gistfile1.diff

diff --git a/include/parrot/io.h b/include/parrot/io.h
index 42b9f44..9fdfd2f 100644
--- a/include/parrot/io.h
+++ b/include/parrot/io.h
@@ -63,6 +63,8 @@
 #define PIO_VF_AWAYS_READABLE       0x0010  /* Handle can always be read               */
 #define PIO_VF_MULTI_READABLE       0x0020  /* Can perform multiple low-level read
                                                operations to satisfy a large request   */
+#define PIO_VF_SYNC_IO              0x0040  /* This type needs synchronization
+                                               before r/w operations */ 
 
 /*
  * pioctl argument constants. These don't have to
diff --git a/src/io/filehandle.c b/src/io/filehandle.c
index 8167f3a..a769336 100644
--- a/src/io/filehandle.c
+++ b/src/io/filehandle.c
@@ -228,7 +228,8 @@ io_filehandle_setup_vtable(PARROT_INTERP, ARGMOD_NULLOK(IO_VTABLE *vtable), INTV
     vtable->flags = PIO_VF_DEFAULT_READ_BUF
                   | PIO_VF_DEFAULT_WRITE_BUF
                   | PIO_VF_MULTI_READABLE
-                  | PIO_VF_FLUSH_ON_CLOSE;
+                  | PIO_VF_FLUSH_ON_CLOSE
+                  | PIO_VF_SYNC_IO;
     vtable->name = "FileHandle";
     vtable->read_b = io_filehandle_read_b;
     vtable->write_b = io_filehandle_write_b;
diff --git a/src/io/stringhandle.c b/src/io/stringhandle.c
index ded7af2..5e9b0c5 100644
--- a/src/io/stringhandle.c
+++ b/src/io/stringhandle.c
@@ -230,8 +230,9 @@ io_stringhandle_setup_vtable(PARROT_INTERP, ARGMOD_NULLOK(IO_VTABLE *vtable), IN
     vtable->flags =
         PIO_VF_PATH_NOT_REQUIRED |  /* StringHandle does not require a path to open */
         PIO_VF_MULTI_READABLE |     /* Multiple reads will not hang */
-        PIO_VF_AWAYS_READABLE;      /* StringHandle can be read when closed or not in
+        PIO_VF_AWAYS_READABLE |     /* StringHandle can be read when closed or not in
                                        read mode */
+        PIO_VF_SYNC_IO;
     vtable->name = "StringHandle";
     vtable->read_b = io_stringhandle_read_b;
     vtable->write_b = io_stringhandle_write_b;
diff --git a/src/io/utilities.c b/src/io/utilities.c
index 0891607..d710895 100644
--- a/src/io/utilities.c
+++ b/src/io/utilities.c
@@ -513,7 +513,7 @@ io_sync_buffers_for_read(PARROT_INTERP, ARGMOD(PMC *handle),
        and advance the cursor before we attempt to read. Otherwise we'll be
        reading data from the buffer that has already been overwritten, from a
        position several bytes before where we're supposed to be. */
-    if (write_buffer && !BUFFER_IS_EMPTY(write_buffer)) {
+    if (write_buffer && !BUFFER_IS_EMPTY(write_buffer) && (vtable->flags & PIO_VF_SYNC_IO)) {
         const size_t bytes_written = Parrot_io_buffer_flush(interp, write_buffer,
                                         handle, vtable);
         PARROT_ASSERT(BUFFER_IS_EMPTY(write_buffer));
@@ -548,7 +548,7 @@ io_sync_buffers_for_write(PARROT_INTERP, ARGMOD(PMC *handle),
        current cursor. We need to clear the read buffer and reset the on-disk
        position to match what we expect it to be, so the written data goes
        to the correct place */
-    if (read_buffer && !BUFFER_IS_EMPTY(read_buffer)) {
+    if (read_buffer && !BUFFER_IS_EMPTY(read_buffer) && (vtable->flags & PIO_VF_SYNC_IO)) {
         const size_t buffer_size = BUFFER_USED_SIZE(read_buffer);
         Parrot_io_buffer_clear(interp, read_buffer);
         vtable->seek(interp, handle, -(PIOOFF_T)buffer_size, SEEK_CUR);