test(tail): add scenarios for edge cases flagged in PR #26 review

builtins/tail/builtin_tail_pentest_test.go

@@ -117,21 +117,21 @@ func TestCmdPentestTailLinesNNegativeOne(t *testing.T) {
 }
 
 func TestCmdPentestTailLinesNMinInt64(t *testing.T) {
-	// -(-9223372036854775808) overflows back to itself; must be rejected cleanly.
+	// -(-9223372036854775808) overflows; clamped to MaxCount, so the file is printed (GNU tail behaviour).
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(filepath.Join(dir, "f.txt"), []byte("hello\n"), 0644))
-	_, stderr, code := tailRun(t, "tail -n -9223372036854775808 f.txt", dir)
-	assert.Equal(t, 1, code)
-	assert.Contains(t, stderr, "tail:")
+	stdout, _, code := tailRun(t, "tail -n -9223372036854775808 f.txt", dir)
+	assert.Equal(t, 0, code)
+	assert.Equal(t, "hello\n", stdout)
 }
 
 func TestCmdPentestTailBytesNMinInt64(t *testing.T) {
-	// Same overflow guard for -c.
+	// Same overflow clamp for -c.
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(filepath.Join(dir, "f.txt"), []byte("hello\n"), 0644))
-	_, stderr, code := tailRun(t, "tail -c -9223372036854775808 f.txt", dir)
-	assert.Equal(t, 1, code)
-	assert.Contains(t, stderr, "tail:")
+	stdout, _, code := tailRun(t, "tail -c -9223372036854775808 f.txt", dir)
+	assert.Equal(t, 0, code)
+	assert.Equal(t, "hello\n", stdout)
 }
 
 func TestCmdPentestTailLinesNPlusZero(t *testing.T) {

builtins/tail/tail.go

@@ -528,12 +528,14 @@ func parseCount(s string) (countMode, bool) {
 		return countMode{}, false
 	}
 	// GNU tail silently treats negative counts as their absolute value.
-	// Guard against MinInt64: -(-9223372036854775808) overflows back to itself.
+	// Guard against MinInt64 overflow: -(-9223372036854775808) overflows back
+	// to itself. Clamp to MaxCount (like any other out-of-range value) so that
+	// behaviour matches GNU tail, which accepts this value and prints the file.
 	if n < 0 {
 		n = -n
 		if n < 0 {
-			// Negation overflowed (was MinInt64); treat as invalid.
-			return countMode{}, false
+			// Negation overflowed (was MinInt64); clamp to MaxCount.
+			n = MaxCount
 		}
 	}
 	if n > MaxCount {

interp/api.go

@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"runtime/debug"
 
 	"mvdan.cc/sh/v3/expand"
 	"mvdan.cc/sh/v3/syntax"
@@ -340,16 +339,13 @@ func (s ExitStatus) Error() string { return fmt.Sprintf("exit status %d", s) }
 func (r *Runner) Run(ctx context.Context, node syntax.Node) (retErr error) {
 	defer func() {
 		if rec := recover(); rec != nil {
-			var panicOut io.Writer
-			if r != nil {
+			panicOut := io.Writer(io.Discard)
+			if r != nil && r.stderr != nil {
 				panicOut = r.stderr
 			}
-			if panicOut == nil {
-				panicOut = os.Stderr
-			}
 			func() {
 				defer func() { recover() }()
-				fmt.Fprintf(panicOut, "rshell: internal panic: %v\n%s\n", rec, debug.Stack())
+				fmt.Fprintf(panicOut, "rshell: internal panic: %v\n", rec)
 			}()
 			retErr = fmt.Errorf("internal error")
 		}

interp/runner_exec.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"io/fs"
 	"os"
-	"runtime/debug"
 	"sync"
 	"time"
 
@@ -140,13 +139,13 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 			go func() {
 				defer func() {
 					if rec := recover(); rec != nil {
-						panicOut := rLeft.stderr
-						if panicOut == nil {
-							panicOut = os.Stderr
+						panicOut := io.Writer(io.Discard)
+						if rLeft.stderr != nil {
+							panicOut = rLeft.stderr
 						}
 						func() {
 							defer func() { recover() }()
-							fmt.Fprintf(panicOut, "rshell: internal panic: %v\n%s\n", rec, debug.Stack())
+							fmt.Fprintf(panicOut, "rshell: internal panic: %v\n", rec)
 						}()
 						rLeft.exit.fatal(fmt.Errorf("internal error"))
 					}

tests/scenarios/cmd/tail/errors/malformed_offset_plus_minus.yaml

@@ -0,0 +1,13 @@
+description: tail rejects malformed offset forms like +-3 and ++5, matching GNU tail behaviour.
+setup:
+  files:
+    - path: file.txt
+      content: "hello\n"
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    tail -n +-3 file.txt
+expect:
+  stdout: ""
+  stderr_contains: ["tail:"]
+  exit_code: 1

tests/scenarios/cmd/tail/errors/malformed_offset_plus_plus.yaml

@@ -0,0 +1,13 @@
+description: tail rejects ++N offset form as an invalid number, matching GNU tail behaviour.
+setup:
+  files:
+    - path: file.txt
+      content: "hello\n"
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    tail -c ++5 file.txt
+expect:
+  stdout: ""
+  stderr_contains: ["tail:"]
+  exit_code: 1

tests/scenarios/cmd/tail/errors/minint64_overflow.yaml

@@ -0,0 +1,13 @@
+description: tail -n -9223372036854775808 is treated as a very large count (clamped), matching GNU tail which prints the file rather than erroring.
+setup:
+  files:
+    - path: file.txt
+      content: "hello\n"
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    tail -n -9223372036854775808 file.txt
+expect:
+  stdout: "hello\n"
+  stderr: ""
+  exit_code: 0