[Libguestfs] [nbdkit PATCH] eval: Allow user override of 'missing'

[Richard W.M. Jones]

On Mon, Feb 10, 2020 at 01:44:14PM -0600, Eric Blake wrote:
> A comment in the code mentioned something that didn't actually work,
> but which can be useful for user-directed logging of what other
> callbacks they might want to implement.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
> 
> I haven't pushed this one, becuase I'm not sure if we want it; but it
> was easy enough to whip together after an IRC question earlier today.
> 
>  plugins/eval/eval.c                 | 16 +++++++++++++---
>  plugins/eval/nbdkit-eval-plugin.pod |  6 ++++++
>  tests/test-eval.sh                  |  2 ++
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/plugins/eval/eval.c b/plugins/eval/eval.c
> index 8f1eb6c..094cac5 100644
> --- a/plugins/eval/eval.c
> +++ b/plugins/eval/eval.c
> @@ -1,5 +1,5 @@
>  /* nbdkit
> - * Copyright (C) 2018-2019 Red Hat Inc.
> + * Copyright (C) 2018-2020 Red Hat Inc.
>   *
>   * Redistribution and use in source and binary forms, with or without
>   * modification, are permitted provided that the following conditions are
> @@ -169,6 +169,12 @@ create_script (const char *method, const char *value)
>      return NULL;
>    }
> 
> +  /* Special case for user override of missing */
> +  if (missing && strcmp (script, missing) == 0 && unlink (script) == -1) {
> +    nbdkit_error ("unlink: %m");
> +    return NULL;
> +  }
> +
>    fp = fopen (script, "w");
>    if (fp == NULL) {
>      nbdkit_error ("fopen: %s: %m", script);
> @@ -216,7 +222,7 @@ eval_load (void)
> 
>    /* To make things easier, create a "missing" script which always
>     * exits with code 2.  If a method is missing we call this script
> -   * instead.  It could even be overridden by the user.
> +   * instead.  It can even be overridden by the user.
>     */
>    missing = create_script ("missing", "exit 2\n");
>    if (!missing)
> @@ -255,11 +261,15 @@ static int
>  add_method (const char *key, const char *value)
>  {
>    char *script;
> +  char *tmp = missing; /* Needed to allow user override of missing */
> 
> -  if (get_script (key) != missing) {
> +  missing = NULL;
> +  if (get_script (key) != NULL) {
> +    missing = tmp;
>      nbdkit_error ("method %s defined more than once on the command line", key);
>      return -1;
>    }
> +  missing = tmp;

This leaks the missing global?

But yes the idea is fine, so ACK.

Rich.

>    /* Do a bit of checking to make sure the key isn't malicious.  This
>     * duplicates work already done by nbdkit, but better safe than
> diff --git a/plugins/eval/nbdkit-eval-plugin.pod b/plugins/eval/nbdkit-eval-plugin.pod
> index 88c1488..cbb4133 100644
> --- a/plugins/eval/nbdkit-eval-plugin.pod
> +++ b/plugins/eval/nbdkit-eval-plugin.pod
> @@ -140,6 +140,12 @@ no longer see that key.
> 
>  All of these parameters are optional.
> 
> +=item B<missing=>SCRIPT
> +
> +The parameter C<missing> defines a script that will be called in place
> +of any other callback not explicitly provided.  If omitted, this
> +defaults to the script "exit 2".
> +
>  =back
> 
>  =head1 ENVIRONMENT VARIABLES
> diff --git a/tests/test-eval.sh b/tests/test-eval.sh
> index 206c680..4557b02 100755
> --- a/tests/test-eval.sh
> +++ b/tests/test-eval.sh
> @@ -42,7 +42,9 @@ cleanup_fn rm -f $files
>  nbdkit -U - eval \
>         get_size='echo 64M' \
>         pread='dd if=/dev/zero count=$3 iflag=count_bytes' \
> +       missing='echo "in missing: $@" >> eval.out; exit 2' \
>         --run 'qemu-img info $nbd' > eval.out
> 
>  cat eval.out
>  grep '67108864 bytes' eval.out
> +grep 'in missing' eval.out
> -- 
> 2.24.1
> 
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