Re: [PATCH v2] sha1-file: test the error behavior of alt_odb_usable()

On Fri, Mar 29 2019, Jeff King wrote:

On Thu, Mar 28, 2019 at 09:04:56PM +0100, Ævar Arnfjörð Bjarmason wrote:

quoted

Add a test for the error() case in alt_odb_usable() where an alternate
directory doesn't exist. This behavior has been the same since
26125f6b9b ("detect broken alternates.", 2006-02-22), but if that
error() was turned into die() the entire test suite would still pass.

Perhaps we should die() in that case, but let's start by adding a test
here to assert the long-standing existing behavior.

I think if anything we might go the other direction, and downgrade the
error() to a warning() or even omit it entirely. It's not an error to
have a missing or transient alternate. Unless of course it has objects
you need, but then those generate their own errors.

Yeah that sounds fine. FWIW it's just an "error" in the sense of being
printed out by error(), but we proceed, so it's really a warning,
sort-of.

I actually think in an ideal world we wouldn't say anything at all about
alternates which aren't present, don't appear to contain objects, etc,
on their own. And then when we hit an error because an object is
missing, only _then_ diagnose and say "hey, you have this alternate but
it doesn't have anything in it. Maybe that's an error?". Doing that
diagnosis in the error path helps in two ways:

  - we don't have to worry about it being slow

  - we can be a bit more loose about things that _might_ be an issue.
    E.g., it's not an error to point to an alternate directory that has
    no files in it. It might be a misconfiguration, or it might just not
    have any objects right now. It's hard to justify complaining about
    it in _every_ git command that loads alternates. But after hitting a
    fatal error due to a missing object, it seems like a convenient
    thing to mention to the user.

I suspect that implementing it that way might be a pain, though. Even if
we had a convenient diagnose_missing_object() one-liner, there are
probably dozens of separate places it would need to be called from.

quoted

diff --git a/t/t5613-info-alternate.sh b/t/t5613-info-alternate.sh
index 895f46bb91..d2964c57b7 100755
--- a/t/t5613-info-alternate.sh
+++ b/t/t5613-info-alternate.sh

@@ -136,4 +136,11 @@ test_expect_success CASE_INSENSITIVE_FS 'dup finding can be case-insensitive' '
 	test_cmp expect actual.alternates
 '

+test_expect_success 'print "error" on non-existing alternate' '
+	git init --bare I &&
+	echo DOES_NOT_EXIST >I/objects/info/alternates &&
+	git -C I fsck 2>stderr &&
+	test_i18ngrep "does not exist; check" stderr
+'

All that said, I don't really have an objection against this patch,
since it's just testing the current behavior. Anybody who wants to
change it would find it pretty easy to tweak this test, too.

Yup. Just wanted to get the patch to test what we do *currently* out,
might loop back to finishing up the rest of this.