[CRIU] [RFC] debug: add debug_wait to wait on signal

[Dmitry Safonov]

Currently to debug CRIU problems there is need sometimes to stop
C/R processing and let computer operator to see what have happened.
I.e, to see mount state in the middle of CR process, before error
happens.
So, people use sleep() and send SIGSTOP with ^Z, then do debug.
It's working, but yet not very handy.
If you want to find where problem first occurs, you need to place
sleeps "here, there and everywhere". And each time rebuild CRIU.
I suggest debug_wait() for this purpose.
All normal people may use gdb breakpoints for this achievement,
but as CRIU uses ptrace - it's no good for debugging it.

Here is an example, how it works:
1. Placing debug_wait(SIGUSR1, "mount %s\n", mnt_path); on interested
places in mount.c; (don't forget include debug.h)
2. Run CRUI and open restore.log to see the next line:
(43.621767)      1: Warn  (mount.c:2394): mnt: debug_wait(-10): mount /tmp/cr-tmpfs.hu9dyJ
3. Send SIGUSR1 to restore process and continue debugging, looking in
the log or fetching filesystem/other interesting source.

Note: this patch is RFC, so I may move debug_wait_on() realization to
debug.c, printing format or anything else if this debugging approach
fits CRIU.

Signed-off-by: Dmitry Safonov <dsafonov at odin.com>
---
 include/debug.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 include/debug.h

diff --git a/include/debug.h b/include/debug.h
new file mode 100644
index 0000000..9ef671c
--- /dev/null
+++ b/include/debug.h
@@ -0,0 +1,30 @@
+#ifndef __CR_DEBUG_H__
+#define __CR_DEBUG_H__
+
+#include <signal.h>
+#include "criu-log.h"
+
+#define debug_wait(signum, reason, ...)                                        \
+       pr_warn("debug_wait(-%d): " reason, signum, ##__VA_ARGS__);     \
+       debug_wait_on(signum);
+
+static void __nop_handle(int signr) {}
+
+static inline void debug_wait_on(int signum)
+{
+       sigset_t mask;
+       struct sigaction old_sigaction;
+       struct sigaction new_sigaction = {
+               .sa_handler = __nop_handle
+       };
+
+       sigfillset(&mask);
+       sigdelset(&mask, signum);
+       sigaction(signum, &new_sigaction, &old_sigaction);
+
+       sigsuspend(&mask);
+
+       sigaction(signum, &old_sigaction, NULL);
+}
+
+#endif /*  __CR_DEBUG_H__ */
-- 
2.6.4