Fixes #10541: regrtest -T is broken

* makes test_trace tests restore the tracefunc after they run
 * write_results() in trace module will not terminate if lnotab
   cannot be found.

Lib/test/test_trace.py

@@ -10,7 +10,6 @@ from trace import CoverageResults, Trace
 
 from test.tracedmodules import testmod
 
-
 #------------------------------- Utilities -----------------------------------#
 
 def fix_ext_py(filename):
@@ -224,6 +223,11 @@ class TestFuncs(unittest.TestCase):
         self.addCleanup(sys.settrace, sys.gettrace())
         self.tracer = Trace(count=0, trace=0, countfuncs=1)
         self.filemod = my_file_and_modname()
+        self._saved_tracefunc = sys.gettrace()
+
+    def tearDown(self):
+        if self._saved_tracefunc is not None:
+            sys.settrace(self._saved_tracefunc)
 
     def test_simple_caller(self):
         self.tracer.runfunc(traced_func_simple_caller, 1)

Lib/trace.py

@@ -326,7 +326,7 @@ class CoverageResults:
                 lnotab = _find_executable_linenos(filename)
             else:
                 lnotab = {}
-
+            if lnotab:
                 source = linecache.getlines(filename)
                 coverpath = os.path.join(dir, modulename + ".cover")
                 with open(filename, 'rb') as fp:
@@ -337,6 +337,7 @@ class CoverageResults:
                     percent = int(100 * n_hits / n_lines)
                     sums[modulename] = n_lines, percent, modulename, filename
 
+
         if summary and sums:
             print("lines   cov%   module   (path)")
             for m in sorted(sums):