Bug 337184: feed sniffer sniffs too many things as feeds... need to make sure that feed indicator strings are only used as indicators if they are present at the top level of the document, patch by Ben Goodger <ben@mozilla.org>, r=darin, mconnor

browser/components/feeds/src/nsFeedSniffer.cpp

@@ -163,6 +163,92 @@ HasAttachmentDisposition(nsIHttpChannel* httpChannel)
   return PR_FALSE;
 }
 
+/**
+ *
+ * Determine if a substring is the "documentElement" in the document.
+ *
+ * All of our sniffed substrings: <rss, <feed, <rdf:RDF must be the "document"
+ * element within the XML DOM, i.e. the root container element. Otherwise,
+ * it's possible that someone embedded one of these tags inside a document of
+ * another type, e.g. a HTML document, and we don't want to show the preview
+ * page if the document isn't actually a feed.
+ * 
+ * @param   dataString
+ *          The data being sniffed
+ * @param   substring
+ *          The substring being tested for document-element-ness
+ * @param   indicator
+ *          An iterator initialized to the end of |substring|, located in
+ *          |dataString|
+ * @returns PR_TRUE if the substring is the documentElement, PR_FALSE 
+ *          otherwise.
+ */
+static PRBool
+IsDocumentElement(nsACString& dataString, const nsACString& substring, 
+                  nsACString::const_iterator& indicator)
+{
+  nsACString::const_iterator start, end, endOfString;
+
+  dataString.BeginReading(start);
+  endOfString = end = indicator;
+
+  // For every tag in the buffer, check to see if it's a PI, Doctype or 
+  // comment, our desired substring or something invalid.
+  while (FindCharInReadable('<', start, end)) {
+    ++start;
+    if (start == endOfString)
+      return PR_FALSE;
+
+    // Check to see if the character following the '<' is either '?' or '!'
+    // (processing instruction or doctype or comment)... these are valid nodes
+    // to have in the prologue. 
+    if (*start != '?' && *start != '!') {
+      // Check to see if the string following the '<' is our indicator substring.
+      // If it's not, it's an indication that the indicator substring was 
+      // embedded in some other kind of document, e.g. HTML.
+      return substring.Equals(Substring(--start, indicator));
+    }
+    
+    // Reset end so we can re-scan the entire remaining section of the 
+    // string, and advance start so we don't loop infinitely.
+    dataString.EndReading(end);
+
+    // Now advance the iterator until the '>' (We do this because we don't want
+    // to sniff indicator substrings that are embedded within other nodes, e.g.
+    // comments: <!-- <rdf:RDF .. > -->
+    if (!FindCharInReadable('>', start, end))
+      return PR_FALSE;
+    
+    // Reset end again
+    dataString.EndReading(end);
+  }
+  return PR_TRUE;
+}
+
+/**
+ * Determines whether or not a string exists as the root element in an XML data
+ * string buffer.
+ * @param   dataString
+ *          The data being sniffed
+ * @param   substring
+ *          The substring being tested for existence and root-ness.
+ * @returns PR_TRUE if the substring exists and is the documentElement, PR_FALSE
+ *          otherwise.
+ */
+static PRBool
+ContainsTopLevelSubstring(nsACString& dataString, const nsACString& substring) 
+{
+  nsACString::const_iterator start, end;
+
+  dataString.BeginReading(start);
+  dataString.EndReading(end);
+
+  PRBool isFeed = FindInReadable(substring, start, end);
+
+  // Only do the validation when we find the substring. 
+  return isFeed ? IsDocumentElement(dataString, substring, end) : isFeed;
+}
+
 NS_IMETHODIMP
 nsFeedSniffer::GetMIMETypeFromContent(nsIRequest* request, 
                                       const PRUint8* data, 
@@ -244,23 +330,15 @@ nsFeedSniffer::GetMIMETypeFromContent(nsIRequest* request,
   PRBool isFeed = PR_FALSE;
 
   // RSS 0.91/0.92/2.0
-  dataString.BeginReading(start_iter);
-  dataString.EndReading(end_iter);
-
-  isFeed = FindInReadable(NS_LITERAL_CSTRING("<rss"), start_iter, end_iter);
+  isFeed = ContainsTopLevelSubstring(dataString, NS_LITERAL_CSTRING("<rss"));
 
   // Atom 1.0
-  if (!isFeed) {
-    dataString.BeginReading(start_iter);
-    dataString.EndReading(end_iter);
-    isFeed = FindInReadable(NS_LITERAL_CSTRING("<feed"), start_iter, end_iter);
-  }
+  if (!isFeed)
+    isFeed = ContainsTopLevelSubstring(dataString, NS_LITERAL_CSTRING("<feed"));
 
   // RSS 1.0
   if (!isFeed) {
-    dataString.BeginReading(start_iter);
-    dataString.EndReading(end_iter);
-    isFeed = FindInReadable(NS_LITERAL_CSTRING("<rdf:RDF"), start_iter, end_iter);
+    isFeed = ContainsTopLevelSubstring(dataString, NS_LITERAL_CSTRING("<rdf:RDF"));
     if (isFeed) {
       dataString.BeginReading(start_iter);
       dataString.EndReading(end_iter);