[device-auth] When headless environment is detected, show device login flow instead. (#8756)

When headless environment is detected, show device login flow instead.

codex-rs/core/src/env.rs

@@ -1,5 +1,9 @@
 //! Functions for environment detection that need to be shared across crates.
 
+fn env_var_set(key: &str) -> bool {
+    std::env::var(key).is_ok_and(|v| !v.trim().is_empty())
+}
+
 /// Returns true if the current process is running under Windows Subsystem for Linux.
 pub fn is_wsl() -> bool {
     #[cfg(target_os = "linux")]
@@ -17,3 +21,26 @@ pub fn is_wsl() -> bool {
         false
     }
 }
+
+/// Returns true when Codex is likely running in an environment without a usable GUI.
+///
+/// This is intentionally conservative and is used by frontends to avoid flows that would try to
+/// open a browser (e.g. device-code auth fallback).
+pub fn is_headless_environment() -> bool {
+    if env_var_set("CI")
+        || env_var_set("SSH_CONNECTION")
+        || env_var_set("SSH_CLIENT")
+        || env_var_set("SSH_TTY")
+    {
+        return true;
+    }
+
+    #[cfg(target_os = "linux")]
+    {
+        if !env_var_set("DISPLAY") && !env_var_set("WAYLAND_DISPLAY") {
+            return true;
+        }
+    }
+
+    false
+}