refactor: add drain() seam to threadHandler interface (#2367)

Third step of the split suggested in #2287: land the handler-interface
extension point that later handler types (background workers) need,
without introducing any new behaviour.

Each handler gains a `drain()` method, called by `drainWorkerThreads`
right before `drainChan` is closed. All current implementations
(`regularThread`, `workerThread`, `inactiveThread`, `taskThread`) are
no-ops, so observable behaviour is unchanged. A later handler that
needs to wake up a thread parked in a blocking C call (e.g. by closing
a stop pipe) plugs its signal in here without modifying
`drainWorkerThreads` again.

## What

- `phpthread.go` — interface gains `drain()`.
- `threadregular.go` / `threadworker.go` / `threadinactive.go` /
  `threadtasks_test.go` — empty `drain()` on each handler.
- `worker.go` — `drainWorkerThreads` calls `thread.handler.drain()`
  right before `close(thread.drainChan)`.

Net change: +15 / -0.

Author: nicolas-grekas

phpthread.go

@@ -34,6 +34,12 @@ type threadHandler interface {
 	afterScriptExecution(exitStatus int)
 	context() context.Context
 	frankenPHPContext() *frankenPHPContext
+	// drain is a hook called by drainWorkerThreads right before drainChan is
+	// closed. Handlers that need to wake up a thread parked in a blocking C
+	// call (e.g. by closing a stop pipe) plug their signal in here. All
+	// current handlers are no-ops; this is the seam later handler types use
+	// without having to modify drainWorkerThreads.
+	drain()
 }
 
 func newPHPThread(threadIndex int) *phpThread {

threadinactive.go

@@ -58,3 +58,5 @@ func (handler *inactiveThread) context() context.Context {
 func (handler *inactiveThread) name() string {
 	return "Inactive PHP Thread"
 }
+
+func (handler *inactiveThread) drain() {}

threadregular.go

@@ -83,6 +83,8 @@ func (handler *regularThread) name() string {
 	return "Regular PHP Thread"
 }
 
+func (handler *regularThread) drain() {}
+
 func (handler *regularThread) waitForRequest() string {
 	// max_requests reached: restart the thread to clean up all ZTS state
 	if maxRequestsPerThread > 0 && handler.requestCount >= maxRequestsPerThread {

threadtasks_test.go

@@ -79,6 +79,8 @@ func (handler *taskThread) name() string {
 	return "Task PHP Thread"
 }
 
+func (handler *taskThread) drain() {}
+
 func (handler *taskThread) waitForTasks() {
 	for {
 		select {

threadworker.go

@@ -105,6 +105,8 @@ func (handler *workerThread) name() string {
 	return "Worker PHP Thread - " + handler.worker.fileName
 }
 
+func (handler *workerThread) drain() {}
+
 func setupWorkerScript(handler *workerThread, worker *worker) {
 	metrics.StartWorker(worker.name)
 

worker.go

@@ -189,6 +189,7 @@ func drainWorkerThreads() []*phpThread {
 				continue
 			}
 
+			thread.handler.drain()
 			close(thread.drainChan)
 			drainedThreads = append(drainedThreads, thread)