Re: [PATCH v2 1/4] notes: preserve object type given by "add -C"

[Nguyen Thai Ngoc Duy <pclouds@gmail.com>]

On Sat, May 12, 2012 at 1:12 PM, Junio C Hamano <gitster@pobox.com> wrote:
> Nguyen Thai Ngoc Duy <pclouds@gmail.com> writes:
>
>> On Sat, May 12, 2012 at 4:16 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> ...
>>> It is not automatically "converting"; as far as the notes subsystem is
>>> concerned, the data you throw at it to be associated with an object the
>>> note annotates has always been uninterpreted stream of bytes. If an
>>> application wants to store the raw representation of a commit object
>>> including the log message and its header, it has every right to expect
>>> that it can use "git cat-file commit $argument_to_the_C_option" as the
>>> source of that uninterpreted stream of bytes, doesn't it?
>>
>> Some part of git-notes expects this stream of bytes to be textual,
>> human readable. In that case, "git notes add -C commit/tag"'s stuffing
>> a blob with the given commit/tag content to notes tree may make sense.
>> Personally I'd rather stuff the commit/tag object instead so no new
>> blobs are created. The end result is the same: read_sha1_file() always
>> return correct text, so does "git notes show".
>
> No, the end result is definitely not the same.
>
> There are two important characteristics of "uninterpreted byte stream" the
> above thinking is not taking into consideration:
>
>  (1) we do not interpret what the application stores; and
>  (2) the application is *not* limited by our type system.
>
> Suppose the application happens to want to stuff the contents it took from
> a commit object, and "add -C $objecname" is a convenient way to do so.  We
> have recorded it as "blob" because it is uninterpreted stream of bytes. If
> you record that as a leaf note in the note tree, does that mean the note
> tree now suddenly have a submodule?  Hell, no.
>
> What if the application wanted to record the contents of a tree object
> instead?  How would that affect the fan-out mechanism the note subsystem
> uses to hash the 40-hexadecimal object names?  After descending the notes
> tree to consume the object name to reach the leaf node, it still finds
> even more level hanging below.  Not very careful "list all object names
> that have notes attached in this note tree" implementation may end up
> descending into the tree object, because of this patch.  Another bad
> implication of such a change is that suddenly we would start including all
> the subobjects in that tree object when computing the reachability of the
> notes tree.

Hmm.. you are right. Consider this series dropped.

>
> The application needs to have a way to tell what is in the data it stores
> anyway, because it is not necessarily "enhancing git" kind of application
> that talks about relationships between git objects.  And it may do so
> either by convention (e.g. my "notes/amlog" notes tree only records a
> single-line text that is a Message-Id header of the original patch e-mail
> commits came from) or by having its own way to identify the application
> specific data type (e.g. json, pickle, protobuf, etc.).  It is pointless
> to say "Git object types can be stored natively, but there is only one
> type of blob so the application needs to find a way to coax the types of
> data it wants to store itself."  It needs to do so anyway, and having
> native and standardized way only for git object types does not improve the
> system.  It only ties our hands going forward because we need to define
> what it _means_ to store non-blob types in the notes tree, and support
> that forever.
>
> So this 1/4 patch is _not_ a bugfix at all.  It breaks perfectly good
> current storage semantics without no good reason.
>
> For that matter, as long as $EDITOR is set to something appropriate for
> the application specific data, there is no reason to forbid editing,
> either.
>
> The only sensible safety against "oops, I forgot that this notes tree
> stores binary gunk" I can think of offhand that won't cripple sensible use
> case is to sample the data to see if it is binary and ask confirmation,
> similar to how "less" asks "frotz may be a binary file. See it anyway?",
> and do so only when we are spewing it to the terminal.



-- 
Duy