ifupdown: rewrite state file atomically

By user's request.
Decided to not use fcntl(F_SETLKW) in lieu of problems with locking
on networked filesystems. The existence of /var/run/ifstate.new
is treated as a write lock. rename() provides atomicity.

function                                             old     new   delta
ifupdown_main                                       1019    1122    +103

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 38 insertions, 5 deletions

diff --git a/networking/ifupdown.c b/networking/ifupdown.c
index b0bc0d70f..1d0fc53cf 100644
--- a/networking/ifupdown.c
+++ b/networking/ifupdown.c
@@ -56,6 +56,7 @@
 #endif
 
 #define UDHCPC_CMD_OPTIONS CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS
+#define IFSTATE_FILE_PATH  CONFIG_IFUPDOWN_IFSTATE_PATH
 
 #define debug_noise(args...) /*fprintf(stderr, args)*/
 
@@ -1200,7 +1201,7 @@ static llist_t *find_iface_state(llist_t *state_list, const char *iface)
 static llist_t *read_iface_state(void)
 {
         llist_t *state_list = NULL;
-        FILE *state_fp = fopen_for_read(CONFIG_IFUPDOWN_IFSTATE_PATH);
+        FILE *state_fp = fopen_for_read(IFSTATE_FILE_PATH);
 
         if (state_fp) {
                 char *start, *end_ptr;
@@ -1215,6 +1216,38 @@ static llist_t *read_iface_state(void)
         return state_list;
 }
 
+/* read the previous state from the state file */
+static FILE *open_new_state_file(void)
+{
+        int fd, flags, cnt;
+
+        cnt = 0;
+        flags = (O_WRONLY | O_CREAT | O_EXCL);
+        for (;;) {
+                fd = open(IFSTATE_FILE_PATH".new", flags, 0666);
+                if (fd >= 0)
+                        break;
+                if (errno != EEXIST
+                 || flags == (O_WRONLY | O_CREAT | O_TRUNC)
+                ) {
+                        bb_perror_msg_and_die("can't open '%s'",
+                                        IFSTATE_FILE_PATH".new");
+                }
+                /* Someone else created the .new file */
+                if (cnt > 30 * 1000) {
+                        /* Waited for 30*30/2 = 450 milliseconds, still EEXIST.
+                         * Assuming a stale file, rewriting it.
+                         */
+                        flags = (O_WRONLY | O_CREAT | O_TRUNC);
+                        continue;
+                }
+                usleep(cnt);
+                cnt += 1000;
+        }
+
+        return xfdopen_for_write(fd);
+}
+
 
 int ifupdown_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int ifupdown_main(int argc UNUSED_PARAM, char **argv)
@@ -1348,7 +1381,7 @@ int ifupdown_main(int argc UNUSED_PARAM, char **argv)
                         any_failures = 1;
                 } else if (!NO_ACT) {
                         /* update the state file */
-                        FILE *state_fp;
+                        FILE *new_state_fp = open_new_state_file();
                         llist_t *state;
                         llist_t *state_list = read_iface_state();
                         llist_t *iface_state = find_iface_state(state_list, iface);
@@ -1368,15 +1401,15 @@ int ifupdown_main(int argc UNUSED_PARAM, char **argv)
                         }
 
                         /* Actually write the new state */
-                        state_fp = xfopen_for_write(CONFIG_IFUPDOWN_IFSTATE_PATH);
                         state = state_list;
                         while (state) {
                                 if (state->data) {
-                                        fprintf(state_fp, "%s\n", state->data);
+                                        fprintf(new_state_fp, "%s\n", state->data);
                                 }
                                 state = state->link;
                         }
-                        fclose(state_fp);
+                        fclose(new_state_fp);
+                        xrename(IFSTATE_FILE_PATH".new", IFSTATE_FILE_PATH);
                         llist_free(state_list, free);
                 }
  next: