Avoid ODR violations with flatbuffers::Verifier. (google#8274)

Fix "One Definition Rule" violation when using flatbuffers::Verifier with
FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE defined in some compilation units
and not defined in other compilation units.

The fix is to make Verifier a template class, with a boolean template
parameter replacing the "#ifdef" conditionals; to rename it as
VerifierTemplate; and then to use "#ifdef" only for a "using" declaration
that defines the original name Verifier an an alias for the instantiated
template.  In this way, even if FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE is
defined in some compilation units and not in others, as long as clients
only reference flatbuffers::Verifier in .cc files, not header files, there
will be no ODR violation, since the only part whose definition varies is the
"using" declaration, which does not have external linkage.

There is still some possibility of clients creating ODR violations
if the client header files (rather than .cc files) reference
flatbuffers::Verifier.  To avoid that, this change also deprecates
FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE, and instead introduces
flatbuffers::SizeVerifier as a public name for the template instance with
the boolean parameter set to true, so that clients don't need to define
the macro at all.

include/flatbuffers/verifier.h

@@ -23,7 +23,8 @@
 namespace flatbuffers {
 
 // Helper class to verify the integrity of a FlatBuffer
-class Verifier FLATBUFFERS_FINAL_CLASS {
+template <bool TrackVerifierBufferSize>
+class VerifierTemplate FLATBUFFERS_FINAL_CLASS {
  public:
   struct Options {
     // The maximum nesting of tables and vectors before we call it invalid.
@@ -40,17 +41,18 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     bool assert = false;
   };
 
-  explicit Verifier(const uint8_t *const buf, const size_t buf_len,
-                    const Options &opts)
+  explicit VerifierTemplate(const uint8_t *const buf, const size_t buf_len,
+                            const Options &opts)
       : buf_(buf), size_(buf_len), opts_(opts) {
     FLATBUFFERS_ASSERT(size_ < opts.max_size);
   }
 
-  // Deprecated API, please construct with Verifier::Options.
-  Verifier(const uint8_t *const buf, const size_t buf_len,
-           const uoffset_t max_depth = 64, const uoffset_t max_tables = 1000000,
-           const bool check_alignment = true)
-      : Verifier(buf, buf_len, [&] {
+  // Deprecated API, please construct with VerifierTemplate::Options.
+  VerifierTemplate(const uint8_t *const buf, const size_t buf_len,
+                   const uoffset_t max_depth = 64,
+                   const uoffset_t max_tables = 1000000,
+                   const bool check_alignment = true)
+      : VerifierTemplate(buf, buf_len, [&] {
           Options opts;
           opts.max_depth = max_depth;
           opts.max_tables = max_tables;
@@ -62,25 +64,25 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
   bool Check(const bool ok) const {
     // clang-format off
     #ifdef FLATBUFFERS_DEBUG_VERIFICATION_FAILURE
-       if (opts_.assert) { FLATBUFFERS_ASSERT(ok); }
-    #endif
-    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
-      if (!ok)
-        upper_bound_ = 0;
+      if (opts_.assert) { FLATBUFFERS_ASSERT(ok); }
     #endif
     // clang-format on
+    if (TrackVerifierBufferSize) {
+      if (!ok) {
+        upper_bound_ = 0;
+      }
+    }
     return ok;
   }
 
   // Verify any range within the buffer.
   bool Verify(const size_t elem, const size_t elem_len) const {
-    // clang-format off
-    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+    if (TrackVerifierBufferSize) {
       auto upper_bound = elem + elem_len;
-      if (upper_bound_ < upper_bound)
+      if (upper_bound_ < upper_bound) {
         upper_bound_ =  upper_bound;
-    #endif
-    // clang-format on
+      }
+    }
     return Check(elem_len < size_ && elem <= size_ - elem_len);
   }
 
@@ -210,14 +212,14 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
 
     // Call T::Verify, which must be in the generated code for this type.
     const auto o = VerifyOffset<uoffset_t>(start);
-    return Check(o != 0) &&
-           reinterpret_cast<const T *>(buf_ + start + o)->Verify(*this)
-    // clang-format off
-    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
-           && GetComputedSize()
-    #endif
-        ;
-    // clang-format on
+    if (!Check(o != 0)) return false;
+    if (!(reinterpret_cast<const T *>(buf_ + start + o)->Verify(*this))) {
+      return false;
+    }
+    if (TrackVerifierBufferSize) {
+      if (GetComputedSize() == 0) return false;
+    }
+    return true;
   }
 
   template<typename T, int &..., typename SizeT>
@@ -232,7 +234,8 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     // If there is a nested buffer, it must be greater than the min size.
     if (!Check(buf->size() >= FLATBUFFERS_MIN_BUFFER_SIZE)) return false;
 
-    Verifier nested_verifier(buf->data(), buf->size(), opts_);
+    VerifierTemplate<TrackVerifierBufferSize> nested_verifier(
+        buf->data(), buf->size(), opts_);
     return nested_verifier.VerifyBuffer<T>(identifier);
   }
 
@@ -286,21 +289,27 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     return true;
   }
 
-  // Returns the message size in bytes
+  // Returns the message size in bytes.
+  //
+  // This should only be called after first calling VerifyBuffer or
+  // VerifySizePrefixedBuffer.
+  //
+  // This method should only be called for VerifierTemplate instances
+  // where the TrackVerifierBufferSize template parameter is true,
+  // i.e. for SizeVerifier.  For instances where TrackVerifierBufferSize
+  // is false, this fails at runtime or returns zero.
   size_t GetComputedSize() const {
-    // clang-format off
-    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+    if (TrackVerifierBufferSize) {
       uintptr_t size = upper_bound_;
       // Align the size to uoffset_t
       size = (size - 1 + sizeof(uoffset_t)) & ~(sizeof(uoffset_t) - 1);
       return (size > size_) ?  0 : size;
-    #else
-      // Must turn on FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE for this to work.
-      (void)upper_bound_;
-      FLATBUFFERS_ASSERT(false);
-      return 0;
-    #endif
-    // clang-format on
+    }
+    // Must use SizeVerifier, or (deprecated) turn on
+    // FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE, for this to work.
+    (void)upper_bound_;
+    FLATBUFFERS_ASSERT(false);
+    return 0;
   }
 
   std::vector<uint8_t> *GetFlexReuseTracker() { return flex_reuse_tracker_; }
@@ -323,10 +332,33 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
 
 // Specialization for 64-bit offsets.
 template<>
-inline size_t Verifier::VerifyOffset<uoffset64_t>(const size_t start) const {
+template<>
+inline size_t VerifierTemplate<false>::VerifyOffset<uoffset64_t>(
+    const size_t start) const {
+  return VerifyOffset<uoffset64_t, soffset64_t>(start);
+}
+template<>
+template<>
+inline size_t VerifierTemplate<true>::VerifyOffset<uoffset64_t>(
+    const size_t start) const {
   return VerifyOffset<uoffset64_t, soffset64_t>(start);
 }
 
+// Instance of VerifierTemplate that supports GetComputedSize().
+using SizeVerifier = VerifierTemplate</*TrackVerifierBufferSize = */ true>;
+
+// The FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE build configuration macro is
+// deprecated, and should not be defined, since it is easy to misuse in ways
+// that result in ODR violations. Rather than using Verifier and defining
+// FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE, please use SizeVerifier instead.
+#ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE  // Deprecated, see above.
+using Verifier = SizeVerifier;
+#else
+// Instance of VerifierTemplate that is slightly faster, but does not
+// support GetComputedSize().
+using Verifier = VerifierTemplate</*TrackVerifierBufferSize = */ false>;
+#endif
+
 }  // namespace flatbuffers
 
 #endif  // FLATBUFFERS_VERIFIER_H_