buffet: Add proper XML parsing for XMPP streams

For large command notifications, a single XMPP stanza can be split
into a number of TCP packets. In order to handle large stanzas and
in order to help with implementing TLS support for XMPP, added an
expat-based XML parser on top of XMPP stream, to make sure that
the stanzas are processed when all the data for a complete XML tag
has arrived.

BUG=brillo:458
TEST=`FEATURES=test emerge-link buffet`

Change-Id: I560f40dafb31c6e6b9e645d232453338ee4fbbef
Reviewed-on: https://chromium-review.googlesource.com/271592
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/notification/xml_node.h b/buffet/notification/xml_node.h
new file mode 100644
index 0000000..fc43650
--- /dev/null
+++ b/buffet/notification/xml_node.h
@@ -0,0 +1,124 @@
+// Copyright 2015 The Chromium OS 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 BUFFET_NOTIFICATION_XML_NODE_H_
+#define BUFFET_NOTIFICATION_XML_NODE_H_
+
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include <base/macros.h>
+
+namespace buffet {
+
+class XmlNodeTest;
+class XmppStreamParser;
+
+// XmlNode is a very simple class to represent the XML document element tree.
+// It is used in conjunction with expat XML parser to implement XmppStreamParser
+// class used to parse Xmpp data stream into individual stanzas.
+class XmlNode final {
+ public:
+  XmlNode(const std::string& name,
+          std::map<std::string, std::string> attributes);
+
+  // The node's name. E.g. in <foo bar="baz">quux</foo> this will return "foo".
+  const std::string& name() const;
+  // The node text content. E.g. in <foo bar="baz">quux</foo> this will return
+  // "quux".
+  const std::string& text() const;
+  // The node attribute map. E.g. in <foo bar="baz">quux</foo> this will return
+  // {{"bar", "baz"}}.
+  const std::map<std::string, std::string>& attributes() const;
+  // Returns the list of child nodes, if any.
+  const std::vector<std::unique_ptr<XmlNode>>& children() const;
+
+  // Retrieves the value of the given attribute specified by |name|.
+  // If the attribute doesn't exist, returns false and |value| is not modified.
+  bool GetAttribute(const std::string& name, std::string* value) const;
+  // Returns the value of the given attribute specified by |name|.
+  // Returns empty string if the attribute does not exist. This method should be
+  // used only in limited scopes such as unit tests.
+  std::string GetAttributeOrEmpty(const std::string& name) const;
+
+  // Finds a first occurrence of a child node specified by |name_path|. A name
+  // path is a "/"-separated list of node names to look for. If |recursive| is
+  // set to true, the children are recursively traversed trying to match the
+  // node names. Otherwise only first-level children of the current node are
+  // matched against the top-level name of |name_path|.
+  // This method returns a pointer to the first node that matches the path,
+  // otherwise a nullptr is returned.
+  const XmlNode* FindFirstChild(const std::string& name_path,
+                                bool recursive) const;
+
+  // Finds all the child nodes matching the |name_path|. This returns the list
+  // of pointers to the child nodes matching the criteria. If |recursive| is
+  // set to true, the children are recursively traversed trying to match the
+  // node names. Otherwise only first-level children of the current node are
+  // matched against the top-level name of |name_path|.
+  // For example, if the current node represents the <top> element of the
+  // following XML document:
+  //  <top>
+  //    <node1 id="1"><node2 id="2"><node3 id="3"/></node2></node1>
+  //    <node2 id="4"><node3 id="5"/></node2>
+  //    <node3 id="6"/>
+  //    <node2 id="7"><node4 id="8"><node3 id="9"/></node4></node2>
+  //  </top>
+  // Then recursively searching for nodes will produce the following results
+  // (only the node "id" attributes are listed in the results, for brevity):
+  //    FindChildren("node2/node3", false) -> {"5"}.
+  //    FindChildren("node2/node3", true) -> {"3", "5"}.
+  //    FindChildren("node3", false) -> {"6"}.
+  //    FindChildren("node3", true) -> {"3", "5", "6", "9"}.
+  std::vector<const XmlNode*> FindChildren(const std::string& name_path,
+                                           bool recursive) const;
+
+  // Adds a new child to the bottom of the child list of this node.
+  void AddChild(std::unique_ptr<XmlNode> child);
+
+  // Converts the node tree to XML-like string. Note that this not necessarily
+  // produces a valid XML string. It does not use any character escaping or
+  // canonicalization, which will produce invalid XML if any of the node or
+  // attribute names or values contain special characters such as ", <, >, etc.
+  // This function should be used only for logging/debugging purposes only and
+  // never to generate valid XML from the parsed node tree.
+  std::string ToString() const;
+
+ private:
+  friend class XmlNodeTest;
+  friend class XmppStreamParser;
+
+  // Sets the node's text. Used by XML parser.
+  void SetText(const std::string& text);
+  // Appends the |text| to the node's text string.
+  void AppendText(const std::string& text);
+
+  // Helper method used by FindFirstChild() and FindChildren(). Searches for
+  // child node(s) matching |name_path|.
+  // If |children| is not specified (nullptr), this function find the first
+  // matching node and returns it via return value of the function. If no match
+  // is found, this function will return nullptr.
+  // If |children| parameter is not nullptr, found nodes are added to the
+  // vector pointed to by |children| and search continues until the whole tree
+  // is inspected. In this mode, the function always returns nullptr.
+  const XmlNode* FindChildHelper(const std::string& name_path,
+                                 bool recursive,
+                                 std::vector<const XmlNode*>* children) const;
+
+
+  const XmlNode* parent_{nullptr};  // Weak pointer to the parent node, if any.
+  std::string name_;
+  std::string text_;
+  std::map<std::string, std::string> attributes_;
+  std::vector<std::unique_ptr<XmlNode>> children_;
+
+  DISALLOW_COPY_AND_ASSIGN(XmlNode);
+};
+
+}  // namespace buffet
+
+#endif  // BUFFET_NOTIFICATION_XML_NODE_H_
+