Fix issue #1689458 by teaching frame_setlineno how to jump to the first line of

a code object.

Lib/test/test_trace.py

@@ -740,6 +740,27 @@ class JumpTestCase(unittest.TestCase):
     def test_19_no_jump_without_trace_function(self):
         no_jump_without_trace_function()
 
+    def test_jump_to_firstlineno(self):
+        # This tests that PDB can jump back to the first line in a
+        # file.  See issue #1689458.  It can only be triggered in a
+        # function call if the function is defined on a single line.
+        code = compile("""
+# Comments don't count.
+output.append(2)  # firstlineno is here.
+output.append(3)
+output.append(4)
+""", "<fake module>", "exec")
+        class fake_function:
+            func_code = code
+            jump = (2, 0)
+        tracer = JumpTracer(fake_function)
+        sys.settrace(tracer.trace)
+        namespace = {"output": []}
+        exec code in namespace
+        sys.settrace(None)
+        self.compare_jump_output([2, 3, 2, 3, 4], namespace["output"])
+
+
 def test_main():
     test_support.run_unittest(
         TraceTestCase,

Objects/frameobject.c

@@ -140,10 +140,15 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno)
 			     new_lineno);
 		return -1;
 	}
-
-	/* Find the bytecode offset for the start of the given line, or the
-	 * first code-owning line after it. */
-	PyString_AsStringAndSize(f->f_code->co_lnotab, &lnotab, &lnotab_len);
+	else if (new_lineno == f->f_code->co_firstlineno) {
+		new_lasti = 0;
+		new_lineno = f->f_code->co_firstlineno;
+	}
+	else {
+		/* Find the bytecode offset for the start of the given
+		 * line, or the first code-owning line after it. */
+		PyString_AsStringAndSize(f->f_code->co_lnotab,
+					 &lnotab, &lnotab_len);
 		addr = 0;
 		line = f->f_code->co_firstlineno;
 		new_lasti = -1;
@@ -156,6 +161,7 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno)
 				break;
 			}
 		}
+	}
 
 	/* If we didn't reach the requested line, return an error. */
 	if (new_lasti == -1) {