Add "exec" trigger. (#367)

I'm not 100% sure this is a good idea, as it could easily lead to
performance issues, but in some situations I think it could be quite
useful.

Author: alecthomas

app/exec_cmd.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"fmt"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -65,6 +66,16 @@ func (e *execCmd) Run(l *ui.UI, cache *cache.Cache, sta *state.State, env *hermi
 	if err := pkg.EnsureSupported(); err != nil {
 		return errors.Wrapf(err, "execution failed")
 	}
+
+	// Run any pre-execution triggers.
+	messages, err := env.TriggerForPackage(l, manifest.EventExec, pkg)
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	w := l.WriterAt(ui.LevelInfo)
+	for _, message := range messages {
+		fmt.Fprintln(w, message)
+	}
 	installed, err := env.ListInstalledReferences()
 	if err != nil {
 		return errors.WithStack(err)

docs/docs/packaging/reference.md

@@ -133,6 +133,8 @@ be applied when certain events occur in the package lifecycle. Supported events
 |---------------|-------------|
 | `unpack`      | Triggered when a package is unpacked into the Hermit cache. |
 | `install`     | Triggered when a package is installed into an environment. |
+| `uninstall`   | Triggered when a package is uninstalled from an environment. |
 | `activate`    | Triggered when the environment the package is installed in is activated. |
+| `exec`        | Triggered whenever a binary in the package is executed. <br>**NOTE:** This trigger will run for _every_ execution and can negatively impact performance. |
 
 More triggers may be added in the future.

integration/integration_test.go

@@ -252,6 +252,7 @@ func TestIntegration(t *testing.T) {
 			hermit install testbin1
 			testbin1
 			testbin2
+			assert test -d exec-hook-triggered
 			`,
 			expectations: exp{outputContains("testbin1 1.0.1")},
 		},

integration/testdata/testenv3/packages/testbin1.hcl

@@ -8,4 +8,9 @@ on unpack {
   symlink { from = "${root}/testbin1" to = "${root}/dir/testbin2" }
 }
 
+on exec {
+  // Messages can't be sent to the parent process, so we create a directory to indicate the hook worked instead.
+  mkdir { dir = "${env}/exec-hook-triggered" }
+}
+
 version "1.0.0" "1.0.1" {}

manifest/triggers.go

@@ -25,6 +25,7 @@ const (
 	EventUnpack    Event = "unpack"
 	EventInstall   Event = "install"
 	EventUninstall Event = "uninstall"
+	EventExec      Event = "exec" // Triggered when a binary in a package is executed.
 	// Environment specific events
 	EventEnvActivate Event = "activate"
 )
@@ -35,6 +36,7 @@ var eventMap = map[Event]bool{
 	EventInstall:     true,
 	EventUninstall:   true,
 	EventEnvActivate: true,
+	EventExec:        true,
 }
 
 // A Trigger applied when a lifecycle event occurs.