fix: Use smarter quoting for ProxyCommand in config-ssh (#3755)

* fix: Use smarter quoting for ProxyCommand in config-ssh

This change takes better into account how OpenSSH executes
`ProxyCommand`s and applies quoting accordingly.

This supercedes #3664, which was reverted.

Fixes #2853

* fix: Ensure `~/.ssh` directory exists

cli/configssh_internal_test.go

@@ -0,0 +1,62 @@
+package cli
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// This test tries to mimic the behavior of OpenSSH
+// when executing e.g. a ProxyCommand.
+func Test_sshConfigExecEscape(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		path    string
+		wantErr bool
+		windows bool
+	}{
+		{"no spaces", "simple", false, true},
+		{"spaces", "path with spaces", false, true},
+		{"quotes", "path with \"quotes\"", false, false},
+		{"backslashes", "path with \\backslashes", false, false},
+		{"tabs", "path with \ttabs", false, false},
+		{"newline fails", "path with \nnewline", true, false},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			if runtime.GOOS == "windows" {
+				t.Skip("Windows doesn't typically execute via /bin/sh or cmd.exe, so this test is not applicable.")
+			}
+
+			dir := filepath.Join(t.TempDir(), tt.path)
+			err := os.MkdirAll(dir, 0o755)
+			require.NoError(t, err)
+			bin := filepath.Join(dir, "coder")
+			contents := []byte("#!/bin/sh\necho yay\n")
+			err = os.WriteFile(bin, contents, 0o755) //nolint:gosec
+			require.NoError(t, err)
+
+			escaped, err := sshConfigExecEscape(bin)
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+
+			b, err := exec.Command("/bin/sh", "-c", escaped).CombinedOutput() //nolint:gosec
+			require.NoError(t, err)
+			got := strings.TrimSpace(string(b))
+			require.Equal(t, "yay", got)
+		})
+	}
+}