Add cancel support for file stream download

library/codex_thread_requests/requests/node_download_request.nim

@@ -10,10 +10,10 @@
 ##    - CHUNK: downloads the next chunk of the file
 ##    - CANCEL: cancels the download session
 ## 2. Via stream.
+##    - INIT: initializes the download session
 ##    - STREAM: downloads the file in a streaming manner, calling
 ## the onChunk handler for each chunk and / or writing to a file if filepath is set.
-## Cancel is supported in this mode because the worker will be busy
-## downloading the file so it cannot pickup another request to cancel the download.
+##    - CANCEL: cancels the download session
 
 import std/[options, streams]
 import chronos
@@ -105,11 +105,11 @@ proc init(
       return err("Failed to init the download: " & res.error.msg)
     stream = res.get()
   except CancelledError:
-    downloadSessions.del($cCid)
+    downloadSessions.del($cid)
     return err("Failed to init the download: download cancelled.")
 
   let blockSize = if chunkSize.int > 0: chunkSize.int else: DefaultBlockSize.int
-  downloadSessions[$cCid] = DownloadSession(stream: stream, chunkSize: blockSize)
+  downloadSessions[$cid] = DownloadSession(stream: stream, chunkSize: blockSize)
 
   return ok("")
 
@@ -161,23 +161,13 @@ proc chunk(
 
 proc streamData(
     codex: ptr CodexServer,
-    cid: Cid,
-    local: bool,
+    stream: LPStream,
     onChunk: OnChunkHandler,
     chunkSize: csize_t,
     filepath: cstring,
-): Future[Result[string, string]] {.async: (raises: [CancelledError]).} =
-  let node = codex[].node
-
-  let res = await node.retrieve(cid, local = local)
-  if res.isErr():
-    return err("Failed to retrieve CID: " & res.error.msg)
-
-  let stream = res.get()
-
-  if stream.atEof:
-    return err("Failed to retrieve CID: empty stream.")
-
+): Future[Result[string, string]] {.
+    async: (raises: [CancelledError, LPStreamError, IOError])
+.} =
   let blockSize = if chunkSize.int > 0: chunkSize.int else: DefaultBlockSize.int
   var buf = newSeq[byte](blockSize)
   var read = 0
@@ -189,6 +179,10 @@ proc streamData(
       outputStream = filedest.fileOutput()
 
     while not stream.atEof:
+      ## Yield immediately to the event loop
+      ## It gives a chance to cancel request to be processed
+      await sleepAsync(0)
+
       let read = await stream.readOnce(addr buf[0], buf.len)
       buf.setLen(read)
 
@@ -202,13 +196,9 @@ proc streamData(
 
     if outputStream != nil:
       outputStream.close()
-  except LPStreamError as e:
-    return err("Failed to stream file: " & $e.msg)
-  except IOError as e:
-    return err("Failed to write to file: " & $e.msg)
   finally:
-    await stream.close()
-    downloadSessions.del($cid)
+    if outputStream != nil:
+      outputStream.close()
 
   return ok("")
 
@@ -223,20 +213,37 @@ proc stream(
   ## Stream the file identified by cid, calling the onChunk handler for each chunk
   ## and / or writing to a file if filepath is set.
   ##
-  ## If local is true, the file will be retrived from the local store.
-
-  let node = codex[].node
+  ## If local is true, the file will be retrieved from the local store.
 
   let cid = Cid.init($cCid)
   if cid.isErr:
     return err("Failed to download locally: cannot parse cid: " & $cCid)
 
+  if not downloadSessions.contains($cid):
+    return err("Failed to download chunk: no session for cid " & $cid)
+
+  var session: DownloadSession
   try:
-    let res = await codex.streamData(cid.get(), local, onChunk, chunkSize, filepath)
+    session = downloadSessions[$cid]
+  except KeyError:
+    return err("Failed to download chunk: no session for cid " & $cid)
+
+  let node = codex[].node
+
+  try:
+    let res = await codex.streamData(session.stream, onChunk, chunkSize, filepath)
     if res.isErr:
       return err($res.error)
+  except LPStreamError as e:
+    return err("Failed to stream file: " & $e.msg)
+  except IOError as e:
+    return err("Failed to write to file: " & $e.msg)
   except CancelledError:
     return err("Failed to download locally: download cancelled.")
+  finally:
+    if session.stream != nil:
+      await session.stream.close()
+    downloadSessions.del($cid)
 
   return ok("")
 
@@ -247,14 +254,18 @@ proc cancel(
   ## This operation is not supported when using the stream mode,
   ## because the worker will be busy downloading the file.
 
-  if not downloadSessions.contains($cCid):
-    return err("Failed to download chunk: no session for cid " & $cCid)
+  let cid = Cid.init($cCid)
+  if cid.isErr:
+    return err("Failed to download locally: cannot parse cid: " & $cCid)
+
+  if not downloadSessions.contains($cid):
+    return err("Failed to download chunk: no session for cid " & $cid)
 
   var session: DownloadSession
   try:
-    session = downloadSessions[$cCid]
+    session = downloadSessions[$cid]
   except KeyError:
-    return err("Failed to download chunk: no session for cid " & $cCid)
+    return err("Failed to download chunk: no session for cid " & $cid)
 
   let stream = session.stream
   await stream.close()