Merge pull request swiftlang#33001 from xymus/swiftmodule-files-are-volatile

[Serialization] Load swiftmodule files as volatile to avoid mmap

Author: xymus

lib/Serialization/SerializedModuleLoader.cpp

@@ -347,8 +347,25 @@ std::error_code SerializedModuleLoaderBase::openModuleFile(
   }
 
   // Actually load the file and error out if necessary.
+  //
+  // Use the default arguments except for IsVolatile. Force avoiding the use of
+  // mmap to workaround issues on NFS when the swiftmodule file loaded changes
+  // on disk while it's in use.
+  //
+  // In practice, a swiftmodule file can chane when a client uses a
+  // swiftmodule file from a framework while the framework is recompiled and
+  // installed over existing files. Or when many processes rebuild the same
+  // module interface.
+  //
+  // We have seen these scenarios leading to deserialization errors that on
+  // the surface look like memory corruption.
+  //
+  // rdar://63755989
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> ModuleOrErr =
-      FS.getBufferForFile(ModulePath);
+      FS.getBufferForFile(ModulePath,
+                          /*FileSize=*/-1,
+                          /*RequiresNullTerminator=*/true,
+                          /*IsVolatile=*/true);
   if (!ModuleOrErr)
     return ModuleOrErr.getError();