Add support for parsing a single range header value

This provides infrastructure for #1070 by refactoring an existing CORS-related check.

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>

Author: dlrobertson

fetch.bs

@@ -887,7 +887,8 @@ fetch("https://victim.example/naïve-endpoint", {
     </div>
 
    <dt>`<code>range</code>`
-   <dd><p>If <var>value</var> is not a <a>simple range header value</a>, then return false.
+   <dd><p>If the result of <a>parse a single range header value</a> given <var>value</var> is
+   failure, then return false.
 
    <dt>Otherwise
    <dd><p>Return false.
@@ -1106,45 +1107,52 @@ run these steps:
  <li><p>Return <var>values</var>.
 </ol>
 
-<p>To determine if a <a>byte sequence</a> <var>value</var> is a
-<dfn>simple range header value</dfn>, perform the following steps. They return a <a>boolean</a>.
+<p>To <dfn id=simple-range-header-value>parse a single range header value</dfn> from a
+<a>byte sequence</a> <var>value</var>, run these steps:
 
 <ol>
  <li><p>Let <var>data</var> be the <a>isomorphic decoding</a> of <var>value</var>.
 
  <li><p>If <var>data</var> does not <a for=string>start with</a> "<code>bytes=</code>", then return
- false.
+ failure.
 
  <li><p>Let <var>position</var> be a <a>position variable</a> for <var>data</var>, initially
  pointing at the 6th <a>code point</a> of <var>data</var>.
 
  <li><p>Let <var>rangeStart</var> be the result of <a>collecting a sequence of code points</a> that
  are <a>ASCII digits</a>, from <var>data</var> given <var>position</var>.
 
+ <li><p>Let <var>rangeStartValue</var> be <var>rangeStart</var>, interpreted as decimal number, if
+ <var>rangeStart</var> is not the empty string; otherwise null.
+
  <li><p>If the <a>code point</a> at <var>position</var> within <var>data</var> is not U+002D (-),
- then return false.
+ then return failure.
 
  <li><p>Advance <var>position</var> by 1.
 
  <li><p>Let <var>rangeEnd</var> be the result of <a>collecting a sequence of code points</a> that
  are <a>ASCII digits</a>, from <var>data</var> given <var>position</var>.
 
- <li><p>If <var>position</var> is not past the end of <var>data</var>, then return false.
+ <li><p>Let <var>rangeEndValue</var> be <var>rangeEnd</var>, interpreted as decimal number, if
+ <var>rangeEnd</var> is not the empty string; otherwise null.
 
- <li>
-  <p>If <var>rangeEnd</var>'s <a for=string>length</a> is 0, then return true.
+ <li><p>If <var>position</var> is not past the end of <var>data</var>, then return failure.
 
-  <p class="note">The range end can be omitted, e.g., `<code>bytes=0-</code>` is valid.
+ <li><p>If <var>rangeEndValue</var> and <var>rangeStartValue</var> are null, then return failure.
 
- <li><p>If <var>rangeStart</var>, interpreted as decimal number, is greater than
- <var>rangeEnd</var>, interpreted as decimal number, then return false.
+ <li><p>If <var>rangeStartValue</var> and <var>rangeEndValue</var> are numbers, and
+ <var>rangeStartValue</var> is greater than <var>rangeEndValue</var>, then return failure.
 
- <li><p>Return true.
+ <li>
+  <p>Return (<var>rangeStartValue</var>, <var>rangeEndValue</var>).
+
+  <p class="note">The range end or start can be omitted, e.g., `<code>bytes=0-</code>` or
+  `<code>bytes=-500</code>` are valid ranges.
 </ol>
 
-<p class="note">A <a>simple range header value</a> is a subset of allowed range header values, but
-it is the most common form used by user agents when requesting media or resuming downloads. This
-format of range header value can be set using <a>add a range header</a>.
+<p class=note><a>Parse a single range header value</a> succeeds for a subset of allowed range header
+values, but it is the most common form used by user agents when requesting media or resuming
+downloads. This format of range header value can be set using <a>add a range header</a>.
 
 <hr>