Add SQLite support, fix un-thread-safe DB caches, small performance f… (#172)

* Add SQLite support, fix un-thread-safe DB caches, small performance fixes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add SQLite licenses to README

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* appease the linter, and fix my dumbass-ery

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* make requested changes

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

* add back comment

Signed-off-by: kim (grufwub) <grufwub@gmail.com>

internal/db/bundb/conn.go

@@ -0,0 +1,72 @@
+package bundb
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/sirupsen/logrus"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/uptrace/bun"
+	"github.com/uptrace/bun/dialect"
+)
+
+// dbConn wrapps a bun.DB conn to provide SQL-type specific additional functionality
+type DBConn struct {
+	errProc func(error) db.Error // errProc is the SQL-type specific error processor
+	log     *logrus.Logger       // log is the logger passed with this DBConn
+	*bun.DB                      // DB is the underlying bun.DB connection
+}
+
+// WrapDBConn @TODO
+func WrapDBConn(dbConn *bun.DB, log *logrus.Logger) *DBConn {
+	var errProc func(error) db.Error
+	switch dbConn.Dialect().Name() {
+	case dialect.PG:
+		errProc = processPostgresError
+	case dialect.SQLite:
+		errProc = processSQLiteError
+	default:
+		panic("unknown dialect name: " + dbConn.Dialect().Name().String())
+	}
+	return &DBConn{
+		errProc: errProc,
+		log:     log,
+		DB:      dbConn,
+	}
+}
+
+// ProcessError processes an error to replace any known values with our own db.Error types,
+// making it easier to catch specific situations (e.g. no rows, already exists, etc)
+func (conn *DBConn) ProcessError(err error) db.Error {
+	switch {
+	case err == nil:
+		return nil
+	case err == sql.ErrNoRows:
+		return db.ErrNoEntries
+	default:
+		return conn.errProc(err)
+	}
+}
+
+// Exists checks the results of a SelectQuery for the existence of the data in question, masking ErrNoEntries errors
+func (conn *DBConn) Exists(ctx context.Context, query *bun.SelectQuery) (bool, db.Error) {
+	// Get the select query result
+	count, err := query.Count(ctx)
+
+	// Process error as our own and check if it exists
+	switch err := conn.ProcessError(err); err {
+	case nil:
+		return (count != 0), nil
+	case db.ErrNoEntries:
+		return false, nil
+	default:
+		return false, err
+	}
+}
+
+// NotExists is the functional opposite of conn.Exists()
+func (conn *DBConn) NotExists(ctx context.Context, query *bun.SelectQuery) (bool, db.Error) {
+	// Simply inverse of conn.exists()
+	exists, err := conn.Exists(ctx, query)
+	return !exists, err
+}