Fix a number of issues with the Range header handling for serving files

pkg/http_server/lib/src/virtual_directory.dart

Index: pkg/http_server/lib/src/virtual_directory.dart
diff --git a/pkg/http_server/lib/src/virtual_directory.dart b/pkg/http_server/lib/src/virtual_directory.dart
index 6dfec5d9124d7644a9e4479edcf331e0833a7754..d82fd992c6e30e6bc287f83b15920e3a49e3646e 100644
--- a/pkg/http_server/lib/src/virtual_directory.dart
+++ b/pkg/http_server/lib/src/virtual_directory.dart
@@ -209,54 +209,73 @@ class VirtualDirectory {
       response.headers.set(HttpHeaders.LAST_MODIFIED, lastModified);
       response.headers.set(HttpHeaders.ACCEPT_RANGES, "bytes");
 
-      if (request.method == 'HEAD') {
-        response.close();
-        return null;
-      }
-
       return file.length().then((length) {
-        String range = request.headers.value("range");
+        String range = request.headers.value(HttpHeaders.RANGE);
         if (range != null) {
           // We only support one range, where the standard support several.
           Match matches = new RegExp(r"^bytes=(\d*)\-(\d*)$").firstMatch(range);
           // If the range header have the right format, handle it.
-          if (matches != null) {
+          if (matches != null &&
+              (matches[1].isNotEmpty || matches[2].isNotEmpty)) {
             // Serve sub-range.
-            int start;
-            int end;
+            int start;  // First byte position - inclusive.
+            int end;  // Last byte position - inclusive.
             if (matches[1].isEmpty) {
-              start = matches[2].isEmpty ?
-                  length :
-                  length - int.parse(matches[2]);
-              end = length;
+              start = length - int.parse(matches[2]);
+              if (start < 0) start = 0;
+              end = length - 1;
             } else {
               start = int.parse(matches[1]);
-              end = matches[2].isEmpty ? length : int.parse(matches[2]) + 1;
+              end = matches[2].isEmpty ? length - 1: int.parse(matches[2]);
             }
+            // If the range is syntactically invalid the Range header
+            // MUST be ignored (RFC 2616 section 14.35.1).
+            if (start <= end) {
+              if (end >= length) {
+                end = length - 1;
+              }
 
-            // Override Content-Length with the actual bytes sent.
-            response.headers.set(HttpHeaders.CONTENT_LENGTH, end - start);
+              if (start >= length) {
+                response
+                    ..statusCode = HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE
+                    ..close();
+                return;
+              }
 
-            // Set 'Partial Content' status code.
-            response.statusCode = HttpStatus.PARTIAL_CONTENT;
-            response.headers.set(HttpHeaders.CONTENT_RANGE,
-                                 "bytes $start-${end - 1}/$length");
+              // Override Content-Length with the actual bytes sent.
+              response.headers.set(HttpHeaders.CONTENT_LENGTH, end - start + 1);
 
-            // Pipe the 'range' of the file.
-            file.openRead(start, end)
-                .pipe(new _VirtualDirectoryFileStream(response, file.path))
-                .catchError((_) {
-                  // TODO(kevmoo): log errors
-                });
-            return;
+              // Set 'Partial Content' status code.
+              response
+                  ..statusCode = HttpStatus.PARTIAL_CONTENT
+                  ..headers.set(HttpHeaders.CONTENT_RANGE,
+                                'bytes $start-$end/$length');
+
+              // Pipe the 'range' of the file.
+              if (request.method == 'HEAD') {
+                response.close();
+              } else {
+                file.openRead(start, end + 1)
+                    .pipe(new _VirtualDirectoryFileStream(response, file.path))
+                    .catchError((_) {
+                      // TODO(kevmoo): log errors
+                    });
+              }
+              return;
+            }
           }
         }
 
-        file.openRead()
-            .pipe(new _VirtualDirectoryFileStream(response, file.path))
-            .catchError((_) {
-              // TODO(kevmoo): log errors
-            });
+        response.headers.set(HttpHeaders.CONTENT_LENGTH, length);
+        if (request.method == 'HEAD') {
+          response.close();
+        } else {
+          file.openRead()
+              .pipe(new _VirtualDirectoryFileStream(response, file.path))
+              .catchError((_) {
+                // TODO(kevmoo): log errors
+              });
+        }
       });
     }).catchError((_) {
       response.statusCode = HttpStatus.NOT_FOUND;