test: fix box/net_box hang when run after box/access_escalation

box/access_escalation cancels a fiber that is waiting for a call over
iproto to complete, but the fiber that is executing the call keeps
running. As a result, if box/net_box is run after this test, it will
hang, because it expects that the fiber pool is idle when it starts.
Fix this by making the call function return in box/access_escalation.

test/box/access_escalation.result

@@ -26,7 +26,10 @@ os = require('os')
 --  The invoked function should get "Access denied" error,
 --  there should be no privilege escalation.
 -- define functions
-function setuid() fiber.sleep(1000000) end
+channel = fiber.channel(1)
+---
+...
+function setuid() channel:get() end
 ---
 ...
 function escalation() return box.space._space:get{box.schema.SPACE_ID} ~= nil end
@@ -55,8 +58,9 @@ connection:call("escalation")
 ---
 - error: Read access to space '_space' is denied for user 'guest'
 ...
-fiber.cancel(background)
+channel:put(true)
 ---
+- true
 ...
 --
 -- tear down the functions; the grants are dropped recursively
@@ -112,8 +116,9 @@ connection:call("escalation")
 ---
 - true
 ...
-fiber.cancel(background)
+channel:put(true)
 ---
+- true
 ...
 -- tear down
 box.schema.user.drop('underprivileged')

test/box/access_escalation.test.lua

@@ -20,7 +20,8 @@ os = require('os')
 
 -- define functions
 
-function setuid() fiber.sleep(1000000) end
+channel = fiber.channel(1)
+function setuid() channel:get() end
 
 function escalation() return box.space._space:get{box.schema.SPACE_ID} ~= nil end
 
@@ -37,7 +38,7 @@ connection = net:connect(os.getenv("LISTEN"))
 
 background = fiber.create(function() connection:call("setuid") end)
 connection:call("escalation")
-fiber.cancel(background)
+channel:put(true)
 
 --
 -- tear down the functions; the grants are dropped recursively
@@ -75,7 +76,7 @@ connection = net:connect(os.getenv("LISTEN"))
 
 background = fiber.create(function() connection:call("setuid") end)
 connection:call("escalation")
-fiber.cancel(background)
+channel:put(true)
 
 -- tear down