fallback on older cow version

If a device's source or target cow version is specified at 2 (or
undefined) we should fallback on using the v2 writer.

e.g.
1. v3 -> v2 downgrade OTA
2. v2 -> v3

Bug: 322279333
Test: ota_from_target_files
Change-Id: I8c5139eceeb64ee255ddea66017b3e17bbdc44c0

tools/releasetools/common.py

@@ -479,6 +479,10 @@ class BuildInfo(object):
   def vabc_compression_param(self):
     return self.get("virtual_ab_compression_method", "")
 
+  @property
+  def vabc_cow_version(self):
+    return self.get("virtual_ab_cow_version", "")
+
   @property
   def vendor_api_level(self):
     vendor_prop = self.info_dict.get("vendor.build.prop")

tools/releasetools/ota_from_target_files.py

@@ -908,6 +908,16 @@ def GenerateAbOtaPackage(target_file, output_file, source_file=None):
           source_info.vabc_compression_param, target_info.vabc_compression_param, source_info.vabc_compression_param))
       vabc_compression_param = source_info.vabc_compression_param
 
+    # Virtual AB Cow version 3 is introduced in Android U with improved memory
+    # and install time performance. All OTA's with
+    # both the source build and target build with VIRTUAL_AB_COW_VERSION = 3
+    # can support the new format. Otherwise, fallback on older versions
+    if not source_info.vabc_cow_version or not target_info.vabc_cow_version:
+      logger.info("Source or Target doesn't have VABC_COW_VERSION specified, default to version 2")
+      OPTIONS.vabc_cow_version = 2
+    elif source_info.vabc_cow_version != target_info.vabc_cow_version:
+      logger.info("Source and Target have different cow VABC_COW_VERSION specified, default to minimum version")
+      OPTIONS.vabc_cow_version = min(source_info.vabc_cow_version, target_info.vabc_cow_version)
     # Virtual AB Compression was introduced in Androd S.
     # Later, we backported VABC to Android R. But verity support was not
     # backported, so if VABC is used and we are on Android R, disable