DnsConfigService and a posix implementation

net/dns/dns_config_service.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_DNS_CONFIG_SERVICE_H_
+#define NET_DNS_CONFIG_SERVICE_H_
+#pragma once
+
+#include <list>
+#include <map>
+#include <string>
+#include <vector>
+
+#include "net/base/net_api.h"
+
+namespace net {
+
+class IPEndPoint;
+
+// DnsConfig stores configuration of the system resolver.
+// To be used with AsyncHostResolver.
+// TODO(szym): determine the next query for a given host according to settings.
+struct NET_API DnsConfig {

[cbentzel, 2011/08/01 19:52:26]

I don't know if these will need NET_API - these are likely net stack internal
things which don't need to be exposed to consumers of the network library.

[szym, 2011/08/01 20:22:01]

Done.

+ public:

[cbentzel, 2011/08/01 19:52:26]

Nit: public by default on structs. 

[szym, 2011/08/01 20:22:01]

Done.

+  // resolv.conf stuff -- standard on POSIX/glibc systems
+  // win stores a subset of those in the registry

[cbentzel, 2011/08/01 19:52:26]

I'd want a default constructor due to the int/bool/POD types - make sure they
are initialized to something reasonable.

[szym, 2011/08/01 20:22:01]

Done.

+
+  // list of name server addresses
+  std::vector<IPEndPoint> nameservers;
+  // suffix search list, overrides |domain|,
+  // used on first lookup when #dots in given name < |ndots|
+  std::vector<std::string> search;
+  // use only if |search| is empty
+  std::string domain;
+
+  // options; see man resolv.conf
+  // min. number of dots before global resolution precedes |search|
+  int ndots;
+  // in seconds between retransmissions
+  int timeout;
+  // max. number of retries
+  int attempts;
+  // round robin in |nameservers|
+  bool rotate;
+  // enable EDNS0
+  bool edns0;
+
+  // /etc/hosts stuff
+  // NOTE: HostCache stores these in reverse order: hostname -> AddressList
+  std::map<IPAddressNumber, std::list<std::string>  > hosts;
+
+  // so that we only notify on real change

[cbentzel, 2011/08/01 19:52:26]

Another style thing - we tend to avoid operator overloading in Chrome. 

[szym, 2011/08/01 20:22:01]

This is inspired by ProxyConfig. There's a dozen other classes in net with
operator== overloaded. Would .equals() be preferred? 

I wasn't sure if keeping track of last_config and avoiding OnConfigChanged when
no real change occurred was actually necessary, but I suspected that
AsyncHostResolver might have to flush some cache out in response and that could
be a waste.

+  bool operator==(const DnsConfig& d) const {
+    return (nameservers == d.nameservers) &&
+           (search == d.search) &&
+           (domain == d.domain) &&
+           (ndots == d.ndots) &&
+           (timeout == d.timeout) &&
+           (attempts == d.attempts) &&
+           (rotate == d.rotate) &&
+           (edns0 == d.edns0);
+  }
+
+  bool operator!=(const DnsConfig& d) const {
+    return !(*this == d);
+  }
+};
+
+// Reads the current config now, separate for testability

[cbentzel, 2011/08/01 19:52:26]

This probably doesn't need to be defined here - implementation specific thing
for the POSIX implementation,.

[szym, 2011/08/01 20:22:01]

I planned a Windows implementation that reads the registry settings and writes
them to the DnsConfig. 

Is there no need to have a synchronous "read-on-demand" utility? How will
AsyncHostResolver be initialized?

+class NET_API DnsConfigReader {
+ public:
+  virtual bool Read(DnsConfig*) = 0; // blocking
+};
+
+// DnsConfigService fetches system DNS configuration as DnsConfig
+// Depending on the platform, watches files in /etc/ or win registry.
+class NET_API DnsConfigService {
+ public:
+  // Callback interface for the client. The delegate is called on the same
+  // thread as Watch(). Delegate must outlive the service.
+  class Delegate {
+   public:
+    virtual ~Delegate() {}
+
+    // Called only when |dns_config| is different from the last check.
+    virtual void OnConfigChanged(const DnsConfig& dns_config) = 0;
+
+    // Called when config could not be accessed/parsed.
+    virtual void OnConfigError() = 0;

[cbentzel, 2011/08/01 19:52:26]

Does this need to be exposed? I'd remove it until needed. 

[szym, 2011/08/01 20:22:01]

Not sure how to handle FilePathWatcher::Delegate::OnFilePathError otherwise.
This is a hard failure: FPWatcher stops watching afterwards.

+  };
+
+  // Creates the platform-specific DnsConfigService.
+  static DnsConfigService* CreateSystemService();
+
+  DnsConfigService() {}
+  virtual ~DnsConfigService() {}
+
+  // Starts watching system configuration for changes. Call at most once.
+  // The current thread must have an IO loop (for base::files::FilePathWatcher).
+  virtual bool Watch(Delegate* delegate) = 0;

[cbentzel, 2011/08/01 19:52:26]

What does the return value indicate? When would this fail?

[szym, 2011/08/01 20:22:01]

When FilePathWatcher::Watch fails, possibly when the registry key watch fails to
installs as well. We could postpone the error to be delivered asynchronously via
Delegate::OnConfigError.

+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DnsConfigService);
+};
+
+}  // namespace net
+
+#endif  // NET_DNS_CONFIG_SERVICE_H_
+