builder: enable the user to set event handlers

Event handlers which have been set in the `aci-manifest.yml` will be written
to the final `manifest` file.

If a "pre-start" handler has been set, then it will not be overwritten by
dgr's default, "/dgr/bin/prestart".

Iff "/dgr/bin" has been excluded from the image, especially using the prefix
"/dgr", then dgr's defaults won't be used at all.
(Such an image would not start anyway, leaving the user confused about the
cause, whether it's app.exec or any pre-start handler.)

dgr/common/aci-manifest.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/appc/spec/aci"
@@ -110,9 +111,24 @@ func WriteAciManifest(m *AciManifest, targetFile string, projectName string, dgr
 	im.Name = *name
 	im.Labels = labels
 
+	for _, exclusion := range m.Build.Exclude {
+		if strings.HasPrefix("/dgr/bin", exclusion) {
+			goto collectionIsComplete
+		}
+	}
 	if len(m.Aci.App.Exec) == 0 {
 		m.Aci.App.Exec = []string{"/dgr/bin/busybox", "sh"}
 	}
+	// Set a pre-start handler, to run dgr's scripts.
+	for i := range m.Aci.App.EventHandlers {
+		if m.Aci.App.EventHandlers[i].Name == "pre-start" {
+			goto collectionIsComplete
+		}
+	}
+	m.Aci.App.EventHandlers = append(m.Aci.App.EventHandlers,
+		types.EventHandler{Name: "pre-start", Exec: []string{"/dgr/bin/prestart"}})
+
+collectionIsComplete:
 
 	isolators, err := ToAppcIsolators(m.Aci.App.Isolators)
 	if err != nil {
@@ -121,7 +137,7 @@ func WriteAciManifest(m *AciManifest, targetFile string, projectName string, dgr
 
 	im.App = &types.App{
 		Exec:              m.Aci.App.Exec,
-		EventHandlers:     []types.EventHandler{{Name: "pre-start", Exec: []string{"/dgr/bin/prestart"}}},
+		EventHandlers:     m.Aci.App.EventHandlers,
 		User:              m.Aci.App.User,
 		Group:             m.Aci.App.Group,
 		WorkingDirectory:  m.Aci.App.WorkingDirectory,

dgr/common/dgr-manifest.go

@@ -78,15 +78,16 @@ type AciDefinition struct {
 }
 
 type DgrApp struct {
-	Exec              types.Exec         `json:"exec,omitempty" yaml:"exec,omitempty"`
-	User              string             `json:"user,omitempty" yaml:"user,omitempty"`
-	Group             string             `json:"group,omitempty" yaml:"group,omitempty"`
-	SupplementaryGIDs []int              `json:"supplementaryGIDs,omitempty" yaml:"supplementaryGIDs,omitempty"`
-	WorkingDirectory  string             `json:"workingDirectory,omitempty" yaml:"workingDirectory,omitempty"`
-	Environment       types.Environment  `json:"environment,omitempty" yaml:"environment,omitempty"`
-	MountPoints       []types.MountPoint `json:"mountPoints,omitempty" yaml:"mountPoints,omitempty"`
-	Ports             []types.Port       `json:"ports,omitempty" yaml:"ports,omitempty"`
-	Isolators         []Isolator         `json:"isolators,omitempty" yaml:"isolators,omitempty"`
+	Exec              types.Exec           `json:"exec,omitempty" yaml:"exec,omitempty"`
+	User              string               `json:"user,omitempty" yaml:"user,omitempty"`
+	Group             string               `json:"group,omitempty" yaml:"group,omitempty"`
+	SupplementaryGIDs []int                `json:"supplementaryGIDs,omitempty" yaml:"supplementaryGIDs,omitempty"`
+	WorkingDirectory  string               `json:"workingDirectory,omitempty" yaml:"workingDirectory,omitempty"`
+	Environment       types.Environment    `json:"environment,omitempty" yaml:"environment,omitempty"`
+	EventHandlers     []types.EventHandler `json:"eventHandlers,omitempty" yaml:"eventHandlers,omitempty"`
+	MountPoints       []types.MountPoint   `json:"mountPoints,omitempty" yaml:"mountPoints,omitempty"`
+	Ports             []types.Port         `json:"ports,omitempty" yaml:"ports,omitempty"`
+	Isolators         types.Isolators      `json:"isolators,omitempty" yaml:"isolators,omitempty"`
 }
 
 type LinuxCapabilitiesSetValue struct {