Add dbx library: PostgreSQL cluster with master/replica routing, retry, health checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tx.go

@@ -0,0 +1,61 @@
+package dbx
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// TxFunc is the callback signature for RunTx.
+type TxFunc func(ctx context.Context, tx pgx.Tx) error
+
+// RunTx executes fn inside a transaction on the master node with default options.
+// The transaction is committed if fn returns nil, rolled back otherwise.
+// Panics inside fn are caught, the transaction is rolled back, and the panic is re-raised.
+func (c *Cluster) RunTx(ctx context.Context, fn TxFunc) error {
+	return c.RunTxOptions(ctx, pgx.TxOptions{}, fn)
+}
+
+// RunTxOptions executes fn inside a transaction with the given options.
+func (c *Cluster) RunTxOptions(ctx context.Context, opts pgx.TxOptions, fn TxFunc) error {
+	tx, err := c.master.BeginTx(ctx, opts)
+	if err != nil {
+		return fmt.Errorf("dbx: begin tx: %w", err)
+	}
+
+	return runTx(ctx, tx, fn)
+}
+
+func runTx(ctx context.Context, tx pgx.Tx, fn TxFunc) (retErr error) {
+	defer func() {
+		if p := recover(); p != nil {
+			_ = tx.Rollback(ctx)
+			panic(p)
+		}
+		if retErr != nil {
+			_ = tx.Rollback(ctx)
+		}
+	}()
+
+	if err := fn(ctx, tx); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
+}
+
+// querierKey is the context key for Querier injection.
+type querierKey struct{}
+
+// InjectQuerier returns a new context carrying q.
+func InjectQuerier(ctx context.Context, q Querier) context.Context {
+	return context.WithValue(ctx, querierKey{}, q)
+}
+
+// ExtractQuerier returns the Querier from ctx, or fallback if none is present.
+func ExtractQuerier(ctx context.Context, fallback Querier) Querier {
+	if q, ok := ctx.Value(querierKey{}).(Querier); ok {
+		return q
+	}
+	return fallback
+}