Verifier computes the buffersize, useful for streaming

Close #3898

include/flatbuffers/flatbuffers.h

@@ -1216,6 +1216,9 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
            size_t _max_tables = 1000000)
     : buf_(buf), end_(buf + buf_len), depth_(0), max_depth_(_max_depth),
       num_tables_(0), max_tables_(_max_tables)
+    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+      , upper_bound_(buf)
+    #endif
     {}
 
   // Central location where any verification failures register.
@@ -1223,11 +1226,20 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     #ifdef FLATBUFFERS_DEBUG_VERIFICATION_FAILURE
       assert(ok);
     #endif
+    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+      if (!ok)
+        upper_bound_ = buf_;
+    #endif
     return ok;
   }
 
   // Verify any range within the buffer.
   bool Verify(const void *elem, size_t elem_len) const {
+    #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+      auto upper_bound = reinterpret_cast<const uint8_t *>(elem) + elem_len;
+      if (upper_bound_ < upper_bound)
+        upper_bound_ =  upper_bound;
+    #endif
     return Check(elem_len <= (size_t) (end_ - buf_) &&
                  elem >= buf_ &&
                  elem <= end_ - elem_len);
@@ -1306,7 +1318,11 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     // Call T::Verify, which must be in the generated code for this type.
     return Verify<uoffset_t>(buf_) &&
       reinterpret_cast<const T *>(buf_ + ReadScalar<uoffset_t>(buf_))->
-        Verify(*this);
+        Verify(*this)
+        #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+          && GetComputedSize()
+        #endif
+            ;
   }
 
   // Called at the start of a table to increase counters measuring data
@@ -1325,6 +1341,16 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     return true;
   }
 
+  #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+  // Returns the message size in bytes
+  size_t GetComputedSize() const {
+    uintptr_t size = upper_bound_ - buf_;
+    // Align the size to uoffset_t
+    size = (size - 1 + sizeof(uoffset_t)) & -uintptr_t(sizeof(uoffset_t));
+    return (buf_  + size > end_) ?  0 : size;
+  }
+  #endif
+
  private:
   const uint8_t *buf_;
   const uint8_t *end_;
@@ -1332,6 +1358,9 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
   size_t max_depth_;
   size_t num_tables_;
   size_t max_tables_;
+  #ifdef FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
+  mutable const uint8_t *upper_bound_;
+  #endif
 };
 
 // Convenient way to bundle a buffer and its length, to pass it around

tests/test.cpp

@@ -15,6 +15,7 @@
  */
 
 #define FLATBUFFERS_DEBUG_VERIFICATION_FAILURE 1
+#define FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE
 
 #include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/idl.h"
@@ -160,6 +161,23 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, size_t length) {
   flatbuffers::Verifier verifier(flatbuf, length);
   TEST_EQ(VerifyMonsterBuffer(verifier), true);
 
+  std::vector<uint8_t> test_buff;
+  test_buff.resize(length * 2);
+  std::memcpy(&test_buff[0], flatbuf , length);
+  std::memcpy(&test_buff[length], flatbuf , length);
+
+  flatbuffers::Verifier verifierl(&test_buff[0], length - 1);
+  TEST_EQ(VerifyMonsterBuffer(verifierl), false);
+  TEST_EQ(verifierl.GetComputedSize(), 0);
+
+  flatbuffers::Verifier verifier1(&test_buff[0], length);
+  TEST_EQ(VerifyMonsterBuffer(verifier1), true);
+  TEST_EQ(verifier1.GetComputedSize(), length);
+
+  flatbuffers::Verifier verifier2(&test_buff[length], length);
+  TEST_EQ(VerifyMonsterBuffer(verifier2), true);
+  TEST_EQ(verifier2.GetComputedSize(), length);
+
   TEST_EQ(strcmp(MonsterIdentifier(), "MONS"), 0);
   TEST_EQ(MonsterBufferHasIdentifier(flatbuf), true);
   TEST_EQ(strcmp(MonsterExtension(), "mon"), 0);