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Date: Wed, 1 Oct 2014 10:52:42 +0100
From: Julian Foad <julianfoad@btopenworld.com>
Reply-To: Julian Foad <julianfoad@btopenworld.com>
Subject: Official way to create an empty revision
To: "dev@subversion.apache.org" <dev@subversion.apache.org>
Cc: Daniel Shahaf <d.s@daniel.shahaf.name>
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Daniel Shahaf wrote in the thread "No no-op changes":=0A> Should we provide=
 an "official" way to create an empty revision?=A0 That=0A> is, a revision =
whose changed-paths list is empty?=0A> =0A> Use-cases:=0A> =0A> 1. Suppose =
last backup is r100 and revisions r101:r105 were lost; then=0A> after resto=
ring the backup, the admin would create 5 empty revisions.=0A> =0A> 2. Forc=
e an empty revision for whatever reason, such as to make the=0A> revnums sy=
nc to something:=0A> 2.1. See r3 of the regression test merge_tests.py#125 =
svnmucc_abuse_1().=0A> 2.2. W hen loading our repository to the ASF reposit=
ory, if Joe had=0A> created 26 empty revisions, then The Offset would have =
been 840100=0A> rather than 840074, which would make our mental math easier=
.=0A=0AHi Daniel. It seems a reasonable tool to have in the svn admin's too=
l kit. Perhaps not often, but people do sometimes want this. I found two we=
b pages where people discussed this. One wrote a script that spits out the =
appropriate few lines of dump file text to represent an empty rev, N times =
[1]; the other is worse, committing N changes to a temporary repo, dumping =
it and filtering everything out [2].=0A=0AI'm assuming this proposal is res=
tricted to the admin side. Your use cases 1. and 2.2 are both admin use cas=
es. Your use case 2.1 is a test which uses a client-side commit to make an =
uninteresting revision, in order to make the subsequent revision numbers ma=
tch (modulo 10) those in the original use case. While people no doubt do th=
is sort of thing sometimes in real life, I can't think of a general behavio=
ur that would make sense from the client side. In a shared repository, you =
never know what revision number your next commit will have.=0A=0AFor a UI, =
I can envisage two useful ways to expose this functionality: commit N empty=
 revisions as a stand-alone operation, and commit N empty revisions before =
the first revision loaded from a dump stream. Obviously the former is suffi=
cient; the latter is convenient but is insufficient on its own unless we al=
so give 'svnadmin load' a convenient way to specify there is no dump stream=
 to load.=0A=0AStand-alone:=0A=0A=A0 svnadmin/svnlook commit-empty-revs N=
=0A=A0=A0=A0 Commit N empty revisions.=0A=0AAs an option to 'svnadmin load'=
:=0A=0A=A0 svnadmin load --prefix-empty-revs N=0A=A0=A0=A0 First commit N e=
mpty revisions.=0A=0Aor, expressing a similar behaviour in a different way:=
=0A=0A=A0 svnadmin load --commit-first-loaded-rev-as X=0A=A0=A0=A0 First co=
mmit enough empty revisions to make the first loaded revision=0A=A0=A0=A0 b=
e committed as revision number X.=0A=0AWhat should the author and log messa=
ge be on the empty revs? I suppose these need to be optionally specified, d=
efaulting to blank?=0A=0AWhat should the date stamps be on the empty revs? =
A thought: it seems cleaner to specify that they should all have the same d=
ate stamp than that they do/don't/may all have different date stamps. (Imag=
ine a future back-end in which we can create millions of 'virtual' empty re=
vs in O(1) time and space as long as their rev-props are all identical.) Th=
e default for 'svnadmin load --prefix-empty-revs' without '--ignore-dates' =
("ignore revision date stamps found in the stream") should, I suppose, be t=
hat all the prefix empty revs have the same date stamp as the first revisio=
n loaded.=0A=0A- Julian=0A=0A[1] <http://www.timj.co.uk/2011/09/generating-=
emptypadding-revisions-in-an-svn-dump/>=0A[2] <http://stackoverflow.com/que=
stions/7030041/can-i-create-a-subversion-repository-starting-at-another-num=
ber>