ssh install ends on a signal while sftp's ssh is still connecting (closes #57)

A signal during ssh install now sends sftp a SIGTERM instead of killing it, as ssh to already does for ssh, so sftp stops the ssh it started. The sftp command also has a WaitDelay of a quarter second, so a child that still holds sftp's output cannot keep the tool waiting on a host that does not answer: it ends with status 1 and removes its working directory. The test's sftp stand-in now leaves such a child.

Judgement call: exec.ErrWaitDelay counts as success, so a run that used ControlPersist does not report a failure for a key it added.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/cli/ssh/install.go

@@ -11,6 +11,8 @@ import (
 	"path/filepath"
 	"slices"
 	"strings"
+	"syscall"
+	"time"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/cli/signals"
@@ -34,6 +36,13 @@ const (
 	localMode     = 0o600
 )
 
+// waitDelay is the WaitDelay sftp runs with: from a signal, or from sftp
+// ending, how long the tool waits for sftp to end and its output to
+// close before it kills sftp and stops reading. That is ample for sftp
+// to stop the ssh it started, and short enough that a signal still ends
+// the tool within a second.
+const waitDelay = 250 * time.Millisecond
+
 // ErrCannotEnter is the refusal of a host whose .ssh is there but
 // cannot be entered, so that nothing in it can be read or written.
 var ErrCannotEnter = errors.New(
@@ -187,8 +196,24 @@ func session(
 	command.Stdout = &said
 	command.Stderr = &said
 
+	// A cancelled context means a signal arrived. Send sftp a SIGTERM
+	// rather than the default kill, so it stops the ssh it started
+	// before it goes. Anything sftp started that still holds its output
+	// keeps the tool waiting no longer than waitDelay.
+	command.Cancel = func() error {
+		return command.Process.Signal(syscall.SIGTERM)
+	}
+	command.WaitDelay = waitDelay
+
 	err := command.Run()
 
+	// sftp ended well and only something it started, such as the
+	// master ssh leaves running for ControlPersist under -v, still held
+	// its output: the session worked.
+	if errors.Is(err, exec.ErrWaitDelay) {
+		err = nil
+	}
+
 	_, _ = cmd.ErrOrStderr().Write(said.Bytes())
 
 	if err != nil {