linux default restore only user xattrs, doc update

* On Linux restore only user.* xattrs by default
* restore all for other OSs
* Update docs and changelog about the new restore
flags --exclude-xattr and --include-xattr

Signed-off-by: Tesshu Flower <tflower@redhat.com>

changelog/unreleased/issue-5089

@@ -0,0 +1,22 @@
+Enhancement: Allow including or excluding extended file attributes
+during restore.
+
+# Describe the problem in the past tense, the new behavior in the present
+# tense. Mention the affected commands, backends, operating systems, etc.
+# If the problem description just says that a feature was missing, then
+# only explain the new behavior.
+# Focus on user-facing behavior, not the implementation.
+# Use "Restic now ..." instead of "We have changed ...".
+#
+Restic restore used to attempt to restore all extended file attributes.
+Now two new command line flags are added to restore to control which
+extended file attributes will be restored.
+
+The new flags are `--exclude-xattr` and `--include-xattr`.
+
+If the flags are not provided, restic will default to restoring
+only `user` namespaced extended file attributes on Linux, and all
+extended file attributes on other operating systems.
+
+https://github.com/restic/restic/issues/5089
+https://github.com/restic/restic/pull/5129

cmd/restic/cmd_restore.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"path/filepath"
+	"runtime"
 	"time"
 
 	"github.com/restic/restic/internal/debug"
@@ -300,6 +301,14 @@ func getXattrSelectFilter(opts RestoreOptions) (func(xattrName string) bool, err
 		}, nil
 	}
 
-	// no includes or excludes, set default of including all xattrs
+	// User has not specified any xattr includes or excludes
+	if runtime.GOOS == "linux" {
+		// For Linux, set default of including only user.* xattrs
+		return func(xattrName string) bool {
+			shouldInclude, _ := filter.IncludeByPattern([]string{"user.*"}, Warnf)(xattrName)
+			return shouldInclude
+		}, nil
+	}
+	// Not linux, default to including all xattrs
 	return func(_ string) bool { return true }, nil
 }

doc/050_restore.rst

@@ -88,6 +88,22 @@ disk space. Note that the exact location of the holes can differ from those in
 the original file, as their location is determined while restoring and is not
 stored explicitly.
 
+Restoring extended file attributes
+----------------------------------
+
+By default, user namespaced extended attributes for files are restored on Linux,
+and all extended attributes are restored for other operating systems.
+
+Use ``--exclude-xattr`` and ``--include-xattr`` to control which extended
+attributes are restored for files in the snapshot. For example, to restore
+user and security namespaced extended attributes for files:
+
+.. code-block:: console
+
+    $ restic -r /srv/restic-repo restore 79766175 --target /tmp/restore-work --include-xattr user.* --include-xattr security.*
+    enter password for repository:
+    restoring <Snapshot of [/home/user/work] at 2015-05-08 21:40:19.884408621 +0200 CEST> to /tmp/restore-work
+
 Restoring in-place
 ------------------