Enforce and await cleanup in StarlarkBaseExternalContext

`StarlarkBaseExternalContext` now implements `AutoCloseable` and, in `close()`:
1. Cancels all pending async tasks.
2. Awaits their termination.
3. Cleans up the working directory (always for module extensions, on failure for repo rules).
4. Fails if there were pending async tasks in an otherwise successful evaluation.

Previously, module extensions didn't do any of those. Repo rules did 1 and 4 and sometimes 3, but not in all cases.

This change required replacing the fixed-size thread pool in `DownloadManager` with virtual threads, thereby resolving a TODO about not using a fixed-size thread pool for the `GrpcRemoteDownloader`.

Work towards bazelbuild#22680
Work towards bazelbuild#22748

Closes bazelbuild#22772.

PiperOrigin-RevId: 644669599
Change-Id: Ib71e5bf346830b92277ac2bd473e11c834cb2624

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionContext.java

@@ -71,6 +71,7 @@ protected ModuleExtensionContext(
         timeoutScaling,
         processWrapper,
         starlarkSemantics,
+        ModuleExtensionEvaluationProgress.moduleExtensionEvaluationContextString(extensionId),
         remoteExecutor,
         /* allowWatchingPathsOutsideWorkspace= */ false);
     this.extensionId = extensionId;
@@ -83,8 +84,10 @@ public Path getWorkingDirectory() {
   }
 
   @Override
-  protected String getIdentifyingStringForLogging() {
-    return ModuleExtensionEvaluationProgress.moduleExtensionEvaluationContextString(extensionId);
+  protected boolean shouldDeleteWorkingDirectoryOnClose(boolean successful) {
+    // The contents of the working directory are purely ephemeral, only the repos instantiated by
+    // the extension are considered its results.
+    return true;
   }
 
   @Override

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/SingleExtensionEvalFunction.java

@@ -891,15 +891,15 @@ public RunModuleExtensionResult run(
               BazelModuleContext.of(bzlLoadValue.getModule()).repoMapping(),
               directories,
               env.getListener());
-      ModuleExtensionContext moduleContext;
       Optional<ModuleExtensionMetadata> moduleExtensionMetadata;
       var repoMappingRecorder = new Label.RepoMappingRecorder();
       repoMappingRecorder.mergeEntries(bzlLoadValue.getRecordedRepoMappings());
       try (Mutability mu =
-          Mutability.create("module extension", usagesValue.getExtensionUniqueName())) {
+              Mutability.create("module extension", usagesValue.getExtensionUniqueName());
+          ModuleExtensionContext moduleContext =
+              createContext(env, usagesValue, starlarkSemantics, extensionId)) {
         StarlarkThread thread = new StarlarkThread(mu, starlarkSemantics);
         thread.setPrintHandler(Event.makeDebugPrintHandler(env.getListener()));
-        moduleContext = createContext(env, usagesValue, starlarkSemantics, extensionId);
         threadContext.storeInThread(thread);
         new BazelStarlarkContext(
                 Phase.WORKSPACE,
@@ -938,39 +938,34 @@ public RunModuleExtensionResult run(
             moduleExtensionMetadata = Optional.empty();
           }
         } catch (NeedsSkyframeRestartException e) {
-          // Clean up and restart by returning null.
-          try {
-            if (moduleContext.getWorkingDirectory().exists()) {
-              moduleContext.getWorkingDirectory().deleteTree();
-            }
-          } catch (IOException e1) {
-            ExternalDepsException externalDepsException =
-                ExternalDepsException.withCauseAndMessage(
-                    ExternalDeps.Code.EXTERNAL_DEPS_UNKNOWN,
-                    e1,
-                    "Failed to clean up module context directory");
-            throw new SingleExtensionEvalFunctionException(
-                externalDepsException, Transience.TRANSIENT);
-          }
+          // Restart by returning null.
           return null;
-        } catch (EvalException e) {
-          env.getListener().handle(Event.error(e.getMessageWithStack()));
-          throw new SingleExtensionEvalFunctionException(
-              ExternalDepsException.withMessage(
-                  ExternalDeps.Code.BAD_MODULE,
-                  "error evaluating module extension %s in %s",
-                  extensionId.getExtensionName(),
-                  extensionId.getBzlFileLabel()),
-              Transience.TRANSIENT);
         }
+        moduleContext.markSuccessful();
+        return RunModuleExtensionResult.create(
+            moduleContext.getRecordedFileInputs(),
+            moduleContext.getRecordedDirentsInputs(),
+            moduleContext.getRecordedEnvVarInputs(),
+            threadContext.getGeneratedRepoSpecs(),
+            moduleExtensionMetadata,
+            repoMappingRecorder.recordedEntries());
+      } catch (EvalException e) {
+        env.getListener().handle(Event.error(e.getMessageWithStack()));
+        throw new SingleExtensionEvalFunctionException(
+            ExternalDepsException.withMessage(
+                ExternalDeps.Code.BAD_MODULE,
+                "error evaluating module extension %s in %s",
+                extensionId.getExtensionName(),
+                extensionId.getBzlFileLabel()),
+            Transience.TRANSIENT);
+      } catch (IOException e) {
+        ExternalDepsException externalDepsException =
+            ExternalDepsException.withCauseAndMessage(
+                ExternalDeps.Code.EXTERNAL_DEPS_UNKNOWN,
+                e,
+                "Failed to clean up module context directory");
+        throw new SingleExtensionEvalFunctionException(externalDepsException, Transience.TRANSIENT);
       }
-      return RunModuleExtensionResult.create(
-          moduleContext.getRecordedFileInputs(),
-          moduleContext.getRecordedDirentsInputs(),
-          moduleContext.getRecordedEnvVarInputs(),
-          threadContext.getGeneratedRepoSpecs(),
-          moduleExtensionMetadata,
-          repoMappingRecorder.recordedEntries());
     }
 
     private ModuleExtensionContext createContext(

src/main/java/com/google/devtools/build/lib/bazel/repository/downloader/DownloadManager.java

@@ -24,7 +24,6 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
-import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.google.devtools.build.lib.authandtls.StaticCredentials;
 import com.google.devtools.build.lib.bazel.repository.cache.RepositoryCache;
 import com.google.devtools.build.lib.bazel.repository.cache.RepositoryCache.KeyType;
@@ -48,7 +47,6 @@
 import java.util.Optional;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import javax.annotation.Nullable;
 
@@ -59,17 +57,6 @@
  * to disk.
  */
 public class DownloadManager {
-  private static final ExecutorService DOWNLOAD_EXECUTOR =
-      Executors.newFixedThreadPool(
-          // There is also GrpcRemoteDownloader so if we set the thread pool to the same size as
-          // the allowed number of HTTP downloads, it might unnecessarily block. No, this is not a
-          // very
-          // principled approach; ideally, we'd grow the thread pool as needed with some generous
-          // upper
-          // limit.
-          2 * HttpDownloader.MAX_PARALLEL_DOWNLOADS,
-          new ThreadFactoryBuilder().setNameFormat("download-manager-%d").build());
-
   private final RepositoryCache repositoryCache;
   private List<Path> distdir = ImmutableList.of();
   private UrlRewriter rewriter;
@@ -115,6 +102,7 @@ public void setCredentialFactory(CredentialFactory credentialFactory) {
   }
 
   public Future<Path> startDownload(
+      ExecutorService executorService,
       List<URL> originalUrls,
       Map<String, List<String>> headers,
       Map<URI, Map<String, List<String>>> authHeaders,
@@ -125,7 +113,7 @@ public Future<Path> startDownload(
       ExtendedEventHandler eventHandler,
       Map<String, String> clientEnv,
       String context) {
-    return DOWNLOAD_EXECUTOR.submit(
+    return executorService.submit(
         () -> {
           try (SilentCloseable c = Profiler.instance().profile("fetching: " + context)) {
             return downloadInExecutor(
@@ -154,33 +142,6 @@ public Path finalizeDownload(Future<Path> download) throws IOException, Interrup
     }
   }
 
-  public Path download(
-      List<URL> originalUrls,
-      Map<String, List<String>> headers,
-      Map<URI, Map<String, List<String>>> authHeaders,
-      Optional<Checksum> checksum,
-      String canonicalId,
-      Optional<String> type,
-      Path output,
-      ExtendedEventHandler eventHandler,
-      Map<String, String> clientEnv,
-      String context)
-      throws IOException, InterruptedException {
-    Future<Path> future =
-        startDownload(
-            originalUrls,
-            headers,
-            authHeaders,
-            checksum,
-            canonicalId,
-            type,
-            output,
-            eventHandler,
-            clientEnv,
-            context);
-    return finalizeDownload(future);
-  }
-
   /**
    * Downloads file to disk and returns path.
    *