Re: merge-recursive thinks symlink's child dirs are "real"

From: Jonathan Nieder <jrnieder@gmail.com>
To: Jonathan Tan <jonathantanmy@google.com>
Cc: git@vger.kernel.org, Elijah Newren <newren@gmail.com>,
	szeder.dev@gmail.com
Subject: Re: merge-recursive thinks symlink's child dirs are "real"
Date: Mon, 16 Sep 2019 17:09:02 -0700	[thread overview]
Message-ID: <20190917000902.GC67467@google.com> (raw)
In-Reply-To: <20190916214707.190171-1-jonathantanmy@google.com>

Jonathan Tan wrote:

> This was raised by a coworker at $DAYJOB. I run the following script:
[reproduction recipe from (*) snipped]
> The cherry-pick must be manually resolved, when I would expect it to
> happen without needing user intervention.
>
> You can see that at the point of the cherry-pick, in the working
> directory, ./foo is a symlink and ./foo/bar is a directory. I traced the
> code that ran during the cherry-pick to process_entry() in
> merge-recursive.c. When processing "foo/bar", control flow correctly
> reaches "Case B: Added in one", but the dir_in_way() invocation returns
> true, since lstat() indeed reveals that "foo/bar" is a directory.

Gábor covered the "what happened", so let me say a little more about
the motivation.

The "foo" symlink is being replaced by a "foo" directory containing a
"bar" file.  We're pretty far along now: we want to write actual files
to disk.  Using the index we know where we were going from and to, but
not everything in the world is tracked in the index: there could be
build outputs under "foo/bar" blocking the operation from moving
forward.

So we check whether there's a directory there.  Once we are writing
things out, there won't be, but the symlink confuses us.  Nice find.

[...]
> Is this use case something that Git should be able to handle, and if
> yes, is the correct solution to teach dir_in_way() that dirs reachable
> from symlinks are not really in the way (presumably the implementation
> would climb directories until we reach the root or we reach a filesystem
> boundary, similar to what we do when we search for the .git directory)?

The crucial detail here is that "foo" is going to be removed before we
write "foo/bar".  We should be able to notice that and skip the
dir_in_way check.

Just my uninformed guess --- I haven't tried it. :)

Thoughts?

Thanks,
Jonathan

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