Cancellable reads; EOF on "read" sets var to ""

- Make reading cancellable. Not all input from stdin, just that done
  directly by the shell (i.e. the "read" builtin). Exec'ed programs
  still read directly from stdin's os.File and are not cancellable.
- If you press ^D (EOF) when reading into a shell variable, set the
  variable to "". This is consistent with bash & zsh.

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/go-quicktest/qt v1.101.0
 	github.com/google/go-cmp v0.6.0
 	github.com/google/renameio/v2 v2.0.0
+	github.com/muesli/cancelreader v0.2.2
 	github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e
 	github.com/rogpeppe/go-internal v1.12.0
 	golang.org/x/sync v0.6.0

go.sum

@@ -11,6 +11,8 @@ github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/muesli/cancelreader v0.2.2 h1:3I4Kt4BQjOR54NavqnDogx/MIoWBFa0StPA8ELUXHmA=
+github.com/muesli/cancelreader v0.2.2/go.mod h1:3XuTXfFS2VjM+HTLZY9Ak0l6eUKfijIfMUZ4EgX0QYo=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e h1:aoZm08cpOy4WuID//EZDgcC4zIxODThtZNPirFr42+A=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=

interp/builtin.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/muesli/cancelreader"
 	"mvdan.cc/sh/v3/expand"
 	"mvdan.cc/sh/v3/syntax"
 )
@@ -589,10 +590,7 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 			r.out(prompt)
 		}
 
-		line, err := r.readLine(raw)
-		if err != nil {
-			return 1
-		}
+		line, err := r.readLine(ctx, raw)
 		if len(args) == 0 {
 			args = append(args, shellReplyVar)
 		}
@@ -606,6 +604,10 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 			r.setVarString(name, val)
 		}
 
+		if err != nil {
+			return 1
+		}
+
 		return 0
 
 	case "getopts":
@@ -917,17 +919,36 @@ func (r *Runner) printOptLine(name string, enabled, supported bool) {
 	r.outf("%s\t%s\t(%q not supported)\n", name, state, r.optStatusText(!enabled))
 }
 
-func (r *Runner) readLine(raw bool) ([]byte, error) {
+func (r *Runner) readLine(ctx context.Context, raw bool) ([]byte, error) {
 	if r.stdin == nil {
 		return nil, errors.New("interp: can't read, there's no stdin")
 	}
 
 	var line []byte
 	esc := false
 
+	stdin := r.stdin
+	if osFile, ok := stdin.(*os.File); ok {
+		cr, err := cancelreader.NewReader(osFile)
+		if err != nil {
+			return nil, err
+		}
+		stdin = cr
+		done := make(chan bool)
+		go func() {
+			select {
+			case <-ctx.Done():
+				cr.Cancel()
+			case <-done:
+			}
+			cr.Close()
+		}()
+		defer close(done)
+	}
+
 	for {
 		var buf [1]byte
-		n, err := r.stdin.Read(buf[:])
+		n, err := stdin.Read(buf[:])
 		if n > 0 {
 			b := buf[0]
 			switch {

interp/runner.go

@@ -515,7 +515,7 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 					}
 					r.errf("%s", ps3)
 
-					line, err := r.readLine(true)
+					line, err := r.readLine(ctx, true)
 					if err != nil {
 						r.exit = 1
 						return nil