testscript: provide way for Setup to pass values to commands

Currently, if Setup sets up something, there's no way for it
to pass values through to custom commands other than
strings (with Setenv). To allow this, we add a map of
arbitrary values to Env that can be accessed by custom commands.
Like context.Context, the keys are `interface{}` values to
allow external helper libraries to add their own keys without
fear of clashing with others.

Author: rogpeppe

testscript/testdata/exec_path_change.txt

@@ -15,6 +15,9 @@ exec go$exe build
 exec go$exe version
 stdout 'This is not go'
 
+-- go/go.mod --
+module example.com/go
+
 -- go/main.go --
 package main
 

testscript/testdata/values.txt

@@ -0,0 +1 @@
+test-values

testscript/testscript.go

@@ -39,13 +39,28 @@ var testWork = flag.Bool("testwork", false, "")
 
 // Env holds the environment to use at the start of a test script invocation.
 type Env struct {
+	// WorkDir holds the path to the root directory of the
+	// extracted files.
 	WorkDir string
-	Vars    []string
-	Cd      string
+	// Vars holds the initial set environment variables that will be passed to the
+	// testscript commands.
+	Vars []string
+	// Cd holds the initial current working directory.
+	Cd string
+	// Values holds a map of arbitrary values for use by custom
+	// testscript commands. This enables Setup to pass arbitrary
+	// values (not just strings) through to custom commands.
+	Values map[interface{}]interface{}
 
 	ts *TestScript
 }
 
+// Value returns a value from Env.Values, or nil if no
+// value was set by Setup.
+func (ts *TestScript) Value(key interface{}) interface{} {
+	return ts.values[key]
+}
+
 // Defer arranges for f to be called at the end
 // of the test. If Defer is called multiple times, the
 // defers are executed in reverse order (similar
@@ -171,23 +186,24 @@ type TestScript struct {
 	params      Params
 	t           T
 	testTempDir string
-	workdir     string            // temporary work dir ($WORK)
-	log         bytes.Buffer      // test execution log (printed at end of test)
-	mark        int               // offset of next log truncation
-	cd          string            // current directory during test execution; initially $WORK/gopath/src
-	name        string            // short name of test ("foo")
-	file        string            // full file name ("testdata/script/foo.txt")
-	lineno      int               // line number currently executing
-	line        string            // line currently executing
-	env         []string          // environment list (for os/exec)
-	envMap      map[string]string // environment mapping (matches env; on Windows keys are lowercase)
-	stdin       string            // standard input to next 'go' command; set by 'stdin' command.
-	stdout      string            // standard output from last 'go' command; for 'stdout' command
-	stderr      string            // standard error from last 'go' command; for 'stderr' command
-	stopped     bool              // test wants to stop early
-	start       time.Time         // time phase started
-	background  []backgroundCmd   // backgrounded 'exec' and 'go' commands
-	deferred    func()            // deferred cleanup actions.
+	workdir     string                      // temporary work dir ($WORK)
+	log         bytes.Buffer                // test execution log (printed at end of test)
+	mark        int                         // offset of next log truncation
+	cd          string                      // current directory during test execution; initially $WORK/gopath/src
+	name        string                      // short name of test ("foo")
+	file        string                      // full file name ("testdata/script/foo.txt")
+	lineno      int                         // line number currently executing
+	line        string                      // line currently executing
+	env         []string                    // environment list (for os/exec)
+	envMap      map[string]string           // environment mapping (matches env; on Windows keys are lowercase)
+	values      map[interface{}]interface{} // values for custom commands
+	stdin       string                      // standard input to next 'go' command; set by 'stdin' command.
+	stdout      string                      // standard output from last 'go' command; for 'stdout' command
+	stderr      string                      // standard error from last 'go' command; for 'stderr' command
+	stopped     bool                        // test wants to stop early
+	start       time.Time                   // time phase started
+	background  []backgroundCmd             // backgrounded 'exec' and 'go' commands
+	deferred    func()                      // deferred cleanup actions.
 
 	ctxt context.Context // per TestScript context
 }
@@ -213,6 +229,7 @@ func (ts *TestScript) setup() string {
 			":=" + string(os.PathListSeparator),
 		},
 		WorkDir: ts.workdir,
+		Values:  make(map[interface{}]interface{}),
 		Cd:      ts.workdir,
 		ts:      ts,
 	}
@@ -242,6 +259,7 @@ func (ts *TestScript) setup() string {
 	}
 	ts.cd = env.Cd
 	ts.env = env.Vars
+	ts.values = env.Values
 
 	ts.envMap = make(map[string]string)
 	for _, kv := range ts.env {

testscript/testscript_test.go

@@ -93,6 +93,11 @@ func TestScripts(t *testing.T) {
 					ts.Fatalf("setup did not see expected files; got %q want %q", setupFilenames, args)
 				}
 			},
+			"test-values": func(ts *TestScript, neg bool, args []string) {
+				if ts.Value("somekey") != 1234 {
+					ts.Fatalf("test-values did not see expected value")
+				}
+			},
 		},
 		Setup: func(env *Env) error {
 			infos, err := ioutil.ReadDir(env.WorkDir)
@@ -103,6 +108,7 @@ func TestScripts(t *testing.T) {
 			for _, info := range infos {
 				setupFilenames = append(setupFilenames, info.Name())
 			}
+			env.Values["somekey"] = 1234
 			return nil
 		},
 	})