fix: --fail-under=100 could report 100 is less than 100.

Use the same rounding rules for the fail-under message that are used for totals
everywhere else, so that it won't say:

    total of 100 is less than fail-under=100

5 files changed, 53 insertions, 10 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 205ef0ab..b15d6298 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -35,11 +35,16 @@ Unreleased
 
 - Warnings generated by coverage.py are now real Python warnings.
 
+- Using ``--fail-under=100`` with coverage near 100% could result in the
+  self-contradictory message :code:`total of 100 is less than fail-under=100`.
+  This bug (`issue 1168`_) is now fixed.
+
 - The ``COVERAGE_DEBUG_FILE`` environment variable now accepts ``stdout`` and
   ``stderr`` to write to those destinations.
 
 .. _Django coverage plugin: https://pypi.org/project/django-coverage-plugin/
 .. _issue 1150: https://github.com/nedbat/coveragepy/issues/1150
+.. _issue 1168: https://github.com/nedbat/coveragepy/issues/1168
 
 
 .. _changes_56b1:
diff --git a/coverage/cmdline.py b/coverage/cmdline.py
index d1e8f283..697d2960 100644
--- a/coverage/cmdline.py
+++ b/coverage/cmdline.py
@@ -20,7 +20,7 @@ from coverage.data import line_counts
 from coverage.debug import info_formatter, info_header, short_stack
 from coverage.exceptions import BaseCoverageException, ExceptionDuringRun, NoSource
 from coverage.execfile import PyRunner
-from coverage.results import should_fail_under
+from coverage.results import Numbers, should_fail_under
 
 
 class Opts:
@@ -655,8 +655,8 @@ class CoverageScript:
             fail_under = self.coverage.get_option("report:fail_under")
             precision = self.coverage.get_option("report:precision")
             if should_fail_under(total, fail_under, precision):
-                msg = "total of {total:.{p}f} is less than fail-under={fail_under:.{p}f}".format(
-                    total=total, fail_under=fail_under, p=precision,
+                msg = "total of {total} is less than fail-under={fail_under:.{p}f}".format(
+                    total=Numbers.display_covered(total), fail_under=fail_under, p=precision,
                 )
                 print("Coverage failure:", msg)
                 return FAIL_UNDER
diff --git a/coverage/results.py b/coverage/results.py
index c60ccac2..f7331b41 100644
--- a/coverage/results.py
+++ b/coverage/results.py
@@ -219,14 +219,24 @@ class Numbers(SimpleReprMixin):
         result in either "0" or "100".
 
         """
-        pc = self.pc_covered
-        if 0 < pc < self._near0:
-            pc = self._near0
-        elif self._near100 < pc < 100:
-            pc = self._near100
+        return self.display_covered(self.pc_covered)
+
+    @classmethod
+    def display_covered(cls, pc):
+        """Return a displayable total percentage, as a string.
+
+        Note that "0" is only returned when the value is truly zero, and "100"
+        is only returned when the value is truly 100.  Rounding can never
+        result in either "0" or "100".
+
+        """
+        if 0 < pc < cls._near0:
+            pc = cls._near0
+        elif cls._near100 < pc < 100:
+            pc = cls._near100
         else:
-            pc = round(pc, self._precision)
-        return "%.*f" % (self._precision, pc)
+            pc = round(pc, cls._precision)
+        return "%.*f" % (cls._precision, pc)
 
     @classmethod
     def pc_str_width(cls):
diff --git a/tests/test_process.py b/tests/test_process.py
index 9c695b2a..04a8a2d5 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -1249,6 +1249,22 @@ class FailUnderTest(CoverageTest):
         expected = "Coverage failure: total of 42.86 is less than fail-under=42.88"
         assert expected == self.last_line_squeezed(out)
 
+    def test_report_99p9_is_not_ok(self):
+        # A file with 99.99% coverage:
+        self.make_file("ninety_nine_plus.py", """\
+            a = 1
+            """ + """
+            b = 2
+            """ * 20000 + """
+            if a > 3:
+                c = 4
+            """)
+        self.run_command("coverage run --source=. ninety_nine_plus.py")
+        st, out = self.run_command_status("coverage report --fail-under=100")
+        assert st == 2
+        expected = "Coverage failure: total of 99 is less than fail-under=100"
+        assert expected == self.last_line_squeezed(out)
+
 
 class FailUnderNoFilesTest(CoverageTest):
     """Test that nothing to report results in an error exit status."""
diff --git a/tests/test_results.py b/tests/test_results.py
index 5811b0c2..fa239e92 100644
--- a/tests/test_results.py
+++ b/tests/test_results.py
@@ -68,6 +68,18 @@ class NumbersTest(CoverageTest):
         assert n10000.pc_covered_str == "0.0"
         Numbers.set_precision(0)
 
+    @pytest.mark.parametrize("prec, pc, res", [
+        (0, 47.87, "48"),
+        (1, 47.87, "47.9"),
+        (0, 99.995, "99"),
+        (2, 99.99995, "99.99"),
+    ])
+    def test_display_covered(self, prec, pc, res):
+        # Numbers._precision is a global, which is bad.
+        Numbers.set_precision(prec)
+        assert Numbers.display_covered(pc) == res
+        Numbers.set_precision(0)
+
     def test_covered_ratio(self):
         n = Numbers(n_files=1, n_statements=200, n_missing=47)
         assert n.ratio_covered == (153, 200)