[C++] Always use setjmp/longjmp for exceptions

We currently throw exceptions from signal handlers (e.g., for
Quit/ctrl-c).  But throwing C++ exceptions from signal handlers is
undefined.  (That doesn't restore signal masks, like siglongjmp does,
and, because asynchronous signals can arrive at any instruction, we'd
have to build _everything_ with -fasync-unwind-tables to make it
reliable.)  It happens to work on x86_64 GNU/Linux at least, but it's
likely broken on other ports.

Until we stop throwing from signal handlers, use setjmp/longjmp based
exceptions in C++ mode as well.

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.h (GDB_XCPT_SJMP, GDB_XCPT_TRY)
	(GDB_XCPT_RAW_TRY, GDB_XCPT): Define.
	Replace __cplusplus checks with GDB_XCPT checks throughout.
	* common/common-exceptions.c: Replace __cplusplus checks with
	GDB_XCPT checks throughout.

2 files changed, 33 insertions, 16 deletions

diff --git a/gdb/common/common-exceptions.c b/gdb/common/common-exceptions.c
index ce476a2..231bab5 100644
--- a/gdb/common/common-exceptions.c
+++ b/gdb/common/common-exceptions.c
@@ -22,7 +22,7 @@
 
 const struct gdb_exception exception_none = { (enum return_reason) 0, GDB_NO_ERROR, NULL };
 
-#ifndef __cplusplus
+#if GDB_XCPT == GDB_XCPT_SJMP
 
 /* Possible catcher states.  */
 enum catcher_state {
@@ -213,7 +213,7 @@ exceptions_state_mc_action_iter_1 (void)
   return exceptions_state_mc (CATCH_ITER_1);
 }
 
-#else /* !__cplusplus */
+#else /* !GDB_XCPT_SJMP */
 
 /* How many nested TRY blocks we have.  See exception_messages and
    throw_it.  */
@@ -261,7 +261,7 @@ gdb_exception_sliced_copy (struct gdb_exception *to, const struct gdb_exception
   *to = *from;
 }
 
-#endif /* !__cplusplus */
+#endif /* !GDB_XCPT_SJMP */
 
 /* Return EXCEPTION to the nearest containing catch_errors().  */
 
@@ -272,7 +272,7 @@ throw_exception (struct gdb_exception exception)
 
   do_cleanups (all_cleanups ());
 
-#ifndef __cplusplus
+#if GDB_XCPT == GDB_XCPT_SJMP
   /* Jump to the containing catch_errors() call, communicating REASON
      to that call via setjmp's return value.  Note that REASON can't
      be zero, by definition in defs.h.  */
@@ -320,7 +320,7 @@ throw_it (enum return_reason reason, enum errors error, const char *fmt,
 {
   struct gdb_exception e;
   char *new_message;
-#ifndef __cplusplus
+#if GDB_XCPT == GDB_XCPT_SJMP
   int depth = catcher_list_size ();
 #else
   int depth = try_scope_depth;
diff --git a/gdb/common/common-exceptions.h b/gdb/common/common-exceptions.h
index 51795b1..def08b1 100644
--- a/gdb/common/common-exceptions.h
+++ b/gdb/common/common-exceptions.h
@@ -116,12 +116,32 @@ struct gdb_exception
   const char *message;
 };
 
+/* The different exception mechanisms that TRY/CATCH can map to.  */
+
+/* Make GDB exceptions use setjmp/longjmp behind the scenes.  This is
+   the only mode supported when GDB is built as a C program.  */
+#define GDB_XCPT_SJMP 1
+
+/* Make GDB exceptions use try/catch behind the scenes.  Can't be made
+   the default until we stop throwing exceptions from signal
+   handlers.  */
+#define GDB_XCPT_TRY 2
+
+/* Specify this mode to build with TRY/CATCH mapped directly to raw
+   try/catch.  GDB won't work correctly, but building that way catches
+   code tryin to break/continue out of the try block, along with
+   spurious code between the TRY and the CATCH block.  */
+#define GDB_XCPT_RAW_TRY 3
+
+/* Always use setjmp/longmp, even in C++ mode.  */
+#define GDB_XCPT GDB_XCPT_SJMP
+
 /* Functions to drive the exceptions state machine.  Though declared
    here by necessity, these functions should be considered internal to
    the exceptions subsystem and not used other than via the TRY/CATCH
    macros defined below.  */
 
-#ifndef __cplusplus
+#if GDB_XCPT == GDB_XCPT_SJMP
 extern SIGJMP_BUF *exceptions_state_mc_init (void);
 extern int exceptions_state_mc_action_iter (void);
 extern int exceptions_state_mc_action_iter_1 (void);
@@ -157,7 +177,7 @@ extern void exception_rethrow (void);
 
   */
 
-#ifndef __cplusplus
+#if GDB_XCPT == GDB_XCPT_SJMP
 
 #define TRY \
      { \
@@ -176,7 +196,9 @@ extern void exception_rethrow (void);
 #define END_CATCH				\
   }
 
-#else
+#endif /* GDB_XCPT_SJMP */
+
+#if GDB_XCPT == GDB_XCPT_TRY || GDB_XCPT == GDB_XCPT_RAW_TRY
 
 /* Prevent error/quit during TRY from calling cleanups established
    prior to here.  This pops out the scope in either case of normal
@@ -195,13 +217,7 @@ struct exception_try_scope
   void *saved_state;
 };
 
-/* Define this to build with TRY/CATCH mapped directly to raw
-   try/catch.  GDB won't work correctly, but building that way catches
-   code tryin to break/continue out of the try block, along with
-   spurious code between the TRY and the CATCH block.  */
-//#define USE_RAW_CXX_TRY
-
-#ifndef USE_RAW_CXX_TRY
+#if GDB_XCPT == GDB_XCPT_TRY
 
 /* We still need to wrap TRY/CATCH in C++ so that cleanups and C++
    exceptions can coexist.  The TRY blocked is wrapped in a
@@ -224,6 +240,7 @@ struct exception_try_scope
   {						\
     exception_rethrow ();			\
   }
+
 #else
 
 #define TRY try
@@ -249,7 +266,7 @@ struct gdb_exception_RETURN_MASK_QUIT : public gdb_exception_RETURN_MASK_ALL
 {
 };
 
-#endif
+#endif /* GDB_XCPT_TRY || GDB_XCPT_RAW_TRY */
 
 /* *INDENT-ON* */