[V2] Add -f option to qemu-nbd

diff --git a/qemu-nbd.c b/qemu-nbd.c
index 291cba2..64892a8 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -32,6 +32,7 @@ 
 #include <signal.h>
 #include <libgen.h>
 #include <pthread.h>
+#include <fcntl.h>
 
 #define SOCKET_PATH    "/var/lock/qemu-nbd-%s"
 
@@ -244,6 +245,60 @@  out:
     return (void *) EXIT_FAILURE;
 }
 
+static int is_nbd_used(int minor)
+{
+    FILE *proc;
+    int NBDMAJOR = 43;
+    char buf[BUFSIZ];
+    int find = 0;
+
+    proc = fopen("/proc/partitions", "r");
+    if (proc != NULL) {
+        while (fgets(buf, sizeof(buf), proc)) {
+            int m, n;
+            unsigned long long sz;
+            char name[16];
+            char *pname = name;
+            char *end;
+
+            if (sscanf(buf, " %d %d %llu %128[^\n ]",
+                    &m, &n, &sz, name) != 4)
+                continue;
+            if (m != NBDMAJOR)
+                continue;
+            if (strncmp(name, "nbd", 3))
+                continue;
+            pname += 3;
+            n = strtol(pname, &end, 10);
+            if (end && end != pname && *end == '\0' && n == minor) {
+                find = 1;
+                break;
+            }
+        }
+        fclose(proc);
+    }
+
+    return find;
+}
+
+static char *find_free_nbd_device(void)
+{
+    int i;
+    int nbds_max = 16;
+    char name[16];
+    char *devname = NULL;
+
+    for (i = 0; i < nbds_max; i++) {
+        if (!is_nbd_used(i)) {
+            snprintf(name, sizeof(name), "/dev/nbd%d", i);
+            devname = strdup(name);
+            break;
+        }
+    }
+
+    return devname;
+}
+
 int main(int argc, char **argv)
 {
     BlockDriverState *bs;
@@ -256,7 +311,7 @@  int main(int argc, char **argv)
     struct sockaddr_in addr;
     socklen_t addr_len = sizeof(addr);
     off_t fd_size;
-    const char *sopt = "hVb:o:p:rsnP:c:dvk:e:t";
+    const char *sopt = "hVb:o:p:rsnP:c:dvk:e:tf";
     struct option lopt[] = {
         { "help", 0, NULL, 'h' },
         { "version", 0, NULL, 'V' },
@@ -273,6 +328,7 @@  int main(int argc, char **argv)
         { "shared", 1, NULL, 'e' },
         { "persistent", 0, NULL, 't' },
         { "verbose", 0, NULL, 'v' },
+        { "find", 0, NULL, 'f' },
         { NULL, 0, NULL, 0 }
     };
     int ch;
@@ -292,6 +348,9 @@  int main(int argc, char **argv)
     int max_fd;
     int persistent = 0;
     pthread_t client_thread;
+    char *devname = NULL;
+    int find = 0;
+    struct flock lock;
 
     /* The client thread uses SIGTERM to interrupt the server.  A signal
      * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
@@ -374,6 +433,18 @@  int main(int argc, char **argv)
         case 'v':
             verbose = 1;
             break;
+        case 'f':
+            find = 1;
+            devname = find_free_nbd_device();
+            if (devname == NULL)
+                exit(1);
+            if (argc == optind) {
+                printf("%s\n", devname);
+                free(devname);
+                exit(0);
+            }
+            device = devname;
+            break;
         case 'V':
             version(argv[0]);
             exit(0);
@@ -464,11 +535,28 @@  int main(int argc, char **argv)
         /* Open before spawning new threads.  In the future, we may
          * drop privileges after opening.
          */
+retry:
         fd = open(device, O_RDWR);
         if (fd == -1) {
             err(EXIT_FAILURE, "Failed to open %s", device);
         }
 
+        memset(&lock, 0, sizeof(lock));
+        lock.l_type = F_WRLCK;
+        lock.l_whence = SEEK_SET;
+        if (fcntl(fd, F_SETLK, &lock) != 0) {
+            if (find) {
+                close(fd);
+                free(device);
+                device = find_free_nbd_device();
+                if (!device)
+                    err(EXIT_FAILURE, "Could not find free nbd device");
+                goto retry;
+            } else {
+                err(EXIT_FAILURE, "Could not lock %s", device);
+            }
+        }
+
         if (sockpath == NULL) {
             sockpath = g_malloc(128);
             snprintf(sockpath, 128, SOCKET_PATH, basename(device));