So the key listing is enclosed in tofu_{begin,end}_batch_update:

#0 tofu_begin_batch_update () at ../../g10/tofu.c:389
#1 0x0000000000452515 in public_key_list (ctrl=0x715870, list=0x0,

locate_mode=0) at ../../g10/keylist.c:137

#2 0x000000000040e496 in main (argc=0, argv=0x7fffffffdc08)

at ../../g10/gpg.c:4153

#0 tofu_begin_batch_update () at ../../g10/tofu.c:389
#1 0x0000000000452515 in public_key_list (ctrl=0x715870, list=0x0,

locate_mode=0) at ../../g10/keylist.c:137

#2 0x000000000040e496 in main (argc=0, argv=0x7fffffffdc08)

    at ../../g10/gpg.c:4153

Then an transaction is started on the email and key db:

#0 begin_transaction (db=0x733bc0, only_batch=0) at ../../g10/tofu.c:278
#1 0x0000000000497365 in record_binding (dbs=0x71cf40,

fingerprint=0x71c980 "362D3527F53AAD1971AAFDE658859975EE37CF96", 
email=0x71cf60 "testing (insecure!)", 
user_id=0x71ab30 "Testing (insecure!)", policy=TOFU_POLICY_AUTO, 
show_old=0) at ../../g10/tofu.c:1202

#2 0x0000000000498e36 in get_trust (dbs=0x71cf40, pk=0x71a8a0,

fingerprint=0x71c980 "362D3527F53AAD1971AAFDE658859975EE37CF96", 
email=0x71cf60 "testing (insecure!)", 
user_id=0x71ab30 "Testing (insecure!)", may_ask=0) at ../../g10/tofu.c:2182

#3 0x000000000049a44d in tofu_get_validity (ctrl=0x715870, pk=0x71a8a0,

user_id=0x71ab30 "Testing (insecure!)", may_ask=0) at ../../g10/tofu.c:2946

#4 0x000000000048f4b2 in tdb_get_validity_core (ctrl=0x715870, pk=0x71a8a0,

uid=0x71aac0, main_pk=0x71a8a0, sig=0x0, may_ask=0)
at ../../g10/trustdb.c:1074

#5 0x000000000048cd9d in get_validity (ctrl=0x715870, pk=0x71a8a0,

uid=0x71aac0, sig=0x0, may_ask=0) at ../../g10/trust.c:338

#6 0x000000000048caeb in uid_trust_string_fixed (ctrl=0x715870, key=0x71a8a0,

uid=0x71aac0) at ../../g10/trust.c:154

#7 0x00000000004544bc in list_keyblock_print (ctrl=0x715870,

keyblock=0x71a9c0, secret=0, fpr=0, listctx=0x7fffffffd560)
at ../../g10/keylist.c:950

#8 0x00000000004567aa in list_keyblock (ctrl=0x715870, keyblock=0x71a9c0,
---Type <return> to continue, or q <return> to quit---

secret=0, has_secret=0, fpr=0, listctx=0x7fffffffd560)
at ../../g10/keylist.c:1604

#9 0x00000000004533bc in list_all (ctrl=0x715870, secret=0, mark_secret=0)

at ../../g10/keylist.c:556

#10 0x000000000045254e in public_key_list (ctrl=0x715870, list=0x0,

locate_mode=0) at ../../g10/keylist.c:143

#11 0x000000000040e496 in main (argc=0, argv=0x7fffffffdc08)

    at ../../g10/gpg.c:4153

... and later ended, but since !! batch_update, it is not actually committed.

Now when tofu_end_batch_update is called and batch_update drops to zero, it iterates
over db_cache and commits all transactions using end_transaction, but db_cache is
empty. This is actually not that surprising, because the only place I see db_cache
being populated is in tofu_closedbs, a few lines after the failing assertion.

I don't have time to look at this immediately, but it looks related to Werner's
recent change to the tofu db code.