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Re: [Qemu-block] [PATCH v1 1/1] block: Add numeric errno field to BLOCK_

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v1 1/1] block: Add numeric errno field to BLOCK_IO_ERROR events
Date: Thu, 21 Dec 2017 19:08:38 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0

On 12/21/2017 06:11 PM, Jack Schwartz wrote:
BLOCK_IO_ERROR events currently contain a "reason" string which is
strerror(errno) of the error.  This enhancement provides those events with
the numeric errno value as well, since it is easier to parse for error type
than a string.

NACK. Numeric errno values are platform-dependent, but QMP must be platform-independent. If you want to expose errno NAMES (not values), then create a QAPI enum and add the enum to the error structure (so that you are still passing names, not int values, over the wire).

+++ b/qapi/block-core.json
@@ -3660,6 +3660,11 @@
  #           io-status is present, please see query-block documentation
  #           for more information (since: 2.2)
+# @errno: int describing the error cause, provided for applications.
+#         (Note: while most errnos are posix compliant between OSs, it
+#         is possible some errno values can vary among different OSs.)
+#         (since 2.12)

The proof is in the pudding - if your documentation has to give this big disclaimer, then what you are adding is not portable and should not be added in that manner.

  # @reason: human readable string describing the error cause.
  #          (This field is a debugging aid for humans, it should not
  #           be parsed by applications) (since: 2.2)
@@ -3675,14 +3680,17 @@
  #      "data": { "device": "ide0-hd1",
  #                "node-name": "#block212",
  #                "operation": "write",
-#                "action": "stop" },
+#                "action": "stop",
+#                "nospace": false,
+#                "errno": 5,

So this should be "errno":"ENOSPC", not 5.

+#                "reason": "Input/output error" },
  #      "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
  { 'event': 'BLOCK_IO_ERROR',
    'data': { 'device': 'str', 'node-name': 'str', 'operation': 
              'action': 'BlockErrorAction', '*nospace': 'bool',
-            'reason': 'str' } }
+            'errno': 'int', 'reason': 'str' } }

and this should be the name of a QAPI enum type, not 'int'.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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