gh-145865: Fix CoverageResults.__init__ to copy the counts dict

Fix typo where self.counter was assigned instead of self.counts,
causing the counts dict to not be copied. This made
CoverageResults.update() mutate the caller's original dict.

The fix changes self.counter = self.counts.copy() to
self.counts = self.counts.copy(), matching the pattern used
for calledfuncs and callers on the following lines.

Also removes the dead self.counter attribute which was never
read anywhere in the codebase.

Author: kimimgo

Lib/test/test_trace.py

@@ -590,5 +590,33 @@ def test_no_source_file(self):
         self.assertIn(f"{filename}({firstlineno + 4})", out[4])
 
 
+class TestCoverageResultsInit(unittest.TestCase):
+    def test_counts_dict_is_copied(self):
+        # gh-145865: CoverageResults.__init__ should copy the counts dict
+        # to avoid mutating the caller's dict on update()
+        from trace import CoverageResults
+
+        counts = {}
+        cr = CoverageResults(counts=counts)
+        cr.update(CoverageResults(counts={("file.py", 1): 5}))
+        self.assertEqual(counts, {})  # original must not be mutated
+
+    def test_calledfuncs_dict_is_copied(self):
+        from trace import CoverageResults
+
+        calledfuncs = {}
+        cr = CoverageResults(calledfuncs=calledfuncs)
+        cr.update(CoverageResults(calledfuncs={("file.py", "mod", "func"): 1}))
+        self.assertEqual(calledfuncs, {})
+
+    def test_callers_dict_is_copied(self):
+        from trace import CoverageResults
+
+        callers = {}
+        cr = CoverageResults(callers=callers)
+        cr.update(CoverageResults(callers={(("a.py", "m", "f"), ("b.py", "m", "g")): 1}))
+        self.assertEqual(callers, {})
+
+
 if __name__ == '__main__':
     unittest.main()

Lib/trace.py

@@ -155,7 +155,7 @@ def __init__(self, counts=None, calledfuncs=None, infile=None,
         self.counts = counts
         if self.counts is None:
             self.counts = {}
-        self.counter = self.counts.copy() # map (filename, lineno) to count
+        self.counts = self.counts.copy()  # map (filename, lineno) to count
         self.calledfuncs = calledfuncs
         if self.calledfuncs is None:
             self.calledfuncs = {}