provision: Manage digests more rigorously.

We now have two functions related to digests
for processes:

    is_digest_obsolete
    write_digest_file

In most cases we now **wait** to write the
digest file until after we've successfully
run a process with its new inputs.

In one place, for database migrations, we
continue to write the digest optimistically.
We'll want to fix this, but it requires a
little more code cleanup.

Here is the typical sequence of events:

    NEVER RUN -
        is_digest_obsolete returns True
        quickly (we don't compute a hash)

        write_digest_file does a write (duh)

    AFTER NO CHANGES -
        is_digest_obsolete returns False
        after reading one file for old
        hash and multiple files to compute
        hash

        most callers skip write_digest_file

        (no files are changed)

    AFTER SOME CHANGES -
        is_digest_obsolete returns False
        after doing full checks

        most callers call write_digest_file
        *after* running a process

scripts/lib/zulip_tools.py

@@ -393,24 +393,38 @@ def path_version_digest(paths: List[str],
 
     return sha1sum.hexdigest()
 
-def file_or_package_hash_updated(hash_name: str,
-                                 paths: List[str],
-                                 package_versions: List[str]=[]) -> bool:
-    # Check whether the files or package_versions passed as arguments
-    # changed compared to the last execution.
-    hash_path = os.path.join(get_dev_uuid_var_path(), hash_name)
+def is_digest_obsolete(hash_name: str,
+                       paths: List[str],
+                       package_versions: List[str]=[]) -> bool:
+    # Check whether the `paths` contents or
+    # `package_versions` have changed.
+
+    last_hash_path = os.path.join(get_dev_uuid_var_path(), hash_name)
+    try:
+        with open(last_hash_path) as f:
+            old_hash = f.read()
+    except FileNotFoundError:
+        # This is normal for a fresh checkout--a missing
+        # digest is an obsolete digest.
+        return True
+
     new_hash = path_version_digest(paths, package_versions)
 
-    with open(hash_path, 'a+') as hash_file:
-        hash_file.seek(0)
-        last_hash = hash_file.read()
+    return new_hash != old_hash
 
-        if (new_hash != last_hash):
-            hash_file.seek(0)
-            hash_file.truncate()
-            hash_file.write(new_hash)
-            return True
-    return False
+def write_new_digest(hash_name: str,
+                     paths: List[str],
+                     package_versions: List[str]=[]) -> None:
+    hash_path = os.path.join(get_dev_uuid_var_path(), hash_name)
+    new_hash = path_version_digest(paths, package_versions)
+    with open(hash_path, 'w') as f:
+        f.write(new_hash)
+
+    # Be a little verbose here--our callers ensure we
+    # only write new digests when things have changed, and
+    # making this system more transparent to developers
+    # can help them troubleshoot provisioning glitches.
+    print('New digest written to: ' + hash_path)
 
 def is_root() -> bool:
     if 'posix' in os.name and os.geteuid() == 0:

tools/lib/provision_inner.py

@@ -11,7 +11,7 @@ ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__f
 
 sys.path.append(ZULIP_PATH)
 from scripts.lib.zulip_tools import run, OKBLUE, ENDC, \
-    get_dev_uuid_var_path, file_or_package_hash_updated
+    get_dev_uuid_var_path, is_digest_obsolete, write_new_digest
 
 from version import PROVISION_VERSION
 from pygments import __version__ as pygments_version
@@ -119,7 +119,7 @@ def need_to_run_build_pygments_data() -> bool:
     if not os.path.exists("static/generated/pygments_data.json"):
         return True
 
-    return file_or_package_hash_updated(
+    return is_digest_obsolete(
         "build_pygments_data_hash",
         build_pygments_data_paths(),
         [pygments_version]
@@ -131,7 +131,7 @@ def need_to_run_compilemessages() -> bool:
         print('Need to run compilemessages due to missing language_name_map.json')
         return True
 
-    return file_or_package_hash_updated(
+    return is_digest_obsolete(
         "last_compilemessages_hash",
         compilemessages_paths(),
     )
@@ -140,7 +140,7 @@ def need_to_run_inline_email_css() -> bool:
     if not os.path.exists('templates/zerver/emails/compiled/'):
         return True
 
-    return file_or_package_hash_updated(
+    return is_digest_obsolete(
         "last_email_source_files_hash",
         inline_email_css_paths(),
     )
@@ -164,11 +164,20 @@ def main(options: argparse.Namespace) -> int:
 
     if options.is_force or need_to_run_build_pygments_data():
         run(["tools/setup/build_pygments_data"])
+        write_new_digest(
+            'build_pygments_data_hash',
+            build_pygments_data_paths(),
+            [pygments_version]
+        )
     else:
         print("No need to run `tools/setup/build_pygments_data`.")
 
     if options.is_force or need_to_run_inline_email_css():
         run(["scripts/setup/inline_email_css.py"])
+        write_new_digest(
+            "last_email_source_files_hash",
+            inline_email_css_paths(),
+        )
     else:
         print("No need to run `scripts/setup/inline_email_css.py`.")
 
@@ -221,6 +230,10 @@ def main(options: argparse.Namespace) -> int:
 
         if options.is_force or need_to_run_compilemessages():
             run(["./manage.py", "compilemessages"])
+            write_new_digest(
+                "last_compilemessages_hash",
+                compilemessages_paths(),
+            )
         else:
             print("No need to run `manage.py compilemessages`.")
 

zerver/lib/test_fixtures.py

@@ -20,8 +20,10 @@ from django.core.management import call_command
 from django.utils.module_loading import module_has_submodule
 
 sys.path.append(os.path.dirname(os.path.dirname(os.path.dirname(__file__))))
-from scripts.lib.zulip_tools import get_dev_uuid_var_path, run, \
-    file_or_package_hash_updated, TEMPLATE_DATABASE_DIR
+from scripts.lib.zulip_tools import (
+    get_dev_uuid_var_path, run, TEMPLATE_DATABASE_DIR,
+    is_digest_obsolete, write_new_digest,
+)
 
 UUID_VAR_DIR = get_dev_uuid_var_path()
 FILENAME_SPLITTER = re.compile(r'[\W\-_]')
@@ -41,6 +43,7 @@ class Database:
             UUID_VAR_DIR,
             self.migration_status_file
         )
+        self.migration_digest_file = "migrations_hash_" + database_name
 
     def run_db_migrations(self) -> None:
         # We shell out to `manage.py` and pass `DJANGO_SETTINGS_MODULE` on
@@ -152,13 +155,20 @@ class Database:
         # changes, we can safely assume we don't need to run
         # migrations without spending a few 100ms parsing all the
         # Python migration code.
-        check_migrations = file_or_package_hash_updated(
-            "migrations_hash_" + database_name
-            migration_paths(),
-        )
-        if not check_migrations:
+        if not self.is_digest_obsolete():
             return 'current'
 
+        '''
+        NOTE:
+            We immediately update the digest, assuming our
+            callers will do what it takes to run the migrations.
+
+            Ideally our callers would just do it themselves
+            AFTER the migrations actually succeeded, but the
+            caller codepaths are kind of complicated here.
+        '''
+        self.write_new_digest()
+
         migration_op = self.what_to_do_with_migrations()
         if migration_op == 'scrap':
             return 'needs_rebuild'
@@ -168,6 +178,18 @@ class Database:
 
         return 'current'
 
+    def is_digest_obsolete(self) -> bool:
+        return is_digest_obsolete(
+            self.migration_digest_file,
+            migration_paths(),
+        )
+
+    def write_new_digest(self) -> None:
+        write_new_digest(
+            self.migration_digest_file,
+            migration_paths(),
+        )
+
 DEV_DATABASE = Database(
     platform='dev',
     database_name='zulip',