add loopback attach support (#39)

go.mod

@@ -3,6 +3,7 @@ module github.com/kraken-hpc/imageapi
 go 1.15
 
 require (
+	github.com/u-root/u-root v7.0.0+incompatible
 	github.com/asaskevich/govalidator v0.0.0-20210307081110-f21760c49a8d // indirect
 	github.com/bensallen/rbd v0.0.0-20210224155049-baf486eceefa
 	github.com/go-openapi/errors v0.20.0
@@ -24,3 +25,5 @@ require (
 	golang.org/x/sys v0.0.0-20210330210617-4fbd30eecc44
 	google.golang.org/genproto v0.0.0-20200825200019-8632dd797987 // indirect
 )
+
+replace github.com/u-root/u-root v7.0.0+incompatible => github.com/u-root/u-root v1.0.1-0.20201119150355-04f343dd1922

internal/api/attach_loopback.go

@@ -0,0 +1,96 @@
+package api
+
+// API operations on rbd maps
+
+import (
+	"os"
+	"path"
+
+	"github.com/kraken-hpc/imageapi/models"
+	"github.com/sirupsen/logrus"
+	"github.com/u-root/u-root/pkg/mount/loop"
+)
+
+func init() {
+	AttachDrivers[models.AttachKindLoopback] = &AttachDriverLoopback{}
+}
+
+type AttachDriverLoopback struct {
+	log *logrus.Entry
+}
+
+func (a *AttachDriverLoopback) Init(log *logrus.Entry) {
+	a.log = log
+	a.log.Trace("initialized")
+}
+
+func (a *AttachDriverLoopback) Attach(att *Attach) (ret *Attach, err error) {
+	// sanity check
+	l := a.log.WithField("operation", "attach")
+	if att.Loopback == nil {
+		l.Trace("attempted to attach loopback with no loopback definition")
+		return nil, ERRINVALDAT
+	}
+	l = l.WithFields(logrus.Fields{
+		"path": att.Loopback.Path,
+		"base": att.Loopback.Base,
+	})
+
+	base := "/"
+	if *att.Loopback.Base == models.MountBindBaseMount {
+		if att.Loopback.Mount == nil {
+			l.Debug("bind mount called with mount base but no mount definition")
+			return nil, ERRINVALDAT
+		}
+		m, err := API.Mounts.GetOrMount((*Mount)(att.Loopback.Mount))
+		if err != nil {
+			l.Error("base mount failed to GetOrMount")
+			return nil, ERRFAIL
+		}
+		att.Loopback.Mount = (*models.Mount)(m)
+		defer func() {
+			if err != nil {
+				API.Store.RefAdd(att.Loopback.Mount.ID, -1)
+			}
+		}()
+		base = att.Loopback.Mount.Mountpoint
+	}
+	fullPath := path.Join(base, *att.Loopback.Path)
+	l.WithField("fullPath", fullPath)
+	info, err := os.Stat(fullPath)
+	if err != nil {
+		l.Debug("loopback attach called on file that doesn't exist")
+		return nil, ERRINVALDAT
+	}
+	if !info.Mode().IsRegular() {
+		l.Debug("loopback attach call on a non-regular file")
+		return nil, ERRINVALDAT
+	}
+
+	att.DeviceFile, err = loop.FindDevice()
+	if err != nil {
+		l.WithError(err).Error("failed to acquire loopback device")
+		return nil, ERRFAIL
+	}
+	if err = loop.SetFile(att.DeviceFile, fullPath); err != nil {
+		l.WithError(err).Debug("failed to assign file to loopback device")
+		return nil, ERRFAIL
+	}
+
+	l.Info("successfully mapped")
+	return att, err
+}
+
+func (a *AttachDriverLoopback) Detach(att *Attach) (ret *Attach, err error) {
+	l := a.log.WithFields(logrus.Fields{
+		"operation": "unmap",
+		"id":        att.ID,
+		"path":      att.Loopback.Path,
+		"base":      att.Loopback.Base,
+	})
+	if err = loop.ClearFile(att.DeviceFile); err != nil {
+		l.Debug("failed to clear loopback association")
+		return nil, ERRFAIL
+	}
+	return att, nil
+}