Something in the test harness. For example, make it easier to run «make check» with client version ≠ server version, to actually test on-the-wire compatibility.
Testing with different client and server versions has been mentioned here several times recently.
I've been giving this some thought. I think this is important given how svn is used in the real world.
How would we do this?
I assume it would be something along these lines:
A test "driver" program would contain a list of versions to be tested. That program would download, configure, make, make check, and make install each listed version to a different prefix. Then, it would iterate through all permutations of client and server versions (except equal client/server versions, since those are already tested by "make check") and run the tests.
Actually, all permutations sounds like overkill and would take an unreasonable length of time. We would probably test the latest client against all listed server versions, and the latest server against all listed client versions.
Is this a reasonable initial concept?
If so, answers / solutions are needed for the following:
(1) Which versions are we interested in cross-testing in this manner?
Do we want to limit ourselves to only cross-testing currently supported versions?
Do we want to test unsupported versions that are likely to be in reasonably widespread use today, including 1.8.x and 1.9.x? 
Do we want to go as far back as some antique version like 1.5 (e.g. test a 1.13 client against a 1.5 server; test a 1.5 client against a 1.13 server)?
Do we want to go for ultimate flexibility and allow testing any two trunk and/or branch revisions against each other (which is different than, say, testing released or rc code from tarballs).
Do we want this to be configurable, i.e. the tester could choose a "shallow" or "deep" test?
(2) How do we handle differences between versions?
For example, newer versions probably contain more features and their associated tests, and more tests in general than older versions.
Is the test driver program supposed to contain knowledge of these differences and prevent some of the tests from running under certain combinations of client and server versions?
(3) How do we handle dependencies? For example IIRC until some recent version we couldn't build against APR 1.7.0. Now we can. Do we try to find a least common denominator version of each dependency and build all versions with that? Or is it better to build each version with the dependency versions as listed in get-deps.sh?
Am I on the right track?
 I'm basing that on what's in a certain popular OS's package manager and recent messages to users@.
Re: Make check with different client and server versions
+1. I'd like to support us doing this.
Nathan Hartman wrote:
> How would we do this?
As a starting point, for each client version to be tested, the new
multi-combination test driver should:
* Run the regression test suite that is supplied with that client
version. (This will be easiest because most variation is associated
with client side changes.)
* Tell the test suite ("make check") which server version to expect.
This is partly done: the Python and C tests take an argument
Set the minor version for the server ('3'..'14')
or, for the C tests, docs apparently not updated recently:
set the minor version for the server ('3', '4', '5', or '6')
I'm not sure exactly how one sets up the server appropriately, before
running with that option, for various kinds of server. Maybe 'make
check' and/or 'make svnserveautocheck' and/or 'make davautocheck' have
ways to specify how to find and run the desired server version. We'll
probably need to check and update that.
Ideally, later, the tests should also be divided or tagged so we can
select sets of tests:
- client-server tests
- client-only tests
- server-only tests
because we could then eliminate running redundant sets. Initially, that
> (1) Which versions are we interested in cross-testing in this manner?
(I suggested 1.10 there because it's the most recent LTS version, but
the important thing is just to start with something.)
> (2) How do we handle differences between versions?
> Is the test driver program supposed to contain knowledge of these
> differences and prevent some of the tests from running under certain
> combinations of client and server versions?
Annotate the tests according to what server versions they require.
This is at least partly done. The test suite already uses conditionals like
if svntest.main.options.server_minor_version < 9:
I'm not sure if this is already done everywhere it needs to be. I would
expect to see some of the Python "decorators" such as
using "server_minor_version" but I don't see any.
> (3) How do we handle dependencies?
Initially: whatever works. Probably neatest to install the dependencies
for each built version of Subversion into a location dedicated to that
version of Subversion, so they don't affect each other and don't affect
the rest of the system.