This may be related to T1792

tl;dr: HKPS handler will die when used with non-HKPS hosts in a given pool.

Using setup directions from https://sks-keyservers.net/overview-of-pools.php I
assumed that configuring my GnuPG client to use ipv4.pool.sks-keyservers.net
would provide an appropriate response. It took me quite some time to determine
that HKPS is totally incompatible with the ipv4 (or other) server pools.

This is further confused by the fact that an older version of the GnuPG skeleton
files which includes a clause with examples that mix HKPS and hkp servers (skel
may not necessarily be updated in a user's directory):

<excerpt>

1. Most users just set the name and type of their preferred keyserver.
2. Note that most servers (with the notable exception of
3. ldap://keyserver.pgp.com) synchronize changes with each other. Note
4. also that a single server name may actually point to multiple
5. servers via DNS round-robin. hkp://subkeys.pgp.net is an example of
6. such a "server", which spreads the load over a number of physical
7. servers.

Keyserver hkps://pool.sks-keyservers.net
Keyserver hkp://subkeys.pgp.net
Keyserver mailto:pgp-public-keys@keys.nl.pgp.net
Keyserver ldap://pgp.surfnet.nl:11370
Keyserver ldap://keyserver.pgp.com
</excerpt>

As a result, I kept encountering the errors reported in
T1792

Here's a simple demonstration of the failure case

$ gpg2 --keyserver hkps://ipv4.pool.sks-keyservers.net --search-keys
2071B08A33BD3F06
gpg: error searching keyserver: General error
gpg: keyserver search failed: General error

Contrast with:

$ gpg2 --keyserver hkps://hkps.pool.sks-keyservers.net --search-keys
2071B08A33BD3F06
gpg: data source: https://mud.stack.nl:443
(1) NIIBE Yutaka (GnuPG Release Key) <gniibe@fsij.org>

	  2048 bit RSA key 2071B08A33BD3F06, created: 2014-10-29, expires: 2020-10-30

PERSISTENT FAILURE CASE:
Now, once the failure condition is encountered, further queries FAIL:

$ pkill dirmngr

$ gpg2 --keyserver hkp://pool.sks-keyservers.net --search-keys 2071B08A33BD3F06
gpg: error searching keyserver: No route to host
gpg: keyserver search failed: No route to host

$ gpg2 --keyserver hkps://hkps.sks-keyservers.net --search-keys 2071B08A33BD3F06
gpg: no keyserver known (use option --keyserver)
gpg: keyserver search failed: No keyserver available

WHAT?! I just specified --keyserver!!!??

Let's see if this can be rectified with clearing the keyserver:

$ gpg-connect-agent --dirmngr keyserver

keyserver --clear

OK

$ gpg2 --keyserver hkps://hkps.sks-keyservers.net --search-keys 2071B08A33BD3F06
gpg: no keyserver known (use option --keyserver)
gpg: keyserver search failed: No keyserver available

Try this with other VALID --keyserver combinations. Bang head against wall.

The ONLY command that seems to fix this persistent failure case:

$ gpg2 --search-keys 2071B08A33BD3F06

Suddenly, I can use --keyserver again, after this.

My gpg.conf:

no-greeting
keyserver hkps://hkps.pool.sks-keyservers.net

In my opinion, gpg should at least fail gracefully, or clue the user in as to
why HKPS is failing. This seems like a "brittle" failure.