tree feb9c166ef63a962ebf9f773df33f4282c33ddaf
parent dbf36a64f67e5bf80d91d21152a655900a1cfc71
author David Greenaway <dgreenaway@google.com> 1558917769 +0000
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1558917769 +0000

[virtualization] Avoid rebuilding guest images when nothing changed.

Currently, a "fx build" involving the "prebuilt_image" target will
always execute the target (and all of its downstream actions), even
if nothing has changed.

The reason is as follows:

The "guest_prebuilt" action needs to perform some trickery to allow a
build to succeed even if no prebuilt image is available. It does this
by dynamically generating a depfile linking the prebuilt image to a
copy of the image in the output directory. (See added comments in this
CL for details).

The current depfile is in an incorrect format, however: GN (or, more
precisely, Ninja) expects the LHS of the depfile to have a path relative
to the output directory, while the current depfile has an absolute path.
Ninja becomes confused, and makes the conservative decision to just
execute the action again.

This CL updates the depfile to be relative to the out directory, as is
expected by Ninja. We also add some additional comments explaining some
of the tricks being carried out by the "prebuilt_image" action.

After this CL, a "fx build" on an unmodified tree performs no actions,
as expected.

Change-Id: I789a7ba7154ee40350278bcea8ad048a587ca857