Backport bpo-44022: Fix http client infinite line reading (DoS) after a HTTP 100 Continue (GH-25916)

Backport the fix from the following commit:

    commit 47895e31b6f626bc6ce47d175fe9d43c1098909d
    Author: Gen Xu <xgbarry@gmail.com>
    Date:   2021-05-06 00:42:41 +0200

        bpo-44022: Fix http client infinite line reading (DoS) after a HTTP 100 Continue (GH-25916)

        Fixes http.client potential denial of service where it could get stuck reading lines from a malicious server after a 100 Continue response.

        Co-authored-by: Gregory P. Smith <greg@krypto.org>

Instead of reusing the header reading code, I have just added explicit
counter to avoid having to refactor the old code.

Plus the improved test from:

    commit e60ab843cbb016fb6ff8b4f418641ac05a9b2fcc
    Author: Gregory P. Smith <greg@krypto.org>
    Date:   2021-06-03 05:43:38 +0200

        bpo-44022: Improve the regression test. (GH-26503)

        It wasn't actually detecting the regression due to the
        assertion being too lenient.

2 files changed, 17 insertions, 1 deletions

diff --git a/Lib/httplib.py b/Lib/httplib.py
index 81a08d5d714..ebfa59ff93c 100644
--- a/Lib/httplib.py
+++ b/Lib/httplib.py
@@ -453,11 +453,14 @@ class HTTPResponse:
             if status != CONTINUE:
                 break
             # skip the header from the 100 response
+            header_count = 0
             while True:
                 skip = self.fp.readline(_MAXLINE + 1)
                 if len(skip) > _MAXLINE:
                     raise LineTooLong("header line")
-                skip = skip.strip()
+                header_count += 1
+                if header_count > _MAXHEADERS:
+                    raise HTTPException("got more than %d headers" % _MAXHEADERS)
                 if not skip:
                     break
                 if self.debuglevel > 0:
diff --git a/Lib/test/test_httplib.py b/Lib/test/test_httplib.py
index e20a0986dce..5f6b1d0b20e 100644
--- a/Lib/test/test_httplib.py
+++ b/Lib/test/test_httplib.py
@@ -675,6 +675,19 @@ class BasicTest(TestCase):
         resp = httplib.HTTPResponse(FakeSocket(body))
         self.assertRaises(httplib.LineTooLong, resp.begin)
 
+    def test_overflowing_header_limit_after_100(self):
+        body = (
+            'HTTP/1.1 100 OK\r\n'
+            'r\n' * 32768
+        )
+        resp = httplib.HTTPResponse(FakeSocket(body))
+        with self.assertRaises(httplib.HTTPException) as cm:
+            resp.begin()
+        # We must assert more because other reasonable errors that we
+        # do not want can also be HTTPException derived.
+        self.assertIn('got more than ', str(cm.exception))
+        self.assertIn('headers', str(cm.exception))
+
     def test_overflowing_chunked_line(self):
         body = (
             'HTTP/1.1 200 OK\r\n'