Support empty Cords with an expected checksum

PiperOrigin-RevId: 484578104
Change-Id: Ie4be3e4de27dc28d88395e16fd075fb10ab7a302

1 files changed, 145 insertions, 3 deletions

diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
index 1fc4be6e..9a72f7be 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc
@@ -1988,6 +1988,12 @@ TEST_P(CordTest, HugeCord) {
 
 // Tests that Append() works ok when handed a self reference
 TEST_P(CordTest, AppendSelf) {
+  // Test the empty case.
+  absl::Cord empty;
+  MaybeHarden(empty);
+  empty.Append(empty);
+  ASSERT_EQ(empty, "");
+
   // We run the test until data is ~16K
   // This guarantees it covers small, medium and large data.
   std::string control_data = "Abc";
@@ -2712,7 +2718,7 @@ class CordMutator {
 
 // clang-format off
 // This array is constant-initialized in conformant compilers.
-CordMutator cord_mutators[] ={
+CordMutator cord_mutators[] = {
   {"clear", [](absl::Cord& c) { c.Clear(); }},
   {"overwrite", [](absl::Cord& c) { c = "overwritten"; }},
   {
@@ -2742,6 +2748,25 @@ CordMutator cord_mutators[] ={
     [](absl::Cord& c) { c.RemoveSuffix(c.size() / 2); }
   },
   {
+    "append empty string",
+    [](absl::Cord& c) { c.Append(""); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "append empty cord",
+    [](absl::Cord& c) { c.Append(absl::Cord()); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "append empty checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_append;
+      to_append.SetExpectedChecksum(999);
+      c.Append(to_append);
+    },
+    [](absl::Cord& c) { }
+  },
+  {
     "prepend string",
     [](absl::Cord& c) { c.Prepend("9876543210"); },
     [](absl::Cord& c) { c.RemovePrefix(10); }
@@ -2763,12 +2788,33 @@ CordMutator cord_mutators[] ={
     [](absl::Cord& c) { c.RemovePrefix(10); }
   },
   {
+    "prepend empty string",
+    [](absl::Cord& c) { c.Prepend(""); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend empty cord",
+    [](absl::Cord& c) { c.Prepend(absl::Cord()); },
+    [](absl::Cord& c) { }
+  },
+  {
+    "prepend empty checksummed cord",
+    [](absl::Cord& c) {
+      absl::Cord to_prepend;
+      to_prepend.SetExpectedChecksum(999);
+      c.Prepend(to_prepend);
+    },
+    [](absl::Cord& c) { }
+  },
+  {
     "prepend self",
     [](absl::Cord& c) { c.Prepend(c); },
     [](absl::Cord& c) { c.RemovePrefix(c.size() / 2); }
   },
-  {"remove prefix", [](absl::Cord& c) { c.RemovePrefix(2); }},
-  {"remove suffix", [](absl::Cord& c) { c.RemoveSuffix(2); }},
+  {"remove prefix", [](absl::Cord& c) { c.RemovePrefix(c.size() / 2); }},
+  {"remove suffix", [](absl::Cord& c) { c.RemoveSuffix(c.size() / 2); }},
+  {"remove 0-prefix", [](absl::Cord& c) { c.RemovePrefix(0); }},
+  {"remove 0-suffix", [](absl::Cord& c) { c.RemoveSuffix(0); }},
   {"subcord", [](absl::Cord& c) { c = c.Subcord(1, c.size() - 2); }},
   {
     "swap inline",
@@ -2834,6 +2880,13 @@ TEST_P(CordTest, ExpectedChecksum) {
         c2.SetExpectedChecksum(24680);
 
         mutator.Mutate(c2);
+
+        if (c1 == c2) {
+          // Not a mutation (for example, appending the empty string).
+          // Whether the checksum is removed is not defined.
+          continue;
+        }
+
         EXPECT_EQ(c2.ExpectedChecksum(), absl::nullopt);
 
         if (mutator.CanUndo()) {
@@ -2903,3 +2956,92 @@ TEST_P(CordTest, ExpectedChecksum) {
     }
   }
 }
+
+// Test the special cases encountered with an empty checksummed cord.
+TEST_P(CordTest, ChecksummedEmptyCord) {
+  absl::Cord c1;
+  EXPECT_FALSE(c1.ExpectedChecksum().has_value());
+
+  // Setting an expected checksum works.
+  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);
+
+  absl::Cord c1_copy_assign;
+  c1_copy_assign = c1;
+  EXPECT_EQ(c1_copy_assign.ExpectedChecksum().value_or(0), 12345);
+
+  absl::Cord c1_move(std::move(c1_copy_assign));
+  EXPECT_EQ(c1_move.ExpectedChecksum().value_or(0), 12345);
+
+  EXPECT_EQ(c1.ExpectedChecksum().value_or(0), 12345);
+
+  // A CRC Cord compares equal to its non-CRC value.
+  EXPECT_EQ(c1, absl::Cord());
+
+  for (const CordMutator& mutator : cord_mutators) {
+    SCOPED_TRACE(mutator.Name());
+
+    // Exercise mutating an empty checksummed cord to catch crashes and exercise
+    // memory sanitizers.
+    absl::Cord c2;
+    c2.SetExpectedChecksum(24680);
+    mutator.Mutate(c2);
+
+    if (c2.empty()) {
+      // Not a mutation
+      continue;
+    }
+    EXPECT_EQ(c2.ExpectedChecksum(), absl::nullopt);
+
+    if (mutator.CanUndo()) {
+      mutator.Undo(c2);
+    }
+  }
+
+  absl::Cord c3;
+  c3.SetExpectedChecksum(999);
+  const absl::Cord& cc3 = c3;
+
+  // Test that all cord reading operations function in the face of an
+  // expected checksum.
+  EXPECT_TRUE(cc3.StartsWith(""));
+  EXPECT_TRUE(cc3.EndsWith(""));
+  EXPECT_TRUE(cc3.empty());
+  EXPECT_EQ(cc3, "");
+  EXPECT_EQ(cc3, absl::Cord());
+  EXPECT_EQ(cc3.size(), 0);
+  EXPECT_EQ(cc3.Compare(absl::Cord()), 0);
+  EXPECT_EQ(cc3.Compare(c1), 0);
+  EXPECT_EQ(cc3.Compare(cc3), 0);
+  EXPECT_EQ(cc3.Compare(""), 0);
+  EXPECT_EQ(cc3.Compare("wxyz"), -1);
+  EXPECT_EQ(cc3.Compare(absl::Cord("wxyz")), -1);
+  EXPECT_EQ(absl::Cord("wxyz").Compare(cc3), 1);
+  EXPECT_EQ(std::string(cc3), "");
+
+  std::string dest;
+  absl::CopyCordToString(cc3, &dest);
+  EXPECT_EQ(dest, "");
+
+  for (absl::string_view chunk : cc3.Chunks()) {  // NOLINT(unreachable loop)
+    static_cast<void>(chunk);
+    GTEST_FAIL() << "no chunks expected";
+  }
+  EXPECT_TRUE(cc3.chunk_begin() == cc3.chunk_end());
+
+  for (char ch : cc3.Chars()) {  // NOLINT(unreachable loop)
+    static_cast<void>(ch);
+    GTEST_FAIL() << "no chars expected";
+  }
+  EXPECT_TRUE(cc3.char_begin() == cc3.char_end());
+
+  EXPECT_EQ(cc3.TryFlat(), "");
+  EXPECT_EQ(absl::HashOf(c3), absl::HashOf(absl::Cord()));
+  EXPECT_EQ(absl::HashOf(c3), absl::HashOf(absl::string_view()));
+}