qemu: Monitor nbdkit process for exit

Adds the ability to monitor the nbdkit process so that we can take
action in case the child exits unexpectedly.

When the nbdkit process exits, we pause the vm, restart nbdkit, and then
resume the vm. This allows the vm to continue working in the event of a
nbdkit failure.

Eventually we may want to generalize this functionality since we may
need something similar for e.g. qemu-storage-daemon, etc.

The process is monitored with the pidfd_open() syscall if it exists
(since linux 5.3). Otherwise it resorts to checking whether the process
is alive once a second. The one-second time period was chosen somewhat
arbitrarily.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

meson.build

@@ -691,6 +691,13 @@ symbols = [
   [ 'sched.h', 'cpu_set_t' ],
 ]
 
+if host_machine.system() == 'linux'
+  symbols += [
+    # process management
+    [ 'sys/syscall.h', 'SYS_pidfd_open' ],
+  ]
+endif
+
 foreach symbol : symbols
   if cc.has_header_symbol(symbol[0], symbol[1], args: '-D_GNU_SOURCE', prefix: symbol.get(2, ''))
     conf.set('WITH_DECL_@0@'.format(symbol[1].to_upper()), 1)
@@ -2011,6 +2018,9 @@ endif
 
 conf.set_quoted('TLS_PRIORITY', get_option('tls_priority'))
 
+if conf.has('WITH_DECL_SYS_PIDFD_OPEN')
+  conf.set('WITH_NBDKIT', 1)
+endif
 
 # Various definitions
 
@@ -2268,6 +2278,7 @@ misc_summary = {
   'firewalld-zone': conf.has('WITH_FIREWALLD_ZONE'),
   'nss': conf.has('WITH_NSS'),
   'numad': conf.has('WITH_NUMAD'),
+  'nbdkit': conf.has('WITH_NBDKIT'),
   'Init script': init_script,
   'Char device locks': chrdev_lock_files,
   'Loader/NVRAM': loader_res,

src/qemu/qemu_domain.c

@@ -11499,6 +11499,7 @@ qemuProcessEventFree(struct qemuProcessEvent *event)
     case QEMU_PROCESS_EVENT_PR_DISCONNECT:
     case QEMU_PROCESS_EVENT_UNATTENDED_MIGRATION:
     case QEMU_PROCESS_EVENT_RESET:
+    case QEMU_PROCESS_EVENT_NBDKIT_EXITED:
     case QEMU_PROCESS_EVENT_LAST:
         break;
     }

src/qemu/qemu_domain.h

@@ -465,6 +465,7 @@ typedef enum {
     QEMU_PROCESS_EVENT_MEMORY_DEVICE_SIZE_CHANGE,
     QEMU_PROCESS_EVENT_UNATTENDED_MIGRATION,
     QEMU_PROCESS_EVENT_RESET,
+    QEMU_PROCESS_EVENT_NBDKIT_EXITED,
 
     QEMU_PROCESS_EVENT_LAST
 } qemuProcessEventType;

src/qemu/qemu_driver.c

@@ -4033,6 +4033,20 @@ processResetEvent(virQEMUDriver *driver,
 }
 
 
+static void
+processNbdkitExitedEvent(virDomainObj *vm,
+                         qemuNbdkitProcess *nbdkit)
+{
+    if (virDomainObjBeginJob(vm, VIR_JOB_MODIFY) < 0)
+        return;
+
+    if (qemuNbdkitProcessRestart(nbdkit, vm) < 0)
+        virDomainObjTaint(vm, VIR_DOMAIN_TAINT_NBDKIT_RESTART);
+
+    virDomainObjEndJob(vm);
+}
+
+
 static void qemuProcessEventHandler(void *data, void *opaque)
 {
     struct qemuProcessEvent *processEvent = data;
@@ -4090,6 +4104,9 @@ static void qemuProcessEventHandler(void *data, void *opaque)
     case QEMU_PROCESS_EVENT_RESET:
         processResetEvent(driver, vm);
         break;
+    case QEMU_PROCESS_EVENT_NBDKIT_EXITED:
+        processNbdkitExitedEvent(vm, processEvent->data);
+        break;
     case QEMU_PROCESS_EVENT_LAST:
         break;
     }

src/qemu/qemu_nbdkit.c

@@ -19,6 +19,7 @@
 
 #include <config.h>
 #include <glib.h>
+#include <sys/syscall.h>
 
 #include "vircommand.h"
 #include "virerror.h"
@@ -33,6 +34,7 @@
 #include "qemu_nbdkit.h"
 #define LIBVIRT_QEMU_NBDKITPRIV_H_ALLOW
 #include "qemu_nbdkitpriv.h"
+#include "qemu_process.h"
 #include "qemu_security.h"
 
 #include <fcntl.h>
@@ -41,6 +43,12 @@
 
 VIR_LOG_INIT("qemu.nbdkit");
 
+#if WITH_NBDKIT
+# define WITHOUT_NBDKIT_UNUSED
+#else
+# define WITHOUT_NBDKIT_UNUSED G_GNUC_UNUSED
+#endif
+
 VIR_ENUM_IMPL(qemuNbdkitCaps,
     QEMU_NBDKIT_CAPS_LAST,
     /* 0 */
@@ -611,6 +619,113 @@ qemuNbdkitCapsCacheNew(const char *cachedir)
 }
 
 
+int
+qemuNbdkitProcessRestart(qemuNbdkitProcess *proc,
+                         virDomainObj *vm)
+{
+    qemuDomainObjPrivate *vmpriv = vm->privateData;
+    virQEMUDriver *driver = vmpriv->driver;
+
+    /* clean up resources associated with process */
+    qemuNbdkitProcessStop(proc);
+
+    return qemuNbdkitProcessStart(proc, vm, driver);
+}
+
+
+#if WITH_NBDKIT
+typedef struct {
+    qemuNbdkitProcess *proc;
+    virDomainObj *vm;
+} qemuNbdkitProcessEventData;
+
+
+static qemuNbdkitProcessEventData*
+qemuNbdkitProcessEventDataNew(qemuNbdkitProcess *proc,
+                              virDomainObj *vm)
+{
+    qemuNbdkitProcessEventData *d = g_new(qemuNbdkitProcessEventData, 1);
+    d->proc = proc;
+    d->vm = virObjectRef(vm);
+    return d;
+}
+
+
+static void
+qemuNbdkitProcessEventDataFree(qemuNbdkitProcessEventData *d)
+{
+    virObjectUnref(d->vm);
+    g_free(d);
+}
+
+
+static void
+qemuNbdkitProcessPidfdCb(int watch G_GNUC_UNUSED,
+                         int fd,
+                         int events G_GNUC_UNUSED,
+                         void *opaque)
+{
+    qemuNbdkitProcessEventData *d = opaque;
+
+    VIR_FORCE_CLOSE(fd);
+    /* submit an event so that it is handled in the per-vm event thread */
+    qemuProcessHandleNbdkitExit(d->proc, d->vm);
+}
+#endif /* WITH_NBDKIT */
+
+
+static int
+qemuNbdkitProcessStartMonitor(qemuNbdkitProcess *proc WITHOUT_NBDKIT_UNUSED,
+                              virDomainObj *vm WITHOUT_NBDKIT_UNUSED)
+{
+#if WITH_NBDKIT
+    int pidfd;
+    qemuNbdkitProcessEventData *data;
+
+    pidfd = syscall(SYS_pidfd_open, proc->pid, 0);
+    if (pidfd < 0) {
+        virReportSystemError(errno, _("pidfd_open failed for %1$i"), proc->pid);
+        return -1;
+    }
+
+    data = qemuNbdkitProcessEventDataNew(proc, vm);
+    if ((proc->eventwatch = virEventAddHandle(pidfd,
+                                              VIR_EVENT_HANDLE_READABLE,
+                                              qemuNbdkitProcessPidfdCb,
+                                              data,
+                                              (virFreeCallback)qemuNbdkitProcessEventDataFree)) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("failed to monitor nbdkit process %1$i"),
+                       proc->pid);
+        VIR_FORCE_CLOSE(pidfd);
+        qemuNbdkitProcessEventDataFree(data);
+        return -1;
+    }
+
+    VIR_DEBUG("Monitoring nbdkit process %i for exit", proc->pid);
+
+    return 0;
+#else
+    /* This should not be reachable */
+    virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                   _("nbdkit support is not enabled"));
+    return -1;
+#endif /* WITH_NBDKIT */
+}
+
+
+static void
+qemuNbdkitProcessStopMonitor(qemuNbdkitProcess *proc WITHOUT_NBDKIT_UNUSED)
+{
+#if WITH_NBDKIT
+    if (proc->eventwatch > 0) {
+        virEventRemoveHandle(proc->eventwatch);
+        proc->eventwatch = 0;
+    }
+#endif /* WITH_NBDKIT */
+}
+
+
 static qemuNbdkitProcess *
 qemuNbdkitProcessNew(virStorageSource *source,
                      const char *pidfile,
@@ -657,29 +772,40 @@ qemuNbdkitReconnectStorageSource(virStorageSource *source,
 }
 
 
-static void
+static int
-qemuNbdkitStorageSourceManageProcessOne(virStorageSource *source)
+qemuNbdkitStorageSourceManageProcessOne(virStorageSource *source,
+                                        virDomainObj *vm)
 {
     qemuDomainStorageSourcePrivate *srcpriv = QEMU_DOMAIN_STORAGE_SOURCE_PRIVATE(source);
+    qemuDomainObjPrivate *vmpriv = vm->privateData;
     qemuNbdkitProcess *proc;
 
     if (!srcpriv)
-        return;
+        return 0;
 
     proc = srcpriv->nbdkitProcess;
 
     if (!proc)
-        return;
+        return 0;
+
+    if (!proc->caps)
+        proc->caps = qemuGetNbdkitCaps(vmpriv->driver);
 
     if (proc->pid <= 0) {
         if (virPidFileReadPath(proc->pidfile, &proc->pid) < 0) {
-            VIR_WARN("Unable to read pidfile '%s'", proc->pidfile);
+            virReportError(VIR_ERR_INTERNAL_ERROR,
-            return;
+                           _("Unable to read pidfile '%1$s'"),
+                           proc->pidfile);
+            return -1;
         }
     }
 
-    if (virProcessKill(proc->pid, 0) < 0)
+    if (virProcessKill(proc->pid, 0) < 0) {
-        VIR_WARN("nbdkit process %i is not alive", proc->pid);
+        VIR_DEBUG("nbdkit process %i is not alive", proc->pid);
+        return qemuNbdkitProcessRestart(proc, vm);
+    }
+
+    return qemuNbdkitProcessStartMonitor(proc, vm);
 }
 
 /**
@@ -691,23 +817,32 @@ qemuNbdkitStorageSourceManageProcessOne(virStorageSource *source)
  * @source. It is intended to be called after libvirt restarts and has loaded its current state from
  * disk and is attempting to re-connect to active domains.
  */
-void
+int
-qemuNbdkitStorageSourceManageProcess(virStorageSource *source)
+qemuNbdkitStorageSourceManageProcess(virStorageSource *source,
+                                     virDomainObj *vm)
 {
     virStorageSource *backing;
     for (backing = source; backing != NULL; backing = backing->backingStore)
-        qemuNbdkitStorageSourceManageProcessOne(backing);
+        if (qemuNbdkitStorageSourceManageProcessOne(backing, vm) < 0)
+            return -1;
+
+    return 0;
 }
 
 
 bool
-qemuNbdkitInitStorageSource(qemuNbdkitCaps *caps,
+qemuNbdkitInitStorageSource(qemuNbdkitCaps *caps WITHOUT_NBDKIT_UNUSED,
-                            virStorageSource *source,
+                            virStorageSource *source WITHOUT_NBDKIT_UNUSED,
-                            char *statedir,
+                            char *statedir WITHOUT_NBDKIT_UNUSED,
-                            const char *alias,
+                            const char *alias WITHOUT_NBDKIT_UNUSED,
-                            uid_t user,
+                            uid_t user WITHOUT_NBDKIT_UNUSED,
-                            gid_t group)
+                            gid_t group WITHOUT_NBDKIT_UNUSED)
 {
+#if !WITH_NBDKIT
+    /* if nbdkit support is not enabled, don't construct the object so the
+     * calling function will fall back to qemu storage drivers */
+    return false;
+#else
     qemuDomainStorageSourcePrivate *srcPriv = qemuDomainStorageSourcePrivateFetch(source);
     g_autofree char *pidname = g_strdup_printf("nbdkit-%s.pid", alias);
     g_autofree char *socketname = g_strdup_printf("nbdkit-%s.socket", alias);
@@ -751,6 +886,7 @@ qemuNbdkitInitStorageSource(qemuNbdkitCaps *caps,
     srcPriv->nbdkitProcess = proc;
 
     return true;
+#endif /* WITH_NBDKIT */
 }
 
 
@@ -968,6 +1104,8 @@ qemuNbdkitProcessBuildCommand(qemuNbdkitProcess *proc)
 void
 qemuNbdkitProcessFree(qemuNbdkitProcess *proc)
 {
+    qemuNbdkitProcessStopMonitor(proc);
+
     g_clear_pointer(&proc->pidfile, g_free);
     g_clear_pointer(&proc->socketfile, g_free);
     g_clear_object(&proc->caps);
@@ -1037,8 +1175,11 @@ qemuNbdkitProcessStart(qemuNbdkitProcess *proc,
         goto error;
 
     while (virTimeBackOffWait(&timebackoff)) {
-        if (virFileExists(proc->socketfile))
+        if (virFileExists(proc->socketfile)) {
+            if (qemuNbdkitProcessStartMonitor(proc, vm) < 0)
+                goto error;
             return 0;
+        }
 
         if (virProcessKill(proc->pid, 0) == 0)
             continue;
@@ -1069,6 +1210,8 @@ qemuNbdkitProcessStart(qemuNbdkitProcess *proc,
 int
 qemuNbdkitProcessStop(qemuNbdkitProcess *proc)
 {
+    qemuNbdkitProcessStopMonitor(proc);
+
     if (proc->pid < 0)
         return 0;
 

src/qemu/qemu_nbdkit.h

@@ -68,8 +68,9 @@ qemuNbdkitStartStorageSource(virQEMUDriver *driver,
 void
 qemuNbdkitStopStorageSource(virStorageSource *src);
 
-void
+int
-qemuNbdkitStorageSourceManageProcess(virStorageSource *src);
+qemuNbdkitStorageSourceManageProcess(virStorageSource *src,
+                                     virDomainObj *vm);
 
 bool
 qemuNbdkitCapsGet(qemuNbdkitCaps *nbdkitCaps,
@@ -91,6 +92,7 @@ struct _qemuNbdkitProcess {
     uid_t user;
     gid_t group;
     pid_t pid;
+    int eventwatch;
 };
 
 int
@@ -98,6 +100,10 @@ qemuNbdkitProcessStart(qemuNbdkitProcess *proc,
                        virDomainObj *vm,
                        virQEMUDriver *driver);
 
+int
+qemuNbdkitProcessRestart(qemuNbdkitProcess *proc,
+                         virDomainObj *vm);
+
 int
 qemuNbdkitProcessStop(qemuNbdkitProcess *proc);
 

src/qemu/qemu_process.c

@@ -9055,10 +9055,12 @@ qemuProcessReconnect(void *opaque)
     }
 
     for (i = 0; i < obj->def->ndisks; i++)
-        qemuNbdkitStorageSourceManageProcess(obj->def->disks[i]->src);
+        if (qemuNbdkitStorageSourceManageProcess(obj->def->disks[i]->src, obj) < 0)
+            goto error;
 
     if (obj->def->os.loader && obj->def->os.loader->nvram)
-        qemuNbdkitStorageSourceManageProcess(obj->def->os.loader->nvram);
+        if (qemuNbdkitStorageSourceManageProcess(obj->def->os.loader->nvram, obj) < 0)
+            goto error;
 
     /* update domain state XML with possibly updated state in virDomainObj */
     if (virDomainObjSave(obj, driver->xmlopt, cfg->stateDir) < 0)
@@ -9512,3 +9514,14 @@ qemuProcessQMPStart(qemuProcessQMP *proc)
 
     return 0;
 }
+
+
+void
+qemuProcessHandleNbdkitExit(qemuNbdkitProcess *nbdkit,
+                            virDomainObj *vm)
+{
+    virObjectLock(vm);
+    VIR_DEBUG("nbdkit process %i died", nbdkit->pid);
+    qemuProcessEventSubmit(vm, QEMU_PROCESS_EVENT_NBDKIT_EXITED, 0, 0, nbdkit);
+    virObjectUnlock(vm);
+}

src/qemu/qemu_process.h

@@ -237,3 +237,6 @@ void qemuProcessRefreshDiskProps(virDomainDiskDef *disk,
                                  struct qemuDomainDiskInfo *info);
 
 int qemuProcessSetupEmulator(virDomainObj *vm);
+
+void qemuProcessHandleNbdkitExit(qemuNbdkitProcess *nbdkit,
+                                 virDomainObj *vm);

tests/meson.build

@@ -456,9 +456,13 @@ if conf.has('WITH_QEMU')
     { 'name': 'qemuvhostusertest', 'link_with': [ test_qemu_driver_lib ], 'link_whole': [ test_file_wrapper_lib ] },
     { 'name': 'qemuxml2argvtest', 'link_with': [ test_qemu_driver_lib, test_utils_qemu_monitor_lib ], 'link_whole': [ test_utils_qemu_lib, test_file_wrapper_lib ] },
     { 'name': 'qemuxml2xmltest', 'link_with': [ test_qemu_driver_lib ], 'link_whole': [ test_utils_qemu_lib, test_file_wrapper_lib ] },
+  ]
+  if conf.has('WITH_NBDKIT')
+    tests += [
       { 'name': 'qemunbdkittest', 'link_with': [ test_qemu_driver_lib ], 'link_whole': [ test_utils_qemu_lib, test_file_wrapper_lib ] },
     ]
   endif
+endif
 
 if conf.has('WITH_REMOTE')
   tests += [

tests/qemuxml2argvtest.c

@@ -778,8 +778,12 @@ mymain(void)
 # define DO_TEST_CAPS_ARCH_VER(name, arch, ver) \
     DO_TEST_CAPS_ARCH_VER_FULL(name, arch, ver, ARG_END)
 
+# if WITH_NBDKIT
 #  define DO_TEST_CAPS_LATEST_NBDKIT(name, ...) \
     DO_TEST_CAPS_ARCH_LATEST_FULL(name, "x86_64", ARG_NBDKIT_CAPS, __VA_ARGS__, QEMU_NBDKIT_CAPS_LAST, ARG_END)
+# else
+#  define DO_TEST_CAPS_LATEST_NBDKIT(name, ...)
+# endif /* WITH_NBDKIT */
 
 # define DO_TEST_CAPS_LATEST(name) \
     DO_TEST_CAPS_ARCH_LATEST(name, "x86_64")