Introduce model.Transaction()

It is a very common pattern that some function needs to access the
database and wants to wrap all the actions into one transaction.

The advantage of a transaction is that it is ACID:

- atomic
- consistent
- isolated
- durable

In Go it is required to request a new transaction, execute functionality
on it and handle rollback or commit of this transaction based on the
success of the operation.

All this and the error handling can be written down in the
model.Transaction() function exactly once. The full signature of it is:

	func Transaction(f func(*sql.Tx) error) error

It requires a function or closure passed as argument which takes the
transaction (*sql.Tx) and returns an error which might be nil.

This is very generic. It is applied to:

- injecting test data
- database migrations
- data read requests
- data write requests

1 files changed, 34 insertions, 28 deletions

diff --git a/model/database.go b/model/database.go
index d816163..44792fe 100644
--- a/model/database.go
+++ b/model/database.go
@@ -4,6 +4,7 @@ import (
 	"database/sql"
 	"embed"
 	"log"
+	"fmt"
 
 	_ "github.com/mattn/go-sqlite3"
 )
@@ -35,37 +36,53 @@ func InitDatabase(path string) {
 		log.Fatal(err)
 	}
 
-	// FIXME roll back migration on error
 	query, err := GetSql(`migrate`)
 	if err != nil {
 		log.Fatal(err)
 	}
-	_, err = db.Exec(query)
+
+	err = Transaction(func(tx *sql.Tx) error {
+		_, err := tx.Exec(query)
+		if err != nil {
+			return err
+		}
+		return nil
+	})
 	if err != nil {
-		log.Fatal(err)
+		log.Fatalf("Migration failed: %v\n", err)
 	}
 }
 
-func InjectTestRecipes() {
-	recipes := RecipeTestData()
-
-	tx, err := NewTx()
+func Transaction(f func(*sql.Tx) error) error {
+	tx, err := db.Begin()
 	if err != nil {
-		log.Fatalf("Failed to inject test recipes: %v\n", err)
+		return err
 	}
 
-	for _, recipe := range recipes {
-		err = recipe.Create(tx)
-		if err != nil {
-			Rollback(tx)
-			log.Fatalf("Failed to inject test recipe: %v\n", err)
+	err = f(tx)
+	if err != nil {
+		rollbackErr := tx.Rollback()
+		if rollbackErr != nil {
+			return fmt.Errorf("Failed rollback '%w' after failed transaction '%w'", rollbackErr, err)
 		}
 	}
 
-	err = tx.Commit()
-	if err != nil {
-		log.Fatalf("Failed to inject test recipe: %v\n", err)
-	}
+	return tx.Commit()
+}
+
+func InjectTestRecipes() {
+	Transaction(func(tx *sql.Tx) error {
+		recipes := RecipeTestData()
+
+		for _, recipe := range recipes {
+			err := recipe.Create(tx)
+			if err != nil {
+				return err
+			}
+		}
+
+		return nil
+	})
 }
 
 func CloseDatabase() {
@@ -76,14 +93,3 @@ func CloseDatabase() {
 		log.Println("Closed database")
 	}
 }
-
-func NewTx() (*sql.Tx, error) {
-	return db.Begin()
-}
-
-func Rollback(tx *sql.Tx) {
-	err := tx.Rollback()
-	if err != nil {
-		log.Printf("Failed to rollback transaction: %v\n", err)
-	}
-}