Fix parsing of folded header field values.

Bug: #53227
Bug: #53185
Change-Id: Ibbdbdf9c8f2875e8f687244982810fffee20e69c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/320920
Commit-Queue: Brian Quinlan <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: brianquinlan

CHANGELOG.md

@@ -88,7 +88,12 @@ constraint][language version] lower bound to 3.2 or greater (`sdk: '^3.2.0'`).
   `HttpClientResponse.headers` and `HttpRequest.headers` no longer include
   trailing whitespace in their values.
 
+- **Breaking change** [#53227][]: Folded headers values returned by
+  `HttpClientResponse.headers` and `HttpRequest.headers` now have a space
+  inserted at the fold point.
+
 [#53005]: https://dartbug.com/53005
+[#53227]: https://dartbug.com/53227
 
 #### `dart:isolate`
 

sdk/lib/_http/http_parser.dart

@@ -680,7 +680,13 @@ class _HttpParser extends Stream<_HttpIncoming> {
 
         case _State.HEADER_VALUE_FOLD_OR_END:
           if (byte == _CharCode.SP || byte == _CharCode.HT) {
-            _state = _State.HEADER_VALUE_START;
+            // This is an obs-fold as defined in RFC 7230 and we should
+            // "...replace each received obs-fold with one or more SP octets
+            // prior to interpreting the field value or forwarding the
+            // message downstream."
+            // See https://www.rfc-editor.org/rfc/rfc7230#section-3.2.4
+            _addWithValidation(_headerValue, _CharCode.SP);
+            _state = _State.HEADER_VALUE_START; // Strips leading whitespace.
           } else {
             String headerField = String.fromCharCodes(_headerField);
             // The field value does not include any leading or trailing whitespace.

tests/standalone/io/http_parser_test.dart

@@ -425,19 +425,42 @@ Empty-Header-2:\t  \t \r
     headers["empty-header-2"] = "";
     _testParseRequestLean(request, "POST", "/test", expectedHeaders: headers);
 
+    // Test folded headers.
     request = """
 POST /test HTTP/1.1\r
-Header-A: \t  AA\r
- A  \t \r
-X-Header-B:           b\r
-  b\r
-\t    b  \t \r
+Header-A: h\r
+ ell\r
+ o\r
+X-Header-B: w\r
+ o\r
+ r\r
+ l\r
+ d\r
 \r
 """;
 
     headers = new Map();
-    headers["header-a"] = "AAA";
-    headers["x-header-b"] = "bbb";
+    headers["header-a"] = "h ell o";
+    headers["x-header-b"] = "w o r l d";
+    _testParseRequestLean(request, "POST", "/test", expectedHeaders: headers);
+
+    // Test folded headers with leading and trailing whitespace.
+    request = """
+POST /test HTTP/1.1\r
+Header-A: \t  h \t \r
+  \t ell \t  \t \r
+\to  \t \r
+X-Header-B:w\r
+\t\to\t\t\r
+\t\tr\t\t\r
+\tl \r
+\td \t\r
+\r
+""";
+
+    headers = new Map();
+    headers["header-a"] = "h \t  ell \t  \t  o";
+    headers["x-header-b"] = "w o\t\t r\t\t l  d";
     _testParseRequestLean(request, "POST", "/test", expectedHeaders: headers);
 
     // _testParseRequestLean encodes the request as ISO-8859-1. Test that the