[clang][Modules] Diagnose mismatching pcm dependencies in explicit builds

In an explicit build, the dependency scanner generates invocations with
dependencies to module files to use during compilation. The pcm's passed
in the invocations should match the ones that were imported by other
modules that share the same dependencies.

We have seen bugs caused from incorrect invocations that mismatch which module file to load.
When this happens report it as a warning, to help with investigations, as
that should never occur in a well behaved build scheduled from the
dependency scanner.

The warning flag is off by default.

Author: cyndyishida

clang/include/clang/Basic/DiagnosticSerializationKinds.td

@@ -139,6 +139,11 @@ def warn_decls_in_multiple_modules : Warning<
   InGroup<DiagGroup<"decls-in-multiple-modules">>,
   DefaultIgnore;
 
+def warn_lazy_pcm_mismatch
+    : Warning<"loaded module file '%0' conflicts with file discovered '%1'">,
+      InGroup<DiagGroup<"lazy-pcm-mismatch">>,
+      DefaultIgnore;
+
 def err_failed_to_find_module_file : Error<
   "failed to find module file for module '%0'">;
 } // let CategoryName

clang/lib/Serialization/ASTReader.cpp

@@ -3286,6 +3286,7 @@ ASTReader::ReadControlBlock(ModuleFile &F,
       time_t StoredModTime = 0;
       ASTFileSignature StoredSignature;
       std::string ImportedFile;
+      std::string StoredFile;
 
       // For prebuilt and explicit modules first consult the file map for
       // an override. Note that here we don't search prebuilt module
@@ -3311,11 +3312,22 @@ ASTReader::ReadControlBlock(ModuleFile &F,
                                                    SignatureBytes.end());
         Blob = Blob.substr(ASTFileSignature::size);
 
+        // Use BaseDirectoryAsWritten to ensure we use the same path in the
+        // ModuleCache as when writing.
+        StoredFile = ReadPathBlob(BaseDirectoryAsWritten, Record, Idx, Blob);
         if (ImportedFile.empty()) {
-          // Use BaseDirectoryAsWritten to ensure we use the same path in the
-          // ModuleCache as when writing.
-          ImportedFile =
-              ReadPathBlob(BaseDirectoryAsWritten, Record, Idx, Blob);
+          ImportedFile = StoredFile;
+        } else {
+          auto ImportedFileRef =
+              PP.getFileManager().getOptionalFileRef(ImportedFile);
+          auto StoredFileRef =
+              PP.getFileManager().getOptionalFileRef(StoredFile);
+          if ((ImportedFileRef && StoredFileRef) &&
+              (*ImportedFileRef != *StoredFileRef)) {
+            Diag(diag::warn_lazy_pcm_mismatch) << ImportedFile << StoredFile;
+            Diag(diag::note_module_file_imported_by)
+                << F.FileName << !F.ModuleName.empty() << F.ModuleName;
+          }
         }
       }
 

clang/test/Modules/invalid-module-dep.c

@@ -0,0 +1,65 @@
+/// This tests the expected error case when there is a mismatch between the pcm dependencies passed in 
+/// the command line with `fmodule-file` and whats encoded in the pcm. 
+
+/// The steps are: 
+/// 1. Build the module A with no dependencies. The first variant to build is A-1.pcm.
+/// 2. Build the same module with files that resolve from different search paths. 
+///     This variant is named A-2.pcm.
+/// 3. Build module B that depends on the earlier module A-1.pcm.
+/// 4. Build client that directly depends on both modules (A & B), 
+///     but depends on a incompatible variant of A (A-2.pcm) for B to use.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -emit-module -x c -fmodules -fno-implicit-modules -isysroot %t/Sysroot \
+// RUN:     -I%t/Sysroot/usr/include \
+// RUN:     -fmodule-name=A %t/Sysroot/usr/include/A/module.modulemap -o %t/A-1.pcm
+
+// RUN: %clang_cc1 -emit-module -x c -fmodules -fno-implicit-modules -isysroot %t/Sysroot \
+// RUN:     -I%t/BuildDir \
+// RUN:     -fmodule-name=A %t/BuildDir/A/module.modulemap -o %t/A-2.pcm
+
+// RUN: %clang_cc1 -emit-module -x c -fmodules -fno-implicit-modules -isysroot %t/Sysroot \
+// RUN:     -I%t/Sysroot/usr/include \
+// RUN:     -fmodule-map-file=%t/Sysroot/usr/include/A/module.modulemap \
+// RUN:     -fmodule-file=A=%t/A-1.pcm \
+// RUN:     -fmodule-name=B %t/Sysroot/usr/include/B/module.modulemap -o %t/B-1.pcm
+
+// RUN: %clang_cc1 -x c -fmodules -fno-implicit-modules -isysroot %t/Sysroot \
+// RUN:     -I%t/BuildDir -I%t/Sysroot/usr/include \
+// RUN:     -fmodule-map-file=%t/BuildDir/A/module.modulemap \
+// RUN:     -fmodule-map-file=%t/Sysroot/usr/include/B/module.modulemap \
+// RUN:     -fmodule-file=A=%t/A-2.pcm -fmodule-file=B=%t/B-1.pcm \
+// RUN:     -Wlazy-pcm-mismatch -verify %s  
+
+
+#include <A/A.h>
+#include <B/B.h> // expected-warning {{conflicts with file discovered}} \
+                 // expected-note {{imported by module 'B'}} \
+                 // expected-error {{out of date and needs to be rebuilt}} \
+                 // expected-note {{imported by module 'B'}}
+
+//--- Sysroot/usr/include/A/module.modulemap
+module A [system] {
+  umbrella header "A.h"
+}
+//--- Sysroot/usr/include/A/A.h
+typedef int A_t;
+
+//--- Sysroot/usr/include/B/module.modulemap
+module B [system] {
+  umbrella header "B.h"
+}
+
+//--- Sysroot/usr/include/B/B.h
+#include <A/A.h>
+typedef int B_t;
+
+//--- BuildDir/A/module.modulemap
+module A [system] {
+  umbrella header "A.h"
+}
+
+//--- BuildDir/A/A.h
+typedef int A_t;