ssh/tailssh: add Plan 9 support for Tailscale SSH

Updates #5794

Change-Id: I7b05cd29ec02085cb503bbcd0beb61bf455002ac
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/osuser/group_ids.go

@@ -19,6 +19,10 @@ import (
 // an error. It will first try to use the 'id' command to get the group IDs,
 // and if that fails, it will fall back to the user.GroupIds method.
 func GetGroupIds(user *user.User) ([]string, error) {
+	if runtime.GOOS == "plan9" {
+		return nil, nil
+	}
+
 	if runtime.GOOS != "linux" {
 		return user.GroupIds()
 	}

util/osuser/user.go

@@ -54,9 +54,18 @@ func lookup(usernameOrUID string, std lookupStd, wantShell bool) (*user.User, st
 	// Skip getent entirely on Non-Unix platforms that won't ever have it.
 	// (Using HasPrefix for "wasip1", anticipating that WASI support will
 	// move beyond "preview 1" some day.)
-	if runtime.GOOS == "windows" || runtime.GOOS == "js" || runtime.GOARCH == "wasm" {
+	if runtime.GOOS == "windows" || runtime.GOOS == "js" || runtime.GOARCH == "wasm" || runtime.GOOS == "plan9" {
+		var shell string
+		if wantShell && runtime.GOOS == "plan9" {
+			shell = "/bin/rc"
+		}
+		if runtime.GOOS == "plan9" {
+			if u, err := user.Current(); err == nil {
+				return u, shell, nil
+			}
+		}
 		u, err := std(usernameOrUID)
-		return u, "", err
+		return u, shell, err
 	}
 
 	// No getent on Gokrazy. So hard-code the login shell.
@@ -78,6 +87,16 @@ func lookup(usernameOrUID string, std lookupStd, wantShell bool) (*user.User, st
 		return u, shell, nil
 	}
 
+	if runtime.GOOS == "plan9" {
+		return &user.User{
+			Uid:      "0",
+			Gid:      "0",
+			Username: "glenda",
+			Name:     "Glenda",
+			HomeDir:  "/",
+		}, "/bin/rc", nil
+	}
+
 	// Start with getent if caller wants to get the user shell.
 	if wantShell {
 		return userLookupGetent(usernameOrUID, std)