[Libguestfs] [PATCH nbdkit] xz: Do not pass can_write through to the plugin.

[Richard W.M. Jones]

Using ‘nbdkit --filter=xz file disk.xz’ and loop mounting eventually
results in this error:

nbdkit: file[1]: debug: xz: pwrite count=4096 offset=1048576 flags=0x0
nbdkit: file[1]: debug: pwrite count=4096 offset=1048576 fua=0
nbdkit: file[1]: error: pwrite: Bad file descriptor
nbdkit: file[1]: debug: sending error reply: Bad file descriptor

Since the filter did not intercept can_write, it was passed through to
the layer below (the file plugin).  The filter opens the file plugin
passing readonly=1, but in the case where there are no filters, nbdkit
core would never call can_write() (assuming that because we opened the
server readonly, it would return false).  However with the filter,
can_write is called and returns the default (true).  This caused
writes to be generated which were also passed through to the plugin
which generates "Bad file descriptor" because:

 EBADF  fd is not a valid file descriptor or is not open for writing.

This change also disables zero and trim because the server logic
disables those when the top backend->can_write returns false.
---
 filters/xz/xz.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/filters/xz/xz.c b/filters/xz/xz.c
index 28a6a81..5ff791e 100644
--- a/filters/xz/xz.c
+++ b/filters/xz/xz.c
@@ -173,6 +173,13 @@ xz_get_size (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle)
   return xzfile_get_size (h->xz);
 }
 
+static int
+xz_can_write (struct nbdkit_next_ops *next_ops, void *nxdata,
+              void *handle)
+{
+  return 0;
+}
+
 /* Read data from the file. */
 static int
 xz_pread (struct nbdkit_next_ops *next_ops, void *nxdata,
@@ -225,6 +232,7 @@ static struct nbdkit_filter filter = {
   .close             = xz_close,
   .prepare           = xz_prepare,
   .get_size          = xz_get_size,
+  .can_write         = xz_can_write,
   .pread             = xz_pread,
 };
 
-- 
2.20.1