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Re: [Qemu-block] [PATCH RFC] qemu-io: Prefer stderr for error messages

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH RFC] qemu-io: Prefer stderr for error messages
Date: Wed, 12 Dec 2018 19:26:27 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 12/12/18 5:52 PM, Nir Soffer wrote:
On Thu, Dec 13, 2018 at 12:13 AM Eric Blake <address@hidden> wrote:

When a qemu-io command fails, it's best if the failure message
goes to stderr rather than stdout.

This makes sense, but it will break users like this:


We need a way to detect qemu-io verification failures, maybe a special
exit code?

0 - verification succeeded
1 - verification failed
2 - other error (e.g no such file)

Or, if qemu-io had a way to read data and write it to stdout, we could
compare the data and avoid the need for special exit code.

@@ -723,8 +725,8 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
          print_cvtnum_err(count, argv[optind]);
          return count;
      } else if (count > BDRV_REQUEST_MAX_BYTES) {
-        printf("length cannot exceed %" PRIu64 ", given %s\n",
-               (uint64_t)BDRV_REQUEST_MAX_BYTES, argv[optind]);
+        fprintf(stderr, "length cannot exceed %" PRIu64 ", given %s\n",
+                (uint64_t)BDRV_REQUEST_MAX_BYTES, argv[optind]);
          return -EINVAL;

@@ -738,19 +740,22 @@ static int read_f(BlockBackend *blk, int argc, char 

      if ((pattern_count < 0) || (pattern_count + pattern_offset > count))  {
-        printf("pattern verification range exceeds end of read data\n");
+        fprintf(stderr,
+                "pattern verification range exceeds end of read data\n");
          return -EINVAL;

Note that 'read -P ...' can fail for both pattern verification failure, and for other reasons, both before and after this patch. The only thing this patch is doing is changing where the failure messages are output. Or is your complaint that your existing code is already catering to older qemu that had 0 exit status, and parsing stdout for a specific string rather than trusting exit status, and now stdout won't have that specific string? qemu-io is not quite as worried about backwards compatibility as qemu-img or qemu proper, but at least knowing what might break might help us design something more user-friendly.

Can you redirect qemu-io to output both stdout and stderr to the same file, if you have to parse the answers to learn if verification has failed?

And your idea of having distinguished error codes for verification fail vs. overall failure makes some sense, but it would require some major rework (right now, returning -errno codes does not really tell the caller what exit status to report).

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

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