qemu-ga: add guest-suspend-disk

As the command name implies, this command suspends the guest to disk.

The suspend operation is implemented by two functions: bios_supports_mode()
and guest_suspend(). Both functions are generic enough to be used by
other suspend modes (introduced by next commits).

Both functions will try to use the scripts provided by the pm-utils
package if it's available. If it's not available, a manual method,
which consists of directly writing to '/sys/power/state', will be used.

To reap terminated children, a new signal handler is installed in the
parent to catch SIGCHLD signals and a non-blocking call to waitpid()
is done to collect their exit statuses. The statuses, however, are
discarded.

The approach used to query the guest for suspend support deserves some
explanation. It's implemented by bios_supports_mode() and shown below:

  qemu-ga
     |
 create pipe
     |
   fork()
     -----------------
     |               |
     |               |
     |             fork()
     |               --------------------------
     |               |                        |
     |               |                        |
     |               |               exec('pm-is-supported')
     |               |
     |              wait()
     |       write exit status to pipe
     |              exit
     |
  read pipe

This might look complex, but the resulting code is quite simple.
The purpose of that approach is to allow qemu-ga to reap its children
(semi-)automatically from its SIGCHLD handler.

Implementing this the obvious way, that's, doing the exec() call from
the first child process, would force us to introduce a more complex way
to reap qemu-ga's children. Like registering PIDs to be reaped and
having a way to wait for them when returning their exit status to
qemu-ga is necessary. The approach explained above avoids that complexity.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/qemu-ga.c b/qemu-ga.c
index ba355d8..1c90e6e 100644
--- a/qemu-ga.c
+++ b/qemu-ga.c
@@ -17,6 +17,7 @@
 #include <getopt.h>
 #ifndef _WIN32
 #include <syslog.h>
+#include <sys/wait.h>
 #endif
 #include "json-streamer.h"
 #include "json-parser.h"
@@ -73,9 +74,16 @@ static void quit_handler(int sig)
 }
 
 #ifndef _WIN32
+/* reap _all_ terminated children */
+static void child_handler(int sig)
+{
+    int status;
+    while (waitpid(-1, &status, WNOHANG) > 0) /* NOTHING */;
+}
+
 static gboolean register_signal_handlers(void)
 {
-    struct sigaction sigact;
+    struct sigaction sigact, sigact_chld;
     int ret;
 
     memset(&sigact, 0, sizeof(struct sigaction));
@@ -91,6 +99,15 @@ static gboolean register_signal_handlers(void)
         g_error("error configuring signal handler: %s", strerror(errno));
         return false;
     }
+
+    memset(&sigact_chld, 0, sizeof(struct sigaction));
+    sigact_chld.sa_handler = child_handler;
+    sigact_chld.sa_flags = SA_NOCLDSTOP;
+    ret = sigaction(SIGCHLD, &sigact_chld, NULL);
+    if (ret == -1) {
+        g_error("error configuring signal handler: %s", strerror(errno));
+    }
+
     return true;
 }
 #endif