[Backtracing] Use stderr by default unless interactive.

This means we don't need to specify `output-to` in CI.

rdar://110371557

Author: al45tair

include/swift/Runtime/Backtrace.h

@@ -109,6 +109,7 @@ enum class SanitizePaths {
 };
 
 enum class OutputTo {
+  Auto = -1,
   Stdout = 0,
   Stderr = 2,
 };

stdlib/public/runtime/Backtrace.cpp

@@ -123,7 +123,7 @@ SWIFT_RUNTIME_STDLIB_INTERNAL BacktraceSettings _swift_backtraceSettings = {
   true,
 
   // outputTo,
-  OutputTo::Stdout,
+  OutputTo::Auto,
 
   // swiftBacktracePath
   NULL,
@@ -322,6 +322,13 @@ BacktraceInitializer::BacktraceInitializer() {
       _swift_backtraceSettings.preset = Preset::Full;
   }
 
+  if (_swift_backtraceSettings.outputTo == OutputTo::Auto) {
+    if (_swift_backtraceSettings.interactive == OnOffTty::On)
+      _swift_backtraceSettings.outputTo = OutputTo::Stdout;
+    else
+      _swift_backtraceSettings.outputTo = OutputTo::Stderr;
+  }
+
 #if !defined(SWIFT_RUNTIME_FIXED_BACKTRACER_PATH)
   if (_swift_backtraceSettings.enabled == OnOffTty::On
       && !_swift_backtraceSettings.swiftBacktracePath) {
@@ -648,7 +655,9 @@ _swift_processBacktracingSetting(llvm::StringRef key,
   } else if (key.equals_insensitive("cache")) {
     _swift_backtraceSettings.cache = parseBoolean(value);
   } else if (key.equals_insensitive("output-to")) {
-    if (value.equals_insensitive("stdout"))
+    if (value.equals_insensitive("auto"))
+      _swift_backtraceSettings.outputTo = OutputTo::Auto;
+    else if (value.equals_insensitive("stdout"))
       _swift_backtraceSettings.outputTo = OutputTo::Stdout;
     else if (value.equals_insensitive("stderr"))
       _swift_backtraceSettings.outputTo = OutputTo::Stderr;

stdlib/public/runtime/CrashHandlerMacOS.cpp

@@ -428,6 +428,7 @@ run_backtracer()
   case OutputTo::Stdout:
     backtracer_argv[30] = "stdout";
     break;
+  case OutputTo::Auto: // Shouldn't happen, but if it does pick stderr
   case OutputTo::Stderr:
     backtracer_argv[30] = "stderr";
     break;

test/lit.cfg

@@ -494,7 +494,7 @@ if backtracing is not None:
 
 backtrace_on_crash = lit_config.params.get('backtrace_on_crash', None)
 if backtrace_on_crash is not None:
-    config.environment['SWIFT_BACKTRACE'] = 'enable=on,output-to=stderr'
+    config.environment['SWIFT_BACKTRACE'] = 'enable=on'
 
 config.available_features.add('lld_lto')