Pure refactor of `MULTIPLE_CONTENT_LENGTH_KEYS` logic

Make the cases that can lead to this error more clear since we'll be changing one of them in a follow up CL. Also add a test case for when we already received a valid content-length but encounter an invalid one (it fails with MULTIPLE_CONTENT_LENGTH_KEYS instead of the overflow). Also delete some repetitive invocations in test that don't add any value.

Protected by pure refactor.

PiperOrigin-RevId: 875760693

quiche/balsa/balsa_frame.cc

diff --git a/quiche/balsa/balsa_frame.cc b/quiche/balsa/balsa_frame.cc
index 80baeb1..ded79f7 100644
--- a/quiche/balsa/balsa_frame.cc
+++ b/quiche/balsa/balsa_frame.cc
@@ -797,14 +797,30 @@
         content_length_remaining_ = length;
         continue;
       }
-      if ((headers->content_length_status_ != content_length_status) ||
-          ((headers->content_length_status_ ==
-            BalsaHeadersEnums::VALID_CONTENT_LENGTH) &&
-           (http_validation_policy().disallow_multiple_content_length ||
-            length != headers->content_length_))) {
+
+      // There are multiple content-length keys in this request.
+
+      // First case: the second CL header conflicts with the first CL. This
+      // request _must_ be rejected.
+      bool found_new_invalid_content_length =
+          headers->content_length_status_ != content_length_status;
+      bool found_new_content_length_with_different_value =
+          length != headers->content_length_;
+      if (found_new_invalid_content_length ||
+          found_new_content_length_with_different_value) {
         HandleError(BalsaFrameEnums::MULTIPLE_CONTENT_LENGTH_KEYS);
         return;
       }
+
+      // Second case: CL is duplicated but the value is valid and the same.
+      // Optionally, reject this per the RFC or simply keep one value.
+      if (headers->content_length_status_ ==
+          BalsaHeadersEnums::VALID_CONTENT_LENGTH) {
+        if (http_validation_policy().disallow_multiple_content_length) {
+          HandleError(BalsaFrameEnums::MULTIPLE_CONTENT_LENGTH_KEYS);
+          return;
+        }
+      }
       continue;
     }
     if (absl::EqualsIgnoreCase(key, kTransferEncoding)) {

quiche/balsa/balsa_frame_test.cc

diff --git a/quiche/balsa/balsa_frame_test.cc b/quiche/balsa/balsa_frame_test.cc
index c1bf58c..65db151 100644
--- a/quiche/balsa/balsa_frame_test.cc
+++ b/quiche/balsa/balsa_frame_test.cc
@@ -3622,6 +3622,19 @@
   }
 }
 
+TEST_F(HTTPBalsaFrameTest, TwoContentLengthFirstValidSecondOverflow) {
+  std::string header =
+      "HTTP/1.1 200 OK\r\n"
+      "content-length: 12\r\n"
+      "content-length: 9223372036854775807999999\r\n"
+      "\r\n";
+  balsa_frame_.set_is_request(false);
+  balsa_frame_.ProcessInput(header.data(), header.size());
+  EXPECT_TRUE(balsa_frame_.Error());
+  EXPECT_EQ(BalsaFrameEnums::MULTIPLE_CONTENT_LENGTH_KEYS,
+            balsa_frame_.ErrorCode());
+}
+
 TEST_F(HTTPBalsaFrameTest, TwoDifferentContentLengthHeadersIsAnError) {
   std::string header =
       "HTTP/1.1 200 OK\r\n"
@@ -3649,6 +3662,7 @@
   EXPECT_EQ(BalsaFrameEnums::BALSA_NO_ERROR, balsa_frame_.ErrorCode());
   EXPECT_FALSE(balsa_frame_.Error());
   EXPECT_TRUE(balsa_frame_.MessageFullyRead());
+  EXPECT_THAT(headers_.GetAllOfHeaderAsString("Content-Length"), "1,1");
 }
 
 TEST_F(HTTPBalsaFrameTest, TwoSameContentLengthHeadersIsAnError) {