libxl: fork: Provide libxl_childproc_sigchld_occurred

Applications exist which don't keep track of all their child processes
in a manner suitable for coherent dispatch of their termination.  In
such a situation, nothing in the whole process may call wait, or
waitpid(-1,,).  Doing so reaps processes belonging to other parts of
the application and there is then no way to deliver the exit status to
the right place.

To facilitate this, provide a facility for such an application to ask
libxl to call waitpid on each of its children individually.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_event.h

@@ -482,9 +482,10 @@ typedef enum {
      * all children, including those not spawned by libxl. */
     libxl_sigchld_owner_libxl,
 
-    /* Application promises to call libxl_childproc_exited but NOT
-     * from within a signal handler.  libxl will not itself arrange to
-     * (un)block or catch SIGCHLD. */
+    /* Application promises to discover when SIGCHLD occurs and call
+     * libxl_childproc_exited or libxl_childproc_sigchld_occurred (but
+     * NOT from within a signal handler).  libxl will not itself
+     * arrange to (un)block or catch SIGCHLD. */
     libxl_sigchld_owner_mainloop,
 
     /* libxl owns SIGCHLD all the time, and the application is
@@ -542,7 +543,8 @@ void libxl_childproc_setmode(libxl_ctx *ctx, const libxl_childproc_hooks *hooks,
 
 /*
  * This function is for an application which owns SIGCHLD and which
- * therefore reaps all of the process's children.
+ * reaps all of the process's children, and dispatches the exit status
+ * to the correct place inside the application.
  *
  * May be called only by an application which has called setmode with
  * chldowner == libxl_sigchld_owner_mainloop.  If pid was a process started
@@ -558,6 +560,25 @@ void libxl_childproc_setmode(libxl_ctx *ctx, const libxl_childproc_hooks *hooks,
 int libxl_childproc_reaped(libxl_ctx *ctx, pid_t, int status)
                            LIBXL_EXTERNAL_CALLERS_ONLY;
 
+/*
+ * This function is for an application which owns SIGCHLD but which
+ * doesn't keep track of all of its own children in a manner suitable
+ * for reaping all of them and then dispatching them.
+ *
+ * Such an the application must notify libxl, by calling this
+ * function, that a SIGCHLD occurred.  libxl will then check all its
+ * children, reap any that are ready, and take any action necessary -
+ * but it will not reap anything else.
+ *
+ * May be called only by an application which has called setmode with
+ * chldowner == libxl_sigchld_owner_mainloop.
+ *
+ * May NOT be called from within a signal handler which might
+ * interrupt any libxl operation (just like libxl_childproc_reaped).
+ */
+void libxl_childproc_sigchld_occurred(libxl_ctx *ctx)
+                           LIBXL_EXTERNAL_CALLERS_ONLY;
+
 
 /*
  * An application which initialises a libxl_ctx in a parent process

tools/libxl/libxl_fork.c

@@ -330,6 +330,51 @@ int libxl_childproc_reaped(libxl_ctx *ctx, pid_t pid, int status)
     return rc;
 }
 
+static void childproc_checkall(libxl__egc *egc)
+{
+    EGC_GC;
+    libxl__ev_child *ch;
+
+    for (;;) {
+        int status;
+        pid_t got;
+
+        LIBXL_LIST_FOREACH(ch, &CTX->children, entry) {
+            got = checked_waitpid(egc, ch->pid, &status);
+            if (got)
+                goto found;
+        }
+        /* not found */
+        return;
+
+    found:
+        if (got == -1) {
+            LIBXL__EVENT_DISASTER
+                (egc, "waitpid() gave ECHILD but we have a child",
+                 ECHILD, 0);
+            /* it must have finished but we don't know its status */
+            status = 255<<8; /* no wait.h macro for this! */
+            assert(WIFEXITED(status));
+            assert(WEXITSTATUS(status)==255);
+            assert(!WIFSIGNALED(status));
+            assert(!WIFSTOPPED(status));
+        }
+        childproc_reaped_ours(egc, ch, status);
+        /* we need to restart the loop, as children may have been edited */
+    }
+}
+
+void libxl_childproc_sigchld_occurred(libxl_ctx *ctx)
+{
+    EGC_INIT(ctx);
+    CTX_LOCK;
+    assert(CTX->childproc_hooks->chldowner
+           == libxl_sigchld_owner_mainloop);
+    childproc_checkall(egc);
+    CTX_UNLOCK;
+    EGC_FREE;
+}
+
 static void sigchld_selfpipe_handler(libxl__egc *egc, libxl__ev_fd *ev,
                                      int fd, short events, short revents)
 {