paycheck: truncate partitions resulting from applying a payload

Since the correctness of the result only encompasses the filesystem (or
otherwise "meaningful data") on the target partition, it is desirable to
actually get rid of whatever is past that point. There are different
reasons for the presence of such extra space in delta updates, including
remnants from a source partition that served as baseline for a delta
update, or scratch space used by MOVE operations for breaking cycles.

We make truncation the default behavior, although it can be suppressed
by passing the right flag (truncate_to_expected_size=False). Note that
this change is necessary for comparing the results of applying a payload
to the partitions as they are extracted from a target image, which is to
be attempted during payload generation.

This also fixes tiny gpylint complaints.

BUG=chromium:241283
TEST=Emitted partition files truncated as expected

Change-Id: Ibb71e4f2305ec41224afdc503168ae02c312f6fe
Reviewed-on: https://gerrit.chromium.org/gerrit/56532
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/scripts/update_payload/applier.py b/scripts/update_payload/applier.py
index f5ae6b73..52a70f91 100644
--- a/scripts/update_payload/applier.py
+++ b/scripts/update_payload/applier.py
@@ -192,18 +192,23 @@ class PayloadApplier(object):
 
   """
 
-  def __init__(self, payload, bsdiff_in_place=True):
+  def __init__(self, payload, bsdiff_in_place=True,
+               truncate_to_expected_size=True):
     """Initialize the applier.
 
     Args:
       payload: the payload object to check
       bsdiff_in_place: whether to perform BSDIFF operation in-place (optional)
+      truncate_to_expected_size: whether to truncate the resulting partitions
+                                 to their expected sizes, as specified in the
+                                 payload (optional)
 
     """
     assert payload.is_init, 'uninitialized update payload'
     self.payload = payload
     self.block_size = payload.manifest.block_size
     self.bsdiff_in_place = bsdiff_in_place
+    self.truncate_to_expected_size = truncate_to_expected_size
 
   def _ApplyReplaceOperation(self, op, op_name, out_data, part_file, part_size):
     """Applies a REPLACE{,_BZ} operation.
@@ -430,6 +435,12 @@ class PayloadApplier(object):
     with open(new_part_file_name, 'r+b') as new_part_file:
       self._ApplyOperations(operations, base_name, new_part_file,
                             new_part_info.size)
+      # Truncate the result, if so instructed.
+      if self.truncate_to_expected_size:
+        new_part_file.seek(0, 2)
+        if new_part_file.tell() > new_part_info.size:
+          new_part_file.seek(new_part_info.size)
+          new_part_file.truncate()
 
     # Verify the resulting partition.
     with open(new_part_file_name, 'rb') as new_part_file: