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Re: libcurl3 vs libcurl4

On Fri, 08 Jun 2018 at 10:06:47 +0200, Harald Dunkel wrote:
> maybe a stupid question, but how comes libcurl4 doesn't
> provide a new soname, making it possible to install both
> libcurl3 and libcurl4 in parallel?

To be compatible with other distributions, Debian and other distributions
should both follow upstream's SONAME policy. When the ABI breaks for
reasons that don't involve upstream code changes, we can't usually
unilaterally invent a new SONAME, so instead we rename the package and add
Conflicts/Replaces on older versions, as seen during C++ ABI transitions
(for instance the package containing libasprintf.so.0 being renamed from
libasprintf0 to libasprintf0c2 at some point before jessie, and then to
libasprintf0v5 between jessie and stretch).

With the transition from OpenSSL 1.0 to 1.1, we encountered
a similar problem: the upstream developer does not alter the
SONAME according to the version of OpenSSL used (and they can't
really, because it's the same source code), so if you find either
libcurl.so.N in some distribution (Debian or otherwise), you can't
know whether CURLOPT_SSL_CTX_FUNCTION works with an OpenSSL 1.0
SSL_CTX or an OpenSSL 1.1 SSL_CTX without further context.

A further complication with libcurl is that for historical reasons,
we shipped a libcurl3 package containing a library whose SONAME
is libcurl.so.4, with a compatibility symlink libcurl.so.3 for
older binaries. It isn't immediately clear why; this might have been
caused by an unnecessary SONAME change by upstream at some point in
the past? Instead of renaming libcurl3 to libcurl3a or something, the
maintainer took the opportunity to sync up the SONAME with the package
name and use libcurl4.

Because libcurl3 and libcurl4 both contain libcurl.so.4, they cannot
be co-installed, and they have to conflict; and because libcurl4
doesn't contain the libcurl.so.3 compat symlink, it can't have
Provides: libcurl3 either. (That symlink wouldn't be useful, because
older binaries that have not been recompiled against the new,
OpenSSL-1.1-using libcurl expect CURLOPT_SSL_CTX_FUNCTION to work
with an OpenSSL 1.0 or older SSL_CTX, so they are not compatible
with the new libcurl anyway.)

See <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=858398>.