[Xen-changelog] Fix the log reopening by moving the code with all the races out of the signal

[Xen patchbot -unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User emellor@xxxxxxxxxxxxxxxxxxxxxx
# Node ID 975aa9e4def35fa8059755129f03f078677355b6
# Parent  4982559d8fde3d0a4401766deb54c5cc1e1a0ec5
Fix the log reopening by moving the code with all the races out of the signal
handler and into the main loop.  The "self-pipe trick" is used to awaken the
select() call on SIGHUP.

Signed-off-by: Ewan Mellor <ewan@xxxxxxxxxxxxx>

diff -r 4982559d8fde -r 975aa9e4def3 tools/xenstore/xenstored_core.c
--- a/tools/xenstore/xenstored_core.c   Thu Nov 10 14:16:01 2005
+++ b/tools/xenstore/xenstored_core.c   Thu Nov 10 15:20:27 2005
@@ -56,6 +56,7 @@
 static bool verbose;
 LIST_HEAD(connections);
 static int tracefd = -1;
+static int reopen_log_pipe[2];
 static char *tracefile = NULL;
 static TDB_CONTEXT *tdb_ctx;
 
@@ -243,20 +244,34 @@
        talloc_free(str);
 }
 
-void reopen_log()
-{
-       if (!tracefile)
-               return;
-
-       if (tracefd > 0)
-               close(tracefd);
-       tracefd = open(tracefile, O_WRONLY|O_CREAT|O_APPEND, 0600);
-       if (tracefd < 0) {
-               perror("Could not open tracefile");
-               return;
-       }
-       write(tracefd, "\n***\n", strlen("\n***\n"));
-}
+
+/**
+ * Signal handler for SIGHUP, which requests that the trace log is reopened
+ * (in the main loop).  A single byte is written to reopen_log_pipe, to awaken
+ * the select() in the main loop.
+ */
+static void trigger_reopen_log(int signal __attribute__((unused)))
+{
+       char c = 'A';
+       write(reopen_log_pipe[1], &c, 1);
+}
+
+
+static void reopen_log()
+{
+       if (tracefile) {
+               if (tracefd > 0)
+                       close(tracefd);
+
+               tracefd = open(tracefile, O_WRONLY|O_CREAT|O_APPEND, 0600);
+
+               if (tracefd < 0)
+                       perror("Could not open tracefile");
+               else
+                       write(tracefd, "\n***\n", strlen("\n***\n"));
+       }
+}
+
 
 static bool write_messages(struct connection *conn)
 {
@@ -331,29 +346,33 @@
        return 0;
 }
 
+
+static void set_fd(int fd, fd_set *set, int *max)
+{
+       FD_SET(fd, set);
+       if (fd > *max)
+               *max = fd;
+}
+
+
 static int initialize_set(fd_set *inset, fd_set *outset, int sock, int ro_sock)
 {
        struct connection *i;
-       int max;
+       int max = -1;
 
        FD_ZERO(inset);
        FD_ZERO(outset);
-       FD_SET(sock, inset);
-       max = sock;
-       FD_SET(ro_sock, inset);
-       if (ro_sock > max)
-               max = ro_sock;
-       FD_SET(eventchn_fd, inset);
-       if (eventchn_fd > max)
-               max = eventchn_fd;
+
+       set_fd(sock,               inset, &max);
+       set_fd(ro_sock,            inset, &max);
+       set_fd(eventchn_fd,        inset, &max);
+       set_fd(reopen_log_pipe[0], inset, &max);
        list_for_each_entry(i, &connections, list) {
                if (i->domain)
                        continue;
-               FD_SET(i->fd, inset);
+               set_fd(i->fd, inset, &max);
                if (!list_empty(&i->out_list))
                        FD_SET(i->fd, outset);
-               if (i->fd > max)
-                       max = i->fd;
        }
        return max;
 }
@@ -1570,6 +1589,10 @@
            || listen(*ro_sock, 1) != 0)
                barf_perror("Could not listen on sockets");
 
+       if (pipe(reopen_log_pipe)) {
+               barf_perror("pipe");
+       }
+
        /* Setup the database */
        setup_structure();
 
@@ -1592,7 +1615,7 @@
                close(STDERR_FILENO);
        }
 
-       signal(SIGHUP, reopen_log);
+       signal(SIGHUP, trigger_reopen_log);
 
 #ifdef TESTING
        signal(SIGUSR1, stop_failtest);
@@ -1610,6 +1633,12 @@
                        if (errno == EINTR)
                                continue;
                        barf_perror("Select failed");
+               }
+
+               if (FD_ISSET(reopen_log_pipe[0], &inset)) {
+                       char c;
+                       read(reopen_log_pipe[0], &c, 1);
+                       reopen_log();
                }
 
                if (FD_ISSET(*sock, &inset))

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