[clang] Fix VFS creation crash with missing DiagnosticConsumer

[jansvoboda11]

For convenience, the CompilerInstance::createVirtualFileSystem() API allows omitting the diagnostic consumer for clients that don't care about missing overlay files and other VFS creation errors. However, even in that case, the temporary DiagnosticsEngine created internally within the function does need a consumer. This PR sets it up.

rdar://176754115

[llvmorg-github-actions]

@llvm/pr-subscribers-clang

Author: Jan Svoboda (jansvoboda11)

Changes

For convenience, the CompilerInstance::createVirtualFileSystem() API allows omitting the diagnostic consumer for clients that don't care about missing overlay files and other VFS creation errors. However, even in that case, the temporary DiagnosticsEngine created internally within the function does need a consumer. This PR sets it up.

rdar://176754115

---

Full diff: https://github.com/llvm/llvm-project/pull/201397.diff

2 Files Affected:

• (modified) clang/lib/Frontend/CompilerInstance.cpp (+7-1)
• (modified) clang/unittests/Frontend/CompilerInstanceTest.cpp (+11)

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 9e88abbece7f2..8aee45b5dc644 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -287,9 +287,15 @@ static void collectVFSEntries(CompilerInstance &CI,
 
 void CompilerInstance::createVirtualFileSystem(
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> BaseFS, DiagnosticConsumer *DC) {
+  bool ShouldOwnClient = false;
+  if (!DC) {
+    DC = new DiagnosticConsumer;
+    ShouldOwnClient = true;
+  }
+
   DiagnosticOptions DiagOpts;
   DiagnosticsEngine Diags(DiagnosticIDs::create(), DiagOpts, DC,
-                          /*ShouldOwnClient=*/false);
+                          ShouldOwnClient);
 
   VFS = createVFSFromCompilerInvocation(getInvocation(), Diags,
                                         std::move(BaseFS));
diff --git a/clang/unittests/Frontend/CompilerInstanceTest.cpp b/clang/unittests/Frontend/CompilerInstanceTest.cpp
index 68578a93d75b5..41908e9c0f254 100644
--- a/clang/unittests/Frontend/CompilerInstanceTest.cpp
+++ b/clang/unittests/Frontend/CompilerInstanceTest.cpp
@@ -82,6 +82,17 @@ TEST(CompilerInstance, DefaultVFSOverlayFromInvocation) {
   ASSERT_TRUE(Instance.getFileManager().getOptionalFileRef("vfs-virtual.file"));
 }
 
+TEST(CompilerInstance, CreateVFSWithoutDiagnosticConsumer) {
+  auto Invocation = std::make_shared<CompilerInvocation>();
+  Invocation->getHeaderSearchOpts().VFSOverlayFiles.push_back("/missing.yaml");
+  auto BaseFS = llvm::makeIntrusiveRefCnt<llvm::vfs::InMemoryFileSystem>();
+  CompilerInstance Instance(std::move(Invocation));
+  // Check that omitting the DiagnosticConsumer doesn't crash (e.g. by
+  // dereferencing the null pointer).
+  ASSERT_NO_FATAL_FAILURE(
+      Instance.createVirtualFileSystem(std::move(BaseFS), /*DC=*/nullptr));
+}
+
 TEST(CompilerInstance, AllowDiagnosticLogWithUnownedDiagnosticConsumer) {
   DiagnosticOptions DiagOpts;
   // Tell the diagnostics engine to emit the diagnostic log to STDERR. This