[WIP] Fix #2869: Avoid unnecessary updates

[hensou]

• Keep output in a memory stream
• Only write to file if there is a change or if the file is new

Fix #2869

[alfonsogarciacaro]

As we are using a single buffer, if count = 0 we just return a reference to the buffer. But in the lines below:

                    let! nextBytes1 = readBytesAsync memoryStream
                    let! nextBytes2 = readBytesAsync fileStream 

nextBytes1 is going to be same as nextBytes2 (if count is 0 in both cases), because readBytesAsync returns the same array reference in both cases and the second readBytesAsync will overwrite the buffer.

[alfonsogarciacaro]

This is fantastic @hensou, thanks a lot for this! I've rewritten a bit the IsMemoryStreamEqualToFileAsync function to fix the issue with the shared buffer and simplify a bit the code. I hope that's ok with you.

        let IsMemoryStreamEqualToFileAsync() = async {
            let areStreamsEqualAsync (stream1: IO.Stream) (stream2: IO.Stream) =
                let buffer1 = Array.zeroCreate<byte> 1024
                let buffer2 = Array.zeroCreate<byte> 1024

                let rec areStreamsEqualAsync() = async {
                    let! count1 = stream1.AsyncRead(buffer1, 0, buffer1.Length)
                    let! count2 = stream2.AsyncRead(buffer2, 0, buffer2.Length)
                    match count1, count2 with
                    | 0, 0 -> return true
                    | count1, count2 when count1 = count2 && buffer1 = buffer2 ->
                        return! areStreamsEqualAsync()
                    | _ -> return false
                }

                areStreamsEqualAsync()

            memoryStream.Seek(0, IO.SeekOrigin.Begin) |> ignore
            use fileStream = IO.File.OpenRead(targetPath)

            return! areStreamsEqualAsync memoryStream fileStream
        }

[hensou]

This is fantastic @hensou, thanks a lot for this! I've rewritten a bit the IsMemoryStreamEqualToFileAsync function to fix the issue with the shared buffer and simplify a bit the code. I hope that's ok with you.

        let IsMemoryStreamEqualToFileAsync() = async {
            let areStreamsEqualAsync (stream1: IO.Stream) (stream2: IO.Stream) =
                let buffer1 = Array.zeroCreate<byte> 1024
                let buffer2 = Array.zeroCreate<byte> 1024

                let rec areStreamsEqualAsync() = async {
                    let! count1 = stream1.AsyncRead(buffer1, 0, buffer1.Length)
                    let! count2 = stream2.AsyncRead(buffer2, 0, buffer2.Length)
                    match count1, count2 with
                    | 0, 0 -> return true
                    | count1, count2 when count1 = count2 && buffer1 = buffer2 ->
                        return! areStreamsEqualAsync()
                    | _ -> return false
                }

                areStreamsEqualAsync()

            memoryStream.Seek(0, IO.SeekOrigin.Begin) |> ignore
            use fileStream = IO.File.OpenRead(targetPath)

            return! areStreamsEqualAsync memoryStream fileStream
        }

Awesome, I'm glad I could help.🙂
The rewritten it's totally fine, the way you did is way more simpler and elegant.
Thanks again.