[OE-core] [PATCH] externalsrc: fix ExpansionError if the source dir does not exist yet

[Luca Ceresoli]

The externalsrc class code assumes that the source directory
(EXTERNALSRC) exists before bitbake is called. Otherwise do_configure
will fail obviously since externalsrc does not fetch anything.

Commit 3ca6085729d9 ("externalsrc: Handle .git not being a directory")
changed this behaviour. Now on a missing EXTERNALSRC directory we get
a bb.data_smart.ExpansionError during _parsing_, way before
do_configure can be run.

This new behaviour creates two problems:

 * First, there error message is very cryptic (and it's hard to
   provide a better message since no task is ever run):

     ERROR: ExpansionError during parsing /<...>/<...>.bb
     Traceback (most recent call last):
     bb.data_smart.ExpansionError: Failure expanding variable do_compile[file-checksums], expression was ${@srctree_hash_files(d)} which triggered exception FileNotFoundError: [Errno 2] No such file or directory: '<...>'

 * Second, this prevents creating a class based on externalsrc that
   automatically fetches the code in EXTERNALSRC before do_compile
   runs.

Fix both problems by simply calling git with '-C ${EXTERNALSRC}'
instead of calling git inside the non-existing directory. This changes
from a bb.data_smart.ExpansionError to a
subprocess.CalledProcessError, which is in line with what's actually
going on: git is telling us it can't find the git dir.

Also remove a comment that does not apply anymore.

Signed-off-by: Luca Ceresoli <luca at lucaceresoli.net>
Cc: Joshua Watt <jpewhacker at gmail.com>
---
 meta/classes/externalsrc.bbclass | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/meta/classes/externalsrc.bbclass b/meta/classes/externalsrc.bbclass
index 08e6e479010f..65dd13ddc1f7 100644
--- a/meta/classes/externalsrc.bbclass
+++ b/meta/classes/externalsrc.bbclass
@@ -188,10 +188,8 @@ def srctree_hash_files(d, srcdir=None):
     git_dir = None
 
     try:
-        # git rev-parse returns the path relative to the current working
-        # directory
         git_dir = os.path.join(s_dir,
-            subprocess.check_output(['git', 'rev-parse', '--git-dir'], cwd=s_dir).decode("utf-8").rstrip())
+            subprocess.check_output(['git', '-C', s_dir, 'rev-parse', '--git-dir']).decode("utf-8").rstrip())
     except subprocess.CalledProcessError:
         pass
 
-- 
2.7.4