Allow setting a parameter to raise gdb.GdbError

A convention in the Python layer is that raising a gdb.GdbError will
not print the Python stack -- instead the exception is treated as any
other gdb exception.

PR python/18852 asks that this treatment be extended the the
get_set_value method of gdb.Parameter.  This makes sense, because it
lets Python-created parameters act like gdb parameters.

2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/18852:
	* python/py-param.c (get_set_value): Use gdbpy_handle_exception.

gdb/doc/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/18852:
	* python.texi (Parameters In Python): Document exception behavior
	of get_set_string.

gdb/testsuite/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/18852:
	* gdb.python/py-parameter.exp: Add test for parameter that throws
	on "set".

6 files changed, 57 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index cbbd146fc94..e8005c02554 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2018-09-23  Tom Tromey  <tom@tromey.com>
 
+	PR python/18852:
+	* python/py-param.c (get_set_value): Use gdbpy_handle_exception.
+
+2018-09-23  Tom Tromey  <tom@tromey.com>
+
 	* python/py-function.c (fnpy_call): Use gdbpy_handle_exception.
 	* python/py-cmd.c (cmdpy_function): Use gdbpy_handle_exception.
 	* python/python-internal.h (gdbpy_handle_exception): Declare.
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 6d12553a28c..4285a4d8bb6 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,9 @@
+2018-09-23  Tom Tromey  <tom@tromey.com>
+
+	PR python/18852:
+	* python.texi (Parameters In Python): Document exception behavior
+	of get_set_string.
+
 2018-09-18  John Baldwin  <jhb@FreeBSD.org>
 
 	* gdb.texinfo (info proc): Remove "running".
diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
index e98178a1216..1035be33f00 100644
--- a/gdb/doc/python.texi
+++ b/gdb/doc/python.texi
@@ -3828,6 +3828,30 @@ example, @kbd{set foo off}).  The @code{value} attribute has already
 been populated with the new value and may be used in output.  This
 method must return a string.  If the returned string is not empty,
 @value{GDBN} will present it to the user.
+
+If this method raises the @code{gdb.GdbError} exception
+(@pxref{Exception Handling}), then @value{GDBN} will print the
+exception's string and the @code{set} command will fail.  Note,
+however, that the @code{value} attribute will not be reset in this
+case.  So, if your parameter must validate values, it should store the
+old value internally and reset the exposed value, like so:
+
+@smallexample
+class ExampleParam (gdb.Parameter):
+   def __init__ (self, name):
+      super (ExampleParam, self).__init__ (name,
+                   gdb.COMMAND_DATA,
+                   gdb.PARAM_BOOLEAN)
+      self.value = True
+      self.saved_value = True
+   def validate(self):
+      return False
+   def get_set_string (self):
+      if not self.validate():
+        self.value = self.saved_value
+        raise gdb.GdbError('Failed to validate')
+      self.saved_value = self.value
+@end smallexample
 @end defun
 
 @defun Parameter.get_show_string (self, svalue)
diff --git a/gdb/python/py-param.c b/gdb/python/py-param.c
index 0f0214befe0..bff5d60fc26 100644
--- a/gdb/python/py-param.c
+++ b/gdb/python/py-param.c
@@ -396,10 +396,7 @@ get_set_value (const char *args, int from_tty,
     {
       set_doc_string = call_doc_function (obj, set_doc_func.get (), NULL);
       if (! set_doc_string)
-	{
-	  gdbpy_print_stack ();
-	  return;
-	}
+	gdbpy_handle_exception ();
     }
 
   const char *str = set_doc_string.get ();
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 4a624ddd662..877edc0cb5c 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2018-09-23  Tom Tromey  <tom@tromey.com>
 
+	PR python/18852:
+	* gdb.python/py-parameter.exp: Add test for parameter that throws
+	on "set".
+
+2018-09-23  Tom Tromey  <tom@tromey.com>
+
 	PR python/17284:
 	* gdb.python/py-template.exp (test_template_arg): Add test for
 	negative template argument number.
diff --git a/gdb/testsuite/gdb.python/py-parameter.exp b/gdb/testsuite/gdb.python/py-parameter.exp
index 0508544f9d9..9a36e6372de 100644
--- a/gdb/testsuite/gdb.python/py-parameter.exp
+++ b/gdb/testsuite/gdb.python/py-parameter.exp
@@ -203,3 +203,18 @@ foreach kind {PARAM_ZUINTEGER PARAM_ZUINTEGER_UNLIMITED} {
 	    "check that PARAM_ZUINTEGER value is -1 after setting"
     }
 }
+
+gdb_py_test_multiple "Throwing gdb parameter" \
+    "python" "" \
+    "class TestThrowParam (gdb.Parameter):" "" \
+    "   def __init__ (self, name):" "" \
+    "      super (TestThrowParam, self).__init__ (name, gdb.COMMAND_DATA, gdb.PARAM_STRING)" "" \
+    "      self.value = True" "" \
+    "   def get_set_string (self):" "" \
+    "      raise gdb.GdbError('Ordinary gdb error')" "" \
+    "test_throw_param = TestThrowParam ('print test-throw-param')" ""\
+    "end"
+
+gdb_test "set print test-throw-param whoops" \
+    "Ordinary gdb error" \
+    "gdb.GdbError does not show Python stack"