enable lockfile by default

README.md

@@ -186,16 +186,18 @@ It is expected that the CO will connect to the plugin through the unix socket an
 
 ### Locking
 
-If the `-lockfile` parameter is specified or the `CSILVM_LOCKFILE_PATH` environment variable is set,
-an exclusive `flock()` is held on that file whenever any lvm2 command-line utility is invoked. This prevents
-multiple CSILVM processes from performing concurrent lvm operations that can
-lead to deadlocks in the underlying lvm2 implementation. For example,
+Any command-line invocations executed by the `csilvm` process first acquires a
+lock on a lockfile. The path to the lockfile can be overridden using the
+`-lockfile=` option or the `CSILVM_LOCKFILE_PATH` environment variable. The
+locking behaviour can be disabled by setting the `-lockfile=` option to the
+empty string. The purpose of locking around command-line invocations is to
+prevent multiple `csilvm` processes from executing concurrent `lvm2` commands.
+This works around deadlocks in lvm2. For example,
 https://jira.mesosphere.com/browse/DCOS_OSS-5434 and
 https://github.com/lvmteam/lvm2/issues/23.
 
-By default no lock file is specified and no inter-process synchronization is
-performed. You are encouraged to enable locking by setting the `-lockfile=`
-option, for example `-lockfile=/run/csilvm.lock`.
+By default the lock file is created at `/run/csilvm.lock` so it is assumed that
+the `/run` directory exists and is writable by the `csilvm` process.
 
 
 ### Logging

cmd/csilvm/csilvm.go

@@ -59,7 +59,7 @@ func main() {
 	var probeModulesF stringsFlag
 	flag.Var(&probeModulesF, "probe-module", "Probe checks that the kernel module is loaded")
 	nodeIDF := flag.String("node-id", "", "The node ID reported via the CSI Node gRPC service")
-	lockFilePathF := flag.String("lockfile", "", "The path to the lock file used to prevent concurrent lvm invocation by multiple csilvm instances")
+	lockFilePathF := flag.String("lockfile", "/run/csilvm.lock", "The path to the lock file used to prevent concurrent lvm invocation by multiple csilvm instances")
 	// Metrics-related flags
 	statsdUDPHostEnvVarF := flag.String("statsd-udp-host-env-var", "", "The name of the environment variable containing the host where a statsd service is listening for stats over UDP")
 	statsdUDPPortEnvVarF := flag.String("statsd-udp-port-env-var", "", "The name of the environment variable containing the port where a statsd service is listening for stats over UDP")
@@ -95,7 +95,7 @@ func main() {
 	// Unlink the domain socket in case it is left lying around from a
 	// previous run. err return is not really interesting because it is
 	// normal for this to fail if the process is starting for the first time.
-	logger.Printf("Unlinking %s", sock)
+	logger.Printf("Unlinking socket file: %q", sock)
 	syscall.Unlink(sock)
 	// Setup socket listener
 	lis, err := net.Listen("unix", sock)

pkg/lvm/lock.go

@@ -1,6 +1,8 @@
 package lvm
 
 import (
+	"fmt"
+
 	"github.com/gofrs/flock"
 )
 
@@ -13,5 +15,16 @@ var lvmlock *flock.Flock
 // - https://jira.mesosphere.com/browse/DCOS_OSS-5434
 // - https://github.com/lvmteam/lvm2/issues/23
 func SetLockFilePath(filepath string) {
+	log.Printf("using lock file at %q", filepath)
 	lvmlock = flock.New(filepath)
+	log.Printf("checking if lock can be acquired")
+	err := lvmlock.Lock()
+	if err != nil {
+		panic(fmt.Sprintf("cannot acquire lock: %v", err))
+	}
+	log.Printf("lock acquired, releasing lock")
+	if err := lvmlock.Unlock(); err != nil {
+		panic(fmt.Sprintf("cannot release lock: %v", err))
+	}
+	log.Printf("configured lock file")
 }