backend: Implement next_ops->reopen call.

This is intended for use by the forthcoming retry filter to close and
reopen the backend chain.  It is handled entirely by server/backend.c
as no cooperation is needed with the plugin.

Note the explicit readonly parameter: An alternative would be to store
the previous readonly setting in the b_conn_handle struct.  However
passing it explicitly allows the retry filter to retry as readonly,
which might be useful.  This design does however require any filter
which might call .reopen to save the original readonly parameter from
the .open call.

docs/nbdkit-filter.pod

@@ -161,6 +161,10 @@ C<nbdkit_set_error> or setting C<errno>), whereas
 C<next_ops-E<gt>pread> exposes this via an explicit parameter,
 allowing a filter to learn or modify this error if desired.
 
+There is also a C<next_ops-E<gt>reopen> function which is used by
+L<nbdkit-retry-filter(3)> to close and reopen the underlying plugin.
+It should be used with caution because it is difficult to use safely.
+
 =head2 Other considerations
 
 You can modify parameters when you call the C<next> function.  However

include/nbdkit-filter.h

@@ -68,6 +68,12 @@ typedef int nbdkit_next_open (void *nxdata,
                               int readonly);
 
 struct nbdkit_next_ops {
+  /* Performs close + open on the underlying chain.
+   * Used by the retry filter.
+   */
+  int (*reopen) (void *nxdata, int readonly);
+
+  /* The rest of the next ops are the same as normal plugin operations. */
   int64_t (*get_size) (void *nxdata);
 
   int (*can_write) (void *nxdata);

server/backend.c

@@ -233,6 +233,18 @@ backend_valid_range (struct backend *b, struct connection *conn,
 
 /* Wrappers for all callbacks in a filter's struct nbdkit_next_ops. */
 
+int
+backend_reopen (struct backend *b, struct connection *conn, int readonly)
+{
+  struct b_conn_handle *h = &conn->handles[b->i];
+
+  debug ("%s: reopen readonly=%d", b->name, readonly);
+
+  if (h->handle != NULL)
+    backend_close (b, conn);
+  return backend_open (b, conn, readonly);
+}
+
 int64_t
 backend_get_size (struct backend *b, struct connection *conn)
 {

server/filters.c

@@ -237,6 +237,13 @@ filter_close (struct backend *b, struct connection *conn)
  * single ‘void *nxdata’ struct pointer (‘b_conn’).
  */
 
+static int
+next_reopen (void *nxdata, int readonly)
+{
+  struct b_conn *b_conn = nxdata;
+  return backend_reopen (b_conn->b, b_conn->conn, readonly);
+}
+
 static int64_t
 next_get_size (void *nxdata)
 {
@@ -373,6 +380,7 @@ next_cache (void *nxdata, uint32_t count, uint64_t offset,
 }
 
 static struct nbdkit_next_ops next_ops = {
+  .reopen = next_reopen,
   .get_size = next_get_size,
   .can_write = next_can_write,
   .can_flush = next_can_flush,

server/internal.h

@@ -360,6 +360,9 @@ extern bool backend_valid_range (struct backend *b, struct connection *conn,
                                  uint64_t offset, uint32_t count)
   __attribute__((__nonnull__ (1, 2)));
 
+extern int backend_reopen (struct backend *b, struct connection *conn,
+                           int readonly)
+  __attribute__((__nonnull__ (1, 2)));
 extern int64_t backend_get_size (struct backend *b, struct connection *conn)
   __attribute__((__nonnull__ (1, 2)));
 extern int backend_can_write (struct backend *b, struct connection *conn)