&
 	(