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os/session_policy.h: add 'with_matching_policy'

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This patch makes the server-side policy-matching logic available outside
the 'Session_policy' class. Given that the new 'with_matching_policy'
function does not throw any exception, it gives server implementations
the freedom to avoid the C++ exception mechanism for the policy handling.
This commit is contained in:
Norman Feske 2022-02-04 12:04:16 +01:00
parent bb285bf758
commit 6eaeb61d58
1 changed files with 56 additions and 23 deletions
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79
repos/os/include/os/session_policy.h
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@ -23,6 +23,11 @@
namespace Genode {
struct Xml_node_label_score;
template <size_t N, typename MATCH_FN, typename NO_MATCH_FN>
void with_matching_policy(String<N> const &, Xml_node const &,
MATCH_FN const &, NO_MATCH_FN const &);
class Session_policy;
}
@ -155,6 +160,49 @@ struct Genode::Xml_node_label_score
};
/**
* Call 'match_fn' with the policy that matches best the given 'label'
*
* \param policies XML node that contains potentially many '<policy>'
* nodes and an optional '<default-policy>' node.
* \param match_fn functor called with best matching policy XML node
* argmument
* \param no_match_fn functor called if no matching policy exists
*/
template <Genode::size_t N, typename MATCH_FN, typename NO_MATCH_FN>
void Genode::with_matching_policy(String<N> const &label,
Xml_node const &policies,
MATCH_FN const &match_fn,
NO_MATCH_FN const &no_match_fn)
{
/*
* Find policy node that matches best
*/
Xml_node best_match("<none/>");
Xml_node_label_score best_score;
policies.for_each_sub_node("policy", [&] (Xml_node const &policy) {
Xml_node_label_score const score(policy, label);
if (score.stronger(best_score)) {
best_match = policy;
best_score = score;
}
});
/* fall back to default policy if no match exists */
if (best_match.has_type("none"))
policies.with_sub_node("default-policy", [&] (Xml_node const &policy) {
best_match = policy; });
if (best_match.has_type("none"))
no_match_fn();
else
match_fn(best_match);
}
/**
* Query server-side policy for a session request
*/
@ -175,33 +223,18 @@ class Genode::Session_policy : public Xml_node
template <size_t N>
static Xml_node _query_policy(String<N> const &label, Xml_node config)
{
/*
* Find policy node that matches best
*/
Xml_node best_match("<none/>");
Xml_node_label_score best_score;
Xml_node result("<none/>");
/*
* Functor to be applied to each policy node
*/
auto lambda = [&] (Xml_node policy) {
Xml_node_label_score const score(policy, label);
if (score.stronger(best_score)) {
best_match = policy;
best_score = score;
}
};
with_matching_policy(label, config,
config.for_each_sub_node("policy", lambda);
[&] (Xml_node const &policy) {
result = policy; },
if (!best_match.has_type("none"))
return best_match;
[&] () {
warning("no policy defined for label '", label, "'");
throw No_policy_defined(); });
try { return config.sub_node("default-policy"); }
catch (...) { }
warning("no policy defined for label '", label, "'");
throw No_policy_defined();
return result;
}
public: