Clean up XMLHttpRequest::FindCharsetInMediaType() and fix a bug in a while-loop condition in it

Bug: 674329
Change-Id: Ia815eb26487c09b69dabf61db018eb075c6b19e1
Reviewed-on: https://chromium-review.googlesource.com/773759
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517749}

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp

@@ -117,22 +117,23 @@ class ScopedEventDispatchProtect final {
 // TODO(tyoshino): Switch XHR to use HttpUtil. See crbug.com/743311.
 void FindCharsetInMediaType(const String& media_type,
                             unsigned& charset_pos,
-                            unsigned& charset_len,
-                            unsigned start) {
-  charset_pos = start;
+                            unsigned& charset_len) {
   charset_len = 0;
 
-  size_t pos = start;
+  size_t pos = charset_pos;
   unsigned length = media_type.length();
 
   while (pos < length) {
     pos = media_type.FindIgnoringASCIICase("charset", pos);
-    if (pos == kNotFound || !pos) {
-      charset_len = 0;
+
+    if (pos == kNotFound)
+      return;
+
+    // Give up if we find "charset" at the head.
+    if (!pos)
       return;
-    }
 
-    // is what we found a beginning of a word?
+    // Now check that "charset" is not a substring of some longer word.
     if (media_type[pos - 1] > ' ' && media_type[pos - 1] != ';') {
       pos += 7;
       continue;
@@ -140,55 +141,46 @@ void FindCharsetInMediaType(const String& media_type,
 
     pos += 7;
 
-    // skip whitespace
-    while (pos != length && media_type[pos] <= ' ')
+    while (pos < length && media_type[pos] <= ' ')
       ++pos;
 
-    // this "charset" substring wasn't a parameter
-    // name, but there may be others
-    if (media_type[pos++] != '=')
-      continue;
+    // Treat this as a charset parameter.
+    if (media_type[pos++] == '=')
+      break;
+  }
 
-    while (pos != length && (media_type[pos] <= ' ' || media_type[pos] == '"' ||
-                             media_type[pos] == '\''))
-      ++pos;
+  while (pos < length && (media_type[pos] <= ' ' || media_type[pos] == '"' ||
+                          media_type[pos] == '\''))
+    ++pos;
 
-    // we don't handle spaces within quoted parameter values, because charset
-    // names cannot have any
-    unsigned endpos = pos;
-    while (pos != length && media_type[endpos] > ' ' &&
-           media_type[endpos] != '"' && media_type[endpos] != '\'' &&
-           media_type[endpos] != ';')
-      ++endpos;
+  charset_pos = pos;
 
-    charset_pos = pos;
-    charset_len = endpos - pos;
-    return;
-  }
+  // we don't handle spaces within quoted parameter values, because charset
+  // names cannot have any
+  while (pos < length && media_type[pos] > ' ' && media_type[pos] != '"' &&
+         media_type[pos] != '\'' && media_type[pos] != ';')
+    ++pos;
+
+  charset_len = pos - charset_pos;
 }
 String ExtractCharsetFromMediaType(const String& media_type) {
-  unsigned pos, len;
-  FindCharsetInMediaType(media_type, pos, len, 0);
+  unsigned pos = 0;
+  unsigned len = 0;
+  FindCharsetInMediaType(media_type, pos, len);
   return media_type.Substring(pos, len);
 }
 
 void ReplaceCharsetInMediaType(String& media_type,
                                const String& charset_value) {
-  unsigned pos = 0, len = 0;
-
-  FindCharsetInMediaType(media_type, pos, len, 0);
+  unsigned pos = 0;
 
-  if (!len) {
-    // When no charset found, do nothing.
-    return;
-  }
-
-  // Found at least one existing charset, replace all occurrences with new
-  // charset.
-  while (len) {
+  while (true) {
+    unsigned len = 0;
+    FindCharsetInMediaType(media_type, pos, len);
+    if (!len)
+      return;
     media_type.replace(pos, len, charset_value);
-    unsigned start = pos + charset_value.length();
-    FindCharsetInMediaType(media_type, pos, len, start);
+    pos += charset_value.length();
   }
 }