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@@ -5,12 +5,11 @@
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 For efficiency reasons, Passenger keeps a pool spawned Rails/Ruby applications.
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 Please read the C++ API documentation for the ApplicationPool class for a full
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-introduction. This document describes an algorithm for managing the pool.
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+introduction. This document describes an algorithm for managing the pool, in a
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+high-level way.
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 The algorithm should strive to keep spawning to a minimum.
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-TODO: check whether the algorithm has thrashing behavior.
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-
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 == Definitions
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@@ -61,6 +60,11 @@ explicitly define some special types:
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               containers[i + 1].app is active
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   * size (unsigned integer): The number of items in _instances_.
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+  
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+  * max_requests (unsigned integer): The maximum number of requests that each
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+    application instance in this domain may process. After having processed this
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+    many requests, the application instance will be shut down.
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+    A value of 0 indicates that there is no maximum.
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 - AppContainer
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   A compound type (class) which contains an application instance, as well as
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@@ -85,6 +89,16 @@ explicitly define some special types:
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     inactive_apps. This iterator is only valid if this AppContainer really is
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     in that list.
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+- SpawnOptions
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+  A structure containing additional information, used by the application instance
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+  spawning process.
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+  
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+  A SpawnOptions has the following members:
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+  * max_requests (unsigned integer) - The maximum number of requests that the
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+    application instance may process. After having processed this many requests,
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+    the application instance will be shut down. A value of 0 indicates that there
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+    is no maximum.
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+
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 === Special functions
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 - spawn(app_root)
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@@ -168,14 +182,18 @@ Here's an UML diagram in ASCII art:
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 # Thread-safetiness notes:
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 # - All wait commands are to unlock the lock during waiting.
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-function get(app_root):
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+# Connect to an existing application instance or to a newly spawned application instance.
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+# 'app_root' specifies the application root folder of the application. 'options' is an
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+# object of type 'SpawnOptions', which contains additional information which may be
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+# relevant for spawning.
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+function get(app_root, options):
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   MAX_ATTEMPTS = 10
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   attempt = 0
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   time_limit = now() + 5 seconds
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   lock.synchronize:
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     while (true):
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       attempt++
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-			container, domain = spawn_or_use_existing(app_root)
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+			container, domain = spawn_or_use_existing(app_root, options)
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       container.last_used = current_time()
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       container.sessions++
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       try:
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@@ -199,7 +217,7 @@ function get(app_root):
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 # Returns a pair of [AppContainer, Domain] that matches the given application
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 # root. If no such AppContainer exists, then it is created and a new
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 # application instance is spawned. All exceptions that occur are propagated.
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-function spawn_or_use_existing(app_root):
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+function spawn_or_use_existing(app_root, options):
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   domain = domains[app_root]
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   if (domain != nil) and (needs_restart(app_root)):
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@@ -284,6 +302,7 @@ function spawn_or_use_existing(app_root):
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     if domain == nil:
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       domain = new Domain
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       domain.size = 1
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+			domain.max_requests = options.max_requests
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       domains[app_root] = domain
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     else:
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       domain.size++
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@@ -301,13 +320,26 @@ function session_has_been_closed(container):
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   lock.synchronize:
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     domain = domains[container.app.app_root]
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     if domain != nil:
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-			container.last_used = current_time()
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-			container.sessions--
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+			instances = domain.instances
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       container.processed++
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-			if container.sessions == 0:
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-				domain.instances.move_to_front(container.iterator)
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-				container.ia_iterator = inactive_apps.add_to_back(container.app)
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+			
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+			if (domain.max_requests) > 0 and (container.processed >= domain.max_requests):
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+				# The application instance has processed its maximum allowed
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+				# number of requests, so we shut it down.
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+				instances.remove(container.iterator)
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+				domain.size--
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+				if instances.empty():
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+					domains.remove(app_root)
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+				count--
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         active--
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+			else:
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+				container.last_used = current_time()
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+				container.sessions--
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+				container.processed++
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+				if container.sessions == 0:
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+					instances.move_to_front(container.iterator)
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+					container.ia_iterator = inactive_apps.add_to_back(container.app)
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+					active--
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 function needs_restart(app_root):

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@@ -113,6 +113,7 @@ private:
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   struct Domain {
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     AppContainerList instances;
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     unsigned int size;
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+		unsigned long maxRequests;
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   };
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   struct AppContainer {
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@@ -192,19 +193,32 @@ private:
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       DomainMap::iterator it;
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       it = data->domains.find(container->app->getAppRoot());
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       if (it != data->domains.end()) {
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-				AppContainerList *instances = &it->second->instances;
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-				container->lastUsed = time(NULL);
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-				container->sessions--;
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+				Domain *domain = it->second.get();
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+				AppContainerList *instances = &domain->instances;
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+				
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         container->processed++;
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-				if (container->sessions == 0) {
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+				if (domain->maxRequests > 0 && container->processed >= domain->maxRequests) {
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           instances->erase(container->iterator);
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-					instances->push_front(container);
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-					container->iterator = instances->begin();
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-					data->inactiveApps.push_back(container);
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-					container->ia_iterator = data->inactiveApps.end();
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-					container->ia_iterator--;
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+					domain->size--;
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+					if (instances->empty()) {
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+						data->domains.erase(container->app->getAppRoot());
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+					}
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+					data->count--;
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           data->active--;
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           data->activeOrMaxChanged.notify_all();
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+				} else {
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+					container->lastUsed = time(NULL);
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+					container->sessions--;
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+					if (container->sessions == 0) {
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+						instances->erase(container->iterator);
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+						instances->push_front(container);
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+						container->iterator = instances->begin();
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+						data->inactiveApps.push_back(container);
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+						container->ia_iterator = data->inactiveApps.end();
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+						container->ia_iterator--;
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+						data->active--;
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+						data->activeOrMaxChanged.notify_all();
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+					}
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         }
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       }
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     }
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@@ -522,6 +536,7 @@ private:
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         if (it == domains.end()) {
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           domain = new Domain();
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           domain->size = 1;
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+					domain->maxRequests = spawnOptions.maxRequests;
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           domains[appRoot] = ptr(domain);
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         } else {
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           domain = it->second.get();

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@@ -364,6 +364,39 @@
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     // TODO: how do we test this?
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   }
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+	TEST_METHOD(18) {
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+		// Application instance is shutdown after 'maxRequests' requests.
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+		SpawnOptions options("stub/railsapp");
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+		int reader;
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+		pid_t originalPid;
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+		Application::SessionPtr session;
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+		
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+		options.maxRequests = 4;
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+		pool->setMax(1);
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+		session = pool->get(options);
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+		originalPid = session->getPid();
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+		session.reset();
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+		
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+		for (unsigned int i = 0; i < 4; i++) {
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+			session = pool->get(options);
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+			session->sendHeaders(createRequestHeaders());
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+			session->shutdownWriter();
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+			reader = session->getStream();
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+			readAll(reader);
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+			// Must explicitly call reset() here because we
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+			// want to close the session right now.
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+			session.reset();
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+			// In case of ApplicationPoolServer, we sleep here
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+			// for a little while to force a context switch to
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+			// the server, so that the session close event may
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+			// be processed.
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+			usleep(100000);
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+		}
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+		
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+		session = pool->get(options);
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+		ensure(session->getPid() != originalPid);
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+	}
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+	
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   // TODO: test maxIdleTime == 0
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 #endif /* USE_TEMPLATE */

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@@ -2,4 +2,8 @@ class FooController < ActionController::Base
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   def new
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     render :text => 'hello world'
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   end
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+
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+	def pid
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+		render :text => Process.pid.to_s
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+	end
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 end