Add note about Rosetta for old protoc-gen-rpc-java

CONTRIBUTING.md

@@ -60,3 +60,20 @@ Build with:
 ```bash
 ./gradlew build
 ```
+
+## Note on Rosetta
+Newer Apple Silicon macs do not ship with Rosetta by default, and the version of `protoc-gen-rpc-java` we use (1.34.1) does not ship Apple Silicon binaries.
+
+So Gradle is set to hardcode the download of the x86_64 binaries on MacOS, but this depends on Rosetta to function. Make sure Rosetta is installed with
+
+```bash
+/usr/bin/pgrep oahd
+```
+
+which should return a PID of the Rosetta process. If it doesn't, you'll need to run
+
+```bash
+softwareupdate --install-rosetta
+```
+
+for builds to complete successfully.

temporal-serviceclient/build.gradle

@@ -78,6 +78,9 @@ protobuf {
     // version/variables substitution is not supported in protobuf section.
     // protoc and protoc-gen-grpc-java versions are selected to be compatible
     // with the oldest supported versions of protoc and grpc artifacts.
+    //
+    // The ternary operators look backward, be assured they are not. There are no aarch64 binaries
+    // for this version and so we are relying on Rosetta. See CONTRIBUTING.md if this breaks for you.
     protoc {
         artifact = 'com.google.protobuf:protoc:3.25.5' + (System.getProperty("os.arch") == 'aarch64' && System.getProperty("os.name") == 'Mac OS X' ? ':osx-x86_64' : '')
     }

temporal-test-server/build.gradle

@@ -54,6 +54,9 @@ protobuf {
     // version/variables substitution is not supported in protobuf section.
     // protoc and protoc-gen-grpc-java versions are selected to be compatible
     // with the oldest supported versions of protoc and grpc artifacts.
+    //
+    // The ternary operators look backward, be assured they are not. There are no aarch64 binaries
+    // for this version and so we are relying on Rosetta. See CONTRIBUTING.md if this breaks for you.
     protoc {
         artifact = 'com.google.protobuf:protoc:3.25.5' + (System.getProperty("os.arch") == 'aarch64' && System.getProperty("os.name") == 'Mac OS X' ? ':osx-x86_64' : '')
     }