[Backtracing] Massage the output a little in various cases.

We shouldn't try to erase the message when in non-color mode; mostly
that's intended for redirected output scenarios anyway, and in that case
we don't really want garbage in the output.

Also, improve the failure messages when the backtracer itself goes
wrong or can't be executed.

Finally, change the behaviour slightly so that *if* we're explicitly
enabled, *and* the backtracer path wasn't explicitly specified, *and*
we can't find the backtracer, we print a warning on start-up.  We
don't do that in any other case because we don't want spurious warnings
everywhere.

rdar://118055527

Author: al45tair

stdlib/public/libexec/swift-backtrace/main.swift

@@ -652,11 +652,11 @@ Generate a backtrace for the parent process.
       description = "Program crashed: \(target.signalDescription) at \(hex(target.faultAddress))"
     }
 
-    // Clear the message written by the crash handler
+    // Clear (or complete) the message written by the crash handler
     if args.color {
       write("\r\u{1b}[0K")
     } else {
-      write("\r" + String(repeating:" ", count: 80) + "\r")
+      write(" done ***\n\n")
     }
 
     writeln(theme.crashReason(description))

stdlib/public/runtime/Backtrace.cpp

@@ -76,13 +76,7 @@ SWIFT_RUNTIME_STDLIB_INTERNAL BacktraceSettings _swift_backtraceSettings = {
   UnwindAlgorithm::Auto,
 
   // enabled
-#if TARGET_OS_OSX
-  OnOffTty::TTY,
-#elif defined(__linux__) // || defined(_WIN32)
-  OnOffTty::On,
-#else
-  OnOffTty::Off,
-#endif
+  OnOffTty::Default,
 
   // demangle
   true,
@@ -232,6 +226,7 @@ const char *algorithmToString(UnwindAlgorithm algorithm) {
 
 const char *onOffTtyToString(OnOffTty oot) {
   switch (oot) {
+  case OnOffTty::Default: return "Default";
   case OnOffTty::On: return "On";
   case OnOffTty::Off: return "Off";
   case OnOffTty::TTY: return "TTY";
@@ -279,6 +274,32 @@ BacktraceInitializer::BacktraceInitializer() {
   if (backtracing)
     _swift_parseBacktracingSettings(backtracing);
 
+#if !defined(SWIFT_RUNTIME_FIXED_BACKTRACER_PATH)
+  if (!_swift_backtraceSettings.swiftBacktracePath) {
+    _swift_backtraceSettings.swiftBacktracePath
+      = swift_copyAuxiliaryExecutablePath("swift-backtrace");
+
+    if (!_swift_backtraceSettings.swiftBacktracePath) {
+      if (_swift_backtraceSettings.enabled == OnOffTty::On) {
+        swift::warning(0,
+                       "swift runtime: unable to locate swift-backtrace; "
+                       "disabling backtracing.\n");
+      }
+      _swift_backtraceSettings.enabled = OnOffTty::Off;
+    }
+  }
+#endif
+
+  if (_swift_backtraceSettings.enabled == OnOffTty::Default) {
+#if TARGET_OS_OSX
+    _swift_backtraceSettings.enabled = OnOffTty::TTY;
+#elif defined(__linux__) // || defined(_WIN32)
+    _swift_backtraceSettings.enabled = OnOffTty::On;
+#else
+    _swift_backtraceSettings.enabled = OnOffTty::Off;
+#endif
+  }
+
 #if !SWIFT_BACKTRACE_ON_CRASH_SUPPORTED
   if (_swift_backtraceSettings.enabled != OnOffTty::Off) {
     swift::warning(0,
@@ -331,22 +352,6 @@ BacktraceInitializer::BacktraceInitializer() {
       _swift_backtraceSettings.outputTo = OutputTo::Stderr;
   }
 
-#if !defined(SWIFT_RUNTIME_FIXED_BACKTRACER_PATH)
-  if (_swift_backtraceSettings.enabled == OnOffTty::On
-      && !_swift_backtraceSettings.swiftBacktracePath) {
-    _swift_backtraceSettings.swiftBacktracePath
-      = swift_copyAuxiliaryExecutablePath("swift-backtrace");
-
-    if (!_swift_backtraceSettings.swiftBacktracePath) {
-      // Disabled warning for now - rdar://106813646
-      /* swift::warning(0,
-                     "swift runtime: unable to locate swift-backtrace; "
-                     "disabling backtracing.\n"); */
-      _swift_backtraceSettings.enabled = OnOffTty::Off;
-    }
-  }
-#endif
-
   if (_swift_backtraceSettings.enabled == OnOffTty::On) {
     // Copy the path to swift-backtrace into swiftBacktracePath, then write
     // protect it so that it can't be overwritten easily at runtime.  We do
@@ -1007,7 +1012,7 @@ _swift_displayCrashMessage(int signum, const void *pc)
   if (_swift_backtraceSettings.color == OnOffTty::On) {
     intro = "\n💣 \033[91mProgram crashed: ";
   } else {
-    intro = "\n*** Program crashed: ";
+    intro = "\n*** ";
   }
   write(fd, intro, strlen(intro));
 

stdlib/public/runtime/BacktracePrivate.h

@@ -51,6 +51,7 @@ enum class UnwindAlgorithm {
 };
 
 enum class OnOffTty {
+  Default = -1,
   Off = 0,
   On = 1,
   TTY = 2

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -250,7 +250,8 @@ handle_fatal_signal(int signum,
 
   // Actually start the backtracer
   if (!run_backtracer(fd)) {
-    const char *message = "\n\nBacktracing failed\n";
+    const char *message = _swift_backtraceSettings.color == OnOffTty::On
+      ? " failed\n\n" : " failed ***\n\n";
     if (_swift_backtraceSettings.outputTo == OutputTo::Stderr)
       write(STDERR_FILENO, message, strlen(message));
     else

stdlib/public/runtime/CrashHandlerMacOS.cpp

@@ -248,7 +248,8 @@ handle_fatal_signal(int signum,
   /* Start the backtracer; this will suspend the process, so there's no need
      to try to suspend other threads from here. */
   if (!run_backtracer()) {
-    const char *message = "\n\nBacktracing failed\n";
+    const char *message = _swift_backtraceSettings.color == OnOffTty::On
+      ? " failed\n\n" : " failed ***\n\n";
     if (_swift_backtraceSettings.outputTo == OutputTo::Stderr)
       write(STDERR_FILENO, message, strlen(message));
     else