internal/impl: check for size cache overflow

The size cache is an int32. Store a -1 in it if the message size
overflows, and fall back to recomputing the size if the value is
negative. This means lamentable O(N^2) costs in marshaling,
but that's better than silently producing invalid output.

Also considered: Return an error. Avoids O(N^2) behavior, but gives the
user no good choices if they don't care the output being slow. Encoding
costs of messages this large are likely to be dominated by copying the
bytes rather than the size operation anyway, so slow-but-correct seems
like the most generally useful option.

We could store valid values for the range (0x7fffffff,0xfffffffe)
reserving only 0xffffffff as the overflow sentinel, but optimizing this
case seems less important than the code being obviously correct.

Fixes golang/protobuf#970.

Change-Id: I44f59ff81fdfbc8672dd5aec959d5153a081aab9
Reviewed-by: Joe Tsai <>
2 files changed
tree: 711528e35158b00afe276c15b20927f31febb183
  1. .gitignore
  2. .travis.yml
  9. cmd/
  10. compiler/
  11. encoding/
  12. go.mod
  13. go.sum
  14. integration_test.go
  15. internal/
  16. proto/
  17. reflect/
  18. regenerate.bash
  19. release.bash
  20. runtime/
  21. test.bash
  22. testing/
  23. types/

Next Generation Go Protocol Buffers

WARNING: This repository is in active development. There are no guarantees about API stability. Breaking changes will occur until a stable release is made and announced.

This repository is for the development of the next major Go implementation of protocol buffers. This library makes breaking API changes relative to the existing Go protobuf library. Of particular note, this API aims to make protobuf reflection a first-class feature of the API and implements the protobuf ecosystem in terms of reflection.

Design Documents

List of relevant design documents:


We appreciate community contributions. See

Reporting Issues

Issues regarding the new API can be filed at Please use a APIv2: prefix in the title to make it clear that the issue is regarding the new API work.