lib/oe/reproducible: Fix error when no git HEAD

Fixes an error that occurs when attempting to get the timestamp of the
latest commit when there is no HEAD in the git repository. The easiest
way to trigger this condition is to use the 'subdir=' option when
specifying a 'git://' SRC_URI.

(From OE-Core rev: a64caca5b5dbe4a76acd0b5709b2c3e75b245863)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/reproducible.py

@@ -47,14 +47,23 @@ def find_git_folder(d, sourcedir):
     return None
 
 def get_source_date_epoch_from_git(d, sourcedir):
-    source_date_epoch = None
-    if "git://" in d.getVar('SRC_URI'):
-        gitpath = find_git_folder(d, sourcedir)
-        if gitpath:
-            import subprocess
-            source_date_epoch = int(subprocess.check_output(['git','log','-1','--pretty=%ct'], cwd=gitpath))
-            bb.debug(1, "git repository: %s" % gitpath)
-    return source_date_epoch
+    if not "git://" in d.getVar('SRC_URI'):
+        return None
+
+    gitpath = find_git_folder(d, sourcedir)
+    if not gitpath:
+        return None
+
+    # Check that the repository has a valid HEAD; it may not if subdir is used
+    # in SRC_URI
+    p = subprocess.run(['git', 'rev-parse', 'HEAD'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, cwd=gitpath)
+    if p.returncode != 0:
+        bb.debug(1, "%s does not have a valid HEAD: %s" % (gitpath, p.stdout.decode('utf-8')))
+        return None
+
+    bb.debug(1, "git repository: %s" % gitpath)
+    p = subprocess.run(['git','log','-1','--pretty=%ct'], check=True, stdout=subprocess.PIPE, cwd=gitpath)
+    return int(p.stdout.decode('utf-8'))
 
 def get_source_date_epoch_from_youngest_file(d, sourcedir):
     if sourcedir == d.getVar('WORKDIR'):