[clang][cas] Fix error accessing cached failure for dep directive scan

When we fail to scan directives in a file we store an empty blob to
cache the failure, but we didn't correctly handle it when loading that
empty blob from the cache. This was triggering crashes the second time
we try to scan a script file. Note: failure to scan is correct here;
it's not C++ code but we had no way to know that up front.

rdar://106090616
(cherry picked from commit 8fb4d9a)

Author: benlangmuir

clang/lib/Tooling/DependencyScanning/DependencyScanningCASFilesystem.cpp

@@ -165,6 +165,8 @@ void DependencyScanningCASFilesystem::scanForDirectives(
   if (Optional<CASID> OutputID =
           reportAsFatalIfError(Cache.get(*InputID))) {
     if (Optional<ObjectRef> OutputRef = CAS.getReference(*OutputID)) {
+      if (OutputRef == EmptyBlobID)
+        return; // Cached directive scanning failure.
       reportAsFatalIfError(
           loadDepDirectives(CAS, *OutputRef, Tokens, Directives));
       return;

clang/unittests/Tooling/DependencyScanningCASFilesystemTest.cpp

@@ -42,3 +42,25 @@ TEST(DependencyScanningCASFilesystem, FilenameSpelling) {
   ASSERT_TRUE(DirectivesSymlink);
   EXPECT_EQ(DirectivesSymlink->size(), 2u);
 }
+
+TEST(DependencyScanningCASFilesystem, DirectiveScanFailure) {
+  TempDir TestDir("DependencyScanningCASFilesystemTest", /*Unique=*/true);
+  TempFile TestFile(TestDir.path("python"), "", "import sys\n");
+
+  std::unique_ptr<ObjectStore> CAS = llvm::cas::createInMemoryCAS();
+  std::unique_ptr<ActionCache> Cache = llvm::cas::createInMemoryActionCache();
+  auto CacheFS = llvm::cantFail(llvm::cas::createCachingOnDiskFileSystem(*CAS));
+  DependencyScanningCASFilesystem FS(CacheFS, *Cache);
+
+  EXPECT_EQ(FS.status(TestFile.path()).getError(), std::error_code());
+  auto Directives = FS.getDirectiveTokens(TestFile.path());
+  ASSERT_FALSE(Directives);
+
+  // Check the cached failure in the action cache.
+  {
+    DependencyScanningCASFilesystem NewFS(CacheFS, *Cache);
+    EXPECT_EQ(NewFS.status(TestFile.path()).getError(), std::error_code());
+    auto Directives = NewFS.getDirectiveTokens(TestFile.path());
+    ASSERT_FALSE(Directives);
+  }
+}