server/plugin: add CloseUserConn operation to server plugin system

[xb-bxy]

WHY --> Feature Request

When a user connection is accepted, the NewUserConn plugin operation is triggered. However, there is no corresponding lifecycle hook when the user connection is closed. This makes it impossible for plugins to track connection duration, perform cleanup, emit metrics per connection, or audit user session end-events.

Without CloseUserConn, plugins have no way to know when a proxied user connection has terminated.

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Changes

• Add OpCloseUserConn = "CloseUserConn" operation constant to the plugin op set
• Add CloseUserConnContent struct (mirrors NewUserConnContent) with user, proxy_name, proxy_type, and remote_addr fields
• Add closeUserConnPlugins []Plugin field to Manager and register plugins that support OpCloseUserConn
• Add Manager.CloseUserConn() method that fans out the event to all registered plugins (errors are collected and logged as warnings, not blocking)
• In handleUserTCPConnection, add a defer that calls CloseUserConn after the user connection closes, passing the same content as NewUserConn
• Minor fix: remove redundant workConn = nil after workConn.Close() in GetWorkConnFromPool, and improve the closing log with userConn.RemoteAddr

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Server Plugin Config Example

[[httpPlugins]]
name    = "my-plugin"
addr    = "https://plugin.example.com"
path    = "/handler"
ops     = ["NewUserConn", "CloseUserConn"]

Plugin handler receives CloseUserConn POST request with body:

{
  "version": "0.1.0",
  "op": "CloseUserConn",
  "content": {
    "user": { "user": "alice", "metas": {}, "run_id": "xxx" },
    "proxy_name": "my-proxy",
    "proxy_type": "tcp",
    "remote_addr": "1.2.3.4:12345"
  }
}

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Test plan

• Plugin registered with CloseUserConn op receives the event after each user connection closes
• Plugin not registered with CloseUserConn is not called (zero-cost fast path when closeUserConnPlugins is empty)
• CloseUserConn errors are logged as warnings and do not affect the data transfer result
• NewUserConn → data transfer → CloseUserConn lifecycle is correct end-to-end
• go vet plugin. clean
• go build compiles cleanly
• All existing server plugin tests pass (go test plugin.)

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (7 files)

• pkg/config/v1/validation/validation.go
• pkg/plugin/server/manager.go
• pkg/plugin/server/plugin.go
• pkg/plugin/server/types.go
• server/proxy/proxy.go
• test/e2e/legacy/plugin/server.go
• test/e2e/v1/plugin/server.go

[Copilot]

Pull request overview

This PR adds a CloseUserConn lifecycle hook to the frp server plugin system, allowing plugins to be notified when a proxied user TCP connection terminates. This mirrors the existing NewUserConn hook and enables plugins to track connection duration, emit per-connection metrics, and perform cleanup.

Changes:

• Adds OpCloseUserConn constant, CloseUserConnContent struct, closeUserConnPlugins slice, and Manager.CloseUserConn() method to the server plugin package
• Registers OpCloseUserConn in the supported plugin ops validation list
• Adds a defer CloseUserConn(...) call in handleUserTCPConnection and includes a minor improvement to the debug log for closed connections

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/plugin/server/plugin.go	Adds OpCloseUserConn = "CloseUserConn" operation constant
pkg/plugin/server/types.go	Adds CloseUserConnContent struct (identical fields to NewUserConnContent)
pkg/plugin/server/manager.go	Adds closeUserConnPlugins slice, registers plugins for the new op, and implements CloseUserConn() fan-out method
pkg/config/v1/validation/validation.go	Adds OpCloseUserConn to SupportedHTTPPluginOps validation list
server/proxy/proxy.go	Adds defer CloseUserConn(...) after successful NewUserConn; also changes err = … to err := … (shadowing bug) and removes workConn = nil

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[xb-bxy]

add CloseUserConn test cases for plugin server