Auto-skip: improved shell expression parsing (#3514)

A minor improvement over the previous string contents check.

inputgraph/loader.go

@@ -1,6 +1,7 @@
 package inputgraph
 
 import (
+	"bufio"
 	"context"
 	"fmt"
 	"os"
@@ -145,7 +146,28 @@ func (l *loader) handleCopy(ctx context.Context, cmd spec.Command) error {
 }
 
 func containsShellExpr(s string) bool {
-	return strings.Contains(s, "$(") && strings.Contains(s, ")")
+	var (
+		last    string
+		depth   int
+		hasExpr bool
+	)
+	scan := bufio.NewScanner(strings.NewReader(s))
+	scan.Split(bufio.ScanRunes)
+	for scan.Scan() {
+		c := scan.Text()
+		switch {
+		case c == "(" && last == "$":
+			hasExpr = true
+			depth++
+		case c == ")":
+			depth--
+		}
+		if depth < 0 {
+			return false
+		}
+		last = c
+	}
+	return depth == 0 && hasExpr
 }
 
 func (l *loader) handleCopySrc(ctx context.Context, src string, mustExist bool) error {

inputgraph/loader_test.go

@@ -0,0 +1,43 @@
+package inputgraph
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_containsShellExpr(t *testing.T) {
+	cases := []struct {
+		desc string
+		val  string
+		want bool
+	}{
+		{
+			desc: "single",
+			val:  "$(echo 'hello')",
+			want: true,
+		},
+		{
+			desc: "nested",
+			val:  "$(echo -n $(cat /tmp/x))",
+			want: true,
+		},
+		{
+			desc: "invalid 1",
+			val:  "$($()",
+			want: false,
+		},
+		{
+			desc: "invalid 2",
+			val:  ")$(",
+			want: false,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.desc, func(t *testing.T) {
+			got := containsShellExpr(c.val)
+			require.Equal(t, c.want, got)
+		})
+	}
+}