Add debugger nodes and select chapter

debugger/README.md

@@ -2,6 +2,8 @@
 
 Go doesn't ship with a debugger but you can use the delve package instead.
 
+https://www.youtube.com/watch?v=UA0SirX6Siw
+
 ```
 go get github.com/go-delve/delve/cmd/dlv
 ```
@@ -295,3 +297,5 @@ Technically you can edit code with `edit`.
 
 
 ## Debugging goroutines
+
+

learn_go_with_tests/concurrency/CheckWebsites.go

@@ -25,6 +25,8 @@ func CheckWebsites(wc WebsiteChecker, urls []string) map[string]bool {
 		}(url) // or just pass by value to give the func it's own url
 	}
 
+	// Improvement idea. We could use a wait group here. Instead we are blocking
+	// each go routine because we are using an unbuffered channel.
 	for i := 0; i < len(urls); i++ {
 		r := <-resultChannel // receive statement
 		results[r.string] = r.bool

learn_go_with_tests/my_select/racer.go

@@ -1,23 +1,66 @@
 package my_select
 
 import (
+	"fmt"
 	"net/http"
 	"time"
 )
 
-func Racer(a string, b string) (winner string) {
-	aDuration := measureResponseTime(a)
-	bDuration := measureResponseTime(b)
+var defaultTimeout time.Duration = 10 * time.Second
 
-	if aDuration < bDuration {
-		return a
+func Racer(a, b string) (winner string, err error) {
+	return ConfigurableRacer(a, b, defaultTimeout)
 }
 
-	return b
+func ConfigurableRacer(a string, b string, timeout time.Duration) (winner string, err error) {
+	// aDuration := measureResponseTime(a)
+	// bDuration := measureResponseTime(b)
+
+	// if aDuration < bDuration {
+	// 	return a
+	// }
+
+	// return b
+
+	select {
+	// Here we are simply waiting on the channel closing out
+	// The first one that does so will return it's url
+
+	// These are blocking operations because they are unbuffered channels.
+	// This works though because `select` allows us to wait on multiple channels
+	case <-ping(a):
+		return a, nil
+	case <-ping(b):
+		return b, nil
+	// Super handy function during select, returns a channel, waits the timeout, then
+	//sends the current time that it was triggered at.
+	// Helps get us out of a blocking case.
+	case <-time.After(timeout):
+		return "", fmt.Errorf("timed out waiting for %s and %s", a, b)
+	}
 }
 
-func measureResponseTime(url string) time.Duration {
-	start := time.Now()
+// In our case, we don't care what type is sent to
+//the channel, we just want to signal we are done
+// and closing the channel works perfectly!
+
+// a chan struct{} is the smallest data type available
+// from a memory perspective
+
+func ping(url string) chan struct{} {
+	// 	Notice how we have to use make when creating a channel; rather than say var ch chan struct{}. When you use var the variable will be initialised with the "zero" value of the type. So for string it is "", int it is 0, etc.
+
+	// For channels the zero value is nil and if you try and send to it with <- it will block forever because you cannot send to nil channels
+	ch := make(chan struct{})
+	go func() {
 		http.Get(url)
-	return time.Since(start)
+		close(ch)
+	}()
+	return ch
 }
+
+// func measureResponseTime(url string) time.Duration {
+// 	start := time.Now()
+// 	http.Get(url)
+// 	return time.Since(start)
+// }

learn_go_with_tests/my_select/racer_test.go

@@ -9,6 +9,7 @@ import (
 
 func TestRacer(t *testing.T) {
 
+	t.Run("compares speeds of servers, returning url of the fastest one", func(t *testing.T) {
 		slowServer := makeDelayedServer(20 * time.Microsecond)
 		defer slowServer.Close()
 
@@ -19,11 +20,27 @@ func TestRacer(t *testing.T) {
 		fastUrl := fastServer.URL
 
 		want := fastUrl
-	got := Racer(slowUrl, fastUrl)
+		got, _ := Racer(slowUrl, fastUrl)
 
 		if got != want {
 			t.Errorf("got %q, want %q", got, want)
 		}
+	})
+
+	t.Run("returns an error if server doesn't respond within 10s", func(t *testing.T) {
+		serverA := makeDelayedServer(50 * time.Millisecond)
+		serverB := makeDelayedServer(60 * time.Millisecond)
+
+		defer serverA.Close()
+		defer serverB.Close()
+
+		_, err := ConfigurableRacer(serverA.URL, serverB.URL, 10*time.Millisecond)
+
+		if err == nil {
+			t.Error("expected an error but didn't get one")
+		}
+
+	})
 }
 
 func makeDelayedServer(delay time.Duration) *httptest.Server {