Connect: Fix premature proxyClient.Close() when getting db users

[ravicious]

In the previous version, the proxy client would be closed immediately
after addMetadataToRetryableError. This commit makes it so that the proxy
client is closed only after GetAllowedDatabaseUsers finishes.

When running Connect on Windows, Grzegorz ran into a problem where fetching
db users for MSSQL would fail but only on Windows and only for MSSQL:

Failed to fetch current user information: connection error:
desc = "transport: Error while dialing failed to dial: read tcp
10.211.55.4:55519->52.14.45.73:3023: use of closed network
connection". services\role.go:764

Other times the error would be

connection error: desc = "transport: Error while dialing failed
to dial: ssh: unexpected packet in response to channel open:
<nil>"] apiserver\middleware.go:39

Surprisingly, tsh db ls didn't have this problem. So when thinking about
what we're doing differently than tsh and how it might be related to
a closed connection, I noticed that I made a bug in the code that closes
the proxy client.

---

Full error log of the error that Grzegorz encountered on 8f477d4

08-07-22 13:47:58]: DEBU             Failed to fetch current user information: connection error: desc = "transport: Error while dialing failed to dial: read tcp 10.211.55.4:55519->52.14.45.73:3023: use of closed network connection". services\role.go:764
[08-07-22 13:47:58]: ERRO [CONN:APIS] Request failed. error:[
[08-07-22 13:47:58]: ERROR REPORT:
[08-07-22 13:47:58]: Original Error: *trace.ConnectionProblemError connection error: desc = "transport: Error while dialing failed to dial: read tcp 10.211.55.4:55519->52.14.45.73:3023: use of closed network connection"
[08-07-22 13:47:58]: Stack Trace:
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/api/client/client.go:1470 github.com/gravitational/teleport/api/client.(*Client).GetRole
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/services/role.go:725 github.com/gravitational/teleport/lib/services.FetchRoleList
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/services/role.go:739 github.com/gravitational/teleport/lib/services.FetchRoles
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/services/role.go:774 github.com/gravitational/teleport/lib/services.FetchAllClusterRoles
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/teleterm/clusters/cluster_databases.go:176 github.com/gravitational/teleport/lib/teleterm/clusters.(*Cluster).GetAllowedDatabaseUsers
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/teleterm/apiserver/handler/handler_databases.go:58 github.com/gravitational/teleport/lib/teleterm/apiserver/handler.(*Handler).ListDatabaseUsers
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/teleterm/api/protogen/golang/v1/service.pb.go:2591 github.com/gravitational/teleport/lib/teleterm/api/protogen/golang/v1._TerminalService_ListDatabaseUsers_Handler.func1
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/teleterm/apiserver/middleware.go:37 github.com/gravitational/teleport/lib/teleterm/apiserver.withErrorHandling.func1
[08-07-22 13:47:58]: 	C:/Users/grzegorz/Desktop/code/teleport/lib/teleterm/api/protogen/golang/v1/service.pb.go:2593 github.com/gravitational/teleport/lib/teleterm/api/protogen/golang/v1._TerminalService_ListDatabaseUsers_Handler
[08-07-22 13:47:58]: 	C:/Users/grzegorz/go/pkg/mod/google.golang.org/grpc@v1.46.0/server.go:1283 google.golang.org/grpc.(*Server).processUnaryRPC
[08-07-22 13:47:58]: 	C:/Users/grzegorz/go/pkg/mod/google.golang.org/grpc@v1.46.0/server.go:1620 google.golang.org/grpc.(*Server).handleStream
[08-07-22 13:47:58]: 	C:/Users/grzegorz/go/pkg/mod/google.golang.org/grpc@v1.46.0/server.go:922 google.golang.org/grpc.(*Server).serveStreams.func1.2
[08-07-22 13:47:58]: 	C:/Program Files/Go/src/runtime/asm_amd64.s:1571 runtime.goexit
[08-07-22 13:47:58]: User Message: connection error: desc = "transport: Error while dialing failed to dial: read tcp 10.211.55.4:55519->52.14.45.73:3023: use of closed network connection"] apiserver\middleware.go:39

[ravicious]

No idea how this managed to work fine on macOS all this time. 🤔

[ravicious]

Well, it looks like it might not work for some users even on macOS after all: https://gravitational.slack.com/archives/C03FZRWDBR7/p1657294600272999

Edit: It seems that it has problems with Teleport 9.3.8 but not 9.3.6, see the Slack thread.

[ravicious]

It'd be great to include this one in the nearest patch release for v10.

[github-actions]

@ravicious See the table below for backport results.

Branch	Result
branch/v10	Create PR