storage/filesystem/dotgit/dotgit_setref.go

package dotgit

import (
	"fmt"
	"os"

	"gopkg.in/src-d/go-git.v4/plumbing"
	"gopkg.in/src-d/go-git.v4/utils/ioutil"

	"gopkg.in/src-d/go-billy.v4"
)

func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) (err error) {
	if billy.CapabilityCheck(d.fs, billy.ReadAndWriteCapability) {
		return d.setRefRwfs(fileName, content, old)
	}

	return d.setRefNorwfs(fileName, content, old)
}

func (d *DotGit) setRefRwfs(fileName, content string, old *plumbing.Reference) (err error) {
	// If we are not checking an old ref, just truncate the file.
	mode := os.O_RDWR | os.O_CREATE
	if old == nil {
		mode |= os.O_TRUNC
	}

	f, err := d.fs.OpenFile(fileName, mode, 0666)
	if err != nil {
		return err
	}

	defer ioutil.CheckClose(f, &err)

	// Lock is unlocked by the deferred Close above. This is because Unlock
	// does not imply a fsync and thus there would be a race between
	// Unlock+Close and other concurrent writers. Adding Sync to go-billy
	// could work, but this is better (and avoids superfluous syncs).
	err = f.Lock()
	if err != nil {
		return err
	}

	// this is a no-op to call even when old is nil.
	err = d.checkReferenceAndTruncate(f, old)
	if err != nil {
		return err
	}

	_, err = f.Write([]byte(content))
	return err
}

// There are some filesystems that don't support opening files in RDWD mode.
// In these filesystems the standard SetRef function can not be used as it
// reads the reference file to check that it's not modified before updating it.
//
// This version of the function writes the reference without extra checks
// making it compatible with these simple filesystems. This is usually not
// a problem as they should be accessed by only one process at a time.
func (d *DotGit) setRefNorwfs(fileName, content string, old *plumbing.Reference) error {
	_, err := d.fs.Stat(fileName)
	if err == nil && old != nil {
		fRead, err := d.fs.Open(fileName)
		if err != nil {
			return err
		}

		ref, err := d.readReferenceFrom(fRead, old.Name().String())
		fRead.Close()

		if err != nil {
			return err
		}

		if ref.Hash() != old.Hash() {
			return fmt.Errorf("reference has changed concurrently")
		}
	}

	f, err := d.fs.Create(fileName)
	if err != nil {
		return err
	}

	defer f.Close()

	_, err = f.Write([]byte(content))
	return err
}