Re: [RFC][PATCH][cryo] Save/restore state of unnamed pipes

[sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org]

Matt Helsley [matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org] wrote:

<snip>
| 
| > | I don't see anything in the pipe man page, at least, that suggests we
| > | can safely assume pipefds[0] < pipefds[1].
| > | 
| > | 	The solution could be to use "trampoline" fds. Suppose last_fd is the
| > | largest fd that exists in the final checkpointed/restarting application.
| > | We could do (Skipping the PT_FUNC "notation" for clarity):
| > 
| > | 
| > | 
| > |         pipe(pipefds); /* returns 5 and 4 in elements 0 and 1 */
| > |         /* use fds after last_fd as trampolines for fds we want to create */
| > |         dup2(pipefds[0], last_fd + 1);
| > |         dup2(pipefds[1], last_fd + 2);
| > |         close(pipefds[0]);
| > |         close(pipefds[1]);
| > |         dup2(last_fd + 1, <orig pipefd[0]>);
| > |         dup2(last_fd + 2, <orig pipefd[1]>);
| > |         close(last_fd + 1);
| > |         close(last_fd + 2);
| > | 
| > | 
| > | Which is alot more code but should work no matter which fds we get back
| > | from pipe(). Of course this assumes the checkpointed application hasn't
| > | used all of its fds. :(
| > | 

It appears that this last_fd approach will fit in easier with current
design of cryo (where we process one or two fds at a time and don't have
the src_fds and dest_fds handy). 

BTW, we should be able to accomplish the above with a single-unused fd
right (i.e no need for last_fd+2) ?

| > 
| > This sounds like a good idea too, but we could use any fd that has not
| > yet been used in the restart-process right ? It would break if all fds
| 
| Yes, but we don't know which fd is available unless we allocate it
| without dup2().

Right. I was thinking we could find that out at the time of checkpoint
(a brute-force fstat(i, &statbuf) for i = 0..n or something more
efficient).

Well just thought of another approach.

Basically, we have a temporary need for an unused fd for use as a
trampoline.  So, why not 'set-aside' an fd for that purpose and after
all other fds have been created, go back and create this fd ?

i.e lets say the first regular file we open is associated with 'fd = 3'.
We save away the 'fdinfo' for 3 say in a global variable and close(3).
Now use 'fd = 3' in place of last_fd+1 above.

Once all fds have been setup correctly, go back and set up fd = 3
using the saved fdinfo (this would be a simple open of the file
followed by seek and maybe an fcntl).

This would work even if the application was using all its fds ?

If we do need both last_fd+1 and last_fd+2, we would have to set
aside two regular files.

Hmm, would it work even if an application uses all (1024) its fds
for pipes :-), but just a thought at this point.

Suka