Set up Alembic for database migrations

diff --git a/INSTALL b/INSTALL
index 68fe5dcd..7087aca2 100644
--- a/INSTALL
+++ b/INSTALL
@@ -47,8 +47,8 @@  read the instructions below.
 
 4) Install Python modules and dependencies:
 
-    # pacman -S python-mysql-connector python-pygit2 python-srcinfo python-sqlalchemy
-    # pacman -S python-bleach python-markdown
+    # pacman -S python-mysql-connector python-pygit2 python-srcinfo python-sqlalchemy \
+                python-bleach python-markdown python-alembic
     # python3 setup.py install
 
 5) Create a new MySQL database and a user and import the aurweb SQL schema:
diff --git a/TESTING b/TESTING
index 190043f9..4a1e6f4c 100644
--- a/TESTING
+++ b/TESTING
@@ -11,7 +11,8 @@  INSTALL.
 
 2) Install the necessary packages:
 
-    # pacman -S --needed php php-sqlite sqlite words fortune-mod python python-sqlalchemy
+    # pacman -S --needed php php-sqlite sqlite words fortune-mod \
+                         python python-sqlalchemy python-alembic
 
    Ensure to enable the pdo_sqlite extension in php.ini.
 
diff --git a/alembic.ini b/alembic.ini
new file mode 100644
index 00000000..6d3a3929
--- /dev/null
+++ b/alembic.ini
@@ -0,0 +1,86 @@ 
+# A generic, single database configuration.
+
+[alembic]
+# path to migration scripts
+script_location = migrations
+
+# template used to generate migration files
+# file_template = %%(rev)s_%%(slug)s
+
+# timezone to use when rendering the date
+# within the migration file as well as the filename.
+# string value is passed to dateutil.tz.gettz()
+# leave blank for localtime
+# timezone =
+
+# max length of characters to apply to the
+# "slug" field
+# truncate_slug_length = 40
+
+# set to 'true' to run the environment during
+# the 'revision' command, regardless of autogenerate
+# revision_environment = false
+
+# set to 'true' to allow .pyc and .pyo files without
+# a source .py file to be detected as revisions in the
+# versions/ directory
+# sourceless = false
+
+# version location specification; this defaults
+# to alembic/versions.  When using multiple version
+# directories, initial revisions must be specified with --version-path
+# version_locations = %(here)s/bar %(here)s/bat alembic/versions
+
+# the output encoding used when revision files
+# are written from script.py.mako
+# output_encoding = utf-8
+
+# the database URL is generated in env.py
+# sqlalchemy.url = driver://user:pass@localhost/dbname
+
+
+[post_write_hooks]
+# post_write_hooks defines scripts or Python functions that are run
+# on newly generated revision scripts.  See the documentation for further
+# detail and examples
+
+# format using "black" - use the console_scripts runner, against the "black" entrypoint
+# hooks=black
+# black.type=console_scripts
+# black.entrypoint=black
+# black.options=-l 79
+
+# Logging configuration
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S
diff --git a/aurweb/initdb.py b/aurweb/initdb.py
index e3e96503..c02fb961 100644
--- a/aurweb/initdb.py
+++ b/aurweb/initdb.py
@@ -1,6 +1,8 @@ 
 import aurweb.db
 import aurweb.schema
 
+import alembic.command
+import alembic.config
 import argparse
 import sqlalchemy
 
@@ -31,10 +33,17 @@  def feed_initial_data(conn):
 
 
 def run(args):
+    # Ensure Alembic is fine before we do the real work, in order not to fail at
+    # the last step and leave the database in an inconsistent state. The
+    # configuration is loaded lazily, so we query it to force its loading.
+    alembic_config = alembic.config.Config('alembic.ini')
+    alembic_config.get_main_option('script_location')
+
     engine = sqlalchemy.create_engine(aurweb.db.get_sqlalchemy_url(),
                                       echo=(args.verbose >= 1))
     aurweb.schema.metadata.create_all(engine)
     feed_initial_data(engine.connect())
+    alembic.command.stamp(alembic_config, 'head')
 
 
 if __name__ == '__main__':
diff --git a/aurweb/schema.py b/aurweb/schema.py
index b1261e86..fde6512f 100644
--- a/aurweb/schema.py
+++ b/aurweb/schema.py
@@ -1,3 +1,11 @@ 
+"""
+Schema of aurweb's database.
+
+Changes here should always be accompanied by an Alembic migration, which can be
+usually be automatically generated. See `migrations/README` for details.
+"""
+
+
 from sqlalchemy import CHAR, Column, ForeignKey, Index, MetaData, String, TIMESTAMP, Table, Text, text
 from sqlalchemy.dialects.mysql import BIGINT, DECIMAL, INTEGER, TINYINT
 from sqlalchemy.ext.compiler import compiles
diff --git a/migrations/README b/migrations/README
new file mode 100644
index 00000000..5c032d9b
--- /dev/null
+++ b/migrations/README
@@ -0,0 +1,48 @@ 
+This directory contains Alembic’s environment for managing database migrations.
+
+From Alembic’s documentation: Alembic is a lightweight database migration tool
+for usage with the SQLAlchemy Database Toolkit for Python.
+https://alembic.sqlalchemy.org/en/latest/index.html
+
+
+Upgrading to the latest version
+-------------------------------
+
+Simply run `alembic upgrade head` from aurweb’s root.
+
+
+Creating new migrations
+-----------------------
+
+When working with Alembic and SQLAlchemy, you should never edit the database
+schema manually. Please proceed like this instead:
+
+1. Edit `aurweb/schema.py` to your liking.
+2. Run `alembic revision --autogenerate -m "your message"`
+3. Proofread the generated migration.
+4. Run `alembic upgrade head` to apply the changes to the database.
+5. Commit the new migration.
+
+To revert a migration, you may run `alembic downgrade -1` and then manually
+delete the migration file. Note that SQLite is limited and that it’s sometimes
+easier to recreate the database.
+
+For anything more complicated, please read Alembic’s documentation.
+
+
+Troubleshooting
+---------------
+
+- `ModuleNotFoundError: No module named 'aurweb'`
+
+  You may either install the aurweb module with pip, or set PYTHONPATH to your
+  aurweb repository. Since alembic must be run from the aurweb root, you may
+  simply use: `PYTHONPATH=. alembic …`.
+
+- `FAILED: No config file 'alembic.ini' found, or file has no '[alembic]' section`
+
+  You need to run Alembic from the project’s root, and not from `migrations/`.
+
+- `configparser.NoSectionError: No section: 'database'`
+
+  You need to set AUR_CONFIG, as explained in `TESTING`.
diff --git a/migrations/env.py b/migrations/env.py
new file mode 100644
index 00000000..1627e693
--- /dev/null
+++ b/migrations/env.py
@@ -0,0 +1,73 @@ 
+import aurweb.db
+import aurweb.schema
+
+from alembic import context
+import logging.config
+import sqlalchemy
+
+
+# this is the Alembic Config object, which provides
+# access to the values within the .ini file in use.
+config = context.config
+
+# Interpret the config file for Python logging.
+# This line sets up loggers basically.
+logging.config.fileConfig(config.config_file_name)
+
+# model MetaData for autogenerating migrations
+target_metadata = aurweb.schema.metadata
+
+# other values from the config, defined by the needs of env.py,
+# can be acquired:
+# my_important_option = config.get_main_option("my_important_option")
+# ... etc.
+
+
+def run_migrations_offline():
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+
+    """
+    context.configure(
+        url=aurweb.db.get_sqlalchemy_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online():
+    """Run migrations in 'online' mode.
+
+    In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+    connectable = sqlalchemy.create_engine(
+        aurweb.db.get_sqlalchemy_url(),
+        poolclass=sqlalchemy.pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection, target_metadata=target_metadata
+        )
+
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()
diff --git a/migrations/script.py.mako b/migrations/script.py.mako
new file mode 100644
index 00000000..2c015630
--- /dev/null
+++ b/migrations/script.py.mako
@@ -0,0 +1,24 @@ 
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+def upgrade():
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade():
+    ${downgrades if downgrades else "pass"}