Description of problem:
I was running a third instance of dnf as user root and got this backtrace.

Version-Release number of selected component:
dnf-3.6.1-1.fc29

Additional info:
reporter:       libreport-2.9.5
cmdline:        /usr/bin/python3 /usr/bin/dnf debuginfo-install libical webkit2gtk3
crash_function: __init__
exception_type: RuntimeError
executable:     /usr/bin/dnf
interpreter:    python3-3.7.0-9.fc29.x86_64
kernel:         4.18.11-301.fc29.x86_64
runlevel:       N 5
type:           Python3

Truncated backtrace:
transaction.py:729:__init__:RuntimeError: Exec failed: database is locked

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 99, in _main
    return cli_run(cli, base)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 123, in cli_run
    ret = resolving(cli, base)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 146, in resolving
    base.resolve(cli.demands.allow_erasing)
  File "/usr/lib/python3.7/site-packages/dnf/base.py", line 748, in resolve
    self._transaction = self._goal2transaction(goal)
  File "/usr/lib/python3.7/site-packages/dnf/base.py", line 645, in _goal2transaction
    ts.add_install(pkg, obs, reason)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 256, in add_install
    ti_new = self.new(new, libdnf.transaction.TransactionItemAction_INSTALL, reason)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 219, in new
    rpm_item = self._pkg_to_swdb_rpm_item(pkg)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 210, in _pkg_to_swdb_rpm_item
    rpm_item = self.history.swdb.createRPMItem()
  File "/usr/lib/python3.7/site-packages/dnf/db/history.py", line 291, in swdb
    self._swdb = libdnf.transaction.Swdb(self.dbpath)
  File "/usr/lib64/python3.7/site-packages/libdnf/transaction.py", line 729, in __init__
    this = _transaction.new_Swdb(*args)
RuntimeError: Exec failed: database is locked

Local variables in innermost frame:
self: <libdnf.transaction.Swdb;  >
args: ('/var/lib/dnf/history.sqlite',)