add slow SQL query warning

- Databases are one of the most important parts of Forgejo, every
interaction uses the database in one way or another.
Therefore, it is important to maintain the database and recognize when
the server is not doing well with the database. There already is the
option to log *every* SQL query along with its execution time, but
monitoring becomes impractical for larger instances and takes up
unnecessary storage in the logs.
- Add a QoL enhancement that allows instance administrators to specify a
threshold value beyond which query execution time is logged as a warning
in the xorm logger. The default value is a conservative five seconds to
avoid this becoming a source of spam in the logs.
- The use case for this patch is that with an instance the size of Codeberg, monitoring SQL logs is not very fruitful and most of them are uninteresting. Recently, in the context of persistent deadlock issues (https://codeberg.org/forgejo/forgejo/issues/220), I have noticed that certain queries hold locks on tables like comment and issue for several seconds. This patch helps to identify which queries these are and when they happen.
- Added unit test.

(cherry picked from commit 9cf501f1af4cd870221cef6af489618785b71186)

models/db/engine.go

@@ -11,10 +11,13 @@ import (
 	"io"
 	"reflect"
 	"strings"
+	"time"
 
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 
 	"xorm.io/xorm"
+	"xorm.io/xorm/contexts"
 	"xorm.io/xorm/names"
 	"xorm.io/xorm/schemas"
 
@@ -146,6 +149,13 @@ func InitEngine(ctx context.Context) error {
 	xormEngine.SetConnMaxLifetime(setting.Database.ConnMaxLifetime)
 	xormEngine.SetDefaultContext(ctx)
 
+	if setting.Database.SlowQueryTreshold > 0 {
+		xormEngine.AddHook(&SlowQueryHook{
+			Treshold: setting.Database.SlowQueryTreshold,
+			Logger:   log.GetLogger("xorm"),
+		})
+	}
+
 	SetDefaultEngine(ctx, xormEngine)
 	return nil
 }
@@ -299,3 +309,21 @@ func SetLogSQL(ctx context.Context, on bool) {
 		sess.Engine().ShowSQL(on)
 	}
 }
+
+type SlowQueryHook struct {
+	Treshold time.Duration
+	Logger   log.Logger
+}
+
+var _ contexts.Hook = &SlowQueryHook{}
+
+func (SlowQueryHook) BeforeProcess(c *contexts.ContextHook) (context.Context, error) {
+	return c.Ctx, nil
+}
+
+func (h *SlowQueryHook) AfterProcess(c *contexts.ContextHook) error {
+	if c.ExecuteTime >= h.Treshold {
+		h.Logger.Log(8, log.WARN, "[Slow SQL Query] %s %v - %v", c.SQL, c.Args, c.ExecuteTime)
+	}
+	return nil
+}

models/db/engine_test.go

@@ -6,15 +6,19 @@ package db_test
 import (
 	"path/filepath"
 	"testing"
+	"time"
 
 	"code.gitea.io/gitea/models/db"
 	issues_model "code.gitea.io/gitea/models/issues"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 
 	_ "code.gitea.io/gitea/cmd" // for TestPrimaryKeys
 
 	"github.com/stretchr/testify/assert"
+	"xorm.io/xorm"
 )
 
 func TestDumpDatabase(t *testing.T) {
@@ -84,3 +88,37 @@ func TestPrimaryKeys(t *testing.T) {
 		}
 	}
 }
+
+func TestSlowQuery(t *testing.T) {
+	lc, cleanup := test.NewLogChecker("slow-query")
+	lc.StopMark("[Slow SQL Query]")
+	defer cleanup()
+
+	e := db.GetEngine(db.DefaultContext)
+	engine, ok := e.(*xorm.Engine)
+	assert.True(t, ok)
+
+	// It's not possible to clean this up with XORM, but it's luckily not harmful
+	// to leave around.
+	engine.AddHook(&db.SlowQueryHook{
+		Treshold: time.Second * 10,
+		Logger:   log.GetLogger("slow-query"),
+	})
+
+	// NOOP query.
+	e.Exec("SELECT 1 WHERE false;")
+
+	_, stopped := lc.Check(100 * time.Millisecond)
+	assert.False(t, stopped)
+
+	engine.AddHook(&db.SlowQueryHook{
+		Treshold: 0, // Every query should be logged.
+		Logger:   log.GetLogger("slow-query"),
+	})
+
+	// NOOP query.
+	e.Exec("SELECT 1 WHERE false;")
+
+	_, stopped = lc.Check(100 * time.Millisecond)
+	assert.True(t, stopped)
+}

modules/setting/database.go

@@ -44,6 +44,7 @@ var (
 		ConnMaxLifetime   time.Duration
 		IterateBufferSize int
 		AutoMigration     bool
+		SlowQueryTreshold time.Duration
 	}{
 		Timeout:           500,
 		IterateBufferSize: 50,
@@ -86,6 +87,7 @@ func loadDBSetting(rootCfg ConfigProvider) {
 	Database.DBConnectRetries = sec.Key("DB_RETRIES").MustInt(10)
 	Database.DBConnectBackoff = sec.Key("DB_RETRY_BACKOFF").MustDuration(3 * time.Second)
 	Database.AutoMigration = sec.Key("AUTO_MIGRATION").MustBool(true)
+	Database.SlowQueryTreshold = sec.Key("SLOW_QUERY_TRESHOLD").MustDuration(5 * time.Second)
 }
 
 // DBConnStr returns database connection string