address comments from code review

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 IMPROVEMENTS:
 * cli: Added `-monitor` flag to `deployment status` command and automatically monitor deployments from `job run` command. [[GH-10661](https://github.com/hashicorp/nomad/pull/10661)]
-* docker: Tasks using `network.mode = "bridge"` that don't set their `network_mode` will receive a `/etc/hosts` file that includes the pause container's hostname. [[GH-10766](https://github.com/hashicorp/nomad/issues/10766)]
+* docker: Tasks using `network.mode = "bridge"` that don't set their `network_mode` will receive a `/etc/hosts` file that includes the pause container's hostname and any `extra_hosts`. [[GH-10766](https://github.com/hashicorp/nomad/issues/10766)]
 
 BUG FIXES:
 * quotas (Enterprise): Fixed a bug where quotas were evaluated before constraints, resulting in quota capacity being used up by filtered nodes. [[GH-10753](https://github.com/hashicorp/nomad/issues/10753)]

client/allocrunner/network_hook.go

@@ -9,6 +9,9 @@ import (
 	"github.com/hashicorp/nomad/plugins/drivers"
 )
 
+// We create a pause container to own the network namespace, and the
+// NetworkIsolationSpec we get back from CreateNetwork has this label set as
+// the container ID. We'll use this to generate a hostname for the task.
 const dockerNetSpecLabelKey = "docker_sandbox_container_id"
 
 type networkIsolationSetter interface {
@@ -110,6 +113,9 @@ func (h *networkHook) Prerun() error {
 		}
 		if hostname, ok := spec.Labels[dockerNetSpecLabelKey]; ok {
 			if len(hostname) > 12 {
+				// the docker_sandbox_container_id is the full ID of the pause
+				// container, whereas we want the shortened name that dockerd
+				// sets as the pause container's hostname
 				hostname = hostname[:12]
 			}
 			h.spec.HostsConfig = &drivers.HostsConfig{

drivers/shared/hostnames/mount.go

@@ -24,7 +24,8 @@ func GenerateEtcHostsMount(taskDir string, conf *drivers.NetworkIsolationSpec, e
 		return nil, nil
 	}
 
-	content := fmt.Sprintf(`# this file was generated by Nomad
+	var content strings.Builder
+	fmt.Fprintf(&content, `# this file was generated by Nomad
 127.0.0.1 localhost
 ::1 localhost
 ::1 ip6-localhost ip6-loopback
@@ -40,7 +41,7 @@ ff02::3 ip6-allhosts
 `, hostsCfg.Address, hostsCfg.Hostname)
 
 	if len(extraHosts) > 0 {
-		content += "\n# these entries are extra hosts added by the task config"
+		content.WriteString("\n# these entries are extra hosts added by the task config")
 		for _, hostLine := range extraHosts {
 			hostsEntry := strings.SplitN(hostLine, ":", 2)
 			if len(hostsEntry) != 2 {
@@ -49,13 +50,13 @@ ff02::3 ip6-allhosts
 			if net.ParseIP(hostsEntry[1]) == nil {
 				return nil, fmt.Errorf("invalid IP address %q", hostLine)
 			}
-			content += fmt.Sprintf("\n%s %s", hostsEntry[1], hostsEntry[0])
+			content.WriteString(fmt.Sprintf("\n%s %s", hostsEntry[1], hostsEntry[0]))
 		}
-		content += "\n"
+		content.WriteString("\n")
 	}
 
 	path := filepath.Join(taskDir, "hosts")
-	err := ioutil.WriteFile(path, []byte(content), 0755)
+	err := ioutil.WriteFile(path, []byte(content.String()), 0755)
 	if err != nil {
 		return nil, err
 	}