fix: close and delete under re-checked lock to fix race conditions

[cnlangzi]

Background

Follow-up to #79/#80 — two race conditions found in review:

1. Delete race: after releasing lock between collection and deletion, a new server can be created for the same URL — sweeper would delete the new entry.
2. Revive race: getServer() can revive a draining entry (reset DrainedAt to zero) while sweeper is outside the lock — sweeper would close a server that is actually active.

Changes

Before closing: re-verify servers[e.url] == e.srv AND DrainedAt still non-zero.
Before delete: re-verify servers[e.url] == e.srv — skip if a new server was created for that URL.

Both checks are under lock, making the operations effectively race-free.

Acceptance Criteria

• No goroutine accumulation
• Memory stays stable
• Close() and CloseAll() work correctly
• sweeper does not hold mu while calling slow Instance.Close()
• No race between sweeper close/delete and concurrent getServer/setServer

Summary by Sourcery

Prevent race conditions in the xray server sweeper by decoupling collection from closing and re-validating entries under lock before closing and deletion.

Bug Fixes:

• Avoid deleting newly created servers for a URL by re-checking the map entry under lock before removal.
• Prevent closing revived or active servers by verifying drained status and identity under lock before invoking Instance.Close().

[sourcery-ai]

Reviewer's Guide

Refactors the sweeper to first collect expired servers under lock and then perform close and delete operations with additional under-lock rechecks to eliminate race conditions with concurrent server creation and revival.

Sequence diagram for sweeper close/delete with concurrent getServer race conditions

sequenceDiagram
    actor Client
    participant getServer
    participant sweeper
    participant mu
    participant servers
    participant Server

    rect rgb(235, 245, 255)
        sweeper->>mu: Lock()
        sweeper->>servers: Scan for expired entries
        servers-->>sweeper: Return expired srv for url
        sweeper->>mu: Unlock()
    end

    par Concurrent_getServer
        Client->>getServer: Request server for url
        getServer->>mu: Lock()
        alt Existing_draining_server
            getServer->>servers: Lookup url
            servers-->>getServer: Return srv
            getServer->>Server: Reset DrainedAt to zero
        else New_server_for_url
            getServer->>servers: Create and store new *Server for url
        end
        getServer->>mu: Unlock()
    and Sweeper_processing_expired
        sweeper->>mu: Lock()
        alt Entry_changed_or_revived
            sweeper->>servers: Check servers[url]
            sweeper-->>sweeper: servers[url] != srv OR DrainedAt is_zero
            sweeper->>mu: Unlock()
            sweeper-->>sweeper: Skip close and delete
        else Entry_still_same_and_draining
            sweeper->>servers: Confirm servers[url] == srv AND DrainedAt non_zero
            sweeper->>mu: Unlock()
            sweeper->>Server: Instance.Close()

            sweeper->>mu: Lock()
            alt No_new_server_created
                sweeper->>servers: Check servers[url] == srv
                sweeper->>servers: delete(servers, url)
            else New_server_replaced_entry
                sweeper-->>sweeper: servers[url] != srv
                sweeper-->>sweeper: Skip delete to avoid removing new server
            end
            sweeper->>mu: Unlock()
        end
    end

Loading

File-Level Changes

Change	Details	Files
Refactor sweeper to batch expired servers under lock, then close and delete them with under-lock rechecks to avoid races with getServer/setServer.	Introduce an expired slice of {url, *Server} collected while holding mu based on DrainedAt and drainTimeout. After releasing mu, iterate over expired and, before closing, briefly reacquire mu to verify the map entry is still the same server and that its drained state check still matches the expiration condition, skipping if it has changed. Call Instance.Close() outside any lock to avoid blocking other map operations while close runs. After closing, reacquire mu and delete the entry only if the current servers[url] is still the same server, so that newly created servers for the same URL are not removed inadvertently.	xray/xray.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• The re-check condition before closing appears inverted relative to the comment/intent: to skip closing when an entry has been revived you probably want to continue when servers[e.url] != e.srv || e.srv.DrainedAt.IsZero() rather than || !e.srv.DrainedAt.IsZero().
• Within the per-entry loop you can reduce lock churn and make the control flow clearer by holding mu for the entire re-check block per phase (e.g., lock, validate conditions and capture a local flag, unlock, then close) instead of multiple short lock/unlock pairs around individual conditionals.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The re-check condition before closing appears inverted relative to the comment/intent: to skip closing when an entry has been revived you probably want to continue when `servers[e.url] != e.srv || e.srv.DrainedAt.IsZero()` rather than `|| !e.srv.DrainedAt.IsZero()`.
- Within the per-entry loop you can reduce lock churn and make the control flow clearer by holding `mu` for the entire re-check block per phase (e.g., lock, validate conditions and capture a local flag, unlock, then close) instead of multiple short lock/unlock pairs around individual conditionals.

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[codecov]

Codecov Report

❌ Patch coverage is 89.36170% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.71%. Comparing base (294a699) to head (88a0413).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xray/xray.go	89.36%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##            main      #81      +/-   ##
=========================================
+ Coverage   8.93%   12.71%   +3.77%     
=========================================
  Files         27       27              
  Lines       1533     1565      +32     
=========================================
+ Hits         137      199      +62     
+ Misses      1381     1351      -30     
  Partials      15       15              

Flag	Coverage Δ
Tests	12.71% <89.36%> (+3.77%)	⬆️

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