[libnbd PATCH 02/13] block_status: Refactor array storage

[Eric Blake]

For 32-bit block status, we were able to cheat and use an array with
an odd number of elements, with array[0] holding the context id, and
passing &array[1] to the user's callback.  But once we have 64-bit
extents, we can no longer abuse array element 0 like that.  Split out
a new state to receive the context id separately from the extents
array.  No behavioral change, other than the rare possibility of
landing in the new state.
---
 lib/internal.h                      |  1 +
 generator/state_machine.ml          | 11 +++++-
 generator/states-reply-structured.c | 58 ++++++++++++++++++++---------
 3 files changed, 51 insertions(+), 19 deletions(-)

diff --git a/lib/internal.h b/lib/internal.h
index 0e205aba..7e96e8e9 100644
--- a/lib/internal.h
+++ b/lib/internal.h
@@ -274,6 +274,7 @@ struct nbd_handle {
   size_t querynum;

   /* When receiving block status, this is used. */
+  uint32_t bs_contextid;
   uint32_t *bs_entries;

   /* Commands which are waiting to be issued [meaning the request
diff --git a/generator/state_machine.ml b/generator/state_machine.ml
index 3bc77f24..99652948 100644
--- a/generator/state_machine.ml
+++ b/generator/state_machine.ml
@@ -1,6 +1,6 @@
 (* hey emacs, this is OCaml code: -*- tuareg -*- *)
 (* nbd client library in userspace: state machine definition
- * Copyright (C) 2013-2020 Red Hat Inc.
+ * Copyright (C) 2013-2021 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -862,10 +862,17 @@ and
     external_events = [];
   };

+  State {
+    default_state with
+    name = "RECV_BS_CONTEXTID";
+    comment = "Receive contextid of structured reply block-status payload";
+    external_events = [];
+  };
+
   State {
     default_state with
     name = "RECV_BS_ENTRIES";
-    comment = "Receive a structured reply block-status payload";
+    comment = "Receive entries array of structured reply block-status payload";
     external_events = [];
   };

diff --git a/generator/states-reply-structured.c 
b/generator/states-reply-structured.c
index 70010474..e1da850d 100644
--- a/generator/states-reply-structured.c
+++ b/generator/states-reply-structured.c
@@ -1,5 +1,5 @@
 /* nbd client library in userspace: state machine
- * Copyright (C) 2013-2019 Red Hat Inc.
+ * Copyright (C) 2013-2021 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -185,19 +185,10 @@ STATE_MACHINE {
       set_error (0, "not expecting NBD_REPLY_TYPE_BLOCK_STATUS here");
       return 0;
     }
-    /* We read the context ID followed by all the entries into a
-     * single array and deal with it at the end.
-     */
-    free (h->bs_entries);
-    h->bs_entries = malloc (length);
-    if (h->bs_entries == NULL) {
-      SET_NEXT_STATE (%.DEAD);
-      set_error (errno, "malloc");
-      return 0;
-    }
-    h->rbuf = h->bs_entries;
-    h->rlen = length;
-    SET_NEXT_STATE (%RECV_BS_ENTRIES);
+    /* Start by reading the context ID. */
+    h->rbuf = &h->bs_contextid;
+    h->rlen = sizeof h->bs_contextid;
+    SET_NEXT_STATE (%RECV_BS_CONTEXTID);
     return 0;
   }
   else {
@@ -452,9 +443,41 @@ STATE_MACHINE {
   }
   return 0;

+ REPLY.STRUCTURED_REPLY.RECV_BS_CONTEXTID:
+  struct command *cmd = h->reply_cmd;
+  uint32_t length;
+
+  switch (recv_into_rbuf (h)) {
+  case -1: SET_NEXT_STATE (%.DEAD); return 0;
+  case 1:
+    save_reply_state (h);
+    SET_NEXT_STATE (%.READY);
+    return 0;
+  case 0:
+    length = be32toh (h->sbuf.sr.structured_reply.length);
+
+    assert (cmd); /* guaranteed by CHECK */
+    assert (cmd->type == NBD_CMD_BLOCK_STATUS);
+    assert (length >= 12);
+    length -= sizeof h->bs_contextid;
+
+    free (h->bs_entries);
+    h->bs_entries = malloc (length);
+    if (h->bs_entries == NULL) {
+      SET_NEXT_STATE (%.DEAD);
+      set_error (errno, "malloc");
+      return 0;
+    }
+    h->rbuf = h->bs_entries;
+    h->rlen = length;
+    SET_NEXT_STATE (%RECV_BS_ENTRIES);
+  }
+  return 0;
+
  REPLY.STRUCTURED_REPLY.RECV_BS_ENTRIES:
   struct command *cmd = h->reply_cmd;
   uint32_t length;
+  uint32_t count;
   size_t i;
   uint32_t context_id;
   struct meta_context *meta_context;
@@ -473,15 +496,16 @@ STATE_MACHINE {
     assert (CALLBACK_IS_NOT_NULL (cmd->cb.fn.extent));
     assert (h->bs_entries);
     assert (length >= 12);
+    count = (length - sizeof h->bs_contextid) / sizeof *h->bs_entries;

     /* Need to byte-swap the entries returned, but apart from that we
      * don't validate them.
      */
-    for (i = 0; i < length/4; ++i)
+    for (i = 0; i < count; ++i)
       h->bs_entries[i] = be32toh (h->bs_entries[i]);

     /* Look up the context ID. */
-    context_id = h->bs_entries[0];
+    context_id = be32toh (h->bs_contextid);
     for (meta_context = h->meta_contexts;
          meta_context;
          meta_context = meta_context->next)
@@ -494,7 +518,7 @@ STATE_MACHINE {

       if (CALL_CALLBACK (cmd->cb.fn.extent,
                          meta_context->name, cmd->offset,
-                         &h->bs_entries[1], (length-4) / 4,
+                         h->bs_entries, count,
                          &error) == -1)
         if (cmd->error == 0)
           cmd->error = error ? error : EPROTO;
-- 
2.33.1