[Concurrency] Fix too eager early return in checkIsolation mode detecting (#73495)

Author: ktoso

include/swift/Runtime/EnvironmentVariables.h

@@ -53,6 +53,10 @@ SWIFT_RUNTIME_STDLIB_SPI bool concurrencyEnableJobDispatchIntegration();
 // Concurrency library can call.
 SWIFT_RUNTIME_STDLIB_SPI bool concurrencyValidateUncheckedContinuations();
 
+// Wrapper around SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE that the
+// Concurrency library can call.
+SWIFT_RUNTIME_STDLIB_SPI const char *concurrencyIsCurrentExecutorLegacyModeOverride();
+
 } // end namespace environment
 } // end namespace runtime
 } // end namespace swift

stdlib/public/Concurrency/Actor.cpp

@@ -30,6 +30,7 @@
 #include "swift/Runtime/Bincompat.h"
 #include "swift/Runtime/Casting.h"
 #include "swift/Runtime/DispatchShims.h"
+#include "swift/Runtime/EnvironmentVariables.h"
 #include "swift/Threading/Mutex.h"
 #include "swift/Threading/Once.h"
 #include "swift/Threading/Thread.h"
@@ -351,15 +352,16 @@ static void checkIsCurrentExecutorMode(void *context) {
 
   // Potentially, override the platform detected mode, primarily used in tests.
 #if SWIFT_STDLIB_HAS_ENVIRON
-  const char *modeStr = getenv("SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE");
-  if (!modeStr)
-    return;
-  
-  if (strcmp(modeStr, "nocrash") == 0) {
-    useLegacyMode = Legacy_NoCheckIsolated_NonCrashing;
-  } else if (strcmp(modeStr, "crash") == 0)  {
-    useLegacyMode = Default_UseCheckIsolated_AllowCrash;
-  } // else, just use the platform detected mode
+  if (const char *modeStr =
+          runtime::environment::concurrencyIsCurrentExecutorLegacyModeOverride()) {
+    if (modeStr) {
+      if (strcmp(modeStr, "nocrash") == 0) {
+        useLegacyMode = true;
+      } else if (strcmp(modeStr, "crash") == 0) {
+        useLegacyMode = false;
+      } // else, just use the platform detected mode
+    }
+  }
 #endif // SWIFT_STDLIB_HAS_ENVIRON
   isCurrentExecutorMode = useLegacyMode ? Legacy_NoCheckIsolated_NonCrashing
                                         : Default_UseCheckIsolated_AllowCrash;

stdlib/public/runtime/EnvironmentVariables.cpp

@@ -272,3 +272,7 @@ SWIFT_RUNTIME_STDLIB_SPI bool concurrencyEnableJobDispatchIntegration() {
 SWIFT_RUNTIME_STDLIB_SPI bool concurrencyValidateUncheckedContinuations() {
   return runtime::environment::SWIFT_DEBUG_VALIDATE_UNCHECKED_CONTINUATIONS();
 }
+
+SWIFT_RUNTIME_STDLIB_SPI const char *concurrencyIsCurrentExecutorLegacyModeOverride() {
+  return runtime::environment::SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE();
+}

stdlib/public/runtime/EnvironmentVariables.def

@@ -112,4 +112,16 @@ VARIABLE(SWIFT_BACKTRACE, string, "",
          "crash catching and backtracing support in the runtime. "
          "See docs/Backtracing.rst in the Swift repository for details.")
 
+VARIABLE(SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE, string, "",
+         "Allows for suppressing 'is current executor' equality check crashes. "
+         "As since Swift 6.0 checking for current executor equality, may crash "
+         "and will never return 'false' because we are calling into library "
+         "implemented SerialExecutor.checkIsolation which should crash if the "
+         "isolation is not the expected one. Some old code may rely on the "
+         "non-crashing behavior. This flag enables temporarily restoring the "
+         "legacy 'nocrash' behavior until adopting code has been adjusted. "
+         "Legal values are: "
+         " 'nocrash' (Legacy behavior), "
+         " 'crash' (Swift 6.0+ behavior)")
+
 #undef VARIABLE

test/abi/macOS/arm64/stdlib.swift

@@ -567,3 +567,6 @@ Added: _swift_updatePureObjCClassMetadata
 
 // Runtime bincompat functions for Concurrency runtime to detect legacy mode
 Added: _swift_bincompat_useLegacyNonCrashingExecutorChecks
+
+// Add add SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE
+Added: _concurrencyIsCurrentExecutorLegacyModeOverride

test/abi/macOS/x86_64/stdlib.swift

@@ -567,3 +567,6 @@ Added: _swift_updatePureObjCClassMetadata
 
 // Runtime bincompat functions for Concurrency runtime to detect legacy mode
 Added: _swift_bincompat_useLegacyNonCrashingExecutorChecks
+
+// Add add SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE
+Added: _concurrencyIsCurrentExecutorLegacyModeOverride