[Bug]: pkg/proxy coverage UT crashes on concurrent test-server map writes

[gouhongshen]

Branch and build

• Branch: main
• Baseline commit: c34f6ed28bc9587e9413d3d2468336fb82ff1407
• Observed PR head: 65ec7eb16a580e1534e41f368f2d7cc561e7612b
• Workflow: MatrixOne ALL CI, run 33126283893, attempt 1
• Failed job: https://github.com/matrixorigin/matrixone/actions/runs/33126283893/job/98705296897

Actual behavior

The Ubuntu coverage suite crashes pkg/proxy while running
TestHandler_HandleEventKill:

fatal error: concurrent map writes
github.com/matrixorigin/matrixone/pkg/proxy.(*testHandler).handleKillConn(...)
    pkg/proxy/server_conn_test.go:385
github.com/matrixorigin/matrixone/pkg/proxy.testHandle(...)
    pkg/proxy/server_conn_test.go:358
github.com/matrixorigin/matrixone/pkg/proxy.(*testCNServer).Start.func1(...)
    pkg/proxy/server_conn_test.go:318
created by github.com/matrixorigin/matrixone/pkg/proxy.(*testCNServer).Start
    pkg/proxy/server_conn_test.go:313
FAIL github.com/matrixorigin/matrixone/pkg/proxy 3.491s

testCNServer.Start creates one handler goroutine per backend connection.
handleKillConn lets those goroutines write the shared
testCNServer.globalVars map without synchronization:

h.server.globalVars["killed"] = "yes"

The same shared map is also iterated without synchronization by
handleShowGlobalVar.

Expected behavior

The proxy test server must serialize or otherwise encapsulate all access to
its shared state so concurrent kill handling cannot crash the test process or
race with readers.

Steps to reproduce

1. Check out the observed head or the baseline above.
2. Run the repository's Ubuntu all-package coverage command with
   -tags matrixone_test -covermode=set.
3. Observe TestHandler_HandleEventKill occasionally dispatch the kill to
   concurrent test-server handlers and crash on the shared map write.

The failure is timing-sensitive. The linked attempt is the exact reproduction
and contains the complete goroutine stack.

Scope and attribution

• Searches for TestHandler_HandleEventKill, handleKillConn,
  pkg/proxy plus concurrent map, and globalVars found no matching issue.
• pkg/proxy/server_conn_test.go has the same SHA-256
  (ac41d30b42d0d94a90b7f066cd6511936cdcbe9acaab3d4d88c9b3e3e4ac2733)
  on the baseline and failing PR head.
• The unsafe map and write were introduced in commit 90f59ec7a3c in 2023.
• PR fix: preserve clone auto-increment continuation #27154 changes only clone allocator construction/tests and clone BVT
  expectations. Although pkg/sql/compile is transitively linked into the
  proxy test binary through pkg/frontend, its changed
  constructTableClone path is not executed by this test.

This is therefore an unrelated latent test-fixture concurrency defect exposed
by coverage scheduling, not a clone allocator regression.

[gouhongshen]

PR #27772 is trying to fix this issue.

[gouhongshen]

Resolution evidence for PR #27154 only: the unrelated coverage crash did not recur on exact head f8b7984758c59fd88343959540eef8aad2012616.

• Workflow: 33132557017 attempt 1
• Ubuntu coverage UT: job 98725343978, completed successfully after 29m19s
• The ordinary Ubuntu UT suite also passed on the same head.

This removes #27771 from PR #27154's unresolved CI-blocker set. The broader timing-sensitive fixture defect remains open and PR #27772 continues to track its fix.

[gouhongshen]

PR #27772 has merged and is awaiting manual validation. The issue remains open.