bpo-42398: Fix "make regen-all" race condition (GH-23362)

Fix a race condition in "make regen-all" when make -jN option is used
to run jobs in parallel. The clinic.py script now only use atomic
write to write files. Moveover, generated files are now left
unchanged if the content does not change, to not change the file
modification time.

The "make regen-all" command runs "make clinic" and "make
regen-importlib" targets:

* "make regen-importlib" builds object files (ex: Modules/_weakref.o)
  from source files (ex: Modules/_weakref.c) and clinic files (ex:
  Modules/clinic/_weakref.c.h)
* "make clinic" always rewrites all clinic files
  (ex: Modules/clinic/_weakref.c.h)

Since there is no dependency between "clinic" and "regen-importlib"
Makefile targets, these two targets can be run in parallel. Moreover,
half of clinic.py file writes are not atomic and so there is a race
condition when "make regen-all" runs jobs in parallel using make -jN
option (which can be passed in MAKEFLAGS environment variable).

Fix clinic.py to make all file writes atomic:

* Add write_file() function to ensure that all file writes are
  atomic: write into a temporary file and then use os.replace().
* Moreover, write_file() doesn't recreate or modify the file if the
  content does not change to avoid modifying the file modification
  file.
* Update test_clinic to verify these assertions with a functional
  test.
* Remove Clinic.force attribute which was no longer used, whereas
  Clinic.verify remains useful.

Lib/test/test_clinic.py

@@ -795,17 +795,29 @@ class ClinicExternalTest(TestCase):
     maxDiff = None
 
     def test_external(self):
+        # bpo-42398: Test that the destination file is left unchanged if the
+        # content does not change. Moreover, check also that the file
+        # modification time does not change in this case.
         source = support.findfile('clinic.test')
         with open(source, 'r', encoding='utf-8') as f:
-            original = f.read()
+            orig_contents = f.read()
-        with os_helper.temp_dir() as testdir:
+
-            testfile = os.path.join(testdir, 'clinic.test.c')
+        with os_helper.temp_dir() as tmp_dir:
+            testfile = os.path.join(tmp_dir, 'clinic.test.c')
             with open(testfile, 'w', encoding='utf-8') as f:
-                f.write(original)
+                f.write(orig_contents)
-            clinic.parse_file(testfile, force=True)
+            old_mtime_ns = os.stat(testfile).st_mtime_ns
+
+            clinic.parse_file(testfile)
+
             with open(testfile, 'r', encoding='utf-8') as f:
-                result = f.read()
+                new_contents = f.read()
-            self.assertEqual(result, original)
+            new_mtime_ns = os.stat(testfile).st_mtime_ns
+
+        self.assertEqual(new_contents, orig_contents)
+        # Don't change the file modification time
+        # if the content does not change
+        self.assertEqual(new_mtime_ns, old_mtime_ns)
 
 
 if __name__ == "__main__":

Misc/NEWS.d/next/Build/2020-11-18-11-58-44.bpo-42398.Yt5wO8.rst

@@ -0,0 +1,4 @@
+Fix a race condition in "make regen-all" when make -jN option is used to run
+jobs in parallel. The clinic.py script now only use atomic write to write
+files. Moveover, generated files are now left unchanged if the content does not
+change, to not change the file modification time.

Tools/clinic/clinic.py

@@ -1777,6 +1777,30 @@ legacy_converters = {}
 # The callable should not call builtins.print.
 return_converters = {}
 
+
+def write_file(filename, new_contents):
+    try:
+        with open(filename, 'r', encoding="utf-8") as fp:
+            old_contents = fp.read()
+
+        if old_contents == new_contents:
+            # no change: avoid modifying the file modification time
+            return
+    except FileNotFoundError:
+        pass
+
+    # Atomic write using a temporary file and os.replace()
+    filename_new = f"{filename}.new"
+    with open(filename_new, "w", encoding="utf-8") as fp:
+        fp.write(new_contents)
+
+    try:
+        os.replace(filename_new, filename)
+    except:
+        os.unlink(filename_new)
+        raise
+
+
 clinic = None
 class Clinic:
 
@@ -1823,7 +1847,7 @@ impl_definition block
 
 """
 
-    def __init__(self, language, printer=None, *, force=False, verify=True, filename=None):
+    def __init__(self, language, printer=None, *, verify=True, filename=None):
         # maps strings to Parser objects.
         # (instantiated from the "parsers" global.)
         self.parsers = {}
@@ -1832,7 +1856,6 @@ impl_definition block
             fail("Custom printers are broken right now")
         self.printer = printer or BlockPrinter(language)
         self.verify = verify
-        self.force = force
         self.filename = filename
         self.modules = collections.OrderedDict()
         self.classes = collections.OrderedDict()
@@ -1965,8 +1988,7 @@ impl_definition block
                     block.input = 'preserve\n'
                     printer_2 = BlockPrinter(self.language)
                     printer_2.print_block(block)
-                    with open(destination.filename, "wt") as f:
+                    write_file(destination.filename, printer_2.f.getvalue())
-                        f.write(printer_2.f.getvalue())
                     continue
         text = printer.f.getvalue()
 
@@ -2018,7 +2040,10 @@ impl_definition block
         return module, cls
 
 
-def parse_file(filename, *, force=False, verify=True, output=None, encoding='utf-8'):
+def parse_file(filename, *, verify=True, output=None):
+    if not output:
+        output = filename
+
     extension = os.path.splitext(filename)[1][1:]
     if not extension:
         fail("Can't extract file type for file " + repr(filename))
@@ -2028,7 +2053,7 @@ def parse_file(filename, *, force=False, verify=True, output=None, encoding='utf
     except KeyError:
         fail("Can't identify file type for file " + repr(filename))
 
-    with open(filename, 'r', encoding=encoding) as f:
+    with open(filename, 'r', encoding="utf-8") as f:
         raw = f.read()
 
     # exit quickly if there are no clinic markers in the file
@@ -2036,19 +2061,10 @@ def parse_file(filename, *, force=False, verify=True, output=None, encoding='utf
     if not find_start_re.search(raw):
         return
 
-    clinic = Clinic(language, force=force, verify=verify, filename=filename)
+    clinic = Clinic(language, verify=verify, filename=filename)
     cooked = clinic.parse(raw)
-    if (cooked == raw) and not force:
-        return
 
-    directory = os.path.dirname(filename) or '.'
+    write_file(output, cooked)
-
-    with tempfile.TemporaryDirectory(prefix="clinic", dir=directory) as tmpdir:
-        bytes = cooked.encode(encoding)
-        tmpfilename = os.path.join(tmpdir, os.path.basename(filename))
-        with open(tmpfilename, "wb") as f:
-            f.write(bytes)
-        os.replace(tmpfilename, output or filename)
 
 
 def compute_checksum(input, length=None):
@@ -5105,7 +5121,7 @@ For more information see https://docs.python.org/3/howto/clinic.html""")
                 path = os.path.join(root, filename)
                 if ns.verbose:
                     print(path)
-                parse_file(path, force=ns.force, verify=not ns.force)
+                parse_file(path, verify=not ns.force)
         return
 
     if not ns.filename:
@@ -5121,7 +5137,7 @@ For more information see https://docs.python.org/3/howto/clinic.html""")
     for filename in ns.filename:
         if ns.verbose:
             print(filename)
-        parse_file(filename, output=ns.output, force=ns.force, verify=not ns.force)
+        parse_file(filename, output=ns.output, verify=not ns.force)
 
 
 if __name__ == "__main__":