Fix Cygwin gdb build

Simon pointed out that the windows-nat sharing series broke the Cygwin
build.  This patch fixes the problem, by moving the Cygwin-specific
code to a new handler function.  This approach is taken because this
code calls find_pc_partial_function, which isn't available in
gdbserver.

gdb/ChangeLog
2020-04-16  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat::handle_access_violation): New
	function.
	* nat/windows-nat.h (handle_access_violation): Declare.
	* nat/windows-nat.c (handle_exception): Move Cygwin code to
	windows-nat.c.  Call handle_access_violation.

gdbserver/ChangeLog
2020-04-16  Tom Tromey  <tromey@adacore.com>

	* win32-low.cc (windows_nat::handle_access_violation): New
	function.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2020-04-16  Tom Tromey  <tromey@adacore.com>
+
+	* windows-nat.c (windows_nat::handle_access_violation): New
+	function.
+	* nat/windows-nat.h (handle_access_violation): Declare.
+	* nat/windows-nat.c (handle_exception): Move Cygwin code to
+	windows-nat.c.  Call handle_access_violation.
+
 2020-04-16  Tom de Vries  <tdevries@suse.de>
 
 	PR symtab/25791

gdb/nat/windows-nat.c

@@ -184,26 +184,8 @@ handle_exception (struct target_waitstatus *ourstatus, bool debug_exceptions)
     case EXCEPTION_ACCESS_VIOLATION:
       DEBUG_EXCEPTION_SIMPLE ("EXCEPTION_ACCESS_VIOLATION");
       ourstatus->value.sig = GDB_SIGNAL_SEGV;
-#ifdef __CYGWIN__
+      if (handle_access_violation (rec))
-      {
+	return HANDLE_EXCEPTION_UNHANDLED;
-	/* See if the access violation happened within the cygwin DLL
-	   itself.  Cygwin uses a kind of exception handling to deal
-	   with passed-in invalid addresses.  gdb should not treat
-	   these as real SEGVs since they will be silently handled by
-	   cygwin.  A real SEGV will (theoretically) be caught by
-	   cygwin later in the process and will be sent as a
-	   cygwin-specific-signal.  So, ignore SEGVs if they show up
-	   within the text segment of the DLL itself.  */
-	const char *fn;
-	CORE_ADDR addr = (CORE_ADDR) (uintptr_t) rec->ExceptionAddress;
-
-	if ((!cygwin_exceptions && (addr >= cygwin_load_start
-				    && addr < cygwin_load_end))
-	    || (find_pc_partial_function (addr, &fn, NULL, NULL)
-		&& startswith (fn, "KERNEL32!IsBad")))
-	  return HANDLE_EXCEPTION_UNHANDLED;
-      }
-#endif
       break;
     case STATUS_STACK_OVERFLOW:
       DEBUG_EXCEPTION_SIMPLE ("STATUS_STACK_OVERFLOW");

gdb/nat/windows-nat.h

@@ -157,6 +157,13 @@ extern void handle_unload_dll ();
 
 extern bool handle_ms_vc_exception (const EXCEPTION_RECORD *rec);
 
+/* When EXCEPTION_ACCESS_VIOLATION is processed, we give the embedding
+   application a chance to change it to be considered "unhandled".
+   This function must be supplied by the embedding application.  If it
+   returns true, then the exception is "unhandled".  */
+
+extern bool handle_access_violation (const EXCEPTION_RECORD *rec);
+
 
 /* Currently executing process */
 extern HANDLE current_process_handle;

gdb/windows-nat.c

@@ -1230,6 +1230,31 @@ windows_nat::handle_ms_vc_exception (const EXCEPTION_RECORD *rec)
   return false;
 }
 
+/* See nat/windows-nat.h.  */
+
+bool
+windows_nat::handle_access_violation (const EXCEPTION_RECORD *rec)
+{
+#ifdef __CYGWIN__
+  /* See if the access violation happened within the cygwin DLL
+     itself.  Cygwin uses a kind of exception handling to deal with
+     passed-in invalid addresses.  gdb should not treat these as real
+     SEGVs since they will be silently handled by cygwin.  A real SEGV
+     will (theoretically) be caught by cygwin later in the process and
+     will be sent as a cygwin-specific-signal.  So, ignore SEGVs if
+     they show up within the text segment of the DLL itself.  */
+  const char *fn;
+  CORE_ADDR addr = (CORE_ADDR) (uintptr_t) rec->ExceptionAddress;
+
+  if ((!cygwin_exceptions && (addr >= cygwin_load_start
+			      && addr < cygwin_load_end))
+      || (find_pc_partial_function (addr, &fn, NULL, NULL)
+	  && startswith (fn, "KERNEL32!IsBad")))
+    return true;
+#endif
+  return false;
+}
+
 /* Resume thread specified by ID, or all artificially suspended
    threads, if we are continuing execution.  KILLED non-zero means we
    have killed the inferior, so we should ignore weird errors due to

gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-16  Tom Tromey  <tromey@adacore.com>
+
+	* win32-low.cc (windows_nat::handle_access_violation): New
+	function.
+
 2020-04-15  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* win32-low.cc (get_child_debug_event): Fix format string warning.

gdbserver/win32-low.cc

@@ -1198,6 +1198,14 @@ windows_nat::handle_ms_vc_exception (const EXCEPTION_RECORD *rec)
   return false;
 }
 
+/* See nat/windows-nat.h.  */
+
+bool
+windows_nat::handle_access_violation (const EXCEPTION_RECORD *rec)
+{
+  return false;
+}
+
 /* A helper function that will, if needed, set
    'stopped_at_software_breakpoint' on the thread and adjust the
    PC.  */