sstate: apply proper umask when fetching from SSTATE_MIRROR

Currently, files and directories created under ${SSTATE_DIR} when
fetching from an sstate mirror are not created with group write,
unlike when the sstate artifacts are generated locally. That's
inconsistent, and problematic when the local sstate dir is shared
among multiple users.

Wrap the fetching in a bb.utils.umask() context manager, and for simplicity
move the mkdir of SSTATE_DIR inside that.

(From OE-Core rev: e56aa6c62e41667d0eeec5a862b7004d21b4da48)

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a6038553aaef3b88b834a09018c524c4fa41e625)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/classes-global/sstate.bbclass

@@ -726,7 +726,6 @@ def pstaging_fetch(sstatefetch, d):
     localdata = bb.data.createCopy(d)
 
     dldir = localdata.expand("${SSTATE_DIR}")
-    bb.utils.mkdirhier(dldir)
 
     localdata.delVar('MIRRORS')
     localdata.setVar('FILESPATH', dldir)
@@ -746,16 +745,19 @@ def pstaging_fetch(sstatefetch, d):
     if bb.utils.to_boolean(d.getVar("SSTATE_VERIFY_SIG"), False):
         uris += ['file://{0}.sig;downloadfilename={0}.sig'.format(sstatefetch)]
 
-    for srcuri in uris:
-        localdata.delVar('SRC_URI')
-        localdata.setVar('SRC_URI', srcuri)
-        try:
-            fetcher = bb.fetch2.Fetch([srcuri], localdata, cache=False)
-            fetcher.checkstatus()
-            fetcher.download()
+    with bb.utils.umask(0o002):
+        bb.utils.mkdirhier(dldir)
 
-        except bb.fetch2.BBFetchException:
-            pass
+        for srcuri in uris:
+            localdata.delVar('SRC_URI')
+            localdata.setVar('SRC_URI', srcuri)
+            try:
+                fetcher = bb.fetch2.Fetch([srcuri], localdata, cache=False)
+                fetcher.checkstatus()
+                fetcher.download()
+
+            except bb.fetch2.BBFetchException:
+                pass
 
 def sstate_setscene(d):
     shared_state = sstate_state_fromvars(d)