Use RAII to set the per-thread SIGSEGV handler

This avoids using a thread-local extern variable, which causes link errors
on some platforms, notably Cygwin.  But I think this is a better pattern
even outside of working around linker bugs because it encapsulates direct
access to the variable inside the class, instead of having a global extern
variable.

The cygwin link error is:
cp-support.o: in function `gdb_demangle(char const*, int)':
/home/Christian/binutils-gdb/obj/gdb/../../gdb/cp-support.c:1619:(.text+0x6472): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init function for thread_local_segv_handler'
/home/Christian/binutils-gdb/obj/gdb/../../gdb/cp-support.c:1619:(.text+0x648b): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init function for thread_local_segv_handler'
collect2: error: ld returned 1 exit status

2021-03-12  Christian Biesinger  <cbiesinger@google.com>

	PR threads/27239
	* cp-support.c: Use scoped_segv_handler_restore.
	* event-top.c (thread_local_segv_handler): Made static.
	(scoped_segv_handler_restore::scoped_segv_handler_restore):
	New function.
	(scoped_segv_handler_restore::~scoped_segv_handler_restore): New
	function.
	* event-top.h (class scoped_segv_handler_restore): New class.
	(thread_local_segv_handler): Removed.

1 files changed, 21 insertions, 2 deletions

diff --git a/gdb/event-top.c b/gdb/event-top.c
index a33f6425140..fb0df943f65 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -850,9 +850,17 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
 }
 
 
-/* See event-top.h.  */
+/* The SIGSEGV handler for this thread, or NULL if there is none.  GDB
+   always installs a global SIGSEGV handler, and then lets threads
+   indicate their interest in handling the signal by setting this
+   thread-local variable.
 
-thread_local void (*thread_local_segv_handler) (int);
+   This is a static variable instead of extern because on various platforms
+   (notably Cygwin) extern thread_local variables cause link errors.  So
+   instead, we have scoped_segv_handler_restore, which also makes it impossible
+   to accidentally forget to restore it to the original value.  */
+
+static thread_local void (*thread_local_segv_handler) (int);
 
 static void handle_sigsegv (int sig);
 
@@ -1288,6 +1296,17 @@ gdb_disable_readline (void)
   delete_file_handler (ui->input_fd);
 }
 
+scoped_segv_handler_restore::scoped_segv_handler_restore (segv_handler_t new_handler)
+{
+  m_old_handler = thread_local_segv_handler;
+  thread_local_segv_handler = new_handler;
+}
+
+scoped_segv_handler_restore::~scoped_segv_handler_restore()
+{
+  thread_local_segv_handler = m_old_handler;
+}
+
 static const char debug_event_loop_off[] = "off";
 static const char debug_event_loop_all_except_ui[] = "all-except-ui";
 static const char debug_event_loop_all[] = "all";