block/nbd-client: use traces instead of noisy error_report_err

Reduce extra noise of nbd-client, change 083 correspondingly.

In various commits (be41c100 in 2.10, f140e300 in 2.11, 78a33ab
in 2.12), we added spots where qemu as an NBD client would report
problems communicating with the server to stderr, because there
was no where else to send the error to.  However, this is racy,
particularly since the most common source of these errors is when
either the client or the server abruptly hangs up, leaving one
coroutine to report the error only if it wins (or loses) the
race in attempting the read from the server before another
thread completes its cleanup of a protocol error that caused the
disconnect in the first place.  The race is also apparent in the
fact that differences in the flush behavior of the server can
alter the frequency of encountering the race in the client (see
commit 6d39db96).

Rather than polluting stderr, it's better to just trace these
situations, for use by developers debugging a flaky connection,
particularly since the real error that either triggers the abrupt
disconnection in the first place, or that results from the EIO
when a request can't receive a reply, DOES make it back to the
user in the normal Error propagation channels.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181102151152.288399-4-vsementsov@virtuozzo.com>
[eblake: drop depedence on error hint, enhance commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>

2 files changed, 23 insertions, 4 deletions

diff --git a/block/nbd-client.c b/block/nbd-client.c
index fc5b7eda8e..ef32075971 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -28,6 +28,8 @@
  */
 
 #include "qemu/osdep.h"
+
+#include "trace.h"
 #include "qapi/error.h"
 #include "nbd-client.h"
 
@@ -79,7 +81,8 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
         assert(s->reply.handle == 0);
         ret = nbd_receive_reply(s->ioc, &s->reply, &local_err);
         if (local_err) {
-            error_report_err(local_err);
+            trace_nbd_read_reply_entry_fail(ret, error_get_pretty(local_err));
+            error_free(local_err);
         }
         if (ret <= 0) {
             break;
@@ -771,7 +774,11 @@ static int nbd_co_request(BlockDriverState *bs, NBDRequest *request,
 
     ret = nbd_co_receive_return_code(client, request->handle, &local_err);
     if (local_err) {
-        error_report_err(local_err);
+        trace_nbd_co_request_fail(request->from, request->len, request->handle,
+                                  request->flags, request->type,
+                                  nbd_cmd_lookup(request->type),
+                                  ret, error_get_pretty(local_err));
+        error_free(local_err);
     }
     return ret;
 }
@@ -802,7 +809,11 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
     ret = nbd_co_receive_cmdread_reply(client, request.handle, offset, qiov,
                                        &local_err);
     if (local_err) {
-        error_report_err(local_err);
+        trace_nbd_co_request_fail(request.from, request.len, request.handle,
+                                  request.flags, request.type,
+                                  nbd_cmd_lookup(request.type),
+                                  ret, error_get_pretty(local_err));
+        error_free(local_err);
     }
     return ret;
 }
@@ -925,7 +936,11 @@ int coroutine_fn nbd_client_co_block_status(BlockDriverState *bs,
     ret = nbd_co_receive_blockstatus_reply(client, request.handle, bytes,
                                            &extent, &local_err);
     if (local_err) {
-        error_report_err(local_err);
+        trace_nbd_co_request_fail(request.from, request.len, request.handle,
+                                  request.flags, request.type,
+                                  nbd_cmd_lookup(request.type),
+                                  ret, error_get_pretty(local_err));
+        error_free(local_err);
     }
     if (ret < 0) {
         return ret;
diff --git a/block/trace-events b/block/trace-events
index 3e8c47bb24..693c14c443 100644
--- a/block/trace-events
+++ b/block/trace-events
@@ -156,3 +156,7 @@ nvme_cmd_map_qiov_iov(void *s, int i, void *page, int pages) "s %p iov[%d] %p pa
 
 # block/iscsi.c
 iscsi_xcopy(void *src_lun, uint64_t src_off, void *dst_lun, uint64_t dst_off, uint64_t bytes, int ret) "src_lun %p offset %"PRIu64" dst_lun %p offset %"PRIu64" bytes %"PRIu64" ret %d"
+
+# block/nbd-client.c
+nbd_read_reply_entry_fail(int ret, const char *err) "ret = %d, err: %s"
+nbd_co_request_fail(uint64_t from, uint32_t len, uint64_t handle, uint16_t flags, uint16_t type, const char *name, int ret, const char *err) "Request failed { .from = %" PRIu64", .len = %" PRIu32 ", .handle = %" PRIu64 ", .flags = 0x%" PRIx16 ", .type = %" PRIu16 " (%s) } ret = %d, err: %s"