all: Add copy-on-write filesystem support (fixes #4271) (#6746)

lib/osutil/osutil.go

@@ -8,7 +8,6 @@
 package osutil
 
 import (
-	"io"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -24,7 +23,7 @@ var renameLock = sync.NewMutex()
 // RenameOrCopy renames a file, leaving source file intact in case of failure.
 // Tries hard to succeed on various systems by temporarily tweaking directory
 // permissions and removing the destination file when necessary.
-func RenameOrCopy(src, dst fs.Filesystem, from, to string) error {
+func RenameOrCopy(method fs.CopyRangeMethod, src, dst fs.Filesystem, from, to string) error {
 	renameLock.Lock()
 	defer renameLock.Unlock()
 
@@ -59,7 +58,7 @@ func RenameOrCopy(src, dst fs.Filesystem, from, to string) error {
 			}
 		}
 
-		err := copyFileContents(src, dst, from, to)
+		err := copyFileContents(method, src, dst, from, to)
 		if err != nil {
 			_ = dst.Remove(to)
 			return err
@@ -74,9 +73,9 @@ func RenameOrCopy(src, dst fs.Filesystem, from, to string) error {
 // Copy copies the file content from source to destination.
 // Tries hard to succeed on various systems by temporarily tweaking directory
 // permissions and removing the destination file when necessary.
-func Copy(src, dst fs.Filesystem, from, to string) (err error) {
+func Copy(method fs.CopyRangeMethod, src, dst fs.Filesystem, from, to string) (err error) {
 	return withPreparedTarget(dst, from, to, func() error {
-		return copyFileContents(src, dst, from, to)
+		return copyFileContents(method, src, dst, from, to)
 	})
 }
 
@@ -107,7 +106,7 @@ func withPreparedTarget(filesystem fs.Filesystem, from, to string, f func() erro
 // by dst. The file will be created if it does not already exist. If the
 // destination file exists, all its contents will be replaced by the contents
 // of the source file.
-func copyFileContents(srcFs, dstFs fs.Filesystem, src, dst string) (err error) {
+func copyFileContents(method fs.CopyRangeMethod, srcFs, dstFs fs.Filesystem, src, dst string) (err error) {
 	in, err := srcFs.Open(src)
 	if err != nil {
 		return
@@ -123,7 +122,11 @@ func copyFileContents(srcFs, dstFs fs.Filesystem, src, dst string) (err error) {
 			err = cerr
 		}
 	}()
-	_, err = io.Copy(out, in)
+	inFi, err := in.Stat()
+	if err != nil {
+		return
+	}
+	err = fs.CopyRange(method, in, out, 0, 0, inFi.Size())
 	return
 }
 

lib/osutil/osutil_test.go

@@ -139,7 +139,7 @@ func TestRenameOrCopy(t *testing.T) {
 			content = string(buf)
 		}
 
-		err := osutil.RenameOrCopy(test.src, test.dst, test.file, "new")
+		err := osutil.RenameOrCopy(fs.CopyRangeMethodStandard, test.src, test.dst, test.file, "new")
 		if err != nil {
 			t.Fatal(err)
 		}