Thanks Dan for providing the log!

Since the upgrade process fails not long after touching the rpmdb via krb5-libs it would seem the environment got rebuilt at that point even with all checks in place via -21.fc25 (check for libdb's lock and check for rpm's transaction log)

Keep in mind it works successfully when I upgrade without krb5-libs by adding the option: exclude=krb5-libs
see description of https://bugzilla.redhat.com/show_bug.cgi?id=1443415#c0

And I got a hung on ppc64 as well, but on a different place (much earlier)

...
D: %triggerpostun(systemd-231-15.fc25.ppc64): running <lua> scriptlet.
D: %triggerpostun(systemd-233-3.fc26.ppc64): running <lua> scriptlet.
fdio:       2 writes,      162 total bytes in 0.000018 secs
D: %postun(ebtables-2.0.10-21.fc25.ppc64): scriptlet start
D: %postun(ebtables-2.0.10-21.fc25.ppc64): execv(/bin/sh) pid 4240
D: Plugin: calling hook scriptlet_fork_post in selinux plugin
D: setexecfilecon: (/bin/sh) 
+ '[' 1 -ge 1 ']'
+ systemctl try-restart ebtables.service
+ /sbin/ldconfig
D: %postun(ebtables-2.0.10-21.fc25.ppc64): waitpid(4240) rc 4240 status 0
  Cleanup     : e2fsprogs-libs-1.43.3-1.fc25.ppc64                      517/730 
D:   --- h#     256 ebtables-2.0.10-21.fc25.ppc64
D: adding "ebtables" to Name index.
D: adding 38 entries to Basenames index.
D: adding "System Environment/Base" to Group index.
D: adding 40 entries to Requirename index.
D: adding 26 entries to Providename index.
D: adding 1 entries to Conflictname index.
D: adding 9 entries to Dirnames index.
D: adding 1 entries to Installtid index.
D: adding 1 entries to Sigmd5 index.
D: adding "a43cf5611f45537556cc133fe32f99b6c945057a" to Sha1header index.
D: ========== +++ e2fsprogs-libs-1.43.3-1.fc25 ppc64-linux 0x2
D:     erase: e2fsprogs-libs-1.43.3-1.fc25.ppc64 has 4 files
D: Plugin: calling hook psm_pre in selinux plugin
D: skip       100755  1 (   0,   0)532864 /usr/lib64/libext2fs.so.2.4
D: skip       120777  1 (   0,   0)    16 /usr/lib64/libext2fs.so.2
D: skip       100755  1 (   0,   0) 70320 /usr/lib64/libe2p.so.2.3
D: skip       120777  1 (   0,   0)    13 /usr/lib64/libe2p.so.2
D: %postun(e2fsprogs-libs-1.43.3-1.fc25.ppc64): scriptlet start
D: %postun(e2fsprogs-libs-1.43.3-1.fc25.ppc64): execv(/sbin/ldconfig) pid 4246
D: Plugin: calling hook scriptlet_fork_post in selinux plugin
D: setexecfilecon: (/sbin/ldconfig) 
D: %postun(e2fsprogs-libs-1.43.3-1.fc25.ppc64): waitpid(4246) rc 4246 status 0

I think I have found the root of the problem.

In the case the libdb -21 update would be installed before the upgrade to F26 takes place and the environment gets recreated with new libdb internal structures (through another rpm/dnf command call), scriplets run during the upgrade could still access the rpmdb even though glibc's pthread internals were changed...
I have modified libdb's checks to be more restrictive and return a failure when a possible pthread version mismatch is encountered and libdb cannot safely rebuild its environment. Dan, Guy, could you try reproducing the issue with this build?

https://koji.fedoraproject.org/koji/taskinfo?taskID=19926708

My ppc64 test looks good now, it went over the e2fsprogs-libs-1.43.3-1.fc25.ppc64 cleanup and issues the expecetd warnings at krb5-libs

...
  Cleanup     : libmount-2.28.2-2.fc25.ppc64                                                                                                                                          610/706 
  Cleanup     : libblkid-2.28.2-2.fc25.ppc64                                                                                                                                          611/706 
  Cleanup     : crypto-policies-20160921-4.gitf3018dd.fc25.noarch                                                                                                                     612/706 
error: db5 error(-30969) from dbenv->open: BDB0091 DB_VERSION_MISMATCH: Database environment version mismatch
error: cannot open Packages index using db5 -  (-30969)
error: cannot open Packages database in /var/lib/rpm
error: db5 error(-30969) from dbenv->open: BDB0091 DB_VERSION_MISMATCH: Database environment version mismatch
error: cannot open Packages index using db5 -  (-30969)
error: cannot open Packages database in /var/lib/rpm
error: db5 error(-30969) from dbenv->open: BDB0091 DB_VERSION_MISMATCH: Database environment version mismatch
error: cannot open Packages index using db5 -  (-30969)
error: cannot open Packages database in /var/lib/rpm
error: db5 error(-30969) from dbenv->open: BDB0091 DB_VERSION_MISMATCH: Database environment version mismatch
error: cannot open Packages index using db5 -  (-30969)
error: cannot open Packages database in /var/lib/rpm
  Cleanup     : krb5-libs-1.14.4-7.fc25.ppc64                                                                                                                                         613/706 
  Cleanup     : coreutils-8.25-17.fc25.ppc64                                                                                                                                          614/706 
...

If I update my f25 ppc64le VM with your libdb -22 before the upgrade to f26, everything works well. "dnf -y --releasever=26 system-upgrade download"  and "dnf system-upgrade reboot" are ok and the system is upgraded successfully.
Thanks

Petr: could this issue also be the cause of the RPM database corruption that people are now reporting sometimes happens after updating to -21 on F24/F25? See https://bugzilla.redhat.com/show_bug.cgi?id=1397087 discussion...

No, I do not think so. This should only be caused by updating to glibc (with new internal structures) after a separate transaction updating libdb to -21 is already completed.

I have yet to look into what happened with the F24/25 updates but afaics there should not be any reason to drop the current patches since the issue happens after the update goes through and is easily fixable with "rpmdb --rebuilddb" ...

I would've liked to discuss it with you, but it only became clear there were still issues on Friday evening (your time) and it didn't seem practical to do nothing until today.

For a stable release of an operating system, "oh don't worry that you can't install anything and your system is printing terrifying errors, you can just run this cryptic command as root at a console to fix it" really isn't a good look.  Given that we had to make a call with the people who were around at the time, trying to prevent this happening to more people seemed like the best idea. Fixing upgrades to a pre-release seems substantially less important than not causing highly alarming bugs for people running stable releases.

Fixing this still needs some more work.
Plain out failing when a possible pthread version mismatch is encountered (which is based on comparing the modify timestamp of libpthread.so file with what is saved in libdb's environment) makes the dnf update process fail in the "verify" step even though it should be able to access the environment as it still has the old version of libpthread loaded.

libdb-5.3.28-24.fc26 has been submitted as an update to Fedora 26. https://bodhi.fedoraproject.org/updates/FEDORA-2017-2b68e14594

libdb-5.3.28-24.fc24 has been pushed to the Fedora 24 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-014d67fa9d

libdb-5.3.28-24.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-372bb1edb3

libdb-5.3.28-24.fc26 has been pushed to the Fedora 26 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-2b68e14594

The fixed version on F24 ppc64le is not available, the version available in updates-testing is libdb-5.3.28-21.fc24.ppc64le.

(In reply to Jakub Vavra from comment #15)
> The fixed version on F24 ppc64le is not available, the version available in
> updates-testing is libdb-5.3.28-21.fc24.ppc64le.

I'm building f25+f24 updates-testing in ppc koji now, should be available soon for download

With the packages libdb-5.3.28-24.fc24.ppc64le.rpm and libdb-utils-5.3.28-24.fc24.ppc64le.rpm built and provided by pkubat the upgrade worked fine.

Upgrade from f25->f26 completed on armhfp with libdb-5.3.28-24.

with f26 compose 20170701 and 
fc24: 4.11.6-101.fc24.ppc64le or
fc25: 4.11.6-201.fc25.ppc64le  
still hang in cleanup phase.

to complete my previous comment #19, the libdb versions used when failure is:
===
$grep -HnE libdb ~/Desktop/upgrade_run-dnf.rpm.log 
/home/normand/Desktop/upgrade_run-dnf.rpm.log:14:Jul 02 01:37:44 INFO Upgraded: libdb-5.3.28-21.fc24.ppc64le
/home/normand/Desktop/upgrade_run-dnf.rpm.log:43:Jul 02 01:38:17 INFO Upgraded: libdb-utils-5.3.28-21.fc24.ppc64le
/home/normand/Desktop/upgrade_run-dnf.rpm.log:66:Jul 02 01:38:26 INFO Cleanup: libdb-utils-5.3.28-14.fc24.ppc64le
/home/normand/Desktop/upgrade_run-dnf.rpm.log:78:Jul 02 01:38:26 INFO Cleanup: libdb-5.3.28-14.fc24.ppc64le
/home/normand/Desktop/upgrade_run-dnf.rpm.log:140:Jul 02 01:42:13 INFO Upgraded: libdb-5.3.28-21.fc26.ppc64le
/home/normand/Desktop/upgrade_run-dnf.rpm.log:248:Jul 02 01:42:13 INFO Upgraded: libdb-utils-5.3.28-21.fc26.ppc64le
===

libdb-5.3.28-24.fc26 has been pushed to the Fedora 26 stable repository. If problems still persist, please make note of it in this bug report.

last tested ppc64/ppc64le compose 20170707 still hangs while cleanup
similar as previous comment #19.
And the grep of libdb is similar as comment #20  with older version
versus expected libdb-5.3.28-24 of comment #21

(In reply to Michel Normand from comment #22)
> last tested ppc64/ppc64le compose 20170707 still hangs while cleanup
> similar as previous comment #19.
> And the grep of libdb is similar as comment #20  with older version
> versus expected libdb-5.3.28-24 of comment #21

I am not sure if I understand the comment correctly but this issue is fixed in libdb-5.3.28-24. If you are trying to update to libdb-5.3.28-21 instead (as per comment 20) then yes the update will most likely hang.

As for the updates for F24/25, I have submitted them to stable earlier today so they should be available soon.

libdb-5.3.28-24.fc24 has been pushed to the Fedora 24 stable repository. If problems still persist, please make note of it in this bug report.

libdb-5.3.28-24.fc25 has been pushed to the Fedora 25 stable repository. If problems still persist, please make note of it in this bug report.

The mirrors for f24  are not yet in sync today,
will have to wait more time. (same pb for f25)
===
[michel@fc24ppc64le ~]$ dnf list upgrade libdb
Last metadata expiration check: 0:00:13 ago on Wed Jul 12 07:42:16 2017.
Installed Packages
libdb.ppc64le      5.3.28-14.fc24 @anaconda
Available Packages
libdb.ppc64le      5.3.28-21.fc24  updates
===