vtgate: query arguments missing for __vtdbname

[mcronce]

Overview of the Issue

When testing diesel migration run with one of Diesel's examples, I'm hitting the error from the title:

vtgate: http://78bab1349f40:33574/: query arguments missing for __vtdbname

The query that aggravated it was SELECT database() but when I execute that from the mysql CLI, it returns as normal. I tried setting all the variables that Diesel sets, but it still succeeds. I'll attach a packet capture.

Reproduction Steps

docker run --rm -it --entrypoint=bash gcr.io/planetscale-vitess-testing/frameworks/rust/diesel -c 'export DATABASE_URL=mysql://USERNAME:PASSWORD@HOST:PORT/DATABASE?ssl-mode=DISABLED; cd getting_started_step_1; diesel migration run'

Binary version

Using vttestserver, which doesn't support -version, but here's output from vtcombo in the same container:

$ /vt/bin/vtcombo -version
ERROR: logging before flag.Parse: E0125 18:43:12.741403     754 syslogger.go:149] can't connect to syslog
Version: acb3b4e1d (Git branch 'master') built on Mon Jan 25 16:57:11 UTC 2021 by vitess@6c41847a305c using go1.15.6 linux/amd64

Operating system and Environment details

Inside the vttestserver container:

PRETTY_NAME="Debian GNU/Linux 10 (buster)"
NAME="Debian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
vitess@3e5d5c6a960a:/$ uname -sr 
Linux 5.10.2-2-MANJARO
vitess@3e5d5c6a960a:/$ uname -m
x86_64

Log Fragments

With query logging enabled, I get this:

Prepare 172.17.0.4:48530        root    'userData1'     'root'  2021-01-25 22:16:12.025626      2021-01-25 22:16:12.025678      0.000052        0.000040     0.000012 0.000000        SELECT  "select :__vtdbname as `database()` from dual"  map[]   0       0       "query arguments missing for __vtdbname"        ""   ""       ""

but nothing else

[mcronce]

https://drive.google.com/file/d/1DHgCTgTWuPPoyOBPJjnr9b6vkaImpoG1/view has the pcap - packet 79 is the request that causes the error

[harshit-gangal]

I think it is related to how we handle that query when database is no selected.

mysql:

mysql [localhost:5729] {msandbox} ((none)) > select database();
+------------+
| database() |
+------------+
| NULL       |
+------------+
1 row in set (0.00 sec)

vitess:

+------------+
| database() |
+------------+
|            |
+------------+
1 row in set (0.00 sec)