good good study, day day up!

vendor/gopl.io/ch5/wait/wait.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 130.
+
+// The wait program waits for an HTTP server to start responding.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"time"
+)
+
+//!+
+// WaitForServer attempts to contact the server of a URL.
+// It tries for one minute using exponential back-off.
+// It reports an error if all attempts fail.
+func WaitForServer(url string) error {
+	const timeout = 1 * time.Minute
+	deadline := time.Now().Add(timeout)
+	for tries := 0; time.Now().Before(deadline); tries++ {
+		_, err := http.Head(url)
+		if err == nil {
+			return nil // success
+		}
+		log.Printf("server not responding (%s); retrying...", err)
+		time.Sleep(time.Second << uint(tries)) // exponential back-off
+	}
+	return fmt.Errorf("server %s failed to respond after %s", url, timeout)
+}
+
+//!-
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "usage: wait url\n")
+		os.Exit(1)
+	}
+	url := os.Args[1]
+	//!+main
+	// (In function main.)
+	if err := WaitForServer(url); err != nil {
+		fmt.Fprintf(os.Stderr, "Site is down: %v\n", err)
+		os.Exit(1)
+	}
+	//!-main
+}