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

[Eric Blake]

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;

   /* 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