database/sql: make Rows.Scan properly wrap underlying errors

The prior implementation used the format verb %v which unfortunately
improperly wrapped any underlying scanner errors, and we couldn't use
errors.Is nor errors.As. This change fixes that by using the %w verb.

Added a unit to ensure that both error sub string matching works, but
also that errors.Is works as expected.

Fixes #38099

Change-Id: Iea667041dd8081d961246f77f2542330417292dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/248337
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql_test.go

@@ -4149,6 +4149,41 @@ func TestQueryExecContextOnly(t *testing.T) {
 	}
 }
 
+type alwaysErrScanner struct{}
+
+var errTestScanWrap = errors.New("errTestScanWrap")
+
+func (alwaysErrScanner) Scan(interface{}) error {
+	return errTestScanWrap
+}
+
+// Issue 38099: Ensure that Rows.Scan properly wraps underlying errors.
+func TestRowsScanProperlyWrapsErrors(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	rows, err := db.Query("SELECT|people|age|")
+	if err != nil {
+		t.Fatalf("Query: %v", err)
+	}
+
+	var res alwaysErrScanner
+
+	for rows.Next() {
+		err = rows.Scan(&res)
+		if err == nil {
+			t.Fatal("expecting back an error")
+		}
+		if !errors.Is(err, errTestScanWrap) {
+			t.Fatalf("errors.Is mismatch\n%v\nWant: %v", err, errTestScanWrap)
+		}
+		// Ensure that error substring matching still correctly works.
+		if !strings.Contains(err.Error(), errTestScanWrap.Error()) {
+			t.Fatalf("Error %v does not contain %v", err, errTestScanWrap)
+		}
+	}
+}
+
 // badConn implements a bad driver.Conn, for TestBadDriver.
 // The Exec method panics.
 type badConn struct{}