Concurrent file write causes content overlap

[mykytamykhailenko]

I have developed an app, which executes a few heavy queries, which yield a lot of data so I decided to write the query results to multiple files concurrently.

I create a FileChannel for each separate query and write the results concurrently.

    def acquireOutputStream(path: Path) = ZIO.acquireRelease(FileChannel.open(path, options: _*)) { stream =>
      stream.flatMapBlocking(_.force(metadata = true)).orDie *>
        stream.close.orDie
    }

    // Writes data to file 
    def writeStream(stream: Stream[Throwable, Result], os: FileChannel) = ???

    def runQuery(query: String, path: Path): RIO[DatabaseProvider, Path] = ZIO.scoped {
        acquireOutputStream(path).flatMap { os =>
            ZIO.fromStreamingDBIO(SQLActionBuilder(query, (_, _) => ()).as[Result]).map(writeStream(_, os)).runDrain
        }
    }

   // Some code
   val concurrentQueries: List[RIO[DatabaseProvider, Path]] = ???

   ZIO.collectAllPar(concurrentQueries)

When I execute the code sequentially, everything works as expected (using ZIO.collectAll), but once I use ZIO.collectAllPar, content from one file can appear in the other.

The question is: even though I do not leak FileChannel anywhere, the content from one file can appear in there other (they will be mixed together as if a few streams use the same FileChannel), how can it be fixed?

Thanks for help in advance!

[mykytamykhailenko]

@pablf if you have a spare minute, could you help please🙏