[scan-build-py] Increase precision of timestamp in report directory name

This commit improves compatibility with the perl version of scan-build.

The perl version of scan-build produces output report directories with
increasing lexicographic ordering. This ordering is relied on by the CmpRuns.py
tool in utils/analyzer when comparing results for build commands with multiple
steps. That tool tries to line up the output directory for each step between
different runs of the analyzer based on the increasing directory name.

The python version of scan-build uses file.mkdtemp() with a time stamp
prefix to create report directories. The timestamp has a 1-second precision.
This means that when analysis of a single build step takes less than a second
the ordering property that CmpRuns.py expects will sometimes not hold,
depending on the timing and the random suffix generated by mkdtemp(). Ultimately
this causes CmpRuns to incorrectly correlate results from build steps and report
spurious differences between runs.

This commit increases the precision of the timestamp used in scan-build-py to
the microsecond level. This approach still has the same underlying issue -- but
in practice analysis of any build step is unlikely to take less than a
millisecond.

Differential Revision: https://reviews.llvm.org/D24163

llvm-svn: 280768

clang/tools/scan-build-py/libscanbuild/report.py

@@ -21,6 +21,7 @@
 import json
 import logging
 import contextlib
+import datetime
 from libscanbuild import duplicate_check
 from libscanbuild.clang import get_version
 
@@ -34,7 +35,8 @@ def report_directory(hint, keep):
     hint -- could specify the parent directory of the output directory.
     keep -- a boolean value to keep or delete the empty report directory. """
 
-    stamp = time.strftime('scan-build-%Y-%m-%d-%H%M%S-', time.localtime())
+    stamp_format = 'scan-build-%Y-%m-%d-%H-%M-%S-%f-'
+    stamp = datetime.datetime.now().strftime(stamp_format)
 
     parentdir = os.path.abspath(hint)
     if not os.path.exists(parentdir):

clang/tools/scan-build-py/tests/unit/test_report.py

@@ -146,3 +146,16 @@ def test_with_single_file(self):
     def test_empty(self):
         self.assertEqual(
             sut.commonprefix([]), '')
+
+class ReportDirectoryTest(unittest.TestCase):
+
+    # Test that successive report directory names ascend in lexicographic
+    # order. This is required so that report directories from two runs of
+    # scan-build can be easily matched up to compare results.
+    def test_directory_name_comparison(self):
+        with libear.TemporaryDirectory() as tmpdir, \
+             sut.report_directory(tmpdir, False) as report_dir1, \
+             sut.report_directory(tmpdir, False) as report_dir2, \
+             sut.report_directory(tmpdir, False) as report_dir3:
+            self.assertLess(report_dir1, report_dir2)
+            self.assertLess(report_dir2, report_dir3)