[ModuleInterfaces] Clear dependencies before checking a different module

Sometimes, if one module we tried failed, we would leave the dependencies from the previous check and fail a different module which would have succeeded.

This manifested as the prebuilt-module-cache-forwarding test testing the wrong thing, because it retained a mod-time based dependency in the prebuilt cache, which should have been hash based, and used it.

Author: Harlan Haskins

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -913,7 +913,12 @@ class ParseableInterfaceModuleLoaderImpl {
   bool serializedASTBufferIsUpToDate(
       StringRef path, const llvm::MemoryBuffer &buf,
       SmallVectorImpl<FileDependency> &allDeps) {
-    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << modulePath << "\n");
+
+    // Clear the existing dependencies, because we're going to re-fill them
+    // in validateSerializedAST.
+    allDeps.clear();
+
+    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << path << "\n");
     auto validationInfo = serialization::validateSerializedAST(
         buf.getBuffer(), /*ExtendedValidationInfo=*/nullptr, &allDeps);
 
@@ -944,6 +949,13 @@ class ParseableInterfaceModuleLoaderImpl {
       StringRef path, const ForwardingModule &fwd,
       SmallVectorImpl<FileDependency> &deps,
       std::unique_ptr<llvm::MemoryBuffer> &moduleBuffer) {
+
+    // Clear the existing dependencies, because we're going to re-fill them
+    // from the forwarding module.
+    deps.clear();
+
+    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << path << "\n");
+
     // First, make sure the underlying module path exists and is valid.
     auto modBuf = fs.getBufferForFile(fwd.underlyingModulePath);
     if (!modBuf || !serializedASTLooksValid(*modBuf.get()))

test/ParseableInterface/ModuleCache/prebuilt-module-cache-forwarding.swift

@@ -12,16 +12,18 @@
 
 // Make sure we installed a forwarding module.
 // RUN: %{python} %S/Inputs/check-is-forwarding-module.py %t/MCP/Lib-*.swiftmodule
+// RUN: cat %t/MCP/Lib-*.swiftmodule
 
-// Make sure it passes:
-// RUN: sleep 1
+// Now invalidate a dependency of the prebuilt module, by changing the hash of the .swiftinterface.
+// RUN: cp %t/Lib.swiftinterface %t/Lib.swiftinterface.moved-aside
+// RUN: echo ' ' >> %t/Lib.swiftinterface
 
-// Now invalidate a dependency of the prebuilt module, and make sure the forwarding file is replaced with a real module.
-// RUN: %{python} %S/Inputs/make-old.py %t/Lib.swiftinterface
+// Make sure the forwarding file is replaced with a real module.
 // RUN: not %target-swift-frontend -typecheck -parse-stdlib -module-cache-path %t/MCP -sdk %S/Inputs -I %S/Inputs/prebuilt-module-cache/ -prebuilt-module-cache-path %t/prebuilt-cache %s 2>&1 | %FileCheck -check-prefix=FROM-INTERFACE %s
 
-// Delete the cached module we just created, and create the forwarding module again
+// Delete the cached module we just created, put the old .swiftinterface back, and create the forwarding module again
 // RUN: %empty-directory(%t/MCP)
+// RUN: mv %t/Lib.swiftinterface.moved-aside %t/Lib.swiftinterface
 // RUN: not %target-swift-frontend -typecheck -parse-stdlib -module-cache-path %t/MCP -sdk %S/Inputs -I %S/Inputs/prebuilt-module-cache/ -prebuilt-module-cache-path %t/prebuilt-cache %s 2>&1 | %FileCheck -check-prefix=FROM-PREBUILT %s
 
 // Move the prebuilt module out of the way, so the forwarding module points to nothing.