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/siggen.py

@@ -120,6 +120,9 @@ class SignatureGenerator(object):
     def save_unitaskhashes(self):
         return
 
+    def copy_unitaskhashes(self, targetdir):
+        return
+
     def set_setscene_tasks(self, setscene_tasks):
         return
 
@@ -358,6 +361,9 @@ class SignatureGeneratorBasic(SignatureGenerator):
     def save_unitaskhashes(self):
         self.unihash_cache.save(self.unitaskhashes)
 
+    def copy_unitaskhashes(self, targetdir):
+        self.unihash_cache.copyfile(targetdir)
+
     def dump_sigtask(self, fn, task, stampbase, runtime):
 
         tid = fn + ":" + task