feat(hooks): add OnTxBegin and runInTx for resolvespec and restheadspec

Every transaction the handlers open now fires OnTxBegin first, with the
transaction in hookCtx.Tx, via common.RunRequestTx.

pkg/common/txhook.go

@@ -0,0 +1,27 @@
+package common
+
+import "context"
+
+// TxHookName is the shared value of every spec's OnTxBegin HookType.
+const TxHookName = "on_tx_begin"
+
+// TxContext is implemented by a spec's HookContext so RunRequestTx can point
+// it at the transaction it opens.
+type TxContext interface {
+	SetTx(tx Database)
+}
+
+// RunRequestTx opens a transaction on db, points tc at it, runs onBegin (the
+// spec's OnTxBegin hooks) and then body. An error from onBegin or body rolls
+// the transaction back; body is not run when onBegin fails.
+func RunRequestTx(ctx context.Context, db Database, tc TxContext, onBegin func() error, body func(tx Database) error) error {
+	return db.RunInTransaction(ctx, func(tx Database) error {
+		tc.SetTx(tx)
+		if onBegin != nil {
+			if err := onBegin(); err != nil {
+				return err
+			}
+		}
+		return body(tx)
+	})
+}