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gitea/modules/git/foreachref/parser.go

132 lines
3.6 KiB

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.
3 years ago
// 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
import (
"bufio"
"bytes"
"fmt"
"io"
"strings"
)
// Parser parses 'git for-each-ref' output according to a given output Format.
type Parser struct {
// tokenizes 'git for-each-ref' output into "reference paragraphs".
scanner *bufio.Scanner
// format represents the '--format' string that describes the expected
// 'git for-each-ref' output structure.
format Format
// err holds the last encountered error during parsing.
err error
}
// NewParser creates a 'git for-each-ref' output parser that will parse all
// references in the provided Reader. The references in the output are assumed
// to follow the specified Format.
func NewParser(r io.Reader, format Format) *Parser {
scanner := bufio.NewScanner(r)
// in addition to the reference delimiter we specified in the --format,
// `git for-each-ref` will always add a newline after every reference.
refDelim := make([]byte, 0, len(format.refDelim)+1)
refDelim = append(refDelim, format.refDelim...)
refDelim = append(refDelim, '\n')
// Split input into delimiter-separated "reference blocks".
scanner.Split(
func(data []byte, atEOF bool) (advance int, token []byte, err error) {
// Scan until delimiter, marking end of reference.
delimIdx := bytes.Index(data, refDelim)
if delimIdx >= 0 {
token := data[:delimIdx]
advance := delimIdx + len(refDelim)
return advance, token, nil
}
// If we're at EOF, we have a final, non-terminated reference. Return it.
if atEOF {
return len(data), data, nil
}
// Not yet a full field. Request more data.
return 0, nil, nil
})
return &Parser{
scanner: scanner,
format: format,
err: nil,
}
}
// Next returns the next reference as a collection of key-value pairs. nil
// denotes EOF but is also returned on errors. The Err method should always be
// consulted after Next returning nil.
//
// It could, for example return something like:
//
// { "objecttype": "tag", "refname:short": "v1.16.4", "object": "f460b7543ed500e49c133c2cd85c8c55ee9dbe27" }
//
func (p *Parser) Next() map[string]string {
if !p.scanner.Scan() {
return nil
}
fields, err := p.parseRef(p.scanner.Text())
if err != nil {
p.err = err
return nil
}
return fields
}
// Err returns the latest encountered parsing error.
func (p *Parser) Err() error {
return p.err
}
// parseRef parses out all key-value pairs from a single reference block, such as
//
// "objecttype tag\0refname:short v1.16.4\0object f460b7543ed500e49c133c2cd85c8c55ee9dbe27"
//
func (p *Parser) parseRef(refBlock string) (map[string]string, error) {
if refBlock == "" {
// must be at EOF
return nil, nil
}
fieldValues := make(map[string]string)
fields := strings.Split(refBlock, p.format.fieldDelimStr)
if len(fields) != len(p.format.fieldNames) {
return nil, fmt.Errorf("unexpected number of reference fields: wanted %d, was %d",
len(fields), len(p.format.fieldNames))
}
for i, field := range fields {
field = strings.TrimSpace(field)
var fieldKey string
var fieldVal string
firstSpace := strings.Index(field, " ")
if firstSpace > 0 {
fieldKey = field[:firstSpace]
fieldVal = field[firstSpace+1:]
} else {
// could be the case if the requested field had no value
fieldKey = field
}
// enforce the format order of fields
if p.format.fieldNames[i] != fieldKey {
return nil, fmt.Errorf("unexpected field name at position %d: wanted: '%s', was: '%s'",
i, p.format.fieldNames[i], fieldKey)
}
fieldValues[fieldKey] = fieldVal
}
return fieldValues, nil
}