A merge-ort TODO comment, and how to test merge-ort?

On Fri, Jan 01 2021, Elijah Newren via GitGitGadget wrote:

+	else {
+		/* must be the 100644/100755 case */
+		assert(S_ISREG(a->mode));
+		result->mode = a->mode;
+		clean = (b->mode == o->mode);
+		/*
+		 * FIXME: If opt->priv->call_depth && !clean, then we really
+		 * should not make result->mode match either a->mode or
+		 * b->mode; that causes t6036 "check conflicting mode for
+		 * regular file" to fail.  It would be best to use some other
+		 * mode, but we'll confuse all kinds of stuff if we use one
+		 * where S_ISREG(result->mode) isn't true, and if we use
+		 * something like 0100666, then tree-walk.c's calls to
+		 * canon_mode() will just normalize that to 100644 for us and
+		 * thus not solve anything.
+		 *
+		 * Figure out if there's some kind of way we can work around
+		 * this...
+		 */

So if tree-walk.c didn't call canon_mode() you would do:

    if (opt->priv->call_depth && !clean)
        result->mode = 0100666;
    else
        result->mode = a->mode;

I haven't looked at this bit closer, but that doesn't make the test
referenced here pass.

I'm refactoring tree-walk.h to do that in a WIP series, and ran into
this.

As an aside, how does one run the merge-ort tests in such a way as
they'll pass on master now? There's now a bunch of failures with
GIT_TEST_MERGE_ALGORITHM=ort, well, just for t*merge*.sh:
    
    t6409-merge-subtree.sh                        (Wstat: 256 Tests: 12 Failed: 1)
      Failed test:  12
      Non-zero exit status: 1
    t6418-merge-text-auto.sh                      (Wstat: 256 Tests: 10 Failed: 3)
      Failed tests:  4-5, 10
      Non-zero exit status: 1
    t6437-submodule-merge.sh                      (Wstat: 0 Tests: 18 Failed: 0)
      TODO passed:   13, 17
    t6423-merge-rename-directories.sh             (Wstat: 256 Tests: 68 Failed: 4)
      Failed tests:  7, 53, 55, 59
      Non-zero exit status: 1

And both test_expect_merge_algorithm and what seems to be a common
pattern of e.g.:
    
    t6400-merge-df.sh:      if test "$GIT_TEST_MERGE_ALGORITHM" = ort
    t6400-merge-df.sh-      then
    t6400-merge-df.sh-              test 0 -eq $(git ls-files -o | wc -l)
    t6400-merge-df.sh-      else
    t6400-merge-df.sh-              test 1 -eq $(git ls-files -o | wc -l)
    t6400-merge-df.sh-      fi &&

Will not run tests on both backends, I was expecting to find something
so we can the test N times for the backends, and declared if things were
to be skipped on ort or whatever.

I understand that this is still WIP code, but it would be nice to have
it in a state where one can confidently touch merge-ort.c when changing
some API or whatever and have it be tested by default.

Or maybe that's the case, and I've missed how it's happening...