[SourceKit] Don't use clang build sessions when validation is disabled

Don't use '-fbuild-session-timestamp' if
'-disable-modules-validate-system-headers' is specified.

Since system header validation happens under *OR* condition of
'ValidateSystemInputs' and 'ModulesValidateOncePerBuildSession', using
build sessions effectively overrides it.

rdar://problem/61075677

Author: rintaro

test/SourceKit/Sema/sema_build_session.swift

@@ -15,7 +15,8 @@ func test() {
 // UNSUPPORTED: OS=windows-msvc
 
 // -----------------------------------------------------------------------------
-// Test that modifications for frameworks in '-Fsystem' doesn't affect the result.
+// Test that modifications for frameworks in '-Fsystem' doesn't affect the result
+// within a session, and that they are propagated after restarting SourceKit.
 
 // RUN: %empty-directory(%t/ModuleCache)
 // RUN: %empty-directory(%t/System/Frameworks)
@@ -69,7 +70,7 @@ func test() {
 // RUN:   -shell -- cp -R %S/../Inputs/build_session/Frameworks_modified/FooHelper.framework %t/System/Frameworks/ == \
 // RUN:   -shell -- echo '## TWO' == \
 // RUN:   -req=sema %s -- %s -D TWO -F %t/Frameworks -Fsystem %t/System/Frameworks -module-cache-path %t/ModuleCache  \
-// RUN:   | tee %t.reponse | %FileCheck %s --check-prefix=CHECK_USER
+// RUN:   | %FileCheck %s --check-prefix=CHECK_USER
 
 // CHECK_USER-LABEL: ## ONE
 // CHECK_USER-NOT: key.description
@@ -81,3 +82,35 @@ func test() {
 // CHECK_USER: key.severity: source.diagnostic.severity.error,
 // CHECK_USER-NEXT: key.description: "use of unresolved identifier 'fooSubFunc'",
 // CHECK_USER-NOT: key.severity: 
+
+// -----------------------------------------------------------------------------
+// Test that modifications for frameworks in '-Fsystem' doesn't affect the result
+// across SourceKit sessions *if* '-disable-modules-validate-system-headers' is
+// passed.
+
+// RUN: %empty-directory(%t/ModuleCache)
+// RUN: %empty-directory(%t/System/Frameworks)
+// RUN: cp -R %S/../Inputs/build_session/Frameworks/Foo.framework %t/System/Frameworks/
+// RUN: cp -R %S/../Inputs/build_session/Frameworks/FooHelper.framework %t/System/Frameworks/
+// RUN: %sourcekitd-test \
+// RUN:   -shell -- echo '## ONE' == \
+// RUN:   -req=sema %s -- %s -D ONE -Fsystem %t/System/Frameworks -module-cache-path %t/ModuleCache -Xfrontend -disable-modules-validate-system-headers == \
+// RUN:   -shell -- cp -R %S/../Inputs/build_session/Frameworks_modified/Foo.framework %t/System/Frameworks/ == \
+// RUN:   -shell -- cp -R %S/../Inputs/build_session/Frameworks_modified/FooHelper.framework %t/System/Frameworks/ == \
+// RUN:   -shell -- echo '## TWO' == \
+// RUN:   -req=sema %s -- %s -D TWO -Fsystem %t/System/Frameworks -module-cache-path %t/ModuleCache -Xfrontend -disable-modules-validate-system-headers \
+// RUN:   | %FileCheck %s --check-prefix=CHECK_DISABLED
+// RUN: sleep 2
+// RUN: %sourcekitd-test \
+// RUN:   -shell -- echo '## THREE' == \
+// RUN:   -req=sema %s -- %s -D THREE -Fsystem %t/System/Frameworks -module-cache-path %t/ModuleCache -Xfrontend -disable-modules-validate-system-headers \
+// RUN:   | %FileCheck %s --check-prefix=CHECK_DISABLED_2
+
+// CHECK_DISABLED-LABEL: ## ONE
+// CHECK_DISABLED-NOT: key.description
+
+// CHECK_DISABLED-LABEL: ## TWO
+// CHECK_DISABLED-NOT: key.description
+
+// CHECK_DISABLED_2-LABEL: ## THREE
+// CHECK_DISABLED_2-NOT: key.description

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -554,15 +554,20 @@ bool SwiftASTManager::initCompilerInvocation(
 
   // To save the time for module validation, consider the lifetime of ASTManager
   // as a single build session.
-  // NOTE: 'SessionTimestamp - 1' because clang compares it with '<=' that may
-  //       cause unnecessary validations if they happens within one second from
-  //       the SourceKit startup.
-  ImporterOpts.ExtraArgs.push_back("-fbuild-session-timestamp=" +
-                                   std::to_string(Impl.SessionTimestamp - 1));
-  ImporterOpts.ExtraArgs.push_back("-fmodules-validate-once-per-build-session");
-
+  // NOTE: Do this only if '-disable-modules-validate-system-headers' is *not*
+  //       explicitly enabled.
   auto &SearchPathOpts = Invocation.getSearchPathOptions();
-  SearchPathOpts.DisableModulesValidateSystemDependencies = true;
+  if (!SearchPathOpts.DisableModulesValidateSystemDependencies) {
+    // NOTE: 'SessionTimestamp - 1' because clang compares it with '<=' that may
+    //       cause unnecessary validations if they happens within one second
+    //       from the SourceKit startup.
+    ImporterOpts.ExtraArgs.push_back("-fbuild-session-timestamp=" +
+                                     std::to_string(Impl.SessionTimestamp - 1));
+    ImporterOpts.ExtraArgs.push_back(
+        "-fmodules-validate-once-per-build-session");
+
+    SearchPathOpts.DisableModulesValidateSystemDependencies = true;
+  }
 
   // Disable expensive SIL options to reduce time spent in SILGen.
   disableExpensiveSILOptions(Invocation.getSILOptions());