memfs: When using O_CREATE, the file is allowed to exist, unless O_EXCL

OpenFile currently gives an error if O_CREATE is passed and the file
already exists. This is not how O_CREATE is meant to work. O_CREATE
requests the file to be created if it does not exist. However, if O_EXCL
is also set, then the file must not exist.

Implement and test this behavior.

Author: dsharp-pivotal

memfs/memfs.go

@@ -229,13 +229,9 @@ func (fs *MemFS) OpenFile(name string, flag int, perm os.FileMode) (vfs.File, er
 		return nil, &os.PathError{"open", name, err}
 	}
 
-	if hasFlag(os.O_CREATE, flag) {
-		if fiNode != nil {
-
-			// If O_TRUNC is set, existing file is overwritten
-			if !hasFlag(os.O_TRUNC, flag) {
-				return nil, &os.PathError{"open", name, os.ErrExist}
-			}
+	if fiNode == nil {
+		if !hasFlag(os.O_CREATE, flag) {
+			return nil, &os.PathError{"open", name, os.ErrNotExist}
 		}
 		fiNode = &fileInfo{
 			name:    base,
@@ -246,14 +242,15 @@ func (fs *MemFS) OpenFile(name string, flag int, perm os.FileMode) (vfs.File, er
 			fs:      fs,
 		}
 		fiParent.childs[base] = fiNode
-	} else { // find existing
-		if fiNode == nil {
-			return nil, &os.PathError{"open", name, os.ErrNotExist}
+	} else { // file exists
+		if hasFlag(os.O_CREATE|os.O_EXCL, flag) {
+			return nil, &os.PathError{"open", name, os.ErrExist}
 		}
 		if fiNode.dir {
 			return nil, &os.PathError{"open", name, ErrIsDirectory}
 		}
 	}
+
 	if !hasFlag(os.O_RDONLY, flag) {
 		fiNode.modTime = time.Now()
 	}

memfs/memfs_test.go

@@ -26,13 +26,29 @@ func TestCreate(t *testing.T) {
 		}
 	}
 
-	// Create same file twice, no error because os.O_TRUNC is used
+	// Create same file again
+	{
+		_, err := fs.OpenFile("/testfile", os.O_RDWR|os.O_CREATE, 0666)
+		if err != nil {
+			t.Fatalf("Unexpected error creating file: %s", err)
+		}
+
+	}
+
+	// Create same file again, but truncate it
 	{
 		_, err := fs.OpenFile("/testfile", os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)
 		if err != nil {
-			t.Fatalf("Expected error creating file: %s", err)
+			t.Fatalf("Unexpected error creating file: %s", err)
 		}
+	}
 
+	// Create same file again with O_CREATE|O_EXCL, which is an error
+	{
+		_, err := fs.OpenFile("/testfile", os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+		if err == nil {
+			t.Fatalf("Expected error creating file: %s", err)
+		}
 	}
 
 	// Create file with unkown parent