From 30c6146c6c864e40d292e7201185624795986dc2 Mon Sep 17 00:00:00 2001
From: Samuel Remis <remiss@amazon.com>
Date: Wed, 14 Feb 2024 14:46:12 -0500
Subject: [PATCH] Add experimental flag for checking the request body before
 setting 100 continue

---
 botocore/handlers.py        |  8 ++++++++
 tests/functional/test_s3.py | 26 ++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/botocore/handlers.py b/botocore/handlers.py
index aa0ae98c0e..155442424b 100644
--- a/botocore/handlers.py
+++ b/botocore/handlers.py
@@ -346,6 +346,14 @@ def add_expect_header(model, params, **kwargs):
     if 'body' in params:
         body = params['body']
         if hasattr(body, 'read'):
+            check_body = utils.ensure_boolean(
+                os.environ.get(
+                    'BOTO_EXPERIMENTAL__CHECK_BODY_BEFORE_CONTINUE',
+                    '',
+                )
+            )
+            if check_body and utils.determine_content_length(body) == 0:
+                return
             # Any file like object will use an expect 100-continue
             # header regardless of size.
             logger.debug("Adding expect 100 continue header to request.")
diff --git a/tests/functional/test_s3.py b/tests/functional/test_s3.py
index f51f41d264..dab3775cef 100644
--- a/tests/functional/test_s3.py
+++ b/tests/functional/test_s3.py
@@ -3619,3 +3619,29 @@ def test_escape_keys_in_xml_put_bucket_lifecycle_configuration(self):
         self.assertNotIn(b"my\r\n\rprefix", request.body)
         self.assertIn(b"my&#xD;&#xA;&#xD;prefix", request.body)
         self.assert_correct_content_md5(request)
+
+
+class TestExpectContinueBehavior(BaseSessionTest):
+    def test_sets_100_continute_with_body(self):
+        op_kwargs = {
+            "Bucket": "mybucket",
+            "Key": "mykey",
+            "Body": b"foo",
+        }
+        s3 = _create_s3_client()
+        with ClientHTTPStubber(s3) as http_stubber:
+            http_stubber.add_response()
+            s3.put_object(**op_kwargs)
+            expect_header = http_stubber.requests[-1].headers.get("Expect")
+            self.assertIsNotNone(expect_header)
+            self.assertEqual(expect_header, b"100-continue")
+
+    def test_does_not_set_100_continute_with_empty_body(self):
+        self.environ["BOTO_EXPERIMENTAL__CHECK_BODY_BEFORE_CONTINUE"] = "True"
+        op_kwargs = {"Bucket": "mybucket", "Key": "mykey", "Body": ""}
+        s3 = _create_s3_client()
+        with ClientHTTPStubber(s3) as http_stubber:
+            http_stubber.add_response()
+            s3.put_object(**op_kwargs)
+            expect_header = http_stubber.requests[-1].headers.get("Expect")
+            self.assertIsNone(expect_header)