target: Add Target.revertable_write_value()

Same as write_value(), but returns a context manager that will write
back the old value on exit.

Also add batch_revertable_write_value() that takes a list of kwargs
dict, and will call revertable_write_value() on each of them, returning
a single combined context manager.

devlib/target.py

@@ -29,6 +29,7 @@ import threading
 import xml.dom.minidom
 import copy
 from collections import namedtuple, defaultdict
+from contextlib import contextmanager
 from pipes import quote
 from past.builtins import long
 from past.types import basestring
@@ -51,6 +52,7 @@ from devlib.utils.android import AdbConnection, AndroidProperties, LogcatMonitor
 from devlib.utils.misc import memoized, isiterable, convert_new_lines
 from devlib.utils.misc import commonprefix, merge_lists
 from devlib.utils.misc import ABI_MAP, get_cpu_name, ranges_to_list
+from devlib.utils.misc import batch_contextmanager
 from devlib.utils.types import integer, boolean, bitmask, identifier, caseless_string, bytes_regex
 
 
@@ -70,7 +72,6 @@ GOOGLE_DNS_SERVER_ADDRESS = '8.8.8.8'
 
 installed_package_info = namedtuple('installed_package_info', 'apk_path package')
 
-
 class Target(object):
 
     path = None
@@ -481,6 +482,18 @@ class Target(object):
     def read_bool(self, path):
         return self.read_value(path, kind=boolean)
 
+    @contextmanager
+    def revertable_write_value(self, path, value, verify=True):
+        orig_value = self.read_value(path)
+        try:
+            self.write_value(path, value, verify)
+            yield
+        finally:
+            self.write_value(path, orig_value, verify)
+
+    def batch_revertable_write_value(self, kwargs_list):
+        return batch_contextmanager(self.revertable_write_value, kwargs_list)
+
     def write_value(self, path, value, verify=True):
         value = str(value)
         self.execute('echo {} > {}'.format(quote(value), quote(path)), check_exit_code=False, as_root=True)

doc/target.rst

@@ -346,6 +346,18 @@ Target
        some sysfs entries silently failing to set the written value without
        returning an error code.
 
+.. method:: Target.revertable_write_value(path, value [, verify])
+
+   Same as :meth:`Target.write_value`, but as a context manager that will write
+   back the previous value on exit.
+
+.. method:: Target.batch_revertable_write_value(kwargs_list)
+
+   Calls :meth:`Target.revertable_write_value` with all the keyword arguments
+   dictionary given in the list. This is a convenience method to update
+   multiple files at once, leaving them in their original state on exit. If one
+   write fails, all the already-performed writes will be reverted as well.
+
 .. method:: Target.read_tree_values(path, depth=1, dictcls=dict, [, tar [, decode_unicode [, strip_null_char ]]]):
 
    Read values of all sysfs (or similar) file nodes under ``path``, traversing