qemu-nbd: move client to a thread

This avoids that qemu-nbd uses both forking and threads, which do
not behave well together.

qemu-nbd is already Unix only, and there is no qemu_thread_join,
so for now use pthreads.

Since the parent and child no longer have separate file descriptors,
we can open the NBD device before daemonizing, instead of checking
with access(2) and restricting the open to the client only.
Reported-by: NPierre Riteau <pierre.riteau@irisa.fr>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-nbd.c

@@ -31,6 +31,7 @@
 #include <arpa/inet.h>
 #include <signal.h>
 #include <libgen.h>
+#include <pthread.h>
 
 #define SOCKET_PATH    "/var/lock/qemu-nbd-%s"
 
@@ -38,6 +39,9 @@
 
 static int sigterm_wfd;
 static int verbose;
+static char *device;
+static char *srcpath;
+static char *sockpath;
 
 static void usage(const char *name)
 {
@@ -172,21 +176,70 @@ static void termsig_handler(int signum)
     }
 }
 
-static void show_parts(const char *device)
+static void *show_parts(void *arg)
 {
-    if (fork() == 0) {
-        int nbd;
+    int nbd;
+
+    /* linux just needs an open() to trigger
+     * the partition table update
+     * but remember to load the module with max_part != 0 :
+     *     modprobe nbd max_part=63
+     */
+    nbd = open(device, O_RDWR);
+    if (nbd != -1) {
+        close(nbd);
+    }
+    return NULL;
+}
 
-        /* linux just needs an open() to trigger
-         * the partition table update
-         * but remember to load the module with max_part != 0 :
-         *     modprobe nbd max_part=63
-         */
-        nbd = open(device, O_RDWR);
-        if (nbd != -1)
-              close(nbd);
-        exit(0);
+static void *nbd_client_thread(void *arg)
+{
+    int fd = *(int *)arg;
+    off_t size;
+    size_t blocksize;
+    uint32_t nbdflags;
+    int sock;
+    int ret;
+    pthread_t show_parts_thread;
+
+    do {
+        sock = unix_socket_outgoing(sockpath);
+        if (sock == -1) {
+            if (errno != ENOENT && errno != ECONNREFUSED) {
+                goto out;
+            }
+            sleep(1);  /* wait parent */
+        }
+    } while (sock == -1);
+
+    ret = nbd_receive_negotiate(sock, NULL, &nbdflags,
+                                &size, &blocksize);
+    if (ret == -1) {
+        goto out;
+    }
+
+    ret = nbd_init(fd, sock, nbdflags, size, blocksize);
+    if (ret == -1) {
+        goto out;
+    }
+
+    /* update partition table */
+    pthread_create(&show_parts_thread, NULL, show_parts, NULL);
+
+    fprintf(stderr, "NBD device %s is now connected to %s\n",
+            device, srcpath);
+
+    ret = nbd_client(fd);
+    if (ret) {
+        goto out;
     }
+    close(fd);
+    kill(getpid(), SIGTERM);
+    return (void *) EXIT_SUCCESS;
+
+out:
+    kill(getpid(), SIGTERM);
+    return (void *) EXIT_FAILURE;
 }
 
 int main(int argc, char **argv)
@@ -201,8 +254,6 @@ int main(int argc, char **argv)
     struct sockaddr_in addr;
     socklen_t addr_len = sizeof(addr);
     off_t fd_size;
-    char *device = NULL;
-    char *sockpath = NULL;
     const char *sopt = "hVb:o:p:rsnP:c:dvk:e:t";
     struct option lopt[] = {
         { "help", 0, NULL, 'h' },
@@ -238,8 +289,11 @@ int main(int argc, char **argv)
     int nb_fds = 0;
     int max_fd;
     int persistent = 0;
+    pthread_t client_thread;
 
-    /* Set up a SIGTERM handler so that we exit with a nice status code.  */
+    /* The client thread uses SIGTERM to interrupt the server.  A signal
+     * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
+     */
     struct sigaction sa_sigterm;
     int sigterm_fd[2];
     if (qemu_pipe(sigterm_fd) == -1) {
@@ -356,9 +410,10 @@ int main(int argc, char **argv)
 
     bs = bdrv_new("hda");
 
-    if ((ret = bdrv_open(bs, argv[optind], flags, NULL)) < 0) {
+    srcpath = argv[optind];
+    if ((ret = bdrv_open(bs, srcpath, flags, NULL)) < 0) {
         errno = -ret;
-        err(EXIT_FAILURE, "Failed to bdrv_open '%s'", argv[optind]);
+        err(EXIT_FAILURE, "Failed to bdrv_open '%s'", srcpath);
     }
 
     fd_size = bs->total_sectors * 512;
@@ -368,12 +423,14 @@ int main(int argc, char **argv)
         err(EXIT_FAILURE, "Could not find partition %d", partition);
 
     if (device) {
-        pid_t pid;
-        int sock;
+        int ret;
 
-        /* want to fail before daemonizing */
-        if (access(device, R_OK|W_OK) == -1) {
-            err(EXIT_FAILURE, "Could not access '%s'", device);
+        /* Open before spawning new threads.  In the future, we may
+         * drop privileges after opening.
+         */
+        fd = open(device, O_RDWR);
+        if (fd == -1) {
+            err(EXIT_FAILURE, "Failed to open %s", device);
         }
 
         if (!verbose) {
@@ -388,64 +445,14 @@ int main(int argc, char **argv)
             snprintf(sockpath, 128, SOCKET_PATH, basename(device));
         }
 
-        pid = fork();
-        if (pid < 0)
-            return 1;
-        if (pid != 0) {
-            off_t size;
-            size_t blocksize;
-
-            ret = 0;
-            bdrv_close(bs);
-
-            do {
-                sock = unix_socket_outgoing(sockpath);
-                if (sock == -1) {
-                    if (errno != ENOENT && errno != ECONNREFUSED) {
-                        ret = 1;
-                        goto out;
-                    }
-                    sleep(1);	/* wait children */
-                }
-            } while (sock == -1);
-
-            fd = open(device, O_RDWR);
-            if (fd == -1) {
-                ret = 1;
-                goto out;
-            }
-
-            ret = nbd_receive_negotiate(sock, NULL, &nbdflags,
-                                        &size, &blocksize);
-            if (ret == -1) {
-                ret = 1;
-                goto out;
-            }
-
-            ret = nbd_init(fd, sock, nbdflags, size, blocksize);
-            if (ret == -1) {
-                ret = 1;
-                goto out;
-            }
-
-            printf("NBD device %s is now connected to file %s\n",
-                    device, argv[optind]);
-
-	    /* update partition table */
-
-            show_parts(device);
-
-            ret = nbd_client(fd);
-            if (ret) {
-                ret = 1;
-            }
-            close(fd);
- out:
-            kill(pid, SIGTERM);
-
-            return ret;
+        ret = pthread_create(&client_thread, NULL, nbd_client_thread, &fd);
+        if (ret != 0) {
+            errx(EXIT_FAILURE, "Failed to create client thread: %s",
+                 strerror(ret));
         }
-        /* children */
+    } else {
+        /* Shut up GCC warnings.  */
+        memset(&client_thread, 0, sizeof(client_thread));
     }
 
     sharing_fds = g_malloc((shared + 1) * sizeof(int));
@@ -517,5 +524,11 @@ int main(int argc, char **argv)
         unlink(sockpath);
     }
 
-    return 0;
+    if (device) {
+        void *ret;
+        pthread_join(client_thread, &ret);
+        exit(ret != NULL);
+    } else {
+        exit(EXIT_SUCCESS);
+    }
 }