Add explicit OpenSourcesAsVolatile option

Author: z2oh

include/swift/Basic/LangOptions.h

@@ -496,6 +496,12 @@ namespace swift {
     /// Enable verification when every SubstitutionMap is constructed.
     bool VerifyAllSubstitutionMaps = false;
 
+    /// If set to true, the source manager will avoid memory mapping source files
+    /// with the expectation they may change on disk. This is most useful when
+    /// opening files under sourcekitd on Windows, as memory mapping on Windows
+    /// prevents files from being written.
+    bool OpenSourcesAsVolatile = false;
+
     /// Load swiftmodule files in memory as volatile and avoid mmap.
     bool EnableVolatileModules = false;
 

include/swift/Basic/SourceManager.h

@@ -94,7 +94,7 @@ class SourceManager {
 private:
   llvm::SourceMgr LLVMSourceMgr;
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem;
-  bool EditorMode = false;
+  bool OpenSourcesAsVolatile = false;
   unsigned IDEInspectionTargetBufferID = 0U;
   unsigned IDEInspectionTargetOffset;
 
@@ -167,8 +167,10 @@ class SourceManager {
     return FileSystem;
   }
 
-  void setEditorMode() {
-    EditorMode = true;
+  // Setting this prevents SourceManager from memory mapping sources (via opening files
+  // with isVolatile=true);
+  void setOpenSourcesAsVolatile() {
+    OpenSourcesAsVolatile = true;
   }
 
   void setIDEInspectionTarget(unsigned BufferID, unsigned Offset) {

lib/Basic/SourceLoc.cpp

@@ -699,7 +699,7 @@ unsigned SourceManager::getExternalSourceBufferID(StringRef Path) {
       swift::vfs::getFileOrSTDIN(*getFileSystem(), Path,
                                  /* FileSize */ -1,
                                  /* RequiresNullTerminator */ true,
-                                 /* isVolatile */ this->EditorMode);
+                                 /* isVolatile */ this->OpenSourcesAsVolatile);
   if (InputFileOrErr) {
     // This assertion ensures we can look up from the map in the future when
     // using the same Path.

lib/Frontend/Frontend.cpp

@@ -552,11 +552,9 @@ bool CompilerInstance::setup(const CompilerInvocation &Invoke,
   if (LangOpts.EnableModuleLoadingRemarks) {
     Invocation.getSearchPathOptions().dump(LangOpts.Target.isOSDarwin());
   }
-  
-  // If the compiler instance is managed by an IDE, inform the source manager to avoid
-  // memory-mapping source files that are likely to be mutated in an editor.
-  if (LangOpts.DiagnosticsEditorMode) {
-    this->getSourceMgr().setEditorMode();
+
+  if (LangOpts.OpenSourcesAsVolatile) {
+    this->getSourceMgr().setOpenSourcesAsVolatile();
   }
 
   // If we expect an implicit stdlib import, load in the standard library. If we

lib/IDETool/CompilerInvocation.cpp

@@ -225,6 +225,11 @@ bool ide::initCompilerInvocation(
   LangOpts.AttachCommentsToDecls = true;
   LangOpts.DiagnosticsEditorMode = true;
   LangOpts.CollectParsedToken = true;
+  #if defined(_WIN32)
+  // Source files that might be open in an editor should not be memory mapped on Windows,
+  // as they will become read-only.
+  LangOpts.OpenSourcesAsVolatile = true;
+  #endif
   if (LangOpts.PlaygroundTransform) {
     // The playground instrumenter changes the AST in ways that disrupt the
     // SourceKit functionality. Since we don't need the instrumenter, and all we