Re: [PATCH 8/8] sparse-checkout: write escaped patterns in cone mode

[Derrick Stolee <stolee@gmail.com>]

On 1/14/2020 5:48 PM, Jeff King wrote:
> On Tue, Jan 14, 2020 at 05:11:03PM -0500, Derrick Stolee wrote:
> 
>>> Do we need to document these rules somewhere? Naively I'd expect
>>> "--stdin" to take in literal pathnames. But of course it can't represent
>>> a path with a newline. So perhaps it makes sense to take quoted names by
>>> default, and allow literal NUL-separated input with "-z" if anybody
>>> wants it.
>>
>> This is worth thinking about the right way to describe the rules:
>>
>> 1. You don't _need_ quotes. They happen to come along for the ride in
>>   'git ls-tree' so it doesn't mess up shell scripts that iterate on
>>   those entries. At least, that's why I think they are quoted.
> 
> It's not just shell scripts. Without quoting, the syntax becomes
> ambiguous (e.g., imagine a file with a newline in it). So most Git
> output that shows a filename will quote it if necessary, unless
> NUL separators are being used.

Good to know.

>> 2. If you use quotes, the first layer of quotes will be removed.
> 
> I take this to mean that anything starting with a double-quote will have
> the outer layer removed, and backslash escapes inside expanded. And
> anything without a starting double quote (even if it has internal
> backslash escapes!) will be taken literally.

Hm. Perhaps you are right! The ls-tree output for the test example
is:

	deep
	folder1
	folder2
	"zbad\\dir"
	zdoes*exist

so the "zdoes*exist" value is not escaped. I believe the current
logic does something extra: consider supplying this input to
'git sparse-checkout set --stdin':

	deep
	folder1
	folder2
	"zbad\\dir"
	zdoes\*exist

then should we un-escape "\*" to "*"? Or is this not a valid input
because a backslash should have been quoted into C-style quotes?

The behavior in the current series allows this output that would
never be written by "git ls-tree".

> That would match how things like "update-index --index-info" work.
> 
> As far as implementation, I know you're trying to keep some of the
> escaping, but I think it might make more sense to do use
> unquote_c_style() to parse the input (see update-index's use for some
> prior art), and then re-quote as necessary to put things into the
> sparse-checkout file (I guess quoting more than just quote_c_style()
> would do, since you need to quote glob metacharacters like '*' and
> probably "!"). But as much as possible, I think you'd want literal
> strings inside the program, and just quoting/unquoting at the edges.

I was playing around with this, and I think that quote_c_style() is
necessary for the output, but we have a strange in-memory situation
for the other escaping: we both fill the hashsets with the un-escaped
data and fill the pattern list with the escaped patterns.

I'll add a commit with the quote_c_style() calls during the 'list'
subcommand.

>> How much of this needs to be documented explicitly, or how much should
>> we say "The input format matches what we would expect from 'git ls-tree
>> --name-only'"?
> 
> I think it's fine to say that, and maybe call attention to the quoting.
> Like:
> 
>   The input format matches the output of `git ls-tree --name-only`. This
>   includes interpreting pathnames that begin with a double quote (") as
>   C-style quoted strings.
> 
> Disappointingly, update-index does not seem to explain the rules
> anywhere. fast-import does cover it. Maybe it's something that ought to
> be hoisted out into gitcli(7) or similar (or maybe it has been and I
> just can't find it).

I'll start the process by using your recommended language. I noticed
also that the 'set' command doesn't actually document what happens
when in cone mode, so I will correct that, too.

Thanks,
-Stolee