qga: add systemd socket activation support

AF_UNIX and AF_VSOCK listen sockets can be passed in by systemd on
startup.  This allows systemd to manage the listen socket until the
first client connects and between restarts.  Advantages of socket
activation are that parallel startup of network services becomes
possible and that unused daemons do not consume memory.

The key to achieving this is the LISTEN_FDS environment variable, which
is a stable ABI as shown here:
https://www.freedesktop.org/wiki/Software/systemd/InterfacePortabilityAndStabilityChart/

We could link against libsystemd and use sd_listen_fds(3) but it's easy
to implement the tiny LISTEN_FDS ABI so that qemu-ga does not depend on
libsystemd.  Some systems may not have systemd installed and wish to
avoid the dependency.  Other init systems or socket activation servers
may implement the same ABI without systemd involvement.

Test as follows:

  $ cat ~/.config/systemd/user/qga.service
  [Unit]
  Description=qga

  [Service]
  WorkingDirectory=/tmp
  ExecStart=/path/to/qemu-ga --logfile=/tmp/qga.log --pidfile=/tmp/qga.pid --statedir=/tmp

  $ cat ~/.config/systemd/user/qga.socket
  [Socket]
  ListenStream=/tmp/qga.sock

  [Install]
  WantedBy=default.target

  $ systemctl --user daemon-reload
  $ systemctl --user start qga.socket
  $ nc -U /tmp/qga.sock

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

1 files changed, 69 insertions, 5 deletions

diff --git a/qga/main.c b/qga/main.c
index 538e4ee299..675ef81dc2 100644
--- a/qga/main.c
+++ b/qga/main.c
@@ -28,6 +28,7 @@
 #include "qga/channel.h"
 #include "qemu/bswap.h"
 #include "qemu/help_option.h"
+#include "qemu/sockets.h"
 #ifdef _WIN32
 #include "qga/service-win32.h"
 #include "qga/vss-win32.h"
@@ -184,6 +185,37 @@ void reopen_fd_to_null(int fd)
 }
 #endif
 
+/**
+ * get_listen_fd:
+ * @consume: true to prevent future calls from succeeding
+ *
+ * Fetch a listen file descriptor that was passed via systemd socket
+ * activation.  Use @consume to prevent child processes from thinking a file
+ * descriptor was passed.
+ *
+ * Returns: file descriptor or -1 if no fd was passed
+ */
+static int get_listen_fd(bool consume)
+{
+#ifdef _WIN32
+    return -1; /* no fd passing expected, unsetenv(3) not available */
+#else
+    const char *listen_fds = getenv("LISTEN_FDS");
+    int fd = STDERR_FILENO + 1;
+
+    if (!listen_fds || strcmp(listen_fds, "1") != 0) {
+        return -1;
+    }
+
+    if (consume) {
+        unsetenv("LISTEN_FDS");
+    }
+
+    qemu_set_cloexec(fd);
+    return fd;
+#endif /* !_WIN32 */
+}
+
 static void usage(const char *cmd)
 {
     printf(
@@ -648,7 +680,8 @@ static gboolean channel_event_cb(GIOCondition condition, gpointer data)
     return true;
 }
 
-static gboolean channel_init(GAState *s, const gchar *method, const gchar *path)
+static gboolean channel_init(GAState *s, const gchar *method, const gchar *path,
+                             int listen_fd)
 {
     GAChannelMethod channel_method;
 
@@ -666,7 +699,8 @@ static gboolean channel_init(GAState *s, const gchar *method, const gchar *path)
         return false;
     }
 
-    s->channel = ga_channel_new(channel_method, path, channel_event_cb, s);
+    s->channel = ga_channel_new(channel_method, path, listen_fd,
+                                channel_event_cb, s);
     if (!s->channel) {
         g_critical("failed to create guest agent channel");
         return false;
@@ -1025,7 +1059,9 @@ static void config_dump(GAConfig *config)
 
     g_key_file_set_boolean(keyfile, "general", "daemon", config->daemonize);
     g_key_file_set_string(keyfile, "general", "method", config->method);
-    g_key_file_set_string(keyfile, "general", "path", config->channel_path);
+    if (config->channel_path) {
+        g_key_file_set_string(keyfile, "general", "path", config->channel_path);
+    }
     if (config->log_filepath) {
         g_key_file_set_string(keyfile, "general", "logfile",
                               config->log_filepath);
@@ -1294,7 +1330,9 @@ static int run_agent(GAState *s, GAConfig *config)
 #endif
 
     s->main_loop = g_main_loop_new(NULL, false);
-    if (!channel_init(ga_state, config->method, config->channel_path)) {
+
+    if (!channel_init(ga_state, config->method, config->channel_path,
+                      get_listen_fd(true))) {
         g_critical("failed to initialize guest agent channel");
         return EXIT_FAILURE;
     }
@@ -1318,6 +1356,7 @@ int main(int argc, char **argv)
     int ret = EXIT_SUCCESS;
     GAState *s = g_new0(GAState, 1);
     GAConfig *config = g_new0(GAConfig, 1);
+    int listen_fd;
 
     config->log_level = G_LOG_LEVEL_ERROR | G_LOG_LEVEL_CRITICAL;
 
@@ -1339,7 +1378,32 @@ int main(int argc, char **argv)
         config->method = g_strdup("virtio-serial");
     }
 
-    if (config->channel_path == NULL) {
+    listen_fd = get_listen_fd(false);
+    if (listen_fd >= 0) {
+        SocketAddress *addr;
+
+        g_free(config->method);
+        g_free(config->channel_path);
+        config->method = NULL;
+        config->channel_path = NULL;
+
+        addr = socket_local_address(listen_fd, NULL);
+        if (addr) {
+            if (addr->type == SOCKET_ADDRESS_KIND_UNIX) {
+                config->method = g_strdup("unix-listen");
+            } else if (addr->type == SOCKET_ADDRESS_KIND_VSOCK) {
+                config->method = g_strdup("vsock-listen");
+            }
+
+            qapi_free_SocketAddress(addr);
+        }
+
+        if (!config->method) {
+            g_critical("unsupported listen fd type");
+            ret = EXIT_FAILURE;
+            goto end;
+        }
+    } else if (config->channel_path == NULL) {
         if (strcmp(config->method, "virtio-serial") == 0) {
             /* try the default path for the virtio-serial port */
             config->channel_path = g_strdup(QGA_VIRTIO_PATH_DEFAULT);