Dump coredata earlier.

Unfortunately, `time.time` and file timestamps are not guaranteed to be
in sync and due to various kernel caches may be different enough to
cause rebuilds to fail [1]. This was masked by older ninja versions that
could not read sub-second timestamps.

[1] https://travis-ci.org/mesonbuild/meson/jobs/296797872

mesonbuild/environment.py

@@ -349,10 +349,9 @@ class Environment:
     def is_cross_build(self):
         return self.cross_info is not None
 
-    def dump_coredata(self, mtime):
+    def dump_coredata(self):
         cdf = os.path.join(self.get_build_dir(), Environment.coredata_file)
         coredata.save(self.coredata, cdf)
-        os.utime(cdf, times=(mtime, mtime))
         return cdf
 
     def get_script_dir(self):

mesonbuild/mesonmain.py

@@ -193,23 +193,19 @@ class MesonApp:
         mlog.log('Build machine cpu family:', mlog.bold(intr.builtin['build_machine'].cpu_family_method([], {})))
         mlog.log('Build machine cpu:', mlog.bold(intr.builtin['build_machine'].cpu_method([], {})))
         intr.run()
-        coredata_mtime = time.time()
-        g.generate(intr)
-        dumpfile = os.path.join(env.get_scratch_dir(), 'build.dat')
-        with open(dumpfile, 'wb') as f:
-            pickle.dump(b, f)
-        # Write this as late as possible since we use the existence of this
-        # file to check if we generated the build file successfully, so we
-        # don't want an error that pops up during generation, etc to cause us to
-        # incorrectly signal a successful meson run which will cause an error
-        # about an already-configured build directory when the user tries again.
-        #
-        # However, we set the mtime to an earlier value to ensure that doing an
-        # mtime comparison between the coredata dump and other build files
-        # shows the build files to be newer, not older.
-        cdf = env.dump_coredata(coredata_mtime)
-        # Post-conf scripts must be run after writing coredata or else introspection fails.
         try:
+            # We would like to write coredata as late as possible since we use the existence of
+            # this file to check if we generated the build file successfully. Since coredata
+            # includes settings, the build files must depend on it and appear newer. However, due
+            # to various kernel caches, we cannot guarantee that any time in Python is exactly in
+            # sync with the time that gets applied to any files. Thus, we dump this file as late as
+            # possible, but before build files, and if any error occurs, delete it.
+            cdf = env.dump_coredata()
+            g.generate(intr)
+            dumpfile = os.path.join(env.get_scratch_dir(), 'build.dat')
+            with open(dumpfile, 'wb') as f:
+                pickle.dump(b, f)
+            # Post-conf scripts must be run after writing coredata or else introspection fails.
             g.run_postconf_scripts()
         except:
             os.unlink(cdf)