Add the :nom:generate_golden_schema pseudo-task (#718)

Add a "pseudo-task" in nom_build to do the three step process of generating
the golden schema.  In the course of this, add support for pseudo-tasks in
general, improve the database directory readme and make nom_build not call
gradlew if there are no tasks.

config/nom_build.py

@@ -19,6 +19,7 @@ import argparse
 import attr
 import io
 import os
+import shutil
 import subprocess
 import sys
 from typing import List, Union
@@ -58,6 +59,21 @@ PROPERTIES_HEADER = """\
 org.gradle.jvmargs=-Xmx1024m
 """
 
+# Help text to be displayed (in addition to the synopsis and flag help, which
+# are displayed automatically).
+HELP_TEXT = """\
+A wrapper around the gradle build that provides the following features:
+
+-   Converts properties into flags to guard against property name spelling errors
+    and to provide help descriptions for all properties.
+-   Provides pseudo-commands (with the ":nom:" prefix) that encapsulate common
+    actions that are difficult to implement in gradle.
+
+Pseudo-commands:
+    :nom:generate_golden_file - regenerates the golden file from the current
+        set of flyway files.
+"""
+
 # Define all of our special gradle properties here.
 PROPERTIES = [
     Property('mavenUrl',
@@ -256,8 +272,42 @@ def get_root() -> str:
     return cur_dir
 
 
-def main(args):
-    parser = argparse.ArgumentParser('nom_build')
+class Abort(Exception):
+    """Raised to terminate the process with a non-zero error code.
+
+    Parameters are ignored.
+    """
+
+
+def do_pseudo_task(task: str) -> None:
+    root = get_root()
+    if task == ':nom:generate_golden_file':
+        if not subprocess.call([f'{root}/gradlew', ':db:test']):
+            print('\033[33mWARNING:\033[0m Golden schema appears to be '
+                  'up-to-date.  If you are making schema changes,  be sure to '
+                  'add a flyway file for them.')
+            return
+        print('\033[33mWARNING:\033[0m Ignore the above failure, it is '
+              'expected.')
+
+        # Copy the new schema into place.
+        shutil.copy(f'{root}/db/build/resources/test/testcontainer/'
+                    'mount/dump.txt',
+                    f'{root}/db/src/main/resources/sql/schema/'
+                    'nomulus.golden.sql')
+
+        if subprocess.call([f'{root}/gradlew', ':db:test']):
+            print('\033[31mERROR:\033[0m Golden file test failed after '
+                  'copying schema.  Please check your flyway files.')
+            raise Abort()
+    else:
+        print(f'\033[31mERROR:\033[0m Unknown task {task}')
+        raise Abort()
+
+
+def main(args) -> int:
+    parser = argparse.ArgumentParser('nom_build', description=HELP_TEXT,
+                                     formatter_class=argparse.RawTextHelpFormatter)
     for prop in PROPERTIES:
         parser.add_argument('--' + prop.name, default=prop.default,
                             help=prop.desc)
@@ -296,7 +346,7 @@ def main(args):
     if args.generate_gradle_properties:
         with open(f'{root}/gradle.properties', 'w') as dst:
             dst.write(gradle_properties)
-        return
+        return 0
 
     # Verify that the gradle properties file is what we expect it to be.
     with open(f'{root}/gradle.properties') as src:
@@ -321,12 +371,39 @@ def main(args):
             if flag.has_arg:
                 gradle_command.append(arg_val)
 
+    # See if there are any special ":nom:" pseudo-tasks specified.
+    got_non_pseudo_tasks = False
+    for arg in args.non_flag_args[1:]:
+        if arg.startswith(':nom:'):
+            if got_non_pseudo_tasks:
+                # We can't currently deal with the situation of gradle tasks
+                # before pseudo-tasks.  This could be implemented by invoking
+                # gradle for only the set of gradle tasks before the pseudo
+                # task, but that's overkill for now.
+                print(f'\033[31mERROR:\033[0m Pseudo task ({arg}) must be '
+                      'specified prior to all actual gradle tasks.  Aborting.')
+                return 1
+            do_pseudo_task(arg)
+        else:
+            got_non_pseudo_tasks = True
+    non_flag_args = [
+        arg for arg in args.non_flag_args[1:] if not arg.startswith(':nom:')]
+
+    if not non_flag_args:
+        if not got_non_pseudo_tasks:
+            print('\033[33mWARNING:\033[0m No tasks specified.  Not '
+                  'doing anything')
+        return 0
+
     # Add the non-flag args (we exclude the first, which is the command name
     # itself) and run.
-    gradle_command.extend(args.non_flag_args[1:])
-    subprocess.call(gradle_command)
+    gradle_command.extend(non_flag_args)
+    return subprocess.call(gradle_command)
 
 
 if __name__ == '__main__':
-    main(sys.argv)
+    try:
+        sys.exit(main(sys.argv))
+    except Abort as ex:
+        sys.exit(1)