add --backup flag and attempt to address various exceptions that may be raised

Author: mattieb

bin/jsonpatch

@@ -18,6 +18,8 @@ parser.add_argument('PATCH', type=argparse.FileType('r'),
                     help='Patch file (read from stdin if omitted)')
 parser.add_argument('--indent', type=int, default=None,
                     help='Indent output by n spaces')
+parser.add_argument('-b', '--backup', action='store_true',
+		    help='Back up ORIGINAL if modifying in-place')
 parser.add_argument('-i', '--in-place', action='store_true',
 		    help='Modify ORIGINAL in-place instead of to stdout')
 parser.add_argument('-v', '--version', action='version',
@@ -37,18 +39,36 @@ def patch_files():
     doc = json.load(args.ORIGINAL)
     patch = json.load(args.PATCH)
     result = jsonpatch.apply_patch(doc, patch)
+
     if args.in_place:
         dirname = os.path.abspath(os.path.dirname(args.ORIGINAL.name))
-        fd, pathname = tempfile.mkstemp(dir=dirname)
-        fp = os.fdopen(fd, 'w')
+        try:
+            fd, pathname = tempfile.mkstemp(dir=dirname)
+            fp = os.fdopen(fd, 'w')
+            atomic = True
+        except OSError:
+            if args.backup:
+                os.rename(args.ORIGINAL.name, args.ORIGINAL.name + '.orig')
+            fp = open(args.ORIGINAL.name, 'w')
+            atomic = False
+
     else:
         fp = sys.stdout
+
     json.dump(result, fp, indent=args.indent)
+    fp.write('\n')
+
     if args.in_place:
         fp.close()
-        os.link(args.ORIGINAL.name, args.ORIGINAL.name + '.orig')
-        os.chmod(pathname, os.stat(args.ORIGINAL.name).st_mode)
-        os.rename(pathname, args.ORIGINAL.name)
+        if atomic:
+            try:
+                if args.backup:
+                    os.link(args.ORIGINAL.name, args.ORIGINAL.name + '.orig')
+                os.chmod(pathname, os.stat(args.ORIGINAL.name).st_mode)
+                os.rename(pathname, args.ORIGINAL.name)
+            except OSError:
+                os.unlink(args.ORIGINAL.name)
+                os.rename(pathname, args.ORIGINAL.name)
 
 
 if __name__ == "__main__":