reduce function body of Select() further, add context to errors

2 files changed, 35 insertions, 15 deletions

diff --git a/README.md b/README.md
index 4494b49..b40d2b1 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,9 @@ SPDX-License-Identifier: Apache-2.0
 
 # Oblast
 
-My attempt at an ORM library for Go. Inspired by [Gorp](https://pkg.go.dev/gopkg.in/gorp.v3), but without the bits that make Gorp slow.
+My attempt at a minimal ORM library for Go. Inspired by [Gorp](https://pkg.go.dev/gopkg.in/gorp.v3), but without the bits that make Gorp slow.
+
+You may think that the name refers to the type of administrative division that exists in several Slavic countries, but it's actually just an acronym for what this library does: **Ob**ject **L**oading **A**nd **St**oring.
 
 ## Unstructured notes
 
diff --git a/query.go b/query.go
index 03b87b6..a464c9e 100644
--- a/query.go
+++ b/query.go
@@ -11,10 +11,9 @@ import (
 	"go.xyrillian.de/oblast/internal"
 )
 
-func Select[T any](db *DB, query string, args ...any) ([]T, error) {
-	// TODO: minimize function body to avoid binary size blowup from monomorphization
-	// TODO: catch error from rows.Close(), if any
-	// TODO: add context to errors
+func Select[T any](db *DB, query string, args ...any) (result []T, returnedError error) {
+	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
+	//       Any expression that does not depend on type T should be factored out into a reusable function.
 
 	plan, err := db.getPlan(reflect.TypeFor[T]())
 	if err != nil {
@@ -24,17 +23,14 @@ func Select[T any](db *DB, query string, args ...any) ([]T, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() {
+		returnedError = mergeRowsCloseError(returnedError, rows.Close())
+	}()
 
-	var result []T
 	slots := make([]any, len(indexes))
 	for rows.Next() {
 		var target T
-		rvalue := reflect.ValueOf(&target).Elem()
-		for idx, index := range indexes {
-			slots[idx] = rvalue.FieldByIndex(index).Addr().Interface()
-		}
-		err := rows.Scan(slots...)
+		err = db.collectRow(rows, reflect.ValueOf(&target).Elem(), slots, indexes)
 		if err != nil {
 			return nil, err
 		}
@@ -47,20 +43,20 @@ func Select[T any](db *DB, query string, args ...any) ([]T, error) {
 func (db *DB) startQuery(plan internal.Plan, query string, args ...any) (rows *sql.Rows, indexes [][]int, err error) {
 	rows, err = db.Query(query, args...)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("during Query(): %w", err)
 	}
 	defer func() {
 		if err != nil {
 			closeErr := rows.Close()
 			if closeErr != nil {
-				err = fmt.Errorf("%w (additional error during rows.Close: %s)", err, closeErr.Error())
+				err = fmt.Errorf("%w (additional error during rows.Close(): %s)", err, closeErr.Error())
 			}
 		}
 	}()
 
 	columnNames, err := rows.Columns()
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("during rows.Columns(): %w", err)
 	}
 	indexes = make([][]int, len(columnNames))
 	for idx, columnName := range columnNames {
@@ -76,3 +72,25 @@ func (db *DB) startQuery(plan internal.Plan, query string, args ...any) (rows *s
 
 	return rows, indexes, nil
 }
+
+func (db *DB) collectRow(rows *sql.Rows, v reflect.Value, slots []any, indexes [][]int) error {
+	for idx, index := range indexes {
+		slots[idx] = v.FieldByIndex(index).Addr().Interface()
+	}
+	err := rows.Scan(slots...)
+	if err != nil {
+		return fmt.Errorf("during rows.Scan(): %w", err)
+	}
+	return nil
+}
+
+func mergeRowsCloseError(err, closeErr error) error {
+	switch {
+	case closeErr == nil:
+		return err
+	case err == nil:
+		return fmt.Errorf("during rows.Close(): %w", closeErr)
+	default:
+		return fmt.Errorf("%w (additional error during rows.Close(): %s)", err, closeErr.Error())
+	}
+}