unpack-trees: failure to set SKIP_WORKTREE bits always just a warning

Setting and clearing of the SKIP_WORKTREE bit is not only done when
users run 'sparse-checkout'; other commands such as 'checkout' also run
through unpack_trees() which has logic for handling this special bit.
As such, we need to consider how they handle special cases.  A couple
comparison points should help explain the rationale for changing how
unpack_trees() handles these bits:

    Ignoring sparse checkouts for a moment, if you are switching
    branches and have dirty changes, it is only considered an error that
    will prevent the branch switching from being successful if the dirty
    file happens to be one of the paths with different contents.

    SKIP_WORKTREE has always been considered advisory; for example, if
    rebase or merge need or even want to materialize a path as part of
    their work, they have always been allowed to do so regardless of the
    SKIP_WORKTREE setting.  This has been used for unmerged paths, but
    it was often used for paths it wasn't needed just because it made
    the code simpler.  It was a best-effort consideration, and when it
    materialized paths contrary to the SKIP_WORKTREE setting, it was
    never required to even print a warning message.

In the past if you trying to run e.g. 'git checkout' and:
  1) you had a path that was materialized and had some dirty changes
  2) the path was listed in $GITDIR/info/sparse-checkout
  3) this path did not different between the current and target branches
then despite the comparison points above, the inability to set
SKIP_WORKTREE was treated as a *hard* error that would abort the
checkout operation.  This is completely inconsistent with how
SKIP_WORKTREE is handled elsewhere, and rather annoying for users as
leaving the paths materialized in the working copy (with a simple
warning) should present no problem at all.

Downgrade any errors from inability to toggle the SKIP_WORKTREE bit to a
warning and allow the operations to continue.

Reviewed-by: Derrick Stolee <[email protected]>
Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

t/t1011-read-tree-sparse-checkout.sh

@@ -233,18 +233,19 @@ test_expect_success 'read-tree --reset removes outside worktree' '
 	test_must_be_empty result
 '
 
-test_expect_success 'print errors when failed to update worktree' '
+test_expect_success 'print warnings when some worktree updates disabled' '
 	echo sub >.git/info/sparse-checkout &&
 	git checkout -f init &&
 	mkdir sub &&
 	touch sub/added sub/addedtoo &&
-	test_must_fail git checkout top 2>actual &&
+	# Use -q to suppress "Previous HEAD position" and "Head is now at" msgs
+	git checkout -q top 2>actual &&
 	cat >expected <<\EOF &&
-error: The following untracked working tree files would be overwritten by checkout:
+warning: The following paths were already present and thus not updated despite sparse patterns:
 	sub/added
 	sub/addedtoo
-Please move or remove them before you switch branches.
-Aborting
+
+After fixing the above paths, you may want to run `git sparse-checkout reapply`.
 EOF
 	test_i18ncmp expected actual
 '

t/t2018-checkout-branch.sh

@@ -238,4 +238,26 @@ test_expect_success 'checkout -b after clone --no-checkout does a checkout of HE
 	test_path_is_file dest/a.t
 '
 
+test_expect_success 'checkout -b to a new branch preserves mergeable changes despite sparse-checkout' '
+	test_when_finished "
+		git reset --hard &&
+		git checkout branch1-scratch &&
+		test_might_fail git branch -D branch3 &&
+		git config core.sparseCheckout false &&
+		rm .git/info/sparse-checkout" &&
+
+	test_commit file2 &&
+
+	echo stuff >>file1 &&
+	echo file2 >.git/info/sparse-checkout &&
+	git config core.sparseCheckout true &&
+
+	CURHEAD=$(git rev-parse HEAD) &&
+	do_checkout branch3 $CURHEAD &&
+
+	echo file1 >expect &&
+	git diff --name-only >actual &&
+	test_cmp expect actual
+'
+
 test_done

unpack-trees.c

@@ -1701,23 +1701,15 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 			 * correct CE_NEW_SKIP_WORKTREE
 			 */
 			if (ce->ce_flags & CE_ADDED &&
-			    verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
-				if (!o->show_all_errors)
-					goto return_failed;
-				ret = -1;
-			}
+			    verify_absent(ce, WARNING_SPARSE_ORPHANED_NOT_OVERWRITTEN, o))
+				ret = 1;
+
+			if (apply_sparse_checkout(&o->result, ce, o))
+				ret = 1;
 
-			if (apply_sparse_checkout(&o->result, ce, o)) {
-				if (!o->show_all_errors)
-					goto return_failed;
-				ret = -1;
-			}
 			if (!ce_skip_worktree(ce))
 				empty_worktree = 0;
-
 		}
-		if (ret < 0)
-			goto return_failed;
 		/*
 		 * Sparse checkout is meant to narrow down checkout area
 		 * but it does not make sense to narrow down to empty working
@@ -1728,6 +1720,15 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 			ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
 			goto done;
 		}
+		if (ret == 1) {
+			/*
+			 * Inability to sparsify or de-sparsify individual
+			 * paths is not an error, but just a warning.
+			 */
+			if (o->show_all_errors)
+				display_warning_msgs(o);
+			ret = 0;
+		}
 	}
 
 	ret = check_updates(o, &o->result) ? (-2) : 0;
@@ -1759,10 +1760,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	return ret;
 
 return_failed:
-	if (o->show_all_errors) {
+	if (o->show_all_errors)
 		display_error_msgs(o);
-		display_warning_msgs(o);
-	}
 	mark_all_ce_unused(o->src_index);
 	ret = unpack_failed(o, NULL);
 	if (o->exiting_early)