[Libguestfs] [nbdkit PATCH 1/2] nbd: Add retry=N parameter

A common command line use case for the nbd plugin is ( sock=`mktemp -u` && nbdkit --exit-with-parent nbd socket=$sock & exec /path/to/server -k $sock) As long as the first client to nbdkit doesn't try to connect until after /path/to/server has created $sock, we are fine. But on a heavily loaded system, it would be nice to give nbdkit some leeway to try again to connect to the socket if a client connects quickly to nbdkit. This becomes even more useful in the next patch when adding an option for the nbd plugin to connect during .config_complete rather than during .open, as the window of time for the external server to make the socket available is no longer dependent on when nbdkit's first client arrives. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- plugins/nbd/nbdkit-nbd-plugin.pod | 6 ++++++ plugins/nbd/nbd.c | 24 ++++++++++++++++++++++-- 2 files changed, 28 insertions(+), 2 deletions(-) diff --git a/plugins/nbd/nbdkit-nbd-plugin.pod b/plugins/nbd/nbdkit-nbd-plugin.pod index 913d1e5..4f9efa1 100644 --- a/plugins/nbd/nbdkit-nbd-plugin.pod +++ b/plugins/nbd/nbdkit-nbd-plugin.pod @@ -5,6 +5,7 @@ nbdkit-nbd-plugin - nbdkit nbd plugin =head1 SYNOPSIS nbdkit nbd { socket=SOCKNAME | hostname=HOST [port=PORT] } [export=NAME] + [retry=N] =head1 DESCRIPTION @@ -50,6 +51,11 @@ If this parameter is given, and the server speaks new style protocol, then connect to the named export instead of the default export (the empty string). +=item B<retry=>N + +If the initial connection attempt to the server fails, retry up to +B<N> times more after a second delay between tries (default 0). + =back =head1 EXAMPLES diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c index 1195eb8..6665cd9 100644 --- a/plugins/nbd/nbd.c +++ b/plugins/nbd/nbd.c @@ -70,6 +70,9 @@ static char *servname; /* Name of export on remote server, default '', ignored for oldstyle */ static const char *export; +/* Number of retries */ +static unsigned long retry; + static void nbd_unload (void) { @@ -79,11 +82,14 @@ nbd_unload (void) /* Called for each key=value passed on the command line. This plugin * accepts socket=<sockname> or hostname=<hostname>/port=<port> - * (exactly one connection required) and export=<name> (optional). + * (exactly one connection required), and optional parameters + * export=<name>, retry=<n>. */ static int nbd_config (const char *key, const char *value) { + char *end; + if (strcmp (key, "socket") == 0) { /* See FILENAMES AND PATHS in nbdkit-plugin(3) */ free (sockname); @@ -97,6 +103,14 @@ nbd_config (const char *key, const char *value) port = value; else if (strcmp (key, "export") == 0) export = value; + else if (strcmp (key, "retry") == 0) { + errno = 0; + retry = strtoul (value, &end, 0); + if (value == end || errno) { + nbdkit_error ("could not parse retry as integer (%s)", value); + return -1; + } + } else { nbdkit_error ("unknown parameter '%s'", key); return -1; @@ -960,12 +974,18 @@ nbd_open (int readonly) return NULL; } + retry: if (sockname) h->fd = nbd_connect_unix (); else h->fd = nbd_connect_tcp (); - if (h->fd == -1) + if (h->fd == -1) { + if (retry--) { + sleep (1); + goto retry; + } goto err; + } /* old and new handshake share same meaning of first 16 bytes */ if (read_full (h->fd, &old, offsetof (struct old_handshake, exportsize))) { -- 2.20.1