Export of internal Abseil changes

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91d76b3ac9edff91f206d9eee60423c39eeeaf93 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 430442277

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9f8a87bcc5cc5b0fd8b7f0318f37d152fd8bea06 by Evan Brown <ezb@google.com>:

Small refactoring to work towards allowing for node_btree_* containers.

- Change common_params::transfer to rely on slot_policy transfer instead of manually doing construct/destruct. Transfer is not construct/destruct for node containers.
- Move maps' value_compare into btree.h from btree_map.h so it can be reused for node_btree_map.h.
- Also add a test for maps' value_compare protected members.

PiperOrigin-RevId: 430245542

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0126e0b6295342317d9c9f0a66e2d7009b858426 by Martijn Vels <mvels@google.com>:

Add CordRepSubString::Create function with hard checks on IsFlat() | IsExternal()

This hardens internal invariants, IsFlat() || IsExternal() is a cheap, single predicted branch, and removes boilerplate code from cord.cc

PiperOrigin-RevId: 429676041

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ed98a92af49d9e238d9f1d1b69fb4eddcd1ccbc7 by Abseil Team <absl-team@google.com>:

tweaks to status.h documentation to reflect general-purpose communication

PiperOrigin-RevId: 429584104
GitOrigin-RevId: 91d76b3ac9edff91f206d9eee60423c39eeeaf93
Change-Id: I54d6d116a564f86a842b983ca76559bf9b388f72

4 files changed, 60 insertions, 56 deletions

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 6547c2da..b65dc58b 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -151,23 +151,6 @@ void InitializeCordRepExternal(absl::string_view data, CordRepExternal* rep) {
 
 }  // namespace cord_internal
 
-static CordRep* NewSubstring(CordRep* child, size_t offset, size_t length) {
-  // Never create empty substring nodes
-  if (length == 0) {
-    CordRep::Unref(child);
-    return nullptr;
-  } else {
-    CordRepSubstring* rep = new CordRepSubstring();
-    assert(child->IsExternal() || child->IsFlat());
-    assert((offset + length) <= child->length);
-    rep->length = length;
-    rep->tag = cord_internal::SUBSTRING;
-    rep->start = offset;
-    rep->child = child;
-    return VerifyTree(rep);
-  }
-}
-
 // Creates a CordRep from the provided string. If the string is large enough,
 // and not wasteful, we move the string into an external cord rep, preserving
 // the already allocated string contents.
@@ -643,7 +626,7 @@ static CordRep* RemovePrefixFrom(CordRep* node, size_t n) {
       start += node->substring()->start;
       node = node->substring()->child;
     }
-    node = NewSubstring(CordRep::Ref(node), start, len);
+    node = CordRepSubstring::Create(CordRep::Ref(node), start, len);
   }
   while (!rhs_stack.empty()) {
     node = Concat(node, CordRep::Ref(rhs_stack.back()));
@@ -678,7 +661,7 @@ static CordRep* RemoveSuffixFrom(CordRep* node, size_t n) {
       start = node->substring()->start;
       node = node->substring()->child;
     }
-    node = NewSubstring(CordRep::Ref(node), start, len);
+    node = CordRepSubstring::Create(CordRep::Ref(node), start, len);
   }
   while (!lhs_stack.empty()) {
     node = Concat(CordRep::Ref(lhs_stack.back()), node);
@@ -768,7 +751,7 @@ static CordRep* NewSubRange(CordRep* node, size_t pos, size_t n) {
         pos += node->substring()->start;
         node = node->substring()->child;
       }
-      results.push_back(NewSubstring(CordRep::Ref(node), pos, n));
+      results.push_back(CordRepSubstring::Create(CordRep::Ref(node), pos, n));
     }
   } while (!todo.empty());
   assert(results.size() == 1);
@@ -1157,12 +1140,7 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
                                            : payload->flat()->Data();
   const size_t offset = current_chunk_.data() - data;
 
-  CordRepSubstring* tree = new CordRepSubstring();
-  tree->tag = cord_internal::SUBSTRING;
-  tree->length = n;
-  tree->start = offset;
-  tree->child = CordRep::Ref(payload);
-
+  auto* tree = CordRepSubstring::Create(CordRep::Ref(payload), offset, n);
   subcord.contents_.EmplaceTree(VerifyTree(tree), method);
   bytes_remaining_ -= n;
   current_chunk_.remove_prefix(n);
diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
index 9dcc4ce5..f69209de 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc
@@ -56,6 +56,7 @@ using absl::cord_internal::CordRepCrc;
 using absl::cord_internal::CordRepExternal;
 using absl::cord_internal::CordRepFlat;
 using absl::cord_internal::CordRepSubstring;
+using absl::cord_internal::CordzUpdateTracker;
 using absl::cord_internal::kFlatOverhead;
 using absl::cord_internal::kMaxFlatLength;
 
@@ -212,14 +213,9 @@ class CordTestPeer {
     ABSL_RAW_CHECK(src.ExpectedChecksum() == absl::nullopt,
                    "Can not be hardened");
     Cord cord;
-    auto* rep = new cord_internal::CordRepSubstring;
-    rep->tag = cord_internal::SUBSTRING;
-    rep->child = cord_internal::CordRep::Ref(
-        cord_internal::SkipCrcNode(src.contents_.tree()));
-    rep->start = offset;
-    rep->length = length;
-    cord.contents_.EmplaceTree(rep,
-                               cord_internal::CordzUpdateTracker::kSubCord);
+    auto* tree = cord_internal::SkipCrcNode(src.contents_.tree());
+    auto* rep = CordRepSubstring::Create(CordRep::Ref(tree), offset, length);
+    cord.contents_.EmplaceTree(rep, CordzUpdateTracker::kSubCord);
     return cord;
   }
 };
@@ -645,15 +641,6 @@ TEST_P(CordTest, TryFlatSubstrExternal) {
   EXPECT_EQ(sub.TryFlat(), "ell");
 }
 
-TEST_P(CordTest, TryFlatSubstrConcat) {
-  absl::Cord c = absl::MakeFragmentedCord({"hello", " world"});
-  absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
-  MaybeHarden(sub);
-  EXPECT_EQ(sub.TryFlat(), absl::nullopt);
-  c.RemovePrefix(1);
-  EXPECT_EQ(c.TryFlat(), absl::nullopt);
-}
-
 TEST_P(CordTest, TryFlatCommonlyAssumedInvariants) {
   // The behavior tested below is not part of the API contract of Cord, but it's
   // something we intend to be true in our current implementation.  This test
diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
index 06119350..b6b06cfa 100644
--- a/absl/strings/internal/cord_internal.cc
+++ b/absl/strings/internal/cord_internal.cc
@@ -17,11 +17,13 @@
 #include <cassert>
 #include <memory>
 
+#include "absl/base/internal/raw_logging.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/internal/cord_rep_btree.h"
 #include "absl/strings/internal/cord_rep_crc.h"
 #include "absl/strings/internal/cord_rep_flat.h"
 #include "absl/strings/internal/cord_rep_ring.h"
+#include "absl/strings/str_cat.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -33,6 +35,11 @@ ABSL_CONST_INIT std::atomic<bool> shallow_subcords_enabled(
     kCordShallowSubcordsDefault);
 ABSL_CONST_INIT std::atomic<bool> cord_btree_exhaustive_validation(false);
 
+void LogFatalNodeType(CordRep* rep) {
+  ABSL_INTERNAL_LOG(FATAL, absl::StrCat("Unexpected node type: ",
+                                        static_cast<int>(rep->tag)));
+}
+
 void CordRep::Destroy(CordRep* rep) {
   assert(rep != nullptr);
 
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 8db6aa6d..ece3db15 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -21,6 +21,7 @@
 #include <cstdint>
 #include <type_traits>
 
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/endian.h"
 #include "absl/base/internal/invoke.h"
@@ -33,6 +34,19 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+// The overhead of a vtable is too much for Cord, so we roll our own subclasses
+// using only a single byte to differentiate classes from each other - the "tag"
+// byte.  Define the subclasses first so we can provide downcasting helper
+// functions in the base class.
+struct CordRep;
+struct CordRepConcat;
+struct CordRepExternal;
+struct CordRepFlat;
+struct CordRepSubstring;
+struct CordRepCrc;
+class CordRepRing;
+class CordRepBtree;
+
 class CordzInfo;
 
 // Default feature enable states for cord ring buffers
@@ -74,6 +88,9 @@ enum Constants {
   kMaxBytesToCopy = 511
 };
 
+// Emits a fatal error "Unexpected node type: xyz" and aborts the program.
+ABSL_ATTRIBUTE_NORETURN void LogFatalNodeType(CordRep* rep);
+
 // Compact class for tracking the reference count and state flags for CordRep
 // instances.  Data is stored in an atomic int32_t for compactness and speed.
 class RefcountAndFlags {
@@ -156,19 +173,6 @@ class RefcountAndFlags {
   std::atomic<int32_t> count_;
 };
 
-// The overhead of a vtable is too much for Cord, so we roll our own subclasses
-// using only a single byte to differentiate classes from each other - the "tag"
-// byte.  Define the subclasses first so we can provide downcasting helper
-// functions in the base class.
-
-struct CordRepConcat;
-struct CordRepExternal;
-struct CordRepFlat;
-struct CordRepSubstring;
-struct CordRepCrc;
-class CordRepRing;
-class CordRepBtree;
-
 // Various representations that we allow
 enum CordRepKind {
   UNUSED_0 = 0,
@@ -276,6 +280,12 @@ struct CordRep {
 struct CordRepSubstring : public CordRep {
   size_t start;  // Starting offset of substring in child
   CordRep* child;
+
+  // Creates a substring on `child`, adopting a reference on `child`.
+  // Requires `child` to be either a flat or external node, and `pos` and `n` to
+  // form a non-empty partial sub range of `'child`, i.e.:
+  // `n > 0 && n < length && n + pos <= length`
+  static inline CordRepSubstring* Create(CordRep* child, size_t pos, size_t n);
 };
 
 // Type for function pointer that will invoke the releaser function and also
@@ -339,6 +349,28 @@ struct CordRepExternalImpl
   }
 };
 
+inline CordRepSubstring* CordRepSubstring::Create(CordRep* child, size_t pos,
+                                                  size_t n) {
+  assert(child != nullptr);
+  assert(n > 0);
+  assert(n < child->length);
+  assert(pos < child->length);
+  assert(n <= child->length - pos);
+
+  // TODO(b/217376272): Harden internal logic.
+  // Move to strategical places inside the Cord logic and make this an assert.
+  if (ABSL_PREDICT_FALSE(!(child->IsExternal() || child->IsFlat()))) {
+    LogFatalNodeType(child);
+  }
+
+  CordRepSubstring* rep = new CordRepSubstring();
+  rep->length = n;
+  rep->tag = SUBSTRING;
+  rep->start = pos;
+  rep->child = child;
+  return rep;
+}
+
 inline void CordRepExternal::Delete(CordRep* rep) {
   assert(rep != nullptr && rep->IsExternal());
   auto* rep_external = static_cast<CordRepExternal*>(rep);