Add a setRef version that supports non rw fs

There are some filesystems that do not support opening the files in read
and write modes at the same time. The method SetRef is split in files with
an extra version that only writes the reference. It can be activated with
-tags norwfs on building.

Signed-off-by: Javi Fontan <jfontan@gmail.com>

Author: jfontan

storage/filesystem/internal/dotgit/dotgit.go

@@ -323,36 +323,9 @@ func (d *DotGit) SetRef(r, old *plumbing.Reference) error {
 		content = fmt.Sprintln(r.Hash().String())
 	}
 
-	// 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(r.Name().String(), 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
-	}
+	fileName := r.Name().String()
 
-	_, err = f.Write([]byte(content))
-	return err
+	return d.setRef(fileName, content, old)
 }
 
 // Refs scans the git directory collecting references, which it returns.

storage/filesystem/internal/dotgit/dotgit_setref.go

@@ -0,0 +1,43 @@
+// +build !norwfs
+
+package dotgit
+
+import (
+	"os"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/utils/ioutil"
+)
+
+func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) 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
+}

storage/filesystem/internal/dotgit/dotgit_setref_norwfs.go

@@ -0,0 +1,17 @@
+// +build norwfs
+
+package dotgit
+
+import "gopkg.in/src-d/go-git.v4/plumbing"
+
+func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) error {
+	f, err := d.fs.Create(fileName)
+	if err != nil {
+		return err
+	}
+
+	defer f.Close()
+
+	_, err = f.Write([]byte(content))
+	return err
+}