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From Julian Foad <julianf...@apache.org>
Subject Subversion semantics: no no-op changes
Date Fri, 11 Oct 2019 14:56:31 GMT
Hello Eric.

TL;DR:  I explain why I am convinced no-op changes don't belong in the 
Subversion versioning semantics.  With your work on Subversion 
repository and dump stream semantics, is this something you can offer a 
view on?  I have previously failed to convince the developer community [1].


In examining the Subversion versioned data semantics and how the 
protocols and APIs represent them, I have come across a number of kinds 
of what could be called a "no-op change" or perhaps better described as 
"I touched this but did not change its value".

Example:
   - I changed the text of file F from T1 to T1;
   - now "svn log -v" tells me the text of F was "modified";
   - some variants of "svn diff" show no output;
   - some variants of "svn diff" show a diff header with no body.

That is the best known user-visible example.  Other kinds are possible 
too, and a number of examples exist on the server side, e.g. [#4623].

A Subversion client generally does not send no-op changes to a 
repository, but in certain cases it does.  A Subversion repository 
generally does not record and play back any no-op changes that may be 
sent to it, but in certain cases it does.

I am convinced "no-op changes" should be considered meaningless and 
removed from the data model presented to the user.  In protocols and 
APIs, a no-op change should be considered a non-canonical form and a 
transient implementation detail of that particular interface, and 
implementations should not attempt to preserve it.

In the rest of this note I try to explain some angles to the issue.

The Subversion system is built on a main design principle of tree 
snapshots and differencing and merging of trees.  A no-op change is 
out-of-tree metadata about certain pairs of trees.  Carrying such 
metadata around the system in general is fundamentally incompatible with 
that principle.

One practical reason the existing system does not preserve that metadata 
is because, with very few exceptions, the existing interfaces convey 
no-op changes only implicitly, as a side effect of how their explicit 
operations are formulated, and so one differs from another.  For 
example, an interface that represents a file change through multiple 
optional operations, one of which is "on file F, property P changes to 
value V" can convey "property P1's value no-op-changed from V1 to V1, 
while property P2's value was not touched" if we invoke the "change 
property" operation for property P1 but do not invoke it for P2.  On the 
other hand, an interface that represents a file change as a single 
operation, "new file := {text, {properties}}" cannot; the only no-op 
change it can convey is at a coarser granularity, "file F no-op-changed 
its value from {T1,PROPS1} to {T1,PROPS1}".  The kinds of no-op change 
an interface can convey locally is an implementation detail of that 
particular interface, and so cannot be expected to match any other 
interface unless explicitly required and tested, which they mostly are not.

Because the existing interfaces convey no-op-change information only 
incidentally, the system cannot be expected to preserve any particular 
no-op change when data flows through multiple interfaces, through 
commit, checkout, branch, diff, merge, and so on.  Subversion only 
preserves some within very limited scopes (such as the "file changed" 
flag in the "changed paths" list in "svn log -v").

Some of the existing svn protocols and APIs explicitly preserve certain 
no-op changes.  For example, one user reported [2] that in their svn 
history (converted from CVS) they would "hate to lose" the historical 
record that "svn log -v" reports "file text changed" for a certain no-op 
file change.  When I eliminated this no-op change from "dump", without 
due care to backward compatibility, it was considered a regression and 
reverted [#4598].  There are valid arguments for preserving backward 
compatibility in some places.  However, I propose such behaviour should 
be considered obsolete and broken, and a migration path should be 
planned to get away from it.

The snapshots argument is diluted because we already have at least one 
other kind of metadata outside a pure tree-snapshots system: the 
"copy-from" links.  I am not immediately planning to ditch copy-from 
links, though I think there are good reasons, analogous to the reasoning 
about no-op changes, to replace them in a possible future system.  I 
have given some thought to it.  That would be a more visible change to 
the system, of course, though not so much as it might first appear.

The example of a no-op file text change is a simple one.  An example 
with deeper implications is a directory copy combined with replacing one 
of its implicitly copied child with an explicit copy of that child from 
the same source as it was implicitly copied.  Addressing a case like 
this may be as simple as declaring one version as the canonical form, or 
may require further travel down the road of copy-from semantics.

In conclusion, I consider svn would be a better system -- more 
predictable, testable, composable, etc.; more generally dependable -- 
and would lose no significant value at all -- if we were to explicitly 
remove no-op changes.

Does this all ring true and obvious to you, or can you explain better 
what I am getting at and what I'm missing?

- Julian


[1] Email: "No no-op changes", from me to dev@, 2014-09-19,
     https://svn.haxx.se/dev/archive-2014-09/0082.shtml
 
https://mail-archives.apache.org/mod_mbox/subversion-dev/201409.mbox/%3c1411138196.98623.YahooMailNeo@web87703.mail.ir2.yahoo.com%3e

[2] Email: "No-op changes no longer dumped by 'svnadmin dump' in 1.9",
     from Johan Corveleyn to dev@, 2015-09-21,
     https://svn.haxx.se/dev/archive-2015-09/0269.shtml
 
http://mail-archives.apache.org/mod_mbox/subversion-dev/201509.mbox/%3CCAB84uBVe8QnEpbPVAb__yQjiDDoYjFn2+M9mPcdBXZCwMCpOLw@mail.gmail.com%3E

[#4598] "No-op changes no longer dumped by 'svnadmin dump' in 1.9",
     https://subversion.apache.org/issue/4598
     https://issues.apache.org/jira/browse/SVN-4598

[#4623] "no-op prop change not preserved across dump/load"
     https://subversion.apache.org/issue/4623
     https://issues.apache.org/jira/browse/SVN-4623

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