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Re: [PATCH v7 21/21] nbd: assert that Error** is not NULL in nbd_iter_ch

From: Eric Blake
Subject: Re: [PATCH v7 21/21] nbd: assert that Error** is not NULL in nbd_iter_channel_error
Date: Thu, 5 Dec 2019 11:14:30 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 12/5/19 9:20 AM, Vladimir Sementsov-Ogievskiy wrote:
The local_err parameter is not here to return information about
nbd_iter_channel_error failure. Instead it's assumed to be filled when
passed to the function. This is already stressed by its name
(local_err, instead of classic errp). Stress it additionally by

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  block/nbd.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/block/nbd.c b/block/nbd.c
index 5f18f78a94..d085554f21 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -866,6 +866,7 @@ typedef struct NBDReplyChunkIter {
  static void nbd_iter_channel_error(NBDReplyChunkIter *iter,
                                     int ret, Error **local_err)
+    assert(local_err && *local_err);

Why are we forbidding grandparent callers from passing NULL when they want to ignore an error? We are called by several parent functions that get an errp from the grandparent, and use this function to do some common grunt work. Let's look at the possibilities:

1. grandparent passes address of a local error: we want to append to the error message, parent will then deal with the error how it wants (report it, ignore it, propagate it, whatever)

2. grandparent passes &error_fatal: we want to append a hint, but before ERRP_AUTO_PROPAGATE, the parent has already exited. After ERRP_AUTO_PROPAGATE, this looks like case 1.

3. grandparent passes &error_abort: we should never be reached (failure earlier in the parent should have already aborted) - true whether or not ERRP_AUTO_PROPAGATE is applied

4. grandparent passes NULL to ignore the error. Does not currently happen in any of the grandparent callers, because if it did, your assertion in this patch would now fire. After ERRP_AUTO_PROPAGATE, this would look like case 1.

Would it be better to assert(!local_err || *local_err)? The assertion as written is too strict without ERRP_AUTO_PROPAGATE, but you get away with it because none of the grandparents pass NULL; but is appropriate as written for after after the macro conversion so then we wonder if churn on the macro is worth it.

      assert(ret < 0);
if (!iter->ret) {

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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