MailboxProcessor first loop can't run if program immediately fails - asynchronous

Question

I have a command running a SFTP check periodically and logging the result to a file.
let logPath = Path.Combine(config.["SharedFolder"],timestamp)
let sw = new StreamWriter(logPath,true)
//...
[<EntryPoint>]
let main argv =
try
sftpExample config.["SharedFolder"] config.["SFTPFolder"] 22 "usr" "pswd" |> ignore
with
| ex ->
ex.Message |> printerAgent.Post
printfn "%s" ex.Message // <- NOTICE THIS LINE
sw.Close()
sw.Dispose()
0
It loops over a MailboxProcessor
let printerAgent = MailboxProcessor.Start(fun inbox->
// the message processing function
let rec messageLoop() = async{
// read a message
let! msg = inbox.Receive()
// process a message
sw.WriteLine("{0}: {1}", DateTime.UtcNow.ToShortTimeString(), msg)
printfn "%s" msg
// loop to top
return! messageLoop()
}
// start the loop
messageLoop()
)
which is called to write the messages to the log
let sftpExample local host port username (password:string) =
async {
use client = new SftpClient(host, port, username, password)
client.Connect()
sprintf "Connected to %s\nroot dir list" host |> printerAgent.Post
do! downloadDir local client ""
sprintf "Done, disconnecting now" |> printerAgent.Post
client.Disconnect()
} |> Async.RunSynchronously
The file downloads are asynchronous, as well as the corresponding messages, but all appears to work well.
The problem is that - if, for some reasons, the sftp connection immediately fails, the MailboxProcessor has no time to log the exception message.
What I've tried to do - which is working indeed - was adding a printfn "%s" ex.Message before the end: I just wanted to know if someone envisions a better solution.
FYI, the full code is in this gist.

Answer 1

In fact, what you want is for the program to wait until the MailboxProcessor has finished handling all of its message queue before the program exits. Your printfn "%s" ex.Message seems to be working, but it's not guaranteed to work: if the MailboxProcessor had multiple items in its queue, the thread running the printfn function might finish before the MailboxProcessor's thread had had time to get through all of its messages.
The design I would recommend is to change the input of your printerAgent to be a DU like the following:
type printerAgentMsg =
| Message of string
| Shutdown
Then when you want the printer agent to finish sending its messages, use MailboxProcessor.PostAndReply (and note the usage example in the docs) in the main function and send it the Shutdown message. Remember that MailboxProcessor messages are queued: by the time it receives the Shutdown message, it will have already gone through the rest of the messages in the queue. So all it needs to do to handle the Shutdown message is to return a unit reply, and simply not call its loop again. And because you used PostAndReply rather than PostAndReplyAsync, the main function will block until the MailboxProcessor has finished doing all its work. (To avoid any chance of blocking forever, I'd recommend setting a timeout like 10 seconds in your PostAndReply call; the default timeout is -1, meaning wait forever).
EDIT: Here's an example (NOT tested, use at own risk) of what I mean:
type printerAgentMsg =
| Message of string
| Shutdown of AsyncReplyChannel<unit>
let printerAgent = MailboxProcessor.Start(fun inbox->
// the message processing function
let rec messageLoop() = async{
// read a message
let! msg = inbox.Receive()
// process a message
match msg with
| Message text ->
sw.WriteLine("{0}: {1}", DateTime.UtcNow.ToShortTimeString(), text)
printfn "%s" text
// loop to top
return! messageLoop()
| Shutdown replyChannel ->
replyChannel.Reply()
// We do NOT do return! messageLoop() here
}
// start the loop
messageLoop()
)
let logPath = Path.Combine(config.["SharedFolder"],timestamp)
let sw = new StreamWriter(logPath,true)
//...
[<EntryPoint>]
let main argv =
try
sftpExample config.["SharedFolder"] config.["SFTPFolder"] 22 "usr" "pswd" |> ignore
with
| ex ->
ex.Message |> Message |> printerAgent.Post
printfn "%s" ex.Message // <- NOTICE THIS LINE
printerAgent.PostAndReply( (fun replyChannel -> Shutdown replyChannel), 10000) // Timeout = 10000 ms = 10 seconds
sw.Close()
sw.Dispose()

Answer 2

Easiest solution would be to use normal (synchronous) function for logging instead of MailboxProcessor or use some logging framework and flush loggers in the end of the main function. If you want to keep using printingAgent you can implement "synchronous" mode like this:
type Msg =
| Log of string
| LogAndWait of string * AsyncReplyChannel<unit>
let printerAgent = MailboxProcessor.Start(fun inbox ->
let processLogMessage logMessage =
sw.WriteLine("{0}: {1}", DateTime.UtcNow.ToShortTimeString(), logMessage)
printfn "%s" logMessage
let rec messageLoop() = async{
let! msg = inbox.Receive()
match msg with
| Log logMessage ->
processLogMessage logMessage
| LogAndWait (logMessage, replyChannel) ->
processLogMessage logMessage
replyChannel.Reply()
return! messageLoop()
}
messageLoop()
)
Which you would then use either asynchronously
printerAgent.Post(Log "Message")
or synchronously
printerAgent.PostAndReply(fun channel -> LogAndWait("Message", channel))
You should use synchronous alternative when you log exception in the main function.