fix: Range suffixes are not Rust RangeTo

An HTTP Range of `bytes=-100` means a suffix, the last 100 bytes. This
was wrongly parsed as the Rust range `..100`, which means the first 100
bytes.

This has been fixed, but doing so required change `Range::iter` to
accept a length argument, to determine if the ranges are satisfiable.

BREAKING CHANGE: Change `.iter()` calls to `.satisfiable_ranges(len)`.
  Also, the `Range::bytes()` constructor will now return an error if
  pass a `RangeTo` (e.g. `Range::bytes(..100)`).

src/common/range.rs

@@ -48,29 +48,50 @@ impl Range {
     /// Creates a `Range` header from bounds.
     pub fn bytes(bounds: impl RangeBounds<u64>) -> Result<Self, InvalidRange> {
         let v = match (bounds.start_bound(), bounds.end_bound()) {
-            (Bound::Unbounded, Bound::Included(end)) => format!("bytes=-{}", end),
-            (Bound::Unbounded, Bound::Excluded(&end)) => format!("bytes=-{}", end - 1),
             (Bound::Included(start), Bound::Included(end)) => format!("bytes={}-{}", start, end),
             (Bound::Included(start), Bound::Excluded(&end)) => {
                 format!("bytes={}-{}", start, end - 1)
             }
             (Bound::Included(start), Bound::Unbounded) => format!("bytes={}-", start),
+            // These do not directly translate.
+            //(Bound::Unbounded, Bound::Included(end)) => format!("bytes=-{}", end),
+            //(Bound::Unbounded, Bound::Excluded(&end)) => format!("bytes=-{}", end - 1),
+
             _ => return Err(InvalidRange { _inner: () }),
         };
 
         Ok(Range(::HeaderValue::from_str(&v).unwrap()))
     }
 
-    /// Iterate the range sets as a tuple of bounds.
-    pub fn iter<'a>(&'a self) -> impl Iterator<Item = (Bound<u64>, Bound<u64>)> + 'a {
+    /// Iterate the range sets as a tuple of bounds, if valid with length.
+    ///
+    /// The length of the content is passed as an argument, and all ranges
+    /// that can be satisfied will be iterated.
+    pub fn satisfiable_ranges<'a>(&'a self, len: u64) -> impl Iterator<Item = (Bound<u64>, Bound<u64>)> + 'a {
         let s = self
             .0
             .to_str()
             .expect("valid string checked in Header::decode()");
 
-        s["bytes=".len()..].split(',').filter_map(|spec| {
+        s["bytes=".len()..].split(',').filter_map(move |spec| {
             let mut iter = spec.trim().splitn(2, '-');
-            Some((parse_bound(iter.next()?)?, parse_bound(iter.next()?)?))
+            let start = parse_bound(iter.next()?)?;
+            let end = parse_bound(iter.next()?)?;
+
+            // Unbounded ranges in HTTP are actually a suffix
+            // For example, `-100` means the last 100 bytes.
+            if let Bound::Unbounded = start {
+                if let Bound::Included(end) = end {
+                    if len < end {
+                        // Last N bytes is larger than available!
+                        return None;
+                    }
+                    return Some((Bound::Included(len - end), Bound::Unbounded));
+                }
+                // else fall through
+            }
+
+            Some((start, end))
         })
     }
 }
@@ -416,3 +437,11 @@ fn test_byte_range_spec_to_satisfiable_range() {
 bench_header!(bytes_multi, Range, { vec![b"bytes=1-1001,2001-3001,10001-".to_vec()]});
 bench_header!(custom_unit, Range, { vec![b"other=0-100000".to_vec()]});
 */
+
+#[test]
+fn test_to_satisfiable_range_suffix() {
+    let range = super::test_decode::<Range>(&["bytes=-100"]).unwrap();
+    let bounds = range.satisfiable_ranges(350).next().unwrap();
+    assert_eq!(bounds, (Bound::Included(250), Bound::Unbounded));
+}
+