Add REQUEST_TIMEOUT environment variable

docs/environment.rst

@@ -294,11 +294,20 @@ raise them.
   The default value :const:`5,000` bytes, meaning the bandwidth cost assigned to a response
   of 100KB is 20.  If your bandwidth is cheap you should probably raise this.
 
+.. envvar:: REQUEST_TIMEOUT
+
+  An integer number of seconds defaulting to :const:`15`.  If a request takes longer than
+  this to respond to, either because of request limiting or because the request is
+  expensive, the server rejects it and returns a timeout error to the client indicating
+  that the server is busy.
+
+  This can help prevent large backlogs of unprocessed requests building up under heavy load.
+
 .. envvar:: SESSION_TIMEOUT
 
-  An integer number of seconds defaulting to 600.  Sessions that have not sent a request
+  An integer number of seconds defaulting to :const:`600`.  Sessions that have not sent a
-  for longer than this are disconnected.  Properly functioning clients should send a
+  request for longer than this are disconnected.  Properly functioning clients should send
-  :func:`server.ping` request once roughly 450 seconds have passed since the previous
+  a :func:`server.ping` request once roughly 450 seconds have passed since the previous
   request, in order to avoid disconnection.
 
 

electrumx/server/env.py

@@ -73,6 +73,7 @@ class Env(EnvBase):
         self.bw_unit_cost = self.integer('BANDWIDTH_UNIT_COST', 5000)
         self.initial_concurrent = self.integer('INITIAL_CONCURRENT', 10)
         self.request_sleep = self.integer('REQUEST_SLEEP', 2500)
+        self.request_timeout = self.integer('REQUEST_TIMEOUT', 15)
         self.session_timeout = self.integer('SESSION_TIMEOUT', 600)
         self.drop_client = self.custom("DROP_CLIENT", None, re.compile)
         self.blacklist_url = self.default('BLACKLIST_URL', self.coin.BLACKLIST_URL)

electrumx/server/session.py

@@ -487,6 +487,7 @@ class SessionManager(object):
             session_class.bw_cost_per_byte = 1.0 / self.env.bw_unit_cost
             session_class.cost_sleep = self.env.request_sleep / 1000
             session_class.initial_concurrent = self.env.initial_concurrent
+            session_class.processing_timeout = self.env.request_timeout
 
             self.logger.info(f'max session count: {self.env.max_sessions:,d}')
             self.logger.info(f'session timeout: {self.env.session_timeout:,d} seconds')
@@ -494,6 +495,7 @@ class SessionManager(object):
             self.logger.info(f'session cost soft limit {self.env.cost_soft_limit:,d}')
             self.logger.info(f'bandwidth unit cost {self.env.bw_unit_cost:,d}')
             self.logger.info(f'request sleep {self.env.request_sleep:,d}ms')
+            self.logger.info(f'request timeout {self.env.request_timeout:,d}s')
             self.logger.info(f'initial concurrent {self.env.initial_concurrent:,d}')
 
             self.logger.info(f'max response size {self.env.max_send:,d} bytes')

tests/server/test_env.py

@@ -236,6 +236,10 @@ def test_MAX_SESSIONS():
     # Cannot test default as it may be lowered by the open file limit cap
 
 
+def test_REQUEST_TIMEOUT():
+    assert_integer('REQUEST_TIMEOUT', 'request_timeout', 15)
+
+
 def test_SESSION_TIMEOUT():
     assert_integer('SESSION_TIMEOUT', 'session_timeout', 600)