With glibc 2.33 (in Ubuntu already) this fails. The upstream version
would work on more common architectures.

Full string of the backtrace is:
(gdb) printf "%s\n", str_c(bt)
/build/dovecot-bRiyOq/dovecot-2.3.13+dfsg1/src/lib/test-lib(+0x83729) [0x5555555d7729]
-> /build/dovecot-bRiyOq/dovecot-2.3.13+dfsg1/src/lib/test-lib(test_backtrace+0x90) [0x55555558a9e0]
-> /build/dovecot-bRiyOq/dovecot-2.3.13+dfsg1/src/lib/test-lib(+0x7e74e) [0x5555555d274e]
-> /build/dovecot-bRiyOq/dovecot-2.3.13+dfsg1/src/lib/test-lib(test_run_named_with_fatals+0x7d) [0x5555555d530d]
-> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xd5) [0x7ffff7dfa565]
-> /build/dovecot-bRiyOq/dovecot-2.3.13+dfsg1/src/lib/test-lib(_start+0x2e) [0x5555555852ee]

It comes down to glibc 2.33 changing behavior in regard to dovecots
backtrace_append_libc of src/lib/backtrace-string.c. We can't report this
upstream as it only affects the Debian Delta.

Comparing what would be in the backtrace on various architectures and
with odl and new glibc it seems "test_backtrace" is more reliable.
Hence let us adapt the fix for that.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

The test suite does not pass reprotest:

test-buffer-istream.c:54: Assert failed: buffer_append_full_file(result, TEST_FILENAME, SIZE_MAX, &error) == BUFFER_APPEND_READ_ERROR
test-buffer-istream.c:56: Assert failed: error != NULL && *error != '\0'
buffer_append_full_file .............................................. : FAILED

But generally dovecot builds reproducible, see
https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/dovecot.html

Update to upstream version '2.3.13+dfsg1'
with Debian dir b5b702426c1d3b2c5af09d6657286d90b498e0b7

Add libzstd-dev to build-dependencies (Closes: #969165)

See merge request debian/dovecot!17

Enable zstd in zlib_plugin

debian/dovecot!16

debian/dovecot!15

The feature has been removed from the autopkgtest configuration.

In 9692455a, we introduced a "testmails" autopkgtest definition with a
"no-build-needed" feature.  Current autopkgtest does not recognize that as a
valid feature, and lintian emits a warning with it preset.