[POA-1340] Add a mechanism to randomly _not_ start apidump. (#17)

Controlled by a flag and a new environment variable.

The use case here is that we want to run the agent but some, but not
all, of the pods in a critical deployment. This is not a great solution,
but it's much easier than the alternatives considered. The deployment
can include a probability as an environment variable that any given
apidump command will run; those that don't run will just hang forever to
avoid any problems caused by container exit.

One downside is that changing the environment variable causes all the
pods to be restarted.

Author: Mark Gritter

cmd/internal/apidump/apidump.go

@@ -1,6 +1,9 @@
 package apidump
 
 import (
+	"math/rand"
+	"os"
+	"strconv"
 	"time"
 
 	"github.com/akitasoftware/akita-libs/akid"
@@ -44,15 +47,56 @@ var (
 	maxWitnessSize_bytes    int
 	dockerExtensionMode     bool
 	healthCheckPort         int
+	randomizedStart         int
 )
 
+// This function will either startup apidump normally, or never return, with probability
+// determined by randomizedStart/100.  The value of the command-line flag may be
+// overridden by an environment variable to make it easier to apply.
+// This function should be called as early as possible in case termination of
+// the agent causes deployment problems.
+//
+// Negative values are effectively treated as 0 probability, instead of being validated.
+func applyRandomizedStart() {
+	prob := randomizedStart
+
+	if env := os.Getenv("POSTMAN_AGENT_RANDOM_START"); env != "" {
+		override, err := strconv.Atoi(env)
+		if err == nil {
+			prob = override
+		}
+	}
+
+	if prob < 100 {
+		printer.Stdout.Infof("Starting trace with probability %d%%.\n", prob)
+
+		// Pre-1.20, Go does not seed the default Random object :(
+		rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+		r := rng.Intn(100) // in range [0,100),
+		// so 1% probability means < 1, not <= 1:
+		if r < prob {
+			return
+		}
+
+		printer.Stdout.Infof("This agent instance will not begin capturing data.\n")
+
+		// Wait forever
+		select {}
+
+		// Unreachable
+		os.Exit(0)
+	}
+}
+
 var Cmd = &cobra.Command{
 	Use:          "apidump",
 	Short:        "Capture requests/responses from network traffic.",
 	Long:         "Capture and store a sequence of requests/responses to a service by observing network traffic.",
 	SilenceUsage: true,
 	Args:         cobra.ExactArgs(0),
 	RunE: func(cmd *cobra.Command, _ []string) error {
+		applyRandomizedStart()
+
 		traceTags, err := util.ParseTagsAndWarn(tagsFlag)
 		if err != nil {
 			return err
@@ -344,4 +388,12 @@ func init() {
 		"Port to listen on for Docker extension health checks. This is an internal flag used by the Akita Docker extension.",
 	)
 	_ = Cmd.Flags().MarkHidden("health-check-port")
+
+	Cmd.Flags().IntVar(
+		&randomizedStart,
+		"randomized-start",
+		100,
+		"Probability that the apidump command will start intercepting traffic.",
+	)
+	_ = Cmd.Flags().MarkHidden("randomized-start")
 }