[clang] [Modules] Process include files changes with -fmodules-validate-input-files-content and -fno-pch-timestamp options (PR #90319)

[Ivan Murashko via cfe-commits]

https://github.com/ivanmurashko created https://github.com/llvm/llvm-project/pull/90319

There were two diffs that introduced some options useful when you build modules externally and cannot rely on file modification time as the key for detecting input file changes:
- [D67249](https://reviews.llvm.org/D67249) introduced the `-fmodules-validate-input-files-content` option, which allows the use of file content hash in addition to the modification time.
- [D141632](https://reviews.llvm.org/D141632) propagated the use of `-fno-pch-timestamps` with Clang modules.

There is a problem when the size of the input file (header) is not modified but the content is. In this case, Clang cannot detect the file change when the `-fno-pch-timestamps` option is used. The `-fmodules-validate-input-files-content` option should help, but there is an issue with its application: it's not applied when the modification time is stored as zero that is the case for `-fno-pch-timestamps`.

The issue can be fixed using the same trick that was applied during the processing of `ForceCheckCXX20ModulesInputFiles`. The patch suggests the corresponding solution and includes a LIT test to verify it.

>From ee4153f6b828c40a56b5b35d6e42cd193a1b5c31 Mon Sep 17 00:00:00 2001
From: Ivan Murashko <ivanmurashko at meta.com>
Date: Fri, 26 Apr 2024 22:45:26 +0100
Subject: [PATCH] [Modules] Process include files changes with
 -fmodules-validate-input-files-content and -fno-pch-timestamp options

There are two diffs that introduce some options required when you build modules
externally and cannot rely on file modification time as a key for detecting
input file changes.

https://reviews.llvm.org/D67249 introduced the
`-fmodules-validate-input-files-content` option, which allows the use of file
content hash instead of modification time.

https://reviews.llvm.org/D141632 propagated the use of `-fno-pch-timestamps`
with Clang modules.

There is a problem when the size of the input file (header) is not modified but
the content is. In this case, Clang cannot detect the file change when the
-fno-pch-timestamps option is used. The -fmodules-validate-input-files-content
option should help, but there is an issue with its application.

The issue can be fixed using the same trick that was applied during the
processing of ForceCheckCXX20ModulesInputFiles. The patch suggests a solution
and includes a LIT test to verify it.
---
 clang/lib/Serialization/ASTReader.cpp         |  8 +++++
 .../Modules/implicit-module-no-timestamp.cpp  | 33 +++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 clang/test/Modules/implicit-module-no-timestamp.cpp

diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 0ef57a3ea804ef..4609ca3e1428c8 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2630,6 +2630,14 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
       F.StandardCXXModule && FileChange.Kind == Change::None)
     FileChange = HasInputContentChanged(FileChange);
 
+  // When we have StoredTime equal to zero and ValidateASTInputFilesContent,
+  // it is better to check the content of the input files because we cannot rely
+  // on the file modification time, which will be the same (zero) for these
+  // files.
+  if (!StoredTime && ValidateASTInputFilesContent &&
+      FileChange.Kind == Change::None)
+    FileChange = HasInputContentChanged(FileChange);
+
   // For an overridden file, there is nothing to validate.
   if (!Overridden && FileChange.Kind != Change::None) {
     if (Complain && !Diags.isDiagnosticInFlight()) {
diff --git a/clang/test/Modules/implicit-module-no-timestamp.cpp b/clang/test/Modules/implicit-module-no-timestamp.cpp
new file mode 100644
index 00000000000000..457ad3c16e184c
--- /dev/null
+++ b/clang/test/Modules/implicit-module-no-timestamp.cpp
@@ -0,0 +1,33 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: cp a1.h a.h
+// RUN: %clang -fmodules -fmodules-validate-input-files-content -Xclang -fno-pch-timestamp -fimplicit-modules -fmodule-map-file=module.modulemap -fsyntax-only  -fmodules-cache-path=%t test1.cpp
+// RUN: cp a2.h a.h
+// RUN: %clang -fmodules -fmodules-validate-input-files-content -Xclang -fno-pch-timestamp -fimplicit-modules -fmodule-map-file=module.modulemap -fsyntax-only  -fmodules-cache-path=%t test2.cpp
+
+//--- a1.h
+#define FOO
+
+//--- a2.h
+#define BAR
+
+//--- module.modulemap
+module a {
+  header "a.h"
+}
+
+//--- test1.cpp
+#include "a.h"
+
+#ifndef FOO
+#error foo
+#endif
+
+//--- test2.cpp
+#include "a.h"
+
+#ifndef BAR
+#error bar
+#endif