Cord: Avoid leaking a node if SetExpectedChecksum() is called on an

empty cord twice in a row.

PiperOrigin-RevId: 485367641
Change-Id: I6605ff25acbcef7c40b68e15a8888076a2da63be

absl/strings/cord.cc

@@ -857,6 +857,7 @@ inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
 void Cord::SetExpectedChecksum(uint32_t crc) {
   auto constexpr method = CordzUpdateTracker::kSetExpectedChecksum;
   if (empty()) {
+    contents_.MaybeRemoveEmptyCrcNode();
     CordRep* rep = CordRepCrc::New(nullptr, crc);
     contents_.EmplaceTree(rep, method);
   } else if (!contents_.is_tree()) {

absl/strings/cord_test.cc

@@ -2856,6 +2856,12 @@ TEST_P(CordTest, ExpectedChecksum) {
       EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
       EXPECT_EQ(c1, base_value);
 
+      // Test that setting an expected checksum again doesn't crash or leak
+      // memory.
+      c1.SetExpectedChecksum(12345);
+      EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+      EXPECT_EQ(c1, base_value);
+
       // CRC persists through copies, assignments, and moves:
       absl::Cord c1_copy_construct = c1;
       EXPECT_EQ(c1_copy_construct.ExpectedChecksum().value_or(0), 12345);
@@ -2968,6 +2974,12 @@ TEST_P(CordTest, ChecksummedEmptyCord) {
   EXPECT_EQ(c1, "");
   EXPECT_TRUE(c1.empty());
 
+  // Test that setting an expected checksum again doesn't crash or leak memory.
+  c1.SetExpectedChecksum(12345);
+  EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+  EXPECT_EQ(c1, "");
+  EXPECT_TRUE(c1.empty());
+
   // CRC persists through copies, assignments, and moves:
   absl::Cord c1_copy_construct = c1;
   EXPECT_EQ(c1_copy_construct.ExpectedChecksum().value_or(0), 12345);