Use a different flag in mx_open_mailbox_append() when mailbox doesn't exist.

The previous commit re-used MUTT_NEWFOLDER, but the meaning of that
flag is slightly different: it causes mbox to use fopen with mode "w",
and is used only for the case of a brand-new mktemp-generated mbox.

Setting it for other non-existing mbox files leads to a race condition
between the stat and the fopen/lock, and so could end up truncating an
existing mailbox created in-between!

Create a different flag, MUTT_APPENDNEW to notify the open_append()
functions that the mailbox doesn't exist.  Change maildir and mh to
check for that flag to create their directory structures.

1 files changed, 2 insertions, 2 deletions

diff --git a/mh.c b/mh.c
index 3d969f62..f7ce25fc 100644
--- a/mh.c
+++ b/mh.c
@@ -1294,7 +1294,7 @@ static int maildir_open_mailbox_append (CONTEXT *ctx, int flags)
 {
   char tmp[_POSIX_PATH_MAX];
 
-  if (flags & MUTT_NEWFOLDER)
+  if (flags & MUTT_APPENDNEW)
   {
     if (mkdir (ctx->path, S_IRWXU))
     {
@@ -1346,7 +1346,7 @@ static int mh_open_mailbox_append (CONTEXT *ctx, int flags)
   char tmp[_POSIX_PATH_MAX];
   int i;
 
-  if (flags & MUTT_NEWFOLDER)
+  if (flags & MUTT_APPENDNEW)
   {
     if (mkdir (ctx->path, S_IRWXU))
     {