bitbake: cache/siggen: Add unihash cache copy function

We see rare failures in eSDK generation with zero sized unihash cache files. This is
almost certainly due to races in the cache file being updated. Add a copy function
where the cache file can be copied with the lock held to avoid this.

(Bitbake rev: 9e72a3915e36cb843037040cb68a82077436dbef)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cache.py

@@ -24,6 +24,7 @@ from collections.abc import Mapping
 import bb.utils
 from bb import PrefixLoggerAdapter
 import re
+import shutil
 
 logger = logging.getLogger("BitBake.Cache")
 
@@ -998,3 +999,11 @@ class SimpleCache(object):
             p.dump([data, self.cacheversion])
 
         bb.utils.unlockfile(glf)
+
+    def copyfile(self, target):
+        if not self.cachefile:
+            return
+
+        glf = bb.utils.lockfile(self.cachefile + ".lock")
+        shutil.copy(self.cachefile, target)
+        bb.utils.unlockfile(glf)