Optimization of labels handling in issue_search (#4228)

This PR optimizes the SQL query and de-duplicate the labels' ids when generating the query string, on the issue page.

<hr/>

### Background

Some time ago, BingBot and some other crawlers have been putting my instance on its knees with requests containing a lot of label ids, like this one :

```
[07/Aug/2023:11:28:37 +0200] "GET /Dolibarr/sendrecurringinvoicebymail/issues?q=&type=all&sort=&state=closed&labels=1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c2%2c10%2c2%2c1%2c1%2c10%2c10%2c7%2c6%2c10%2c10%2c3%2c2%2c1%2c5%2c10%2c1%2c6%2c2%2c7%2c3%2c7%2c6%2c10%2c1%2c10%2c1%2c1%2c7%2c7%2c1%2c1%2c1%2c1%2c10%2c10%2c1%2c2%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c1%2c2%2c1%2c12%2c6%2c6%2c10&milestone=0&project=-1&poster=0 HTTP/1.1" 499 0 "-" "Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm) Chrome/103.0.5060.134 Safari/537.36"
```

Since each of the label ids implies a join, it grows exponentially expensive for the database engine (at least on PostgreSQL but SQLite suffers a little too).

Thus, this PR proposes two enhancements:

* rewrite the database query to use only one squashed condition,
* deduplicate the label ids when generating the URL.

### Performance comparison

Here are some timings on Postgresql-backed, Forgejo 7.0.4 instances :
```sh
$ time curl -s -o /dev/null "http://localhost:3000/toto/tata/issues?q=&type=all&sort=&labels=19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25&state=open&milestone=0&project=0&assignee=0&poster=0"

real    0m10,491s
user    0m0,017s
sys     0m0,008s
```
...and with the patch:
```sh
$ time curl -s -o /dev/null "http://localhost:3000/toto/tata/issues?q=&type=all&sort=&labels=19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25%2c19%2c25&state=open&milestone=0&project=0&assignee=0&poster=0"

real    0m0,094s
user    0m0,012s
sys     0m0,013s
```

### Annex

This issue was originally proposed to [Gitea](https://github.com/go-gitea/gitea/pull/26460) but didn't get much attention, and I switched to Forgejo in the meantime :)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4228
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Chl <chl@xlii.si>
Co-committed-by: Chl <chl@xlii.si>

models/issues/issue_search.go

@@ -6,6 +6,7 @@ package issues
 import (
 	"context"
 	"fmt"
+	"strconv"
 	"strings"
 
 	"code.gitea.io/gitea/models/db"
@@ -13,6 +14,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/container"
 	"code.gitea.io/gitea/modules/optional"
 
 	"xorm.io/builder"
@@ -116,14 +118,30 @@ func applyLabelsCondition(sess *xorm.Session, opts *IssuesOptions) {
 		if opts.LabelIDs[0] == 0 {
 			sess.Where("issue.id NOT IN (SELECT issue_id FROM issue_label)")
 		} else {
-			for i, labelID := range opts.LabelIDs {
+			// deduplicate the label IDs for inclusion and exclusion
+			includedLabelIDs := make(container.Set[int64])
+			excludedLabelIDs := make(container.Set[int64])
+			for _, labelID := range opts.LabelIDs {
 				if labelID > 0 {
-					sess.Join("INNER", fmt.Sprintf("issue_label il%d", i),
-						fmt.Sprintf("issue.id = il%[1]d.issue_id AND il%[1]d.label_id = %[2]d", i, labelID))
+					includedLabelIDs.Add(labelID)
 				} else if labelID < 0 { // 0 is not supported here, so just ignore it
-					sess.Where("issue.id not in (select issue_id from issue_label where label_id = ?)", -labelID)
+					excludedLabelIDs.Add(-labelID)
 				}
 			}
+			// ... and use them in a subquery of the form :
+			//  where (select count(*) from issue_label where issue_id=issue.id and label_id in (2, 4, 6)) = 3
+			// This equality is guaranteed thanks to unique index (issue_id,label_id) on table issue_label.
+			if len(includedLabelIDs) > 0 {
+				subQuery := builder.Select("count(*)").From("issue_label").Where(builder.Expr("issue_id = issue.id")).
+					And(builder.In("label_id", includedLabelIDs.Values()))
+				sess.Where(builder.Eq{strconv.Itoa(len(includedLabelIDs)): subQuery})
+			}
+			// or (select count(*)...) = 0 for excluded labels
+			if len(excludedLabelIDs) > 0 {
+				subQuery := builder.Select("count(*)").From("issue_label").Where(builder.Expr("issue_id = issue.id")).
+					And(builder.In("label_id", excludedLabelIDs.Values()))
+				sess.Where(builder.Eq{"0": subQuery})
+			}
 		}
 	}
 

models/issues/issue_test.go

@@ -191,6 +191,19 @@ func TestIssues(t *testing.T) {
 			},
 			[]int64{}, // issues with **both** label 1 and 2, none of these issues matches, TODO: add more tests
 		},
+		{
+			issues_model.IssuesOptions{
+				LabelIDs: []int64{-1, 2},
+			},
+			[]int64{5}, // issue without label 1 but with label 2.
+		},
+		{
+			issues_model.IssuesOptions{
+				RepoCond: builder.In("repo_id", 1),
+				LabelIDs: []int64{0},
+			},
+			[]int64{11, 3}, // issues without any label (ordered by creation date desc.)(note: 11 is a pull request)
+		},
 		{
 			issues_model.IssuesOptions{
 				MilestoneIDs: []int64{1},

models/issues/label.go

@@ -7,6 +7,7 @@ package issues
 import (
 	"context"
 	"fmt"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -142,28 +143,38 @@ func (l *Label) CalOpenOrgIssues(ctx context.Context, repoID, labelID int64) {
 
 // LoadSelectedLabelsAfterClick calculates the set of selected labels when a label is clicked
 func (l *Label) LoadSelectedLabelsAfterClick(currentSelectedLabels []int64, currentSelectedExclusiveScopes []string) {
-	var labelQuerySlice []string
+	labelQuerySlice := []int64{}
 	labelSelected := false
-	labelID := strconv.FormatInt(l.ID, 10)
-	labelScope := l.ExclusiveScope()
-	for i, s := range currentSelectedLabels {
-		if s == l.ID {
+	exclusiveScope := l.ExclusiveScope()
+	for i, curSel := range currentSelectedLabels {
+		if curSel == l.ID {
 			labelSelected = true
-		} else if -s == l.ID {
+		} else if -curSel == l.ID {
 			labelSelected = true
 			l.IsExcluded = true
-		} else if s != 0 {
+		} else if curSel != 0 {
 			// Exclude other labels in the same scope from selection
-			if s < 0 || labelScope == "" || labelScope != currentSelectedExclusiveScopes[i] {
-				labelQuerySlice = append(labelQuerySlice, strconv.FormatInt(s, 10))
+			if curSel < 0 || exclusiveScope == "" || exclusiveScope != currentSelectedExclusiveScopes[i] {
+				labelQuerySlice = append(labelQuerySlice, curSel)
 			}
 		}
 	}
+
 	if !labelSelected {
-		labelQuerySlice = append(labelQuerySlice, labelID)
+		labelQuerySlice = append(labelQuerySlice, l.ID)
 	}
 	l.IsSelected = labelSelected
-	l.QueryString = strings.Join(labelQuerySlice, ",")
+
+	// Sort and deduplicate the ids to avoid the crawlers asking for the
+	// same thing with simply a different order of parameters
+	slices.Sort(labelQuerySlice)
+	labelQuerySlice = slices.Compact(labelQuerySlice)
+	// Quick conversion (strings.Join() doesn't accept slices of Int64)
+	labelQuerySliceStrings := make([]string, len(labelQuerySlice))
+	for i, x := range labelQuerySlice {
+		labelQuerySliceStrings[i] = strconv.FormatInt(x, 10)
+	}
+	l.QueryString = strings.Join(labelQuerySliceStrings, ",")
 }
 
 // BelongsToOrg returns true if label is an organization label
@@ -176,7 +187,7 @@ func (l *Label) BelongsToRepo() bool {
 	return l.RepoID > 0
 }
 
-// Return scope substring of label name, or empty string if none exists
+// ExclusiveScope returns scope substring of label name, or empty string if none exists
 func (l *Label) ExclusiveScope() string {
 	if !l.Exclusive {
 		return ""

models/issues/label_test.go

@@ -23,6 +23,28 @@ func TestLabel_CalOpenIssues(t *testing.T) {
 	assert.EqualValues(t, 2, label.NumOpenIssues)
 }
 
+func TestLabel_LoadSelectedLabelsAfterClick(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	// Loading the label id:8 (scope/label2) which have a scope and an
+	// exclusivity with id:7 (scope/label1)
+	label := unittest.AssertExistsAndLoadBean(t, &issues_model.Label{ID: 8})
+
+	// First test : with negative and scope
+	label.LoadSelectedLabelsAfterClick([]int64{1, -8}, []string{"", "scope"})
+	assert.Equal(t, "1", label.QueryString)
+	assert.Equal(t, true, label.IsSelected)
+
+	// Second test : with duplicates
+	label.LoadSelectedLabelsAfterClick([]int64{1, 7, 1, 7, 7}, []string{"", "scope", "", "scope", "scope"})
+	assert.Equal(t, "1,8", label.QueryString)
+	assert.Equal(t, false, label.IsSelected)
+
+	// Third test : empty set
+	label.LoadSelectedLabelsAfterClick([]int64{}, []string{})
+	assert.False(t, label.IsSelected)
+	assert.Equal(t, "8", label.QueryString)
+}
+
 func TestLabel_ExclusiveScope(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 	label := unittest.AssertExistsAndLoadBean(t, &issues_model.Label{ID: 7})