From 0d3f62705aedb08590f7b73b405b317957aae52b Mon Sep 17 00:00:00 2001
From: Googler <wyv@google.com>
Date: Wed, 24 Jan 2024 11:47:33 -0800
Subject: [PATCH] Use reflection to support
 `--experimental_worker_for_repo_fetching=virtual`

Bazel already ships JDK 21, but the codebase can't use JDK 21 features (language OR runtime) because Google hasn't migrated to JDK 21 yet (soon TM). But we can cheat a bit using reflection!

Work towards https://github.com/bazelbuild/bazel/issues/10515

PiperOrigin-RevId: 601188027
Change-Id: I65c1712da0347bef40fc4af94bad4c211687a8b7
---
 .../lib/bazel/BazelRepositoryModule.java      | 20 +++++++++++++++----
 .../shell/bazel/starlark_repository_test.sh   |  6 ++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
index 0a2661fde95dc7..51162dd1491951 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
@@ -340,10 +340,22 @@ public void beforeCommand(CommandEnvironment env) throws AbruptExitException {
           starlarkRepositoryFunction.setWorkerExecutorService(repoFetchingWorkerThreadPool);
           break;
         case VIRTUAL:
-          throw new AbruptExitException(
-              detailedExitCode(
-                  "using a virtual worker thread for repo fetching is not yet supported",
-                  Code.BAD_DOWNLOADER_CONFIG));
+          try {
+            // Since Google hasn't migrated to JDK 21 yet, we can't directly call
+            // Executors.newVirtualThreadPerTaskExecutor here. But a bit of reflection never hurt
+            // anyone... right? (OSS Bazel already ships with a bundled JDK 21)
+            starlarkRepositoryFunction.setWorkerExecutorService(
+                (ExecutorService)
+                    Executors.class
+                        .getDeclaredMethod("newVirtualThreadPerTaskExecutor")
+                        .invoke(null));
+          } catch (ReflectiveOperationException e) {
+            throw new AbruptExitException(
+                detailedExitCode(
+                    "couldn't create virtual worker thread executor for repo fetching",
+                    Code.BAD_DOWNLOADER_CONFIG),
+                e);
+          }
       }
       downloadManager.setDisableDownload(repoOptions.disableDownload);
       if (repoOptions.repositoryDownloaderRetries >= 0) {
diff --git a/src/test/shell/bazel/starlark_repository_test.sh b/src/test/shell/bazel/starlark_repository_test.sh
index d5008c47cd5dd4..8493fe92f86ca0 100755
--- a/src/test/shell/bazel/starlark_repository_test.sh
+++ b/src/test/shell/bazel/starlark_repository_test.sh
@@ -2384,6 +2384,12 @@ EOF
   bazel build @foo//:bar --experimental_worker_for_repo_fetching=platform >& $TEST_log \
     || fail "Expected build to succeed"
   expect_log_n "hello world!" 1
+
+  # virtual worker thread, never restarts
+  bazel shutdown
+  bazel build @foo//:bar --experimental_worker_for_repo_fetching=virtual >& $TEST_log \
+    || fail "Expected build to succeed"
+  expect_log_n "hello world!" 1
 }
 
 function test_duplicate_value_in_environ() {