[Frontend]: Write out .swiftdeps files atomically

Modify ReferenceDependenciesEmitter::emit to write to a temporary file and then atomically rename it in place using the atomicallyWritingToFile helper function.
This resolves SR-8707, where .swiftdeps files could be truncated if the compiler job was killed in the middle of a write.

Author: owenv

Diff for: lib/FrontendTool/ReferenceDependencies.cpp

@@ -22,6 +22,7 @@
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/ReferencedNameTracker.h"
 #include "swift/AST/Types.h"
+#include "swift/Basic/FileSystem.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/ReferenceDependencyKeys.h"
 #include "swift/Frontend/FrontendOptions.h"
@@ -65,11 +66,6 @@ class ReferenceDependenciesEmitter {
                    llvm::raw_ostream &out);
 
 private:
-  /// Opens file for reference dependencies. Emits diagnostic if needed.
-  ///
-  /// \return nullptr on error
-  static std::unique_ptr<llvm::raw_fd_ostream> openFile(DiagnosticEngine &diags,
-                                                        StringRef OutputPath);
   /// Emits all the dependency information.
   void emit() const;
 
@@ -189,35 +185,24 @@ static std::string escape(DeclBaseName name) {
   return llvm::yaml::escape(name.userFacingName());
 }
 
-std::unique_ptr<llvm::raw_fd_ostream>
-ReferenceDependenciesEmitter::openFile(DiagnosticEngine &diags,
-                                       StringRef outputPath) {
+bool ReferenceDependenciesEmitter::emit(DiagnosticEngine &diags,
+                                        SourceFile *const SF,
+                                        const DependencyTracker &depTracker,
+                                        StringRef outputPath) {
   // Before writing to the dependencies file path, preserve any previous file
   // that may have been there. No error handling -- this is just a nicety, it
   // doesn't matter if it fails.
   llvm::sys::fs::rename(outputPath, outputPath + "~");
-
-  std::error_code EC;
-  auto out = llvm::make_unique<llvm::raw_fd_ostream>(outputPath, EC,
-                                                     llvm::sys::fs::F_None);
-
-  if (out->has_error() || EC) {
+  std::error_code EC =
+      swift::atomicallyWritingToFile(outputPath,
+                                     [&](llvm::raw_pwrite_stream &out) {
+    ReferenceDependenciesEmitter::emit(SF, depTracker, out);
+  });
+  if (EC) {
     diags.diagnose(SourceLoc(), diag::error_opening_output, outputPath,
                    EC.message());
-    out->clear_error();
-    return nullptr;
-  }
-  return out;
-}
-
-bool ReferenceDependenciesEmitter::emit(DiagnosticEngine &diags,
-                                        SourceFile *const SF,
-                                        const DependencyTracker &depTracker,
-                                        StringRef outputPath) {
-  const std::unique_ptr<llvm::raw_ostream> out = openFile(diags, outputPath);
-  if (!out.get())
     return true;
-  ReferenceDependenciesEmitter::emit(SF, depTracker, *out);
+  }
   return false;
 }