[Libguestfs] [nbdkit PATCH] error: Tweak filter name for easier debugging

[Laszlo Ersek]

On 02/03/22 22:29, Eric Blake wrote:
> Grepping for the string 'error:' is complicated when the error filter
> reports its name in the same manner.  We can keep the filter .so and
> file names the same, but tweak the filter .name field to change
> messages using nbdkit_debug() to instead produce "error-inject:", so
> that "error:" is reserved for uses of nbdkit_error().
> ---
> 
> I confused myself while trying to debug a new libnbd today, where
> injecting errors was intentional.  And then I noticed that 'man
> nbdkit-error-filter' already tried to warn me.  But we can do one step
> better, as in this patch...
> 
>  filters/error/nbdkit-error-filter.pod | 13 +++++++------
>  filters/error/error.c                 |  6 +++---
>  2 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/filters/error/nbdkit-error-filter.pod b/filters/error/nbdkit-error-filter.pod
> index 5fb93063..47fcd379 100644
> --- a/filters/error/nbdkit-error-filter.pod
> +++ b/filters/error/nbdkit-error-filter.pod
> @@ -129,14 +129,15 @@ settings to NBD cache requests.
>  =head2 Peculiar debug output
> 
>  If you are looking at the debugging output (using C<nbdkit -f -v>)
> -then you may see peculiar "errors" appearing when using this filter,
> -for example:
> +references to the name of this filter show up as C<"error-inject:">,
> +and such lines indicate that the filter is not altering output, for
> +example:
> 
> - nbdkit: file.9: debug: error: pread count=1024 offset=0 flags=0x0
> + nbdkit: file.9: debug: error-inject: pread count=1024 offset=0 flags=0x0
> 
> -In fact these are not errors, nbdkit core is simply printing the name
> -of the filter which happens to be C<"error">.  When this filter
> -injects an error you will see something like:
> +Conversely, references to the string C<"error:"> occur when the
> +L<nbdkit_error(3)> API was used, including when this filter injects an
> +error, as in:
> 
>   nbdkit: file.4: error: injecting ENOSPC error into pwrite
> 
> diff --git a/filters/error/error.c b/filters/error/error.c
> index 4b513a81..95eb5622 100644
> --- a/filters/error/error.c
> +++ b/filters/error/error.c
> @@ -1,5 +1,5 @@
>  /* nbdkit
> - * Copyright (C) 2018-2020 Red Hat Inc.
> + * Copyright (C) 2018-2022 Red Hat Inc.
>   *
>   * Redistribution and use in source and binary forms, with or without
>   * modification, are permitted provided that the following conditions are
> @@ -374,8 +374,8 @@ error_cache (nbdkit_next *next,
>  }
> 
>  static struct nbdkit_filter filter = {
> -  .name              = "error",
> -  .longname          = "nbdkit error filter",
> +  .name              = "error-inject",
> +  .longname          = "nbdkit error injection filter",
>    .load              = error_load,
>    .unload            = error_unload,
>    .config            = error_config,
> 

Very important patch, I'd say; log analysis is very common!

Reviewed-by: Laszlo Ersek <lersek at redhat.com>