Export of internal Abseil changes

--
803abc2dcad8b2354c988e9bf58dac4a17683832 by Gennadiy Rozental <rogeeff@google.com>:

Avoid warning when RTTI is not enabled.

PiperOrigin-RevId: 294247546

--
5a7b0b4d07d1d6e56fbb0b0ffbf4f8fcab772dbf by Derek Mauro <dmauro@google.com>:

Add a public Abseil FAQ

PiperOrigin-RevId: 294226960

--
6945c4a6df7d7679711fea31aacf4fba6ac7baa1 by Gennadiy Rozental <rogeeff@google.com>:

Re-enable type mismatch check, which works in all the cases including shared libraries.

We will use RTTI in case when our hand written approximation of it reports a type mismatch. This way we can ensure that if a flag is defined in one shared object and referenced in another we do not report spurious errors.

PiperOrigin-RevId: 293905563
GitOrigin-RevId: 803abc2dcad8b2354c988e9bf58dac4a17683832
Change-Id: I1a23776d227ed2734c2e7183323786b7a95c3cc7

3 files changed, 69 insertions, 43 deletions

diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index 4bc0c12fc68c..6a0b5fad89c7 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -21,9 +21,11 @@
 
 #include <memory>
 #include <string>
+#include <typeinfo>
 
 #include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/flags/config.h"
 #include "absl/flags/marshalling.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
@@ -41,7 +43,10 @@ enum FlagOp {
   kCopyConstruct,
   kSizeof,
   kParse,
-  kUnparse
+  kUnparse,
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+  kRuntimeTypeId
+#endif
 };
 using FlagOpFn = void* (*)(FlagOp, const void*, void*);
 using FlagMarshallingOpFn = void* (*)(FlagOp, const void*, void*, void*);
@@ -84,6 +89,11 @@ void* FlagOps(FlagOp op, const void* v1, void* v2) {
       return nullptr;
     case kSizeof:
       return reinterpret_cast<void*>(sizeof(T));
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+    case kRuntimeTypeId:
+      return const_cast<std::type_info*>(&typeid(T));
+      break;
+#endif
     default:
       return nullptr;
   }
@@ -146,6 +156,13 @@ inline size_t Sizeof(FlagOpFn op) {
       op(flags_internal::kSizeof, nullptr, nullptr)));
 }
 
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+inline const std::type_info& RuntimeTypeId(FlagOpFn op) {
+  return *static_cast<const std::type_info*>(
+      op(flags_internal::kRuntimeTypeId, nullptr, nullptr));
+}
+#endif
+
 // Handle to FlagState objects. Specific flag state objects will restore state
 // of a flag produced this flag state from method CommandLineFlag::SaveState().
 class FlagStateInterface {
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index 6ce7def2d3d2..cfc0cf4d93d1 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc
@@ -56,6 +56,14 @@ bool ShouldValidateFlagValue(FlagOpFn flag_type_id) {
   return true;
 }
 
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+bool MatchRuntimeTypeId(FlagOpFn lhs_type_id, FlagOpFn rhs_type_id) {
+  return RuntimeTypeId(lhs_type_id) == RuntimeTypeId(rhs_type_id);
+}
+#else
+bool MatchRuntimeTypeId(FlagOpFn, FlagOpFn) { return true; }
+#endif
+
 // RAII helper used to temporarily unlock and relock `absl::Mutex`.
 // This is used when we need to ensure that locks are released while
 // invoking user supplied callbacks and then reacquired, since callbacks may
@@ -133,6 +141,18 @@ void FlagImpl::Destroy() {
   is_data_guard_inited_ = false;
 }
 
+void FlagImpl::AssertValidType(const flags_internal::FlagOpFn op) const {
+  // `op` is the unmarshaling operation corresponding to the declaration
+  // visibile at the call site. `op_` is the Flag's defined unmarshalling
+  // operation. They must match for this operation to be well-defined.
+  if (ABSL_PREDICT_FALSE(op != op_) && !MatchRuntimeTypeId(op, op_)) {
+    ABSL_INTERNAL_LOG(
+        FATAL,
+        absl::StrCat("Flag '", Name(),
+                     "' is defined as one type and declared as another"));
+  }
+}
+
 std::unique_ptr<void, DynValueDeleter> FlagImpl::MakeInitValue() const {
   void* res = nullptr;
   if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
@@ -219,7 +239,7 @@ bool FlagImpl::RestoreState(const void* value, bool modified,
     if (counter_ == counter) return false;
   }
 
-  Write(value, op_);
+  Write(value);
 
   {
     absl::MutexLock l(DataGuard());
@@ -254,18 +274,9 @@ bool FlagImpl::TryParse(void** dst, absl::string_view value,
   return true;
 }
 
-void FlagImpl::Read(void* dst, const flags_internal::FlagOpFn dst_op) const {
+void FlagImpl::Read(void* dst) const {
   absl::ReaderMutexLock l(DataGuard());
 
-  // `dst_op` is the unmarshaling operation corresponding to the declaration
-  // visibile at the call site. `op` is the Flag's defined unmarshalling
-  // operation. They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(dst_op != op_)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", Name(),
-                     "' is defined as one type and declared as another"));
-  }
   CopyConstruct(op_, value_.dynamic, dst);
 }
 
@@ -286,19 +297,9 @@ void FlagImpl::StoreAtomic() {
 #endif
 }
 
-void FlagImpl::Write(const void* src, const flags_internal::FlagOpFn src_op) {
+void FlagImpl::Write(const void* src) {
   absl::MutexLock l(DataGuard());
 
-  // `src_op` is the marshalling operation corresponding to the declaration
-  // visible at the call site. `op` is the Flag's defined marshalling operation.
-  // They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(src_op != op_)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", Name(),
-                     "' is defined as one type and declared as another"));
-  }
-
   if (ShouldValidateFlagValue(op_)) {
     void* obj = Clone(op_, src);
     std::string ignored_error;
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index b5471fa8a734..c6c4a2f7beb5 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -301,41 +301,44 @@ class FlagImpl {
   bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(*DataGuard());
   std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
   std::string CurrentValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
-  void Read(void* dst, const FlagOpFn dst_op) const
-      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void Read(void* dst) const ABSL_LOCKS_EXCLUDED(*DataGuard());
   // Attempts to parse supplied `value` std::string. If parsing is successful, then
   // it replaces `dst` with the new value.
   bool TryParse(void** dst, absl::string_view value, std::string* err) const
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
 
-#ifndef NDEBUG
-  template <typename T>
-  void Get(T* dst) const {
-    Read(dst, &FlagOps<T>);
-  }
-#else
   template <typename T, typename std::enable_if<
                             !IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
   void Get(T* dst) const {
-    Read(dst, &FlagOps<T>);
+    AssertValidType(&flags_internal::FlagOps<T>);
+    Read(dst);
   }
   // Overload for `GetFlag()` for types that support lock-free reads.
   template <typename T, typename std::enable_if<IsAtomicFlagTypeTrait<T>::value,
                                                 int>::type = 0>
   void Get(T* dst) const {
-    using U = BestAtomicType<T>;
-    const typename U::type r = value_.atomics.template load<T>();
+    // For flags of types which can be accessed "atomically" we want to avoid
+    // slowing down flag value access due to type validation. That's why
+    // this validation is hidden behind !NDEBUG
+#ifndef NDEBUG
+    AssertValidType(&flags_internal::FlagOps<T>);
+#endif
+    using U = flags_internal::BestAtomicType<T>;
+    typename U::type r = value_.atomics.template load<T>();
     if (r != U::AtomicInit()) {
       std::memcpy(static_cast<void*>(dst), &r, sizeof(T));
     } else {
-      Read(dst, &FlagOps<T>);
+      Read(dst);
     }
   }
-#endif
+  template <typename T>
+  void Set(const T& src) {
+    AssertValidType(&flags_internal::FlagOps<T>);
+    Write(&src);
+  }
 
   // Mutating access methods
-  void Write(const void* src, const FlagOpFn src_op)
-      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard());
   bool SetFromString(absl::string_view value, FlagSettingMode set_mode,
                      ValueSource source, std::string* err)
       ABSL_LOCKS_EXCLUDED(*DataGuard());
@@ -383,6 +386,13 @@ class FlagImpl {
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()) {
     return static_cast<FlagDefaultKind>(def_kind_);
   }
+  // Used in read/write operations to validate source/target has correct type.
+  // For example if flag is declared as absl::Flag<int> FLAGS_foo, a call to
+  // absl::GetFlag(FLAGS_foo) validates that the type of FLAGS_foo is indeed
+  // int. To do that we pass the "assumed" type id (which is deduced from type
+  // int) as an argument `op`, which is in turn is validated against the type id
+  // stored in flag object by flag definition statement.
+  void AssertValidType(const flags_internal::FlagOpFn op) const;
 
   // Immutable flag's state.
 
@@ -461,9 +471,7 @@ class Flag final : public flags_internal::CommandLineFlag {
     impl_.Get(&u.value);
     return std::move(u.value);
   }
-
-  void Set(const T& v) { impl_.Write(&v, &FlagOps<T>); }
-
+  void Set(const T& v) { impl_.Set(v); }
   void SetCallback(const FlagCallbackFunc mutation_callback) {
     impl_.SetCallback(mutation_callback);
   }
@@ -509,10 +517,10 @@ class Flag final : public flags_internal::CommandLineFlag {
 
   void Destroy() override { impl_.Destroy(); }
 
-  void Read(void* dst) const override { impl_.Read(dst, &FlagOps<T>); }
+  void Read(void* dst) const override { impl_.Read(dst); }
   FlagOpFn TypeId() const override { return &FlagOps<T>; }
 
-  // Flag's implementation with value type abstracted out.
+  // Flag's data
   FlagImpl impl_;
 };