Safe yet correct readpipe closer

Author: ribasushi

core/commands/dag/dag.go

@@ -275,17 +275,17 @@ The output of blocks happens in strict DAG-traversal, first-seen, order.
 	},
 	Run: func(req *cmds.Request, res cmds.ResponseEmitter, env cmds.Environment) error {
 
-		c, err := cid.Decode(req.Arguments[0])
-		if err != nil {
+		c, cidErr := cid.Decode(req.Arguments[0])
+		if cidErr != nil {
 			return fmt.Errorf(
 				"unable to parse root specification (currently only bare CIDs are supported): %s",
-				err,
+				cidErr,
 			)
 		}
 
-		node, err := cmdenv.GetNode(env)
-		if err != nil {
-			return err
+		node, nodeErr := cmdenv.GetNode(env)
+		if nodeErr != nil {
+			return nodeErr
 		}
 
 		// Code disabled until descent-issue in go-ipld-prime is fixed
@@ -307,6 +307,24 @@ The output of blocks happens in strict DAG-traversal, first-seen, order.
 		// if err := car.Write(pipeW); err != nil {}
 
 		pipeR, pipeW := io.Pipe()
+		// this is so convoluted because we:
+		// - want to close the readpipe on error so that the goroutine dies
+		// - can not do so on success, because during a no-deamon process
+		//   scenario the PostRun will not be able to grab the EOF from the
+		//   now-closed pipe
+		//
+		// Skipping this check results in:
+		//
+		// 		cmd/ipfs/ipfs dag export QmdmQXB2mzChmMeKY47C43LxUdg1NDJ5MWcKMKxDu7RgQm >/dev/null
+		//  0s  0 B / ? [------------------------------------------------------------------------=]Error: io: read/write on closed pipe
+		//  1s  107.08 MiB / ? [-----------------------------------------=----------] 106.59 MiB/s
+		//
+		var emittingError error
+		defer func() {
+			if emittingError != nil {
+				pipeR.Close()
+			}
+		}()
 
 		errCh := make(chan error, 2) // we only report the 1st error
 		go func() {
@@ -330,21 +348,20 @@ The output of blocks happens in strict DAG-traversal, first-seen, order.
 			}
 		}()
 
-		if err := res.Emit(pipeR); err != nil {
-			pipeR.Close() // ignore the error if any
-			return err
+		if emittingError = res.Emit(pipeR); emittingError != nil {
+			return emittingError
 		}
 
-		err = <-errCh
+		emittingError = <-errCh
 
 		// minimal user friendliness
-		if err != nil &&
+		if emittingError != nil &&
 			!node.IsOnline &&
-			err == ipld.ErrNotFound {
-			err = fmt.Errorf("%s (currently offline, perhaps retry after attaching to the network)", err)
+			emittingError == ipld.ErrNotFound {
+			emittingError = fmt.Errorf("%s (currently offline, perhaps retry after attaching to the network)", emittingError)
 		}
 
-		return err
+		return emittingError
 	},
 	PostRun: cmds.PostRunMap{
 		cmds.CLI: func(res cmds.Response, re cmds.ResponseEmitter) error {