git-pages/src/fetch.go

package main

import (
	"errors"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"strings"
	"time"

	"github.com/go-git/go-billy/v6/osfs"
	"github.com/go-git/go-git/v6"
	"github.com/go-git/go-git/v6/plumbing"
	"github.com/go-git/go-git/v6/storage/memory"
)

type FetchOutcome int

const (
	FetchError FetchOutcome = iota
	FetchTimeout
	FetchCreated
	FetchUpdated
	FetchNoChange
)

type FetchResult struct {
	outcome FetchOutcome
	head    string
	err     error
}

func splitHash(hash plumbing.Hash) string {
	head := hash.String()
	return filepath.Join(head[:2], head[2:])
}

func fetch(
	dataDir string,
	webRoot string,
	repoURL string,
	branch string,
) FetchResult {
	storer := memory.NewStorage()

	repo, err := git.Clone(storer, nil, &git.CloneOptions{
		URL:           repoURL,
		ReferenceName: plumbing.ReferenceName(branch),
		SingleBranch:  true,
		Depth:         1,
		Tags:          git.NoTags,
	})
	if err != nil {
		return FetchResult{err: fmt.Errorf("git clone: %s", err)}
	}

	ref, err := repo.Head()
	if err != nil {
		return FetchResult{err: fmt.Errorf("git head: %s", err)}
	}
	head := ref.Hash()

	destDir := filepath.Join(dataDir, "tree", splitHash(head))
	if _, err := os.Stat(destDir); errors.Is(err, os.ErrNotExist) {
		// check out to a temporary directory to avoid TOCTTOU race on destDir
		tempDir, err := os.MkdirTemp(dataDir, ".tree")
		if err != nil {
			return FetchResult{err: fmt.Errorf("mkdir temp: %s", err)}
		}
		defer os.RemoveAll(tempDir)

		repo, err = git.Open(storer, osfs.New(tempDir, osfs.WithBoundOS()))
		if err != nil {
			return FetchResult{err: fmt.Errorf("git open: %s", err)}
		}

		worktree, err := repo.Worktree()
		if err != nil {
			return FetchResult{err: fmt.Errorf("git worktree: %s", err)}
		}

		if err := worktree.Checkout(&git.CheckoutOptions{
			Hash: head,
		}); err != nil {
			return FetchResult{err: fmt.Errorf("git checkout: %s", err)}
		}

		if err := os.MkdirAll(filepath.Dir(destDir), 0o755); err != nil {
			return FetchResult{err: fmt.Errorf("mkdir parent dest: %s", err)}
		}

		// commit atomically; assume another fetch has won the race if directory exists
		if err := os.Rename(tempDir, destDir); err != nil && !errors.Is(err, os.ErrExist) {
			return FetchResult{err: fmt.Errorf("rename dest: %s", err)}
		}
	}

	webLink := filepath.Join(dataDir, "www", webRoot)
	destDirRel, _ := filepath.Rel(filepath.Dir(webLink), destDir)

	tempLink := filepath.Join(dataDir,
		fmt.Sprintf(".link.%s.%s", strings.ReplaceAll(webRoot, "/", ".."), head.String()))
	if err := os.Symlink(destDirRel, tempLink); err != nil {
		return FetchResult{err: fmt.Errorf("symlink temp: %s", err)}
	}
	defer os.Remove(tempLink)

	if err := os.MkdirAll(filepath.Dir(webLink), 0o755); err != nil {
		return FetchResult{err: fmt.Errorf("mkdir parent web: %s", err)}
	}

	// this status is advisory only (is subject to race conditions); it's used only
	// to return the correct HTTP status per the spec
	outcome := FetchCreated
	if existingLink, err := os.Readlink(webLink); err == nil {
		if existingLink != destDirRel {
			outcome = FetchUpdated
		} else {
			outcome = FetchNoChange
		}
	}

	// commit atomically; assume another fetch has won the race if symlink exists
	// FIXME: might not have the same target
	if err := os.Rename(tempLink, webLink); err != nil && !errors.Is(err, os.ErrExist) {
		return FetchResult{err: fmt.Errorf("rename web: %s", err)}
	}

	return FetchResult{outcome: outcome, head: head.String(), err: nil}
}

func Fetch(
	dataDir string,
	webRoot string,
	repoURL string,
	branch string,
) FetchResult {
	log.Println("fetch:", webRoot, repoURL, branch)
	result := fetch(dataDir, webRoot, repoURL, branch)
	if result.err == nil {
		status := ""
		switch result.outcome {
		case FetchCreated:
			status = "created"
		case FetchUpdated:
			status = "updated"
		case FetchNoChange:
			status = "unchanged"
		}
		log.Println("fetch ok:", webRoot, result.head, status)
	} else {
		log.Println("fetch err:", fmt.Errorf("%s: %s", webRoot, result.err))
	}
	return result
}

func FetchWithTimeout(
	dataDir string,
	webRoot string,
	repoURL string,
	branch string,
	timeout time.Duration,
) FetchResult {
	// fetch the updated content with a timeout
	c := make(chan FetchResult, 1)
	go func() {
		result := Fetch(dataDir, webRoot, repoURL, branch)
		c <- result
	}()
	select {
	case result := <-c:
		return result
	case <-time.After(timeout):
		return FetchResult{outcome: FetchTimeout, err: fmt.Errorf("fetch timeout")}
	}
}