Fix execution of failpoint should not block deactivation

There are 2 main flows of the gofail library: namely enable/disable and
execution (`Acquire`) of the failpoints.

Currently, a mutex is protecting both flows, thus only one action can
make progress at a time.

This PR proposes a fine-grained mutex, as each failpoint is protected under
a dedicated `RWMutex`.

The existing `failpointsMu` will only be protecting the main shared
data structures, such as `failpoints` map.

Notice that in our current implementation, the execution of the same
failpoint is still sequential (there is a lock within `eval` on the
term being executed)

Reference:
- #64

Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>

runtime/failpoint.go

@@ -16,10 +16,13 @@ package runtime
 
 import (
 	"fmt"
+	"sync"
 )
 
 type Failpoint struct {
 	t *terms
+
+	failpointMu sync.RWMutex
 }
 
 func NewFailpoint(name string) *Failpoint {
@@ -28,14 +31,19 @@ func NewFailpoint(name string) *Failpoint {
 
 // Acquire gets evalutes the failpoint terms; if the failpoint
 // is active, it will return a value. Otherwise, returns a non-nil error.
+//
+// Notice that during the exection of Acquire(), the failpoint can be disabled,
+// but the already in-flight execution won't be terminated
 func (fp *Failpoint) Acquire() (interface{}, error) {
-	failpointsMu.RLock()
-	defer failpointsMu.RUnlock()
+	fp.failpointMu.RLock()
+	// terms are locked during execution, so deepcopy is not required as no change can be made during execution
+	cachedT := fp.t
+	fp.failpointMu.RUnlock()
 
-	if fp.t == nil {
+	if cachedT == nil {
 		return nil, ErrDisabled
 	}
-	result := fp.t.eval()
+	result := cachedT.eval()
 	if result == nil {
 		return nil, ErrDisabled
 	}

runtime/runtime.go

@@ -86,6 +86,10 @@ func enable(name, inTerms string) error {
 		fmt.Printf("failed to enable \"%s=%s\" (%v)\n", name, inTerms, err)
 		return err
 	}
+
+	fp.failpointMu.Lock()
+	defer fp.failpointMu.Unlock()
+
 	fp.t = t
 
 	return nil
@@ -104,6 +108,9 @@ func disable(name string) error {
 		return ErrNoExist
 	}
 
+	fp.failpointMu.Lock()
+	defer fp.failpointMu.Unlock()
+
 	if fp.t == nil {
 		return ErrDisabled
 	}
@@ -116,6 +123,7 @@ func disable(name string) error {
 func Status(failpath string) (string, int, error) {
 	failpointsMu.Lock()
 	defer failpointsMu.Unlock()
+
 	return status(failpath)
 }
 
@@ -125,6 +133,9 @@ func status(failpath string) (string, int, error) {
 		return "", 0, ErrNoExist
 	}
 
+	fp.failpointMu.RLock()
+	defer fp.failpointMu.RUnlock()
+
 	t := fp.t
 	if t == nil {
 		return "", 0, ErrDisabled