storage/filesystem: avoid norwfs build flag

norwfs build flag was used to work on filesystems that do not support neither opening a file in read/write mode or renaming a file (e.f. sivafs).

This had two problems:

- go-git could not be compiled to work properly both with regular filesystems and limited filesystems at the same time.
- the norwfs trick was not available on Windows.

This PR removes the norwfs build flag, as well as the windows conditional flag on the dotgit package.

For the file open mode, we use the new billy capabilities, to check at runtime if the filesystem supports opening a file in read/write mode or not.

For the renaming, we just try and fallback to alternative methods if billy.ErrNotSupported is returned.

Signed-off-by: Santiago M. Mola <santi@mola.io>

Author: smola

storage/filesystem/dotgit/dotgit.go

@@ -469,7 +469,7 @@ func (d *DotGit) openAndLockPackedRefs(doCreate bool) (
 	// File mode is retrieved from a constant defined in the target specific
 	// files (dotgit_rewrite_packed_refs_*). Some modes are not available
 	// in all filesystems.
-	openFlags := openAndLockPackedRefsMode
+	openFlags := d.openAndLockPackedRefsMode()
 	if doCreate {
 		openFlags |= os.O_CREATE
 	}

storage/filesystem/dotgit/dotgit_rewrite_packed_refs.go

@@ -0,0 +1,81 @@
+package dotgit
+
+import (
+	"io"
+	"os"
+	"runtime"
+
+	"gopkg.in/src-d/go-billy.v4"
+	"gopkg.in/src-d/go-git.v4/utils/ioutil"
+)
+
+func (d *DotGit) openAndLockPackedRefsMode() int {
+	if billy.CapabilityCheck(d.fs, billy.ReadAndWriteCapability) {
+		return os.O_RDWR
+	}
+
+	return os.O_RDONLY
+}
+
+func (d *DotGit) rewritePackedRefsWhileLocked(
+	tmp billy.File, pr billy.File) error {
+	// Try plain rename. If we aren't using the bare Windows filesystem as the
+	// storage layer, we might be able to get away with a rename over a locked
+	// file.
+	err := d.fs.Rename(tmp.Name(), pr.Name())
+	if err == nil {
+		return nil
+	}
+
+	// If we are in a filesystem that does not support rename (e.g. sivafs)
+	// a full copy is done.
+	if err == billy.ErrNotSupported {
+		return d.copyNewFile(tmp, pr)
+	}
+
+	if runtime.GOOS != "windows" {
+		return err
+	}
+
+	// Otherwise, Windows doesn't let us rename over a locked file, so
+	// we have to do a straight copy.  Unfortunately this could result
+	// in a partially-written file if the process fails before the
+	// copy completes.
+	return d.copyToExistingFile(tmp, pr)
+}
+
+func (d *DotGit) copyToExistingFile(tmp, pr billy.File) error {
+	_, err := pr.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	err = pr.Truncate(0)
+	if err != nil {
+		return err
+	}
+	_, err = tmp.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(pr, tmp)
+
+	return err
+}
+
+func (d *DotGit) copyNewFile(tmp billy.File, pr billy.File) (err error) {
+	prWrite, err := d.fs.Create(pr.Name())
+	if err != nil {
+		return err
+	}
+
+	defer ioutil.CheckClose(prWrite, &err)
+
+	_, err = tmp.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(prWrite, tmp)
+
+	return err
+}