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RE: [RFE] Inverted sparseness (amended)




On December 3, 2017 6:14 PM, Philip Oakley wrote a nugget of wisdom: 
>From: "Randall S. Becker" <rsbecker@xxxxxxxxxxxxx>
>Sent: Friday, December 01, 2017 6:31 PM
>> On December 1, 2017 1:19 PM, Jeff Hostetler wrote:
>>>On 12/1/2017 12:21 PM, Randall S. Becker wrote:
>>>> I recently encountered a really strange use-case relating to sparse 
>>>> clone/fetch that is really backwards from the discussion that has 
>>>> been going on, and well, I'm a bit embarrassed to bring it up, but 
>>>> I have no good solution including building a separate data store 
>>>> that will end up inconsistent with repositories (a bad solution).  
>>>> The use-case is as
>>>> follows:
>>>>
>>>> Given a backbone of multiple git repositories spread across an 
>>>> organization with a server farm and upstream vendors.
>>>> The vendor delivers code by having the client perform git pull into 
>>>> a specific branch.
>>>> The customer may take the code as is or merge in customizations.
>>>> The vendor wants to know exactly what commit of theirs is installed 
>>>> on each server, in near real time.
>>>> The customer is willing to push the commit-ish to the vendor's 
>>>> upstream repo but does not want, by default, to share the actual 
>>>> commit contents for security reasons.
>>>> Realistically, the vendor needs to know that their own commit id 
>>>> was put somewhere (process exists to track this, so not part of the
>>>> use-case) and whether there is a subsequent commit contributed >by 
>>>> the customer, but the content is not relevant initially.
>>>>
>>>> After some time, the vendor may request the commit contents from 
>>>> the customer in order to satisfy support requirements - a.k.a. a 
>>>> defect was found but has to be resolved.
>>>> The customer would then perform a deeper push that looks a lot like 
>>>> a "slightly" symmetrical operation of a deep fetch following a 
>>>> prior sparse fetch to supply the vendor with the specific commit(s).
>>
>>>Perhaps I'm not understanding the subtleties of what you're 
>>>describing, but could you do this with stock git functionality.
>>
>>>Let the vendor publish a "well known branch" for the client.
>>>Let the client pull that and build.
>>>Let the client create a branch set to the same commit that they fetched.
>>>Let the client push that branch as a client-specific branch to the 
>>>vendor to indicate that that is the official release they are based on.
>>
>>>Then the vendor would know the official commit that the client was using.
>> This is the easy part, and it doesn't require anything sparse to exist.
>>
>>>If the client makes local changes, does the vendor really need the 
>>>SHA of those -- without the actual content?
>>>I mean any SHA would do right?  Perhaps let the client create a 
>>>second client-specific branch (set to  the same commit as the first) 
>>>to indicate they had mods.
>>>Later, when the vendor needs the actual client changes, the client 
>>>does a normal push to this 2nd client-specific branch at the vendor.
>>>This would send everything that the client has done to the code since 
>>>the official release.
>>
>> What I should have added to the use-case was that there is a strong 
>> audit requirement (regulatory, actually) involved that the SHA is 
>> exact, immutable, and cannot be substitute or forged (one of the 
>> reasons git is in such high regard). So, no I can't arrange a fake 
>> SHA to represent a SHA to be named later. It SHA of the installed 
>> commit is part of the official record of what happened on the specific server, so I'm stuck with it.
>>
>>>I'm not sure what you mean about "it is inside a tree".
>>
>> m---a---b---c---H1
>>          `---d---H2
>>
>> d would be at a head. b would be inside. Determining content of c is 
>> problematic if b is sparse, so I'm really unsure that any of this is 
>> possible.

>I think I get the jist of your use case. Would I be right that you 
>don't have a true working solution yet? i.e. that it's a problem that is almost sorted but falls down at the last step.

>If one pretended that this was a single development shop, and the 
>various vendors, clients and customers as being independent devolopers, 
>each of whom is over protective of their code, it may give a better view that maps onto classic feature development diagrams.
>(i.e draw the answer for local devs, then mark where the splits happen)

>In particular, I think you could use a notional regulator's view that 
>the whole code base is part of a large Git heirarchy of branches and 
>merges, and that some of the feature loops are only available via the particular developer that worked on that feature.

>This would mean that from a regulatory overview there is a merge commit in the 'main'
>(master) heirachy that has the main and feature commits listed, and the 
>feature commit is probably an --allow-empty commit (that has an empty 
>tree if they are that paranoid) that says 'function X released' (and 
>probably tagged), and that release commit then has, as its parent, the 
>true release commit, with the true code tree. The latter commit isn't actually being shown to you!

>At this point the potential for using the graft capability comes in (as a regulated method!). 
>Locally the graft records the missing line of pearls for that paranoid dev/vendor/customer/client. 
>The whole git heirachy still works.

>The question is how to get that  release commit with its empty tree, 
>and its tag, to you from the dev. I'd guess that a fast-export of just 
>that tag/commit/empty tree would allow you to bring in that sentinel 
>point to your heirachy (initially as a psuedo --root), and then graft 
>it on. (I haven't checked if fast-export allows such specificity, but 
>it's a method)

>You can now form the merge commit and have regulatory oversight and the 
>full git validation and verification capability, as long as your web of 
>trust extends to the regulator looking effectively across the air gap. It's a fresh way of seeing the web of trust.

>Thus you/they have various "shallow clones", but with gaps and islands in the shallowness.... 
>and those gaps are spanned by grafts (which are audited). The 
>`git-replace` may also be an option, but I don't think it's quite right 
>for this case. You just have a temporary gap in the history, and with 
>fast export

>If using the empty tree part doesn't pass muster (i.e. showing nothing 
>isn't sufficient), then the narrow clone could come into play to limit 
>what parts of the trees are widely visible, but mainly its using the 
>grafts to cover the regulatory gap, and (for the moment) using 
>fast-export to transfer the singleton commit / tags

>Oh Just remembered, there is the newish capability to fetch random blobs, so that may help.

I think you hit the nail on the head pretty well. We're currently at 2.3.7, with a push to 2.15.1 this week, so I'm looking forward to trying this. My two worries are whether the empty tree is acceptable (it should be to the client, and might be to the vendor), and doing this reliably (semi-automated) so the user base does not have to worry about the gory details of doing this. The unit tests for it are undoubtedly going to give me headaches.

Thanks for the advice. Islands of shallowness are a really descriptive image for what this is. So identifying that there are shoals (to extend the metaphor somewhat), will be crucial to this adventure.

These islands of shallowness, however, are also concerns as described in the [Re: How hard would it be to implement sparse fetching/pulling?] thread. The matter of the security audit is important here also:
> I'm just thinking that even if we get a *perfectly working* partial clone/fetch/push/etc. that it would not pass a security audit. 
Not having the capability would similarly cause a failure of a security audit.

Cheers,
Randall

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