Force Attach() to send a SIGWINCH and redraw

Basically, we want to force the application in the container to
(iff the container was made with a terminal) redraw said terminal
immediately after an attach completes, so the fresh Attach
session will be able to see what's going on (e.g. will have a
shell prompt). Our current attach functions are unfortunately
geared more towards `podman run` than `podman attach` and will
start forwarding resize events *immediately* instead of waiting
until the attach session is alive (much safer for short-lived
`podman run` sessions, but broken for the `podman attach` case).
To avoid a major rewrite, let's just manually send a SIGWINCH
after attach succeeds to force a redraw.

Fixes #6253

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

3 files changed, 23 insertions, 4 deletions

diff --git a/libpod/container_api.go b/libpod/container_api.go
index 0d7bbacd0..aef37dd59 100644
--- a/libpod/container_api.go
+++ b/libpod/container_api.go
@@ -9,6 +9,7 @@ import (
 
 	"github.com/containers/podman/v2/libpod/define"
 	"github.com/containers/podman/v2/libpod/events"
+	"github.com/containers/podman/v2/pkg/signal"
 	"github.com/opentracing/opentracing-go"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -129,7 +130,7 @@ func (c *Container) StartAndAttach(ctx context.Context, streams *define.AttachSt
 
 	// Attach to the container before starting it
 	go func() {
-		if err := c.attach(streams, keys, resize, true, startedChan); err != nil {
+		if err := c.attach(streams, keys, resize, true, startedChan, nil); err != nil {
 			attachChan <- err
 		}
 		close(attachChan)
@@ -243,8 +244,23 @@ func (c *Container) Attach(streams *define.AttachStreams, keys string, resize <-
 		return errors.Wrapf(define.ErrCtrStateInvalid, "can only attach to created or running containers")
 	}
 
+	// HACK: This is really gross, but there isn't a better way without
+	// splitting attach into separate versions for StartAndAttach and normal
+	// attaching, and I really do not want to do that right now.
+	// Send a SIGWINCH after attach succeeds so that most programs will
+	// redraw the screen for the new attach session.
+	attachRdy := make(chan bool)
+	if c.config.Spec.Process != nil && c.config.Spec.Process.Terminal {
+		go func() {
+			<-attachRdy
+			if err := c.ociRuntime.KillContainer(c, uint(signal.SIGWINCH), false); err != nil {
+				logrus.Warnf("Unable to send SIGWINCH to container %s after attach: %v", c.ID(), err)
+			}
+		}()
+	}
+
 	c.newContainerEvent(events.Attach)
-	return c.attach(streams, keys, resize, false, nil)
+	return c.attach(streams, keys, resize, false, nil, attachRdy)
 }
 
 // HTTPAttach forwards an attach session over a hijacked HTTP session.
diff --git a/libpod/oci_attach_linux.go b/libpod/oci_attach_linux.go
index 622c613d9..74af449ed 100644
--- a/libpod/oci_attach_linux.go
+++ b/libpod/oci_attach_linux.go
@@ -31,7 +31,7 @@ const (
 // Attach to the given container
 // Does not check if state is appropriate
 // started is only required if startContainer is true
-func (c *Container) attach(streams *define.AttachStreams, keys string, resize <-chan remotecommand.TerminalSize, startContainer bool, started chan bool) error {
+func (c *Container) attach(streams *define.AttachStreams, keys string, resize <-chan remotecommand.TerminalSize, startContainer bool, started chan bool, attachRdy chan<- bool) error {
 	if !streams.AttachOutput && !streams.AttachError && !streams.AttachInput {
 		return errors.Wrapf(define.ErrInvalidArg, "must provide at least one stream to attach to")
 	}
@@ -74,6 +74,9 @@ func (c *Container) attach(streams *define.AttachStreams, keys string, resize <-
 	}
 
 	receiveStdoutError, stdinDone := setupStdioChannels(streams, conn, detachKeys)
+	if attachRdy != nil {
+		attachRdy <- true
+	}
 	return readStdio(streams, receiveStdoutError, stdinDone)
 }
 
diff --git a/libpod/oci_attach_unsupported.go b/libpod/oci_attach_unsupported.go
index cd7c674b2..317dfdc90 100644
--- a/libpod/oci_attach_unsupported.go
+++ b/libpod/oci_attach_unsupported.go
@@ -9,7 +9,7 @@ import (
 	"k8s.io/client-go/tools/remotecommand"
 )
 
-func (c *Container) attach(streams *define.AttachStreams, keys string, resize <-chan remotecommand.TerminalSize, startContainer bool, started chan bool) error {
+func (c *Container) attach(streams *define.AttachStreams, keys string, resize <-chan remotecommand.TerminalSize, startContainer bool, started chan bool, attachRdy chan<- bool) error {
 	return define.ErrNotImplemented
 }