maintain_key_values is not convergent if replacement value has the same length as the old one; set_line_based reccommended

A test file (/tmp/test):

@foo = bar@

promise.cf:

<pre>
body common control
{
bundlesequence =>

{ "example" }

;
inputs =>

{ "$(sys.workdir)/inputs/libraries/cfengine_stdlib.cf" }

;
}

bundle agent example
{
vars:
"v[foo]" string => "abc";

files:

"/tmp/test"
edit_line => maintain_key_values("example.v","=");
}
</pre>

output:
<pre>
$ cf-agent -KI
2014-04-02T15:35:44+0200 error: /example/files/'/tmp/test'/maintain_key_values/replace_patterns/'^{($(keypat[$(index)]))(?!$(ve[$(index)])$).': Promised replacement 'foo = abc' on line 'foo = bar' for pattern '}(\s*foo\s=\s*)(?!abc$).*' is not convergent while editing '/tmp/test'
2014-04-02T15:35:44+0200 error: Because the regular expression '^(\s*foo\s*=\s*)(?!abc$).*' still matches the replacement string 'foo = abc'
2014-04-02T15:35:44+0200 error: Promise belongs to bundle 'maintain_key_values' in file '/home/fabien/.cfagent/inputs/libraries/cfengine_stdlib.cf' near line 565
2014-04-02T15:35:44+0200 error: Comment is 'Replace definition of foo'
</pre>

If the length of the value is different or if there is no space between the separator and the value, it works as expected. I don't know how to correct this bug.

Regards,
Fabien Grumelard