Fix speculativeAuthenticate panic on empty database

[chilagrow]

Description

This panic is only visible on mechanism called MONGODB-X509. But it's nice to fix panic if you come across one 🤗

Run FerretDB with --test-enable-new-auth flag.

diff --git a/Taskfile.yml b/Taskfile.yml
index c50d557f3..8cef1ba5c 100644
--- a/Taskfile.yml
+++ b/Taskfile.yml
@@ -364,6 +364,7 @@ tasks:
         --handler=pg
         --postgresql-url='postgres://username@127.0.0.1:5432/ferretdb?search_path='
         --test-records-dir=tmp/records
+        --test-enable-new-auth

Run mongosh without database, use below mechanism, it won't panic for PLAIN or SCRAM.

$ docker compose exec mongodb mongosh --verbose --shell 'mongodb://host.docker.internal:27017/?authMechanism=MONGODB-X509'
Current Mongosh Log ID: 65f163a2de277b1c0f3c9181                                                      
Connecting to:          mongodb://host.docker.internal:27017/?authMechanism=MONGODB-X509&directConnection=true&appName=mongosh+2.1.4                                                                        
MongoNetworkError: connection 1 to 192.168.65.2:27017 closed                                                     

See panic, it appears because Op query seems to error on command error or lazyerror.

2024-03-13T17:23:46.441+0900    ERROR   // 127.0.0.1:43834 -> 127.0.0.1:27017   clientconn/conn.go:525  Handler error for unhandled response opcode      {"error": "BadValue (2): required parameter \"db\" is miss
ing", "opcode": "OP_REPLY"}                                                                                                                                                                                        
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).route                                                                                                                                                     
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/conn.go:525                   
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).run
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/conn.go:316
github.com/FerretDB/FerretDB/internal/clientconn.acceptLoop.func1
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/listener.go:305               
2024-03-13T17:23:46.441+0900    ERROR   // 127.0.0.1:43834 -> 127.0.0.1:27017   clientconn/conn.go:615  Response header: length:     0, id:    0, response_to:    0, opcode: OP_REPLY
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).logResponse                                     
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/conn.go:615                   
github.com/FerretDB/FerretDB/internal/clientconn.(*conn).run                                                                                                                                                       
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/conn.go:317                   
github.com/FerretDB/FerretDB/internal/clientconn.acceptLoop.func1                                        
        /home/chi/go/src/github.com/chilagrow/FerretDB/internal/clientconn/listener.go:305               
2024-03-13T17:23:46.441+0900    ERROR   // 127.0.0.1:43834 -> 127.0.0.1:27017   clientconn/conn.go:616  Response message:
%!s(<nil>)                   

Since if SpeculativeAuthenticate does not succeed, it calls saslStart. If db is not set, return Op query without SpeculativeAuthenticate field, the error is returned from saslStart.

Closes #4100.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.81%. Comparing base (ea9c5bd) to head (e0bc05e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4178      +/-   ##
==========================================
- Coverage   74.52%   73.81%   -0.72%     
==========================================
  Files         339      339              
  Lines       22785    22786       +1     
==========================================
- Hits        16981    16819     -162     
- Misses       4498     4683     +185     
+ Partials     1306     1284      -22     

Files	Coverage Δ
internal/handler/cmd_query.go	45.94% <0.00%> (-1.28%)	⬇️

... and 19 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.21% <0.00%> (-0.87%)	⬇️
hana-1	?
integration	67.21% <0.00%> (-0.87%)	⬇️
mongodb-1	4.89% <0.00%> (-0.01%)	⬇️
postgresql-1	46.60% <0.00%> (-0.02%)	⬇️
postgresql-2	49.40% <0.00%> (-0.06%)	⬇️
postgresql-3	49.31% <0.00%> (-0.04%)	⬇️
sqlite-1	45.75% <0.00%> (-0.03%)	⬇️
sqlite-2	?
sqlite-3	48.51% <0.00%> (-0.01%)	⬇️
unit	32.54% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[chilagrow]

With this fix an error is returned.

$ docker compose exec mongodb mongosh --verbose --shell 'mongodb://host.docker.internal:27017/?authMechanism=MONGODB-X509'                         [100/9304]
Current Mongosh Log ID: 65f16407349a0891c9b05ba3                                                                                                                                                            
Connecting to:          mongodb://host.docker.internal:27017/?authMechanism=MONGODB-X509&directConnection=true&appName=mongosh+2.1.4                                                                        
MongoServerError: no such command: 'authenticate'     

[rumyantseva]

Nice catch!