classes/buildhistory: fix failures collecting output signatures

It's possible for tasks to stage symlinks that point to non-existent
files; an example is ncurses-native.do_populate_sysroot. There wasn't
any error checking here so this broke the build when "task" was included
in BUILDHISTORY_FEATURES. In any case we shouldn't be following symlinks
and getting the sha256sum of the link target - we need concern ourselves
only with the target path, so check if the file is a link and sha256 the
target path instead if it is. If it's neither a regular file nor a
symlink (perhaps a pipe or a device), just skip it.

(From OE-Core rev: f60520d97f53dafe783f61eb58fe249798a1e1be)

(From OE-Core rev: 66a0d184d8f55a8da03de9fedb18d166b80b198b)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/buildhistory.bbclass

@@ -301,6 +301,8 @@ python buildhistory_emit_outputsigs() {
     if not "task" in (d.getVar('BUILDHISTORY_FEATURES') or "").split():
         return
 
+    import hashlib
+
     taskoutdir = os.path.join(d.getVar('BUILDHISTORY_DIR'), 'task', 'output')
     bb.utils.mkdirhier(taskoutdir)
     currenttask = d.getVar('BB_CURRENTTASK')
@@ -314,7 +316,17 @@ python buildhistory_emit_outputsigs() {
             if fname == 'fixmepath':
                 continue
             fullpath = os.path.join(root, fname)
-            filesigs[os.path.relpath(fullpath, cwd)] = bb.utils.sha256_file(fullpath)
+            try:
+                if os.path.islink(fullpath):
+                    sha256 = hashlib.sha256(os.readlink(fullpath).encode('utf-8')).hexdigest()
+                elif os.path.isfile(fullpath):
+                    sha256 = bb.utils.sha256_file(fullpath)
+                else:
+                    continue
+            except OSError:
+                bb.warn('buildhistory: unable to read %s to get output signature' % fullpath)
+                continue
+            filesigs[os.path.relpath(fullpath, cwd)] = sha256
     with open(taskfile, 'w') as f:
         for fpath, fsig in sorted(filesigs.items(), key=lambda item: item[0]):
             f.write('%s %s\n' % (fpath, fsig))