cli/streams: Out, In: preserve original os.File when available

Preserve the original *os.File, if available, and add a File() method
to return it.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cli/streams/in.go

@@ -3,6 +3,7 @@ package streams
 import (
 	"errors"
 	"io"
+	"os"
 
 	"github.com/moby/term"
 )
@@ -14,7 +15,8 @@ type In struct {
 	cs commonStream
 }
 
-// NewIn returns a new [In] from an [io.ReadCloser].
+// NewIn returns a new [In] from an [io.ReadCloser]. If in is an [*os.File],
+// a reference is kept to the file, and accessible through [In.File].
 func NewIn(in io.ReadCloser) *In {
 	return &In{
 		in: in,
@@ -27,6 +29,13 @@ func (i *In) FD() uintptr {
 	return i.cs.fd
 }
 
+// File returns the underlying *os.File if the stream was constructed from one.
+// If the stream was created from a non-file (e.g., a pipe, buffer, or wrapper),
+// the returned boolean will be false.
+func (i *In) File() (*os.File, bool) {
+	return i.cs.file()
+}
+
 // Read implements the [io.Reader] interface.
 func (i *In) Read(p []byte) (int, error) {
 	return i.in.Read(p)

cli/streams/out.go

@@ -2,6 +2,7 @@ package streams
 
 import (
 	"io"
+	"os"
 
 	"github.com/moby/term"
 )
@@ -14,7 +15,8 @@ type Out struct {
 	cs  commonStream
 }
 
-// NewOut returns a new [Out] from an [io.Writer].
+// NewOut returns a new [Out] from an [io.Writer]. If out is an [*os.File],
+// a reference is kept to the file, and accessible through [Out.File].
 func NewOut(out io.Writer) *Out {
 	return &Out{
 		out: out,
@@ -27,6 +29,13 @@ func (o *Out) FD() uintptr {
 	return o.cs.FD()
 }
 
+// File returns the underlying *os.File if the stream was constructed from one.
+// If the stream was created from a non-file (e.g., a pipe, buffer, or wrapper),
+// the returned boolean will be false.
+func (o *Out) File() (*os.File, bool) {
+	return o.cs.file()
+}
+
 // Write writes to the output stream.
 func (o *Out) Write(p []byte) (int, error) {
 	return o.out.Write(p)

cli/streams/stream.go

@@ -1,3 +1,6 @@
+// FIXME(thaJeztah): remove once we are a module; the go:build directive prevents go from downgrading language version to go1.16:
+//go:build go1.25
+
 package streams
 
 import (
@@ -8,14 +11,21 @@ import (
 )
 
 func newCommonStream(stream any) commonStream {
+	var f *os.File
+	if v, ok := stream.(*os.File); ok {
+		f = v
+	}
+
 	fd, tty := term.GetFdInfo(stream)
 	return commonStream{
+		f:   f,
 		fd:  fd,
 		tty: tty,
 	}
 }
 
 type commonStream struct {
+	f     *os.File
 	fd    uintptr
 	tty   bool
 	state *term.State
@@ -24,6 +34,9 @@ type commonStream struct {
 // FD returns the file descriptor number for this stream.
 func (s *commonStream) FD() uintptr { return s.fd }
 
+// file returns the underlying *os.File if the stream was constructed from one.
+func (s *commonStream) file() (*os.File, bool) { return s.f, s.f != nil }
+
 // isTerminal returns whether this stream is connected to a terminal.
 func (s *commonStream) isTerminal() bool { return s.tty }