Fix offBy1 errors (#17606)

* Fix offBy1 errors

- Partially resolves #17596
- Resolve errors from go-critic `offBy1: Index() can return -1; maybe
you wanted to do Index()+1`.

* Match golang spec

* Remove comments

* Update migrations.go

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

cmd/docs.go

@@ -43,7 +43,11 @@ func runDocs(ctx *cli.Context) error {
 		// Clean up markdown. The following bug was fixed in v2, but is present in v1.
 		// It affects markdown output (even though the issue is referring to man pages)
 		// https://github.com/urfave/cli/issues/1040
-		docs = docs[strings.Index(docs, "#"):]
+		firstHashtagIndex := strings.Index(docs, "#")
+
+		if firstHashtagIndex > 0 {
+			docs = docs[firstHashtagIndex:]
+		}
 	}
 
 	out := os.Stdout

models/migrations/migrations.go

@@ -7,6 +7,7 @@ package migrations
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -791,8 +792,14 @@ func dropTableColumns(sess *xorm.Session, tableName string, columnNames ...strin
 		}
 		tableSQL := string(res[0]["sql"])
 
+		// Get the string offset for column definitions: `CREATE TABLE ( column-definitions... )`
+		columnDefinitionsIndex := strings.Index(tableSQL, "(")
+		if columnDefinitionsIndex < 0 {
+			return errors.New("couldn't find column definitions")
+		}
+
 		// Separate out the column definitions
-		tableSQL = tableSQL[strings.Index(tableSQL, "("):]
+		tableSQL = tableSQL[columnDefinitionsIndex:]
 
 		// Remove the required columnNames
 		for _, name := range columnNames {