Add basic database schema migration system

PluralKit.Core/SchemaService.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Data;
+using System.IO;
+using System.Threading.Tasks;
+using Dapper;
+
+using Npgsql;
+
+using Serilog;
+
+namespace PluralKit {
+    public class SchemaService
+    {
+        private const int TargetSchemaVersion = 1;
+
+        private DbConnectionFactory _conn;
+        private ILogger _logger;
+        
+        public SchemaService(DbConnectionFactory conn, ILogger logger)
+        {
+            _conn = conn;
+            _logger = logger.ForContext<SchemaService>();
+        }
+
+        public async Task ApplyMigrations()
+        {
+            for (var version = 0; version <= TargetSchemaVersion; version++) 
+                await ApplyMigration(version);
+        }
+
+        private async Task ApplyMigration(int migrationId)
+        {
+            // migrationId is the *target* version
+            using var conn = await _conn.Obtain();
+            using var tx = conn.BeginTransaction();
+            
+            // See if we even have the info table... if not, we implicitly define the version as -1
+            // This means migration 0 will get executed, which ensures we're at a consistent state.
+            // *Technically* this also means schema version 0 will be identified as -1, but since we're only doing these
+            // checks in the above for loop, this doesn't matter.
+            var hasInfoTable = await conn.QuerySingleOrDefaultAsync<int>("select count(*) from information_schema.tables where table_name = 'info'") == 1;
+
+            int currentVersion;
+            if (hasInfoTable)
+                currentVersion = await conn.QuerySingleOrDefaultAsync<int>("select schema_version from info");
+            else currentVersion = -1;
+            
+            if (currentVersion >= migrationId)
+                return; // Don't execute the migration if we're already at the target version.
+
+            using var stream = typeof(SchemaService).Assembly.GetManifestResourceStream($"PluralKit.Core.Migrations.{migrationId}.sql");
+            if (stream == null) throw new ArgumentException("Invalid migration ID");
+            
+            using var reader = new StreamReader(stream);
+            var migrationQuery = await reader.ReadToEndAsync();
+            
+            _logger.Information("Current schema version is {CurrentVersion}, applying migration {MigrationId}", currentVersion, migrationId);
+            await conn.ExecuteAsync(migrationQuery, transaction: tx);
+            tx.Commit();
+        }
+    }
+}