[5.3][Serialization] Add option to load swiftmodule files as volatile and avoid mmap

include/swift/Basic/LangOptions.h

@@ -350,6 +350,9 @@ namespace swift {
     /// If set to \c false, fall back to the legacy manual reference name tracking code.
     bool EnableRequestBasedIncrementalDependencies = true;
 
+    /// Load swiftmodule files in memory as volatile and avoid mmap.
+    bool EnableVolatileModules = false;
+
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///

include/swift/Option/FrontendOptions.td

@@ -455,6 +455,9 @@ def warn_long_expression_type_checking_EQ : Joined<["-"], "warn-long-expression-
 def Rmodule_interface_rebuild : Flag<["-"], "Rmodule-interface-rebuild">,
   HelpText<"Emits a remark if an imported module needs to be re-compiled from its module interface">;
 
+def enable_volatile_modules : Flag<["-"], "enable-volatile-modules">,
+  HelpText<"Load Swift modules in memory">;
+
 def solver_expression_time_threshold_EQ : Joined<["-"], "solver-expression-time-threshold=">;
 
 def solver_disable_shrink :

lib/Frontend/CompilerInvocation.cpp

@@ -583,6 +583,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.VerifyAllSubstitutionMaps |= Args.hasArg(OPT_verify_all_substitution_maps);
 
+  Opts.EnableVolatileModules |= Args.hasArg(OPT_enable_volatile_modules);
+
   Opts.UseDarwinPreStableABIBit =
     (Target.isMacOSX() && Target.isMacOSXVersionLT(10, 14, 4)) ||
     (Target.isiOS() && Target.isOSVersionLT(12, 2)) ||

lib/Serialization/SerializedModuleLoader.cpp

@@ -337,8 +337,27 @@ std::error_code SerializedModuleLoaderBase::openModuleFile(
   }
 
   // Actually load the file and error out if necessary.
+  //
+  // Use the default arguments except for IsVolatile that is set by the
+  // frontend option -enable-volatile-modules. If set, we avoid 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
+  bool enableVolatileModules = Ctx.LangOpts.EnableVolatileModules;
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> ModuleOrErr =
-      FS.getBufferForFile(ModulePath);
+      FS.getBufferForFile(ModulePath,
+                          /*FileSize=*/-1,
+                          /*RequiresNullTerminator=*/true,
+                          /*IsVolatile=*/enableVolatileModules);
   if (!ModuleOrErr)
     return ModuleOrErr.getError();