Update ScalaPB wrapper for Scala > 2.11, add notes

Tested by setting the `protobuf` dependencies to a versions I know will
break `ScalaPB` 0.11.17:

- `abseil-cpp`: 20220623.1 => 20240722.0
- `protobuf`: v21.7 => v26.1

Then running the following (since Bazel 6.5.0 can't build `abseil-cpp`
20240722.0 without setting C++14 compiler flags), showing that it will
crash instead of hang. Stashing the temporary changes produced a working
build.

```txt
$ USE_BAZEL_VERSION=7.4.1 bazel build //third_party/test/proto:scala

[ ...snip... ]

ERROR: third_party/test/proto/BUILD.bazel:4:14:
  ProtoScalaPBRule
  third_party/test/proto/proto_jvm_extra_protobuf_generator_scalapb.srcjar
  failed: (Exit 1):
  scalapb_worker failed: error executing ProtoScalaPBRule command
  (from target //third_party/test/proto:proto)
  bazel-out/darwin_arm64-opt-exec-ST-a828a81199fe/bin/src/scala/scripts/scalapb_worker
    ... (remaining 2 arguments skipped)
--jvm_extra_protobuf_generator_out: java.lang.VerifyError: Cannot inherit from final class
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)
    at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1022)
    at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:174)
    at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:800)
    at java.base/jdk.internal.loader.BuiltinClassLoader$4.run(BuiltinClassLoader.java:711)
    at java.base/jdk.internal.loader.BuiltinClassLoader$4.run(BuiltinClassLoader.java:706)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:719)
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:621)
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:579)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
    at scalapb.options.Scalapb.<clinit>(Scalapb.java:24835)
    at scalapb.options.compiler.Scalapb$.registerAllExtensions(Scalapb.scala:8)
    at scalarules.test.extra_protobuf_generator.ExtraProtobufGenerator$.run(ExtraProtobufGenerator.scala:52)
    at protocbridge.frontend.PluginFrontend$.runWithBytes(PluginFrontend.scala:48)
    at protocbridge.frontend.PluginFrontend$.runWithInputStream(PluginFrontend.scala:113)
    at protocbridge.frontend.SocketBasedPluginFrontend.$anonfun$prepare$2(SocketBasedPluginFrontend.scala:31)
    at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
    at scala.concurrent.impl.ExecutionContextImpl$DefaultThreadFactory$$anon$1$$anon$2.block(ExecutionContextImpl.scala:75)
    at java.base/java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3118)
    at scala.concurrent.impl.ExecutionContextImpl$DefaultThreadFactory$$anon$1.blockOn(ExecutionContextImpl.scala:87)
    at scala.concurrent.package$.blocking(package.scala:146)
    at protocbridge.frontend.SocketBasedPluginFrontend.$anonfun$prepare$1(SocketBasedPluginFrontend.scala:23)
    at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
    at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:659)
    at scala.util.Success.$anonfun$map$1(Try.scala:255)
    at scala.util.Success.map(Try.scala:213)
    at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
    at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:42)
    at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:74)
    at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)

java.lang.RuntimeException: Exit with code 1
    at scala.sys.package$.error(package.scala:30)
    at scripts.ScalaPBWorker$.work(ScalaPBWorker.scala:44)
    at io.bazel.rulesscala.worker.Worker.persistentWorkerMain(Worker.java:96)
    at io.bazel.rulesscala.worker.Worker.workerMain(Worker.java:49)
    at scripts.ScalaPBWorker$.main(ScalaPBWorker.scala:39)
    at scripts.ScalaPBWorker.main(ScalaPBWorker.scala)

Target //third_party/test/proto:scala failed to build

ERROR: third_party/test/proto/BUILD.bazel:4:14
  scala @//third_party/test/proto:proto failed:
  (Exit 1): scalapb_worker failed:
  error executing ProtoScalaPBRule command
  (from target //third_party/test/proto:proto)
  bazel-out/darwin_arm64-opt-exec-ST-a828a81199fe/bin/src/scala/scripts/scalapb_worker
    ... (remaining 2 arguments skipped)
```

Author: mbland

scala_proto/scala_proto_toolchain.bzl

@@ -80,6 +80,13 @@ scala_proto_toolchain = rule(
             default = Label("//src/scala/scripts:scalapb_worker"),
             allow_files = True,
         ),
+        # `scripts.ScalaPbCodeGenerator` and `_main_generator_dep` are currently
+        # necessary to support protoc-bridge < 0.9.8, specifically 0.7.14
+        # required by Scala 2.11. See #1647 and scalapb/ScalaPB#1771.
+        #
+        # If we drop 2.11 support, restore `scalapb.ScalaPbCodeGenerator` here,
+        # remove `_main_generator_dep`, and delete
+        # `//src/scala/scripts:scalapb_codegenerator_wrapper` and its files.
         "main_generator": attr.string(
             default = "scripts.ScalaPbCodeGenerator",
         ),

src/scala/scripts/BUILD

@@ -23,6 +23,13 @@ scala_binary(
     ],
 )
 
+# Used by `scala_proto_toolchain()` to support protoc-bridge < 0.9.8,
+# specifically 0.7.14 required by Scala 2.11. See #1647 and
+# scalapb/ScalaPB#1771.
+#
+# If we drop 2.11 support, delete this target and its files, and update
+# `scala_proto_toolchain()` (in `scala_proto/scala_proto_toolchain.bzl`) to use
+# `scalapb.ScalaPbCodeGenerator` as its `main_generator`.
 scala_library(
     name = "scalapb_codegenerator_wrapper",
     srcs = select_for_scala_version(

src/scala/scripts/ScalaPbCodeGeneratorWrapper.scala

@@ -3,15 +3,6 @@ package scripts
 import protocgen.{CodeGenApp,CodeGenRequest,CodeGenResponse}
 
 object ScalaPbCodeGenerator extends CodeGenApp {
-    def process(request: CodeGenRequest): CodeGenResponse = {
-        try {
-            scalapb.ScalaPbCodeGenerator.process(request)
-
-        } catch {
-          case e: Throwable =>
-            val stackStream = new java.io.ByteArrayOutputStream
-            e.printStackTrace(new java.io.PrintStream(stackStream))
-            CodeGenResponse.fail(stackStream.toString())
-        }
-    }
+    def process(request: CodeGenRequest): CodeGenResponse =
+        scalapb.ScalaPbCodeGenerator.process(request)
 }

test/src/main/scala/scalarules/test/extra_protobuf_generator/ExtraProtobufGenerator.scala

@@ -43,7 +43,6 @@ class CustomProtobufGenerator(
 
 }
 
-
 object ExtraProtobufGenerator extends ProtocCodeGenerator {
    override def run(req: Array[Byte]): Array[Byte] = {
     val b = CodeGeneratorResponse.newBuilder
@@ -58,7 +57,12 @@ object ExtraProtobufGenerator extends ProtocCodeGenerator {
       case e: Throwable =>
         // Yes, we want to catch _all_ errors and send them back to the
         // requestor. Otherwise uncaught errors will cause the generator to
-        // die and the worker invoking it to hang.
+        // die and the worker invoking it to hang under protoc-bridge < 0.9.8.
+        // See #1647 and scalapb/ScalaPB#1771.
+        //
+        // Scala 2.11 is stuck at protoc-bridge 0.7.14. If/when we drop
+        // Scala 2.11 support, we can remove this `catch` block (and elide the
+        // `ProtobufAdapter` implementation and delete its files).
         val stackStream = new java.io.ByteArrayOutputStream
         e.printStackTrace(new java.io.PrintStream(stackStream))
         b.setError(stackStream.toString())