If authentication over an SSL connection fails, with sslmode=prefer, libpq will reconnect without SSL and retry. However, we did not clear the variables related to GSS, SSPI, and SASL authentication state, when reconnecting. Because of that, the second authentication attempt would always fail with a "duplicate GSS/SASL authentication request" error. pg_SSPI_startup did not check for duplicate authentication requests like the corresponding GSS and SASL functions, so with SSPI, you would leak some memory instead.
Another way this could manifest itself, on version 10, is if you list multiple hostnames in the "host" parameter. If the first server requests Kerberos or SCRAM authentication, but it fails, the attempts to connect to the other servers will also fail with "duplicate authentication request" errors.
To fix, move the clearing of authentication state from closePGconn to pgDropConnection, so that it is cleared also when re-connecting.
Patch by Michael Paquier, with some kibitzing by me.
Backpatch down to 9.3. 9.2 has the same bug, but the code around closing the connection is somewhat different, so that this patch doesn't apply. To fix this in 9.2, I think we would need to back-port commit 210eb9b743 first, and then apply this patch. However, given that we only bumped into this in our own testing, we haven't heard any reports from users about this, and that 9.2 will be end-of-lifed in a couple of months anyway, it doesn't seem worth the risk and trouble.
e6c33d5 Clear auth context correctly when re-connecting after failed auth attempt.
src/interfaces/libpq/fe-auth.c | 7 +++-
src/interfaces/libpq/fe-connect.c | 80 +++++++++++++++++++++------------------
2 files changed, 50 insertions(+), 37 deletions(-)