Improve sync performance for pull-mirrors (#19125)

This addresses https://github.com/go-gitea/gitea/issues/18352

It aims to improve performance (and resource use) of the `SyncReleasesWithTags` operation for pull-mirrors.

For large repositories with many tags, `SyncReleasesWithTags` can be a costly operation (taking several minutes to complete). The reason is two-fold:
    
1. on sync, every upstream repo tag is compared (for changes) against existing local entries in the release table to ensure that they are up-to-date.
    
2. the procedure for getting _each tag_ involves a series of git operations    
    ```bash
     git show-ref --tags -- v8.2.4477
     git cat-file -t 29ab6ce9f36660cffaad3c8789e71162e5db5d2f
     git cat-file -p 29ab6ce9f36660cffaad3c8789e71162e5db5d2f
     git rev-list --count 29ab6ce9f36660cffaad3c8789e71162e5db5d2f
     ```    

     of which the `git rev-list --count` can be particularly heavy.
    
This PR optimizes performance for pull-mirrors. We utilize the fact that a pull-mirror is always identical to its upstream and rebuild the entire release table on every sync and use a batch `git for-each-ref .. refs/tags` call to retrieve all tags in one go.
    
For large mirror repos, with hundreds of annotated tags, this brings down the duration of the sync operation from several minutes to a few seconds. A few unscientific examples run on my local machine:

- https://github.com/spring-projects/spring-boot (223 tags)
  - before: `0m28,673s`
  - after: `0m2,244s`
- https://github.com/kubernetes/kubernetes (890 tags)
  - before: `8m00s`
  - after: `0m8,520s`
- https://github.com/vim/vim (13954 tags)
  - before: `14m20,383s`
  - after: `0m35,467s`

 

I added a `foreachref` package which contains a flexible way of specifying which reference fields are of interest (`git-for-each-ref(1)`) and to produce a parser for the expected output. These could be reused in other places where `for-each-ref` is used.  I'll add unit tests for those if the overall PR looks promising.

1 files changed, 67 insertions, 0 deletions

diff --git a/modules/git/foreachref/format_test.go b/modules/git/foreachref/format_test.go
new file mode 100644
index 0000000000..5aca10f752
--- /dev/null
+++ b/modules/git/foreachref/format_test.go
@@ -0,0 +1,67 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package foreachref_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/git/foreachref"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFormat_Flag(t *testing.T) {
+	tests := []struct {
+		name string
+
+		givenFormat foreachref.Format
+
+		wantFlag string
+	}{
+		{
+			name: "references are delimited by dual null chars",
+
+			// no reference fields requested
+			givenFormat: foreachref.NewFormat(),
+
+			// only a reference delimiter field in --format
+			wantFlag: "%00%00",
+		},
+
+		{
+			name: "a field is a space-separated key-value pair",
+
+			givenFormat: foreachref.NewFormat("refname:short"),
+
+			// only a reference delimiter field
+			wantFlag: "refname:short %(refname:short)%00%00",
+		},
+
+		{
+			name: "fields are separated by a null char field-delimiter",
+
+			givenFormat: foreachref.NewFormat("refname:short", "author"),
+
+			wantFlag: "refname:short %(refname:short)%00author %(author)%00%00",
+		},
+
+		{
+			name: "multiple fields",
+
+			givenFormat: foreachref.NewFormat("refname:short", "objecttype", "objectname"),
+
+			wantFlag: "refname:short %(refname:short)%00objecttype %(objecttype)%00objectname %(objectname)%00%00",
+		},
+	}
+
+	for _, test := range tests {
+		tc := test // don't close over loop variable
+		t.Run(tc.name, func(t *testing.T) {
+			gotFlag := tc.givenFormat.Flag()
+
+			require.Equal(t, tc.wantFlag, gotFlag, "unexpected for-each-ref --format string. wanted: '%s', got: '%s'", tc.wantFlag, gotFlag)
+		})
+	}
+}