Fix Substrait Build Protoc Version Error

Fix Substrait Build Protoc Version Error | Generated by AI

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Understanding the Error

This build failure is in the substrait crate (v0.58.0), which GreptimeDB depends on for cross-language query planning. The error stems from the protobuf compiler (protoc) rejecting the substrait/algebra.proto file because it uses “optional” fields in proto3 syntax—a feature that requires either:

Protoc version ≥3.21 (where support is stable and no special flag is needed), or
An older protoc (e.g., 3.15–3.20) with the --experimental_allow_proto3_optional flag passed during compilation.

The prost-build tool in substrait’s build script doesn’t pass this flag, so it assumes a compatible protoc version. Your system’s protobuf-compiler (likely from apt on Ubuntu 22.04 or earlier, giving ~3.12.4) is too old and lacks the flag, causing the panic.

GreptimeDB’s docs specify protoc ≥3.15, but for this dependency, ≥3.21 is effectively required.

Quick Fix: Upgrade Protoc to ≥3.21

The easiest, non-root way is to download and install the official binary release (no compilation needed). Here’s how:

Download the Latest Protoc:
- Go to Protocol Buffers Releases.
- Grab the latest protoc-<version>-linux-x86_64.zip (e.g., protoc-28.1-linux-x86_64.zip or whatever is current—anything ≥3.21 works).
- Example direct link (adjust version):
  wget https://github.com/protocolbuffers/protobuf/releases/download/v27.3/protoc-27.3-linux-x86_64.zip

Install It:

unzip protoc-*.zip -d protoc-install
sudo mv protoc-install/bin/protoc /usr/local/bin/
sudo chmod +x /usr/local/bin/protoc
rm -rf protoc-install protoc-*.zip  # Cleanup

Verify:
```
protoc --version
```
Should output libprotoc 27.3 (or higher). If it’s still old, check your PATH (echo $PATH)—/usr/local/bin should come before /usr/bin.

Retry the Build:

make clean  # Clears stale targets
make

This overrides the system protoc without conflicting with apt packages. If you’re on a newer distro like Ubuntu 24.04+, apt install protobuf-compiler might already give ≥3.21—double-check your version first.

If You Can’t Upgrade (Rare Cases)

If stuck on an old system, you’d need to patch substrait’s build.rs to add the flag (via Command::new(protoc).arg("--experimental_allow_proto3_optional")), but this requires forking the crate and updating GreptimeDB’s Cargo.toml—messy and not recommended.
Alternatively, some crates offer a protobuf-src feature to bundle a recent protoc, but substrait doesn’t enable it by default.

Additional Tips

Ensure other prereqs: build-essential (for GCC), libc6-dev (glibc headers).
If the build passes but later fails on C++ deps (e.g., Arrow), install libarrow-dev via conda or apt.
For CI/CD or Docker: Add the download step to your env.

This should resolve it—substrait compiles cleanly on modern setups. If you hit another snag, share protoc --version and your distro.

GreptimeDB Build Docs
Protobuf Releases
Proto3 Optional Fields

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