{-# LANGUAGE ScopedTypeVariables #-}
{-|
Module      : Instana.SDK.Internal.AgentConnection.ConnectLoop
Description : Establishes a connection to the agent.
-}
module Instana.SDK.Internal.AgentConnection.ConnectLoop
    ( initConnectLoop
    ) where


import qualified Control.Concurrent                                   as Concurrent
import qualified Control.Concurrent.STM                               as STM
import           Control.Exception                                    (SomeException,
                                                                       catch)
import           Control.Monad                                        (forever)
import           Data.Maybe                                           (isNothing)
import           Data.Text                                            (Text)
import qualified Data.Text                                            as T
import qualified Data.Text.IO                                         as TextIO
import qualified System.Environment                                   as Environment
import           System.Log.Logger                                    (debugM,
                                                                       infoM,
                                                                       warningM)
import qualified System.Posix.Process                                 as PosixProcess

import qualified Instana.SDK.Internal.AgentConnection.AgentHostLookup as AgentHostLookup
import           Instana.SDK.Internal.AgentConnection.ProcessInfo     (ProcessInfo (ProcessInfo))
import qualified Instana.SDK.Internal.AgentConnection.ProcessInfo     as ProcessInfo
import           Instana.SDK.Internal.AgentConnection.SchedFile       (parsePidFromSchedFile)
import           Instana.SDK.Internal.Context                         (ConnectionState (..),
                                                                       InternalContext)
import qualified Instana.SDK.Internal.Context                         as InternalContext
import           Instana.SDK.Internal.Logging                         (instanaLogger)


{-| Kick of a thread that loops endlessly and checks once in a while if the
agent connection is still up. If not, a connection attempt will be initiated.
The first attempt is made immediately when calling this.
-}
initConnectLoop :: InternalContext -> IO ()
initConnectLoop context = do
  pid <- PosixProcess.getProcessID
  progName <- Environment.getProgName
  execPath <- Environment.getExecutablePath
  args <- Environment.getArgs
  let
    pidStr = show pid
  cpuSetFileContent <- getCpuSetFileContent pidStr
  parentNsPid <- getPidFromParentNamespace pidStr
  let
    processInfo =
      ProcessInfo
        { ProcessInfo.pidString         = pidStr
        , ProcessInfo.programName       = progName
        , ProcessInfo.executablePath    = execPath
        , ProcessInfo.arguments         = args
        , ProcessInfo.cpuSetFileContent = cpuSetFileContent
        , ProcessInfo.parentNsPid       = parentNsPid
        }
  if isNothing parentNsPid then
    warningM instanaLogger $ "Could not parse PID from sched file. " ++
             "Discovery might not work if this process is running inside a " ++
             "container."
  else
    if (Just pidStr) == parentNsPid then
      debugM instanaLogger $
        "PID in sched file matches process PID. Probably not running inside " ++
        "a PID namespace"
    else do
      let
        Just parentPid = parentNsPid
      infoM instanaLogger $ "Changing PID from " ++ pidStr ++ " to " ++
            parentPid ++
            " due to successful identification of PID in parent namespace."
  debugM instanaLogger $ "discovered process info " ++ show processInfo

  -- connection loop works as follows:
  -- - try to connect to an an agent at either the agent host/port received via
  -- configuration, environment variables, default (127.0.0.1:42699) or default
  -- gateway
  -- - establishAgentConnection tries to connect to the agent by issuing
  --   a POST to /com.instana.plugin.haskell.discovery
  -- - establishAgentConnection only ever terminates if it has been successful,
  --   then we have switched to announced state.
  -- - after that, establishAgentConnection is called every 5 seconds,
  -- - if the connection is still up, establishAgentConnection does nothing and
  --   returns immediately,
  -- - should the connection have been lost, the cycle starts again, that is,
  --   establishAgentConnection will retry the POST forever and only terminate
  --   after success.
  forever $ do
    establishAgentConnectionSafe context processInfo
    Concurrent.threadDelay $ 5 * 1000 * 1000


establishAgentConnectionSafe ::
  InternalContext
  -> ProcessInfo
  -> IO ()
establishAgentConnectionSafe context processInfo =
  catch
    (establishAgentConnection context processInfo)
    -- exceptions in establishAgentConnection must not kill the loop, so we just
    -- catch everything
    (\e -> warningM instanaLogger $ show (e :: SomeException))


establishAgentConnection ::
  InternalContext
  -> ProcessInfo
  -> IO ()
establishAgentConnection context processInfo = do
  currentState <- STM.atomically $
    STM.readTVar (InternalContext.connectionState context)

  -- debugM instanaLogger $
  --   "Checking agent connection, current state is " ++ show currentState

  -- Do nothing if a connection attempt is already in progress or connection has
  -- already been established.
  if currentState /= Unconnected
    then
      return ()
    else do
      STM.atomically $ STM.writeTVar
        (InternalContext.connectionState context)
        AgentHostLookup
      debugM instanaLogger $
        "Agent connection is not up, attempting (re)connect"
      -- Initial status: Unconnected
      -- step 1: do agent host looup (retry forever until an agent has
      --         been found)
      -- New status: Unannounced
      -- step 2: announce request (retry 3 times with 200 ms delay)
      -- New status: Announced
      -- step 3: check whether agent is ready to accept data (retry 10 times
      --         with 10 second delay)
      -- New status: Connected
      -- If anything fails in between, go back to "Unconnected"
      AgentHostLookup.lookupAgentHost context processInfo


getCpuSetFileContent :: String -> IO (Maybe Text)
getCpuSetFileContent pidStr = do
  let
    cpuSetPath = "/proc/" ++ pidStr ++ "/cpuset"
  catch
    ( do
      content <- TextIO.readFile cpuSetPath
      -- paranoid check - if the cpusets file for whatever reason is really big,
      -- we don't want to send it to the agent at all.
      if T.length content >= 2000 then
        return Nothing
      else
        return $ Just content
    )
    (\(_ :: SomeException) -> do
      debugM instanaLogger $ "Can't read " ++ cpuSetPath ++ ", process " ++
           "is probably not running in a container."
      return Nothing
    )


getPidFromParentNamespace :: String -> IO (Maybe String)
getPidFromParentNamespace pidStr = do
  let
    schedFilePath = "/proc/" ++ pidStr ++ "/sched"
  catch
    ( do
        schedFileContent <- readFile schedFilePath
        return $ parsePidFromSchedFile schedFileContent
    )
    (\(_ :: SomeException) -> do
      debugM instanaLogger $ "Can't read " ++ schedFilePath ++ "."
      return Nothing
    )