do not block on send channels under high load (#19090)

all send channels must compete with `ctx` if not
they will perpetually stay alive.

cmd/erasure-object.go

@@ -937,7 +937,11 @@ func (er erasureObjects) getObjectFileInfo(ctx context.Context, bucket, object s
 		onlineDisks[i] = nil
 	}
 
-	mrfCheck <- fi.ShallowCopy()
+	select {
+	case mrfCheck <- fi.ShallowCopy():
+	case <-ctx.Done():
+		return fi, onlineMeta, onlineDisks, toObjectErr(ctx.Err(), bucket, object)
+	}
 
 	return fi, onlineMeta, onlineDisks, nil
 }

cmd/metacache-set.go

@@ -219,7 +219,10 @@ func (o *listPathOptions) gatherResults(ctx context.Context, in <-chan metaCache
 				// Do not return io.EOF
 				if resCh != nil {
 					resErr = nil
-					resCh <- results
+					select {
+					case resCh <- results:
+					case <-ctx.Done():
+					}
 					resCh = nil
 					returned = true
 				}

cmd/metacache-walk.go

@@ -299,7 +299,11 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 			// If directory entry on stack before this, pop it now.
 			for len(dirStack) > 0 && dirStack[len(dirStack)-1] < meta.name {
 				pop := dirStack[len(dirStack)-1]
-				out <- metaCacheEntry{name: pop}
+				select {
+				case <-ctx.Done():
+					return ctx.Err()
+				case out <- metaCacheEntry{name: pop}:
+				}
 				if opts.Recursive {
 					// Scan folder we found. Should be in correct sort order where we are.
 					err := scanDir(pop)
@@ -368,7 +372,11 @@ func (s *xlStorage) WalkDir(ctx context.Context, opts WalkDirOptions, wr io.Writ
 				return ctx.Err()
 			}
 			pop := dirStack[len(dirStack)-1]
-			out <- metaCacheEntry{name: pop}
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case out <- metaCacheEntry{name: pop}:
+			}
 			if opts.Recursive {
 				// Scan folder we found. Should be in correct sort order where we are.
 				logger.LogIf(ctx, scanDir(pop))

cmd/xl-storage.go

@@ -2869,14 +2869,22 @@ func (s *xlStorage) ReadMultiple(ctx context.Context, req ReadMultipleReq, resp
 		if req.MaxSize > 0 && int64(len(data)) > req.MaxSize {
 			r.Exists = true
 			r.Error = fmt.Sprintf("max size (%d) exceeded: %d", req.MaxSize, len(data))
-			resp <- r
-			continue
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case resp <- r:
+				continue
+			}
 		}
 		found++
 		r.Exists = true
 		r.Data = data
 		r.Modtime = mt
-		resp <- r
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case resp <- r:
+		}
 		if req.MaxResults > 0 && found >= req.MaxResults {
 			return nil
 		}