spawn: keep new FastCGI, WAS, LHTTP processes on cancellation

Implement the virtual method ShouldContinueOnCancel().  This reduces
pressure and overhead for spawning new child processes that get
canceled quickly.  Instead of killing the not-yet spawned process
right away, it waits for the spawner to complete and then keeps the
process in the "idle" list, because it might be used very soon.  This
is most important during initial server boot when many requests are
slow, but killing and respawning the processes over and over just
leads to an avalanche.

This is similar to commit 7eb75a4 but
for child processes instead of outgoing HTTPS connections.

Author: MaxKellermann

debian/changelog

@@ -1,5 +1,6 @@
 cm4all-beng-proxy (21.20) unstable; urgency=low
 
+  * spawn: keep new FastCGI, WAS, LHTTP processes on cancellation
   * spawn: do not log spawn errors of killed processes
 
  --   

src/fcgi/Stock.cxx

@@ -121,6 +121,13 @@ FcgiStock::PrepareChild(const void *info, PreparedChildProcess &p,
 	params.CopyTo(p, close_fds);
 }
 
+bool
+FcgiStock::ShouldContinueOnCancel(const void *request) const noexcept
+{
+	const auto &params = *static_cast<const CgiChildParams *>(request);
+	return !params.disposable;
+}
+
 void
 FcgiStock::PrepareListenChild(const void *, UniqueSocketDescriptor fd,
 			      PreparedChildProcess &p,

src/fcgi/Stock.hxx

@@ -84,6 +84,7 @@ private:
 	std::string_view GetChildTag(const void *info) const noexcept override;
 	void PrepareChild(const void *info, PreparedChildProcess &p,
 			  FdHolder &close_fds) override;
+	bool ShouldContinueOnCancel(const void *request) const noexcept override;
 
 	/* virtual methods from class ChildStockMapClass */
 	StockOptions GetChildOptions(const void *request,

src/spawn/ChildStock.cxx

@@ -125,6 +125,12 @@ ChildStock::Create(CreateStockItem c, StockRequest request,
 	queue_item->Start(spawn_service);
 }
 
+bool
+ChildStock::ShouldContinueOnCancel(const void *request) const noexcept
+{
+	return cls.ShouldContinueOnCancel(request);
+}
+
 /*
  * interface
  *

src/spawn/ChildStock.hxx

@@ -66,6 +66,10 @@ public:
 	 */
 	virtual void PrepareChild(const void *info, PreparedChildProcess &p,
 				  FdHolder &close_fds) = 0;
+
+	virtual bool ShouldContinueOnCancel([[maybe_unused]] const void *request) const noexcept {
+		return false;
+	}
 };
 
 class ChildStockMapClass : public ChildStockClass {
@@ -159,6 +163,7 @@ private:
 	void Create(CreateStockItem c, StockRequest request,
 		    StockGetHandler &handler,
 		    CancellablePointer &cancel_ptr) override;
+	bool ShouldContinueOnCancel(const void *request) const noexcept override;
 };
 
 /**

src/was/MStock.cxx

@@ -177,6 +177,13 @@ MultiWasStock::PrepareChild(const void *info, PreparedChildProcess &p,
 	params.CopyTo(p, close_fds);
 }
 
+bool
+MultiWasStock::ShouldContinueOnCancel(const void *request) const noexcept
+{
+	const auto &params = *static_cast<const CgiChildParams *>(request);
+	return !params.disposable;
+}
+
 StockItem *
 MultiWasStock::Create(CreateStockItem c, StockItem &shared_item)
 {

src/was/MStock.hxx

@@ -100,4 +100,5 @@ private:
 						    ChildStock &child_stock) override;
 	void PrepareChild(const void *info, PreparedChildProcess &p,
 			  FdHolder &close_fds) override;
+	bool ShouldContinueOnCancel(const void *request) const noexcept override;
 };

src/was/Stock.cxx

@@ -128,6 +128,13 @@ WasStock::Create(CreateStockItem c, StockRequest _request,
 	child->InvokeCreateSuccess(handler);
 }
 
+bool
+WasStock::ShouldContinueOnCancel(const void *request) const noexcept
+{
+	const auto &params = *static_cast<const CgiChildParams *>(request);
+	return !params.disposable;
+}
+
 WasChild::~WasChild() noexcept = default;
 
 /*

src/was/Stock.hxx

@@ -105,4 +105,5 @@ private:
 	void Create(CreateStockItem c, StockRequest request,
 		    StockGetHandler &handler,
 		    CancellablePointer &cancel_ptr) override;
+	bool ShouldContinueOnCancel(const void *request) const noexcept override;
 };