update design doc and move tests

Signed-off-by: ZhouJianMS <zhoujian@microsoft.com>

README.md

@@ -86,6 +86,12 @@ List the failpoints,
 $ curl http://127.0.0.1:1234/SomeFuncString=return("hello")
 ```
 
+Retrieve the execution count of a failpoint,
+
+```sh
+$curl http://127.0.0.1:1234/SomeFuncString/count -XGET
+```
+
 Deactivate a failpoint,
 
 ```sh

doc/design.md

@@ -69,6 +69,16 @@ Similarly, you can set multiple failpoints using endpoint `/failpoints`,
 curl http://127.0.0.1:22381/failpoints -X PUT -d'failpoint1=return("hello");failpoint2=sleep(10)'
 ```
 
+You can get the execution count of the failpoint in the dynamic way,
+```
+$curl http://127.0.0.1:1234/SomeFuncString/count -XGET
+```
+
+To deactivate a failpoint,
+```
+$ curl http://127.0.0.1:1234/SomeFuncString -XDELETE
+```
+
 #### 3.2 Unit test
 Assuming there is a function with a failpoint something like below,
 ```

runtime/terms.go

@@ -105,7 +105,7 @@ func (t *terms) eval() interface{} {
 	for _, term := range t.chain {
 		if term.mods.allow() {
 			t.counter++
-			return term.do(), nil
+			return term.do()
 		}
 	}
 	return nil

runtime/terms_test.go

@@ -71,3 +71,45 @@ func TestTermsTypes(t *testing.T) {
 		}
 	}
 }
+
+func TestTermsCounter(t *testing.T) {
+	tests := []struct {
+		failpointTerm    string
+		runAfterEnabling int
+		wantCount        int
+	}{
+		{
+			failpointTerm:    `10*sleep(10)->1*return("abc")`,
+			runAfterEnabling: 12,
+			// This example tests mods which allows users to restrict the
+			// number of failpoint actions as against their callsite executions.
+			// This is the reason why wantCount < runAfterEnabling
+			// In a real world example you can hit a code spot a million times but
+			// using mods restrict the associated fallpoint actions to run twice.
+			wantCount: 11,
+		},
+		{
+			failpointTerm:    `10*sleep(10)->1*return("abc")`,
+			runAfterEnabling: 3,
+			wantCount:        3,
+		},
+		{
+			failpointTerm:    `10*sleep(10)->1*return("abc")`,
+			runAfterEnabling: 0,
+			wantCount:        0,
+		},
+	}
+	for _, tt := range tests {
+		ter, err := newTerms("test", tt.failpointTerm)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for i := 0; i < tt.runAfterEnabling; i++ {
+			_ = ter.eval()
+		}
+
+		if ter.counter != tt.wantCount {
+			t.Errorf("counter is not properly incremented, got: %d, want: %d", ter.counter, tt.wantCount)
+		}
+	}
+}