[Modules] Do not emit file modification error when -fno-validate-pch is set

[yamaguchi]

Summary

This is a follow-up patch of
rL338503: [Modules] Do not emit relocation error when -fno-validate-pch is set

File modification with modules is also fine for our usecase as we
want to reuse pcms. This enables us to use the same build directory
again without deleting pcm directory.

[rsmith]

The consequence of this will be that we use a stale PCH / PCM file if files have changed, rather than emitting an "out of date" error if / when we decide we need to reference a modified file that was used to build the AST file. That can lead to bad things happening: any time we decide to re-lex or otherwise go back to the original source file, we'll be "surprised" that it's not the same as we remember it being. -fno-validate-pch isn't supposed to allow the files on disk to have changed, it's a promise that they haven't.

clang/lib/Serialization/ASTReader.cpp

if ((!Overridden && !Transient) && SM.isFileOverridden(File)) {
     // size/time.
     FileMgr.modifyFileEntry(const_cast<FileEntry*>(File),
                             StoredSize, StoredTime);
   }
 
   bool IsOutOfDate = false;
 
   // For an overridden file, there is nothing to validate.
-  if (!Overridden && //
+  // Don't emit file modification error if we have -fno-validate-pch
+  if (!Overridden && !PP.getPreprocessorOpts().DisablePCHValidation &&
       (StoredSize != File->getSize() ||
        (StoredTime && StoredTime != File->getModificationTime() &&
         !DisableValidation)
        )) {
     if (Complain) {
       // Build a list of the PCH imports that got us here (in reverse).
       SmallVector<ModuleFile *, 4> ImportStack(1, &F);
       while (!ImportStack.back()->ImportedBy.empty())

clang/test/Modules/module-file-novalidate.c

+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: echo 'int foo = 0;' > %t/a.h
+// RUN: echo 'module A { header "a.h" }' > %t/m.modulemap
+// RUN: %clang_cc1 -fmodules -emit-module -fmodule-name=A -x c %t/m.modulemap -o %t/m.pcm
+// RUN: echo 'int bar;' > %t/a.h
+// RUN: not %clang_cc1 -fmodules -fmodule-file=%t/m.pcm -fmodule-map-file=%t/m.modulemap -fno-validate-pch -x c %s -I%t -fsyntax-only 2>&1 | FileCheck %s
+#include "a.h"
+int foo = 0; // redefinition of 'foo'
+// CHECK-NOT: fatal error: file {{.*}} has been modified since the module file {{.*}} was built
+// REQUIRES: shell