cache: Use configuration's hash value to validate cache

Previously we use the file time stamp to judge if a cache is valid.
Here this commit introduce a new method, which calculates the total
hash value for a certain configuration's key/value paris, and tag
it into cache filename, for example, bb_cache.dat.xxxyyyzzz.

This mechanism also ensures the cache's correctness if user
dynamically setting variables from some frontend GUI, like HOB.

(Bitbake rev: 1c1df03a6c4717bfd5faab144c4f8bbfcbae0b57)

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/data_smart.py

@@ -31,6 +31,7 @@ BitBake build tools.
 import copy, re
 from collections import MutableMapping
 import logging
+import hashlib
 import bb, bb.codeparser
 from bb   import utils
 from bb.COW  import COWDictBase
@@ -459,3 +460,23 @@ class DataSmart(MutableMapping):
 
     def __delitem__(self, var):
         self.delVar(var)
+
+    def get_hash(self):
+        data = ""
+        keys = iter(self)
+        for key in keys:
+            if key in ["TIME", "DATE"]:
+                continue
+            if key == "__depends":
+                deps = list(self.getVar(key, False))
+                deps.sort()
+                value = [deps[i][0] for i in range(len(deps))]
+            elif key == "PATH":
+                path = list(set(self.getVar(key, False).split(':')))
+                path.sort()
+                value = " ".join(path)
+            else:
+                value = self.getVar(key, False) or ""
+            data = data + key + ': ' + str(value) + '\n'
+
+        return hashlib.md5(data).hexdigest()