First cut at run_mksnapshot action for gn

First cut at run_mksnapshot action for gn

BUG=none
R=brettw@chromium.org
LOG=n

Committed: https://code.google.com/p/v8/source/detail?r=21618

[jochen (gone - plz use gerrit), 2014-05-27 15:36:59]

is script = "mksnapshot" enough to make gn build mksnapshot first? And does it
figure out itself that it needs to build mksnapshot for the host arch?

[brettw, 2014-05-27 17:36:03]

On 2014/05/27 15:36:59, jochen wrote:
> is script = "mksnapshot" enough to make gn build mksnapshot first? And does it
> figure out itself that it needs to build mksnapshot for the host arch?

No and no.

To make it build mksnapshot for the host arch before running the action, do:
  deps = [ ":mksnapshot($host_toolchain)" ]

just writing script = "foo" will look in the current dir for "foo", which is not
what you want. Albert has some stuff to make it easier to run generated
binaries, but this isn't quite ready yet. In the meantime, you'll need to write
a python script to run the binary. A bit annoying but not hard:

The host binary will go into a directory that depends on the host arch and also
whether that's the same as the target arch. To get the output dir name do:
get_label_info(":mksnapshot($host_toolchain)", "root_out_dir") which will return
something like "//out/Default" or "//out/Default/x64". The run that through
rebase_path to make it relative to the build dir.

So assuming your Python script takes the binary name as the first param and the
args to the binary as the following ones, do
  args = [ "./" + rebase_path(get_label_info(":mksnapshot($host_toolchain)",
"root_out_dir") + "/mksnapshot"), "root_build_dir"), <other args here> ]

Hope that makes sense, it's currently a bit more complicated than I'd like.
(The "./" at the beginning is to prevent it from finding other "mksnapshot"
binaries that might be on the path).

[jochen (gone - plz use gerrit), 2014-05-27 21:24:55]

ptal

I also added the v8 target. the only problem is that the resulting binary
doesn't work :)

[brettw, 2014-05-27 21:26:23]

lgtm

https://codereview.chromium.org/293363009/diff/20001/BUILD.gn
File BUILD.gn (right):

https://codereview.chromium.org/293363009/diff/20001/BUILD.gn#newcode986
BUILD.gn:986: component("v8") {
CAn you indent the stuff inside the if?

[jochen (gone - plz use gerrit), 2014-06-03 06:50:54]

Committed patchset #2 manually as r21618 (presubmit successful).