Initial working version

Summary:
* Logging is still bad
* test output in a lot of placed
* FIXMEs every where

Test Plan: None, just review

Differential Revision: https://phabricator.cschwarz.com/D2

zfs/diff.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os/exec"
 	"sort"
+	"io"
 )
 
 type fsbyCreateTXG []FilesystemVersion
@@ -245,6 +246,19 @@ func IsPlaceholder(p *DatasetPath, placeholderPropertyValue string) (isPlacehold
 	return
 }
 
+// for nonexistent FS, isPlaceholder == false && err == nil
+func ZFSIsPlaceholderFilesystem(p *DatasetPath) (isPlaceholder bool, err error) {
+	props, err := ZFSGet(p, []string{ZREPL_PLACEHOLDER_PROPERTY_NAME})
+	if err == io.ErrUnexpectedEOF {
+		// interpret this as an early exit of the zfs binary due to the fs not existing
+		return false, nil
+	} else if err != nil {
+		return false, err
+	}
+	isPlaceholder, _  = IsPlaceholder(p, props.Get(ZREPL_PLACEHOLDER_PROPERTY_NAME))
+	return
+}
+
 func ZFSCreatePlaceholderFilesystem(p *DatasetPath) (err error) {
 	v := PlaceholderPropertyValue(p)
 	cmd := exec.Command(ZFS_BINARY, "create",