Recommend/convert to use case-sensitive collation for MySQL/MSSQL (#28662)

Mainly for MySQL/MSSQL.

It is important for Gitea to use case-sensitive database charset
collation. If the database is using a case-insensitive collation, Gitea
will show startup error/warning messages, and show the errors/warnings
on the admin panel's Self-Check page.

Make `gitea doctor convert` work for MySQL to convert the collations of
database & tables & columns.

* Fix #28131

## :warning: BREAKING :warning:

It is not quite breaking, but it's highly recommended to convert the
database&table&column to a consistent and case-sensitive collation.

3 files changed, 203 insertions, 5 deletions

diff --git a/models/db/collation.go b/models/db/collation.go
new file mode 100644
index 0000000000..2f5ff2bf05
--- /dev/null
+++ b/models/db/collation.go
@@ -0,0 +1,191 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package db
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/container"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+type CheckCollationsResult struct {
+	ExpectedCollation        string
+	AvailableCollation       container.Set[string]
+	DatabaseCollation        string
+	IsCollationCaseSensitive func(s string) bool
+	CollationEquals          func(a, b string) bool
+	ExistingTableNumber      int
+
+	InconsistentCollationColumns []string
+}
+
+func findAvailableCollationsMySQL(x *xorm.Engine) (ret container.Set[string], err error) {
+	var res []struct {
+		Collation string
+	}
+	if err = x.SQL("SHOW COLLATION WHERE (Collation = 'utf8mb4_bin') OR (Collation LIKE '%\\_as\\_cs%')").Find(&res); err != nil {
+		return nil, err
+	}
+	ret = make(container.Set[string], len(res))
+	for _, r := range res {
+		ret.Add(r.Collation)
+	}
+	return ret, nil
+}
+
+func findAvailableCollationsMSSQL(x *xorm.Engine) (ret container.Set[string], err error) {
+	var res []struct {
+		Name string
+	}
+	if err = x.SQL("SELECT * FROM sys.fn_helpcollations() WHERE name LIKE '%[_]CS[_]AS%'").Find(&res); err != nil {
+		return nil, err
+	}
+	ret = make(container.Set[string], len(res))
+	for _, r := range res {
+		ret.Add(r.Name)
+	}
+	return ret, nil
+}
+
+func CheckCollations(x *xorm.Engine) (*CheckCollationsResult, error) {
+	dbTables, err := x.DBMetas()
+	if err != nil {
+		return nil, err
+	}
+
+	res := &CheckCollationsResult{
+		ExistingTableNumber: len(dbTables),
+		CollationEquals:     func(a, b string) bool { return a == b },
+	}
+
+	var candidateCollations []string
+	if x.Dialect().URI().DBType == schemas.MYSQL {
+		if _, err = x.SQL("SELECT @@collation_database").Get(&res.DatabaseCollation); err != nil {
+			return nil, err
+		}
+		res.IsCollationCaseSensitive = func(s string) bool {
+			return s == "utf8mb4_bin" || strings.HasSuffix(s, "_as_cs")
+		}
+		candidateCollations = []string{"utf8mb4_0900_as_cs", "uca1400_as_cs", "utf8mb4_bin"}
+		res.AvailableCollation, err = findAvailableCollationsMySQL(x)
+		if err != nil {
+			return nil, err
+		}
+		res.CollationEquals = func(a, b string) bool {
+			// MariaDB adds the "utf8mb4_" prefix, eg: "utf8mb4_uca1400_as_cs", but not the name "uca1400_as_cs" in "SHOW COLLATION"
+			// At the moment, it's safe to ignore the database difference, just trim the prefix and compare. It could be fixed easily if there is any problem in the future.
+			return a == b || strings.TrimPrefix(a, "utf8mb4_") == strings.TrimPrefix(b, "utf8mb4_")
+		}
+	} else if x.Dialect().URI().DBType == schemas.MSSQL {
+		if _, err = x.SQL("SELECT DATABASEPROPERTYEX(DB_NAME(), 'Collation')").Get(&res.DatabaseCollation); err != nil {
+			return nil, err
+		}
+		res.IsCollationCaseSensitive = func(s string) bool {
+			return strings.HasSuffix(s, "_CS_AS")
+		}
+		candidateCollations = []string{"Latin1_General_CS_AS"}
+		res.AvailableCollation, err = findAvailableCollationsMSSQL(x)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		return nil, nil
+	}
+
+	if res.DatabaseCollation == "" {
+		return nil, errors.New("unable to get collation for current database")
+	}
+
+	res.ExpectedCollation = setting.Database.CharsetCollation
+	if res.ExpectedCollation == "" {
+		for _, collation := range candidateCollations {
+			if res.AvailableCollation.Contains(collation) {
+				res.ExpectedCollation = collation
+				break
+			}
+		}
+	}
+
+	if res.ExpectedCollation == "" {
+		return nil, errors.New("unable to find a suitable collation for current database")
+	}
+
+	allColumnsMatchExpected := true
+	allColumnsMatchDatabase := true
+	for _, table := range dbTables {
+		for _, col := range table.Columns() {
+			if col.Collation != "" {
+				allColumnsMatchExpected = allColumnsMatchExpected && res.CollationEquals(col.Collation, res.ExpectedCollation)
+				allColumnsMatchDatabase = allColumnsMatchDatabase && res.CollationEquals(col.Collation, res.DatabaseCollation)
+				if !res.IsCollationCaseSensitive(col.Collation) || !res.CollationEquals(col.Collation, res.DatabaseCollation) {
+					res.InconsistentCollationColumns = append(res.InconsistentCollationColumns, fmt.Sprintf("%s.%s", table.Name, col.Name))
+				}
+			}
+		}
+	}
+	// if all columns match expected collation or all match database collation, then it could also be considered as "consistent"
+	if allColumnsMatchExpected || allColumnsMatchDatabase {
+		res.InconsistentCollationColumns = nil
+	}
+	return res, nil
+}
+
+func CheckCollationsDefaultEngine() (*CheckCollationsResult, error) {
+	return CheckCollations(x)
+}
+
+func alterDatabaseCollation(x *xorm.Engine, collation string) error {
+	if x.Dialect().URI().DBType == schemas.MYSQL {
+		_, err := x.Exec("ALTER DATABASE CHARACTER SET utf8mb4 COLLATE " + collation)
+		return err
+	} else if x.Dialect().URI().DBType == schemas.MSSQL {
+		// TODO: MSSQL has many limitations on changing database collation, it could fail in many cases.
+		_, err := x.Exec("ALTER DATABASE CURRENT COLLATE " + collation)
+		return err
+	}
+	return errors.New("unsupported database type")
+}
+
+// preprocessDatabaseCollation checks database & table column collation, and alter the database collation if needed
+func preprocessDatabaseCollation(x *xorm.Engine) {
+	r, err := CheckCollations(x)
+	if err != nil {
+		log.Error("Failed to check database collation: %v", err)
+	}
+	if r == nil {
+		return // no check result means the database doesn't need to do such check/process (at the moment ....)
+	}
+
+	// try to alter database collation to expected if the database is empty, it might fail in some cases (and it isn't necessary to succeed)
+	// at the moment, there is no "altering" solution for MSSQL, site admin should manually change the database collation
+	// and there is a bug https://github.com/go-testfixtures/testfixtures/pull/182 mssql: Invalid object name 'information_schema.tables'.
+	if !r.CollationEquals(r.DatabaseCollation, r.ExpectedCollation) && r.ExistingTableNumber == 0 && x.Dialect().URI().DBType == schemas.MYSQL {
+		if err = alterDatabaseCollation(x, r.ExpectedCollation); err != nil {
+			log.Error("Failed to change database collation to %q: %v", r.ExpectedCollation, err)
+		} else {
+			_, _ = x.Exec("SELECT 1") // after "altering", MSSQL's session becomes invalid, so make a simple query to "refresh" the session
+			if r, err = CheckCollations(x); err != nil {
+				log.Error("Failed to check database collation again after altering: %v", err) // impossible case
+				return
+			}
+			log.Warn("Current database has been altered to use collation %q", r.DatabaseCollation)
+		}
+	}
+
+	// check column collation, and show warning/error to end users -- no need to fatal, do not block the startup
+	if !r.IsCollationCaseSensitive(r.DatabaseCollation) {
+		log.Warn("Current database is using a case-insensitive collation %q, although Gitea could work with it, there might be some rare cases which don't work as expected.", r.DatabaseCollation)
+	}
+
+	if len(r.InconsistentCollationColumns) > 0 {
+		log.Error("There are %d table columns using inconsistent collation, they should use %q. Please go to admin panel Self Check page", len(r.InconsistentCollationColumns), r.DatabaseCollation)
+	}
+}
diff --git a/models/db/convert.go b/models/db/convert.go
index 112c8575ca..8c124471ab 100644
--- a/models/db/convert.go
+++ b/models/db/convert.go
@@ -14,13 +14,18 @@ import (
 	"xorm.io/xorm/schemas"
 )
 
-// ConvertUtf8ToUtf8mb4 converts database and tables from utf8 to utf8mb4 if it's mysql and set ROW_FORMAT=dynamic
-func ConvertUtf8ToUtf8mb4() error {
+// ConvertDatabaseTable converts database and tables from utf8 to utf8mb4 if it's mysql and set ROW_FORMAT=dynamic
+func ConvertDatabaseTable() error {
 	if x.Dialect().URI().DBType != schemas.MYSQL {
 		return nil
 	}
 
-	_, err := x.Exec(fmt.Sprintf("ALTER DATABASE `%s` CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci", setting.Database.Name))
+	r, err := CheckCollations(x)
+	if err != nil {
+		return err
+	}
+
+	_, err = x.Exec(fmt.Sprintf("ALTER DATABASE `%s` CHARACTER SET utf8mb4 COLLATE %s", setting.Database.Name, r.ExpectedCollation))
 	if err != nil {
 		return err
 	}
@@ -30,11 +35,11 @@ func ConvertUtf8ToUtf8mb4() error {
 		return err
 	}
 	for _, table := range tables {
-		if _, err := x.Exec(fmt.Sprintf("ALTER TABLE `%s` ROW_FORMAT=dynamic;", table.Name)); err != nil {
+		if _, err := x.Exec(fmt.Sprintf("ALTER TABLE `%s` ROW_FORMAT=dynamic", table.Name)); err != nil {
 			return err
 		}
 
-		if _, err := x.Exec(fmt.Sprintf("ALTER TABLE `%s` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;", table.Name)); err != nil {
+		if _, err := x.Exec(fmt.Sprintf("ALTER TABLE `%s` CONVERT TO CHARACTER SET utf8mb4 COLLATE %s", table.Name, r.ExpectedCollation)); err != nil {
 			return err
 		}
 	}
diff --git a/models/db/engine.go b/models/db/engine.go
index 99906813ca..2cd1c36c58 100755
--- a/models/db/engine.go
+++ b/models/db/engine.go
@@ -182,6 +182,8 @@ func InitEngineWithMigration(ctx context.Context, migrateFunc func(*xorm.Engine)
 		return err
 	}
 
+	preprocessDatabaseCollation(x)
+
 	// We have to run migrateFunc here in case the user is re-running installation on a previously created DB.
 	// If we do not then table schemas will be changed and there will be conflicts when the migrations run properly.
 	//