Load --grpc_auth_static_client_creds file once

[timvaillancourt]

Description

This PR causes the --grpc_auth_static_client_creds file to be loaded once in order to avoid high OS-thread usage when vtgate starts up. In the right circumstances, the repeat loading of this file can contribute to vtgate crashing on runtime: program exceeds 10000-thread limit

This stack is seen using a significant amount of blocking syscalls on every spawned healthcheck stream:

(dlv) stack
 0  0x000000000047f120 in syscall.Syscall6
    at syscall/asm_linux_amd64.s:43
 1  0x000000000047bb05 in syscall.openat
    at syscall/zsyscall_linux_amd64.go:68
 2  0x00000000004f7c5b in syscall.Open
    at syscall/syscall_linux.go:155
 3  0x00000000004f7c5b in os.openFileNolog
    at os/file_unix.go:216
 4  0x00000000004f6585 in os.OpenFile
    at os/file.go:338
 5  0x00000000004f694a in os.Open
    at os/file.go:318
 6  0x00000000004f694a in os.ReadFile
    at os/file.go:670
 7  0x0000000000de2f7a in vitess.io/vitess/go/vt/grpcclient.AppendStaticAuth
    at vitess.io/vitess/go/vt/grpcclient/client_auth_static.go:62
 8  0x0000000000de21cc in vitess.io/vitess/go/vt/grpcclient.DialContext
    at vitess.io/vitess/go/vt/grpcclient/client.go:105
 9  0x0000000001150a45 in vitess.io/vitess/go/vt/grpcclient.Dial
    at vitess.io/vitess/go/vt/grpcclient/client.go:63
10  0x0000000001150a45 in vitess.io/vitess/go/vt/vttablet/grpctabletconn.DialTablet
    at vitess.io/vitess/go/vt/vttablet/grpctabletconn/conn.go:80
11  0x0000000000e5a54c in vitess.io/vitess/go/vt/discovery.(*tabletHealthCheck).connectionLocked
    at vitess.io/vitess/go/vt/discovery/tablet_health_check.go:148
12  0x0000000000e5a430 in vitess.io/vitess/go/vt/discovery.(*tabletHealthCheck).Connection
    at vitess.io/vitess/go/vt/discovery/tablet_health_check.go:143
13  0x0000000000e5a2f1 in vitess.io/vitess/go/vt/discovery.(*tabletHealthCheck).stream
    at vitess.io/vitess/go/vt/discovery/tablet_health_check.go:127
14  0x0000000000e5ae9d in vitess.io/vitess/go/vt/discovery.(*tabletHealthCheck).checkConn
    at vitess.io/vitess/go/vt/discovery/tablet_health_check.go:276
15  0x0000000000e508aa in vitess.io/vitess/go/vt/discovery.(*HealthCheckImpl).AddTablet.func2
    at vitess.io/vitess/go/vt/discovery/healthcheck.go:380
16  0x00000000004692c1 in runtime.goexit
    at runtime/asm_amd64.s:1571

At startup vtgate opens healthcheck streams to every tablet it is watching in parallel, and right now each of those goroutines are loading/unmarshalling the static client creds file

A consequence of this change is a process reload is required to change the file, whereas today you could change the file and it would be re-loaded (sadly by N x goroutines) on each loop of the topology watcher .Start(...) ticker, which to me isn't a dealbreaker (I was surprised it's reloaded actually), but I'm curious what everyone thought? I'm happy to adjust this to support SIGHUP and I'm curious if there are helpers for that sort of thing

Related Issue(s)

#15031

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[deepthi]

Nice work! This is a much better way forward than imposing a concurrency limit on vtgate's processing of tablet records. I can re-review and approve once the feedback is addressed. They are all small things, the basic approach is obviously correct and what everyone has agreed on.

[timvaillancourt]

Nice work! This is a much better way forward than imposing a concurrency limit on vtgate's processing of tablet records. I can re-review and approve once the feedback is addressed. They are all small things, the basic approach is obviously correct and what everyone has agreed on.

Thanks @deepthi! I think I've addressed your helpful PR comments now 🙇

[timvaillancourt]

This PR seems to be causing this test failure, but I can't spot why. More 👀 appreciated!

--- FAIL: TestVtctlAuthClient (0.00s)
    client.go:88: failed to get first line: rpc error: code = Unauthenticated desc = username and password must be provided
FAIL
FAIL	vitess.io/vitess/go/vt/vtctl/grpcvtctlclient	0.116s

[timvaillancourt]

@deepthi / @ajm188: I believe this is ready for review!

I unfortunately needed to move most of the logic out of init() because the CLI-library was setting credsFile after the init() was ran

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1bf8dda) 47.36% compared to head (6b23362) 39.76%.
Report is 85 commits behind head on main.

Files	Patch %	Lines
go/vt/grpcclient/client_auth_static.go	88.37%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #15030       +/-   ##
===========================================
- Coverage   47.36%   39.76%    -7.60%     
===========================================
  Files        1144     1639      +495     
  Lines      238995   380084   +141089     
===========================================
+ Hits       113197   151155    +37958     
- Misses     117213   212721    +95508     
- Partials     8585    16208     +7623     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[timvaillancourt]

@deepthi / @ajm188: I believe this is ready for review!

@deepthi / @ajm188: I lied 😅, now it's ready for review!

[ajm188]

one minor bug fix then lgtm

[ajm188]

LGTM