Avoid materializing writable-root carveouts for presence checks (#39812)

## What changed

- Add `FileSystemSandboxPolicy::has_writable_roots_with_cwd` to detect an
  effective writable root without constructing its read-only carveouts.
- Use the helper when classifying permission profiles and selecting patch
  rejection reasons.

## Testing

- Verify the presence check matches the existing materialized-root result
  across read-only, unrestricted, external, writable, denied, and unresolved
  special-path policies.

GitOrigin-RevId: 17ee490aa3180e05732133203f68544d9de5268b

codex-rs/core/src/safety.rs

@@ -102,11 +102,9 @@ fn patch_rejection_reason(
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
     cwd: &PathUri,
 ) -> &'static str {
-    let has_no_writable_roots = cwd.to_abs_path().is_ok_and(|cwd| {
-        file_system_sandbox_policy
-            .get_writable_roots_with_cwd(cwd.as_path())
-            .is_empty()
-    });
+    let has_no_writable_roots = cwd
+        .to_abs_path()
+        .is_ok_and(|cwd| !file_system_sandbox_policy.has_writable_roots_with_cwd(cwd.as_path()));
     match permission_profile {
         PermissionProfile::Managed { .. }
             if !file_system_sandbox_policy.has_full_disk_write_access()

codex-rs/core/src/sandbox_tags.rs

@@ -48,10 +48,7 @@ pub(crate) fn permission_profile_policy_tag(
             let file_system_policy = profile.file_system_sandbox_policy();
             if file_system_policy.has_full_disk_write_access() {
                 "danger-full-access"
-            } else if file_system_policy
-                .get_writable_roots_with_cwd(cwd)
-                .is_empty()
-            {
+            } else if !file_system_policy.has_writable_roots_with_cwd(cwd) {
                 "read-only"
             } else {
                 "workspace-write"

codex-rs/protocol/src/permissions.rs

@@ -1167,6 +1167,18 @@ impl FileSystemSandboxPolicy {
         self.get_writable_roots_with_cwd_impl(cwd, WritableRootPathResolution::Effective)
     }
 
+    /// Returns whether any effective writable root exists without materializing its carveouts.
+    pub fn has_writable_roots_with_cwd(&self, cwd: &Path) -> bool {
+        !self.has_full_disk_write_access()
+            && self
+                .resolved_entries_with_cwd(cwd)
+                .into_iter()
+                .any(|entry| {
+                    entry.access.can_write()
+                        && self.can_write_path_with_cwd(entry.path.as_path(), cwd)
+                })
+    }
+
     /// Returns writable roots without following attacker-mutable path components.
     ///
     /// Trusted top-level aliases such as `/tmp -> /private/tmp` are still
@@ -2173,6 +2185,44 @@ mod tests {
         }
     }
 
+    #[test]
+    fn writable_root_presence_matches_materialized_roots() {
+        let cwd = TempDir::new().expect("tempdir");
+        let writable_root = AbsolutePathBuf::resolve_path_against_base("work", cwd.path());
+        let policies = [
+            FileSystemSandboxPolicy::read_only(),
+            FileSystemSandboxPolicy::unrestricted(),
+            FileSystemSandboxPolicy::external_sandbox(),
+            FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry::new(
+                writable_root.clone().into(),
+                FileSystemAccessMode::Write,
+            )]),
+            FileSystemSandboxPolicy::restricted(vec![
+                FileSystemSandboxEntry::new(
+                    writable_root.clone().into(),
+                    FileSystemAccessMode::Write,
+                ),
+                FileSystemSandboxEntry::new(writable_root.into(), FileSystemAccessMode::Deny),
+            ]),
+            FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry::new(
+                FileSystemPath::Special {
+                    value: FileSystemSpecialPath::unknown(
+                        ":future_special_path",
+                        /*subpath*/ None,
+                    ),
+                },
+                FileSystemAccessMode::Write,
+            )]),
+        ];
+
+        for policy in policies {
+            assert_eq!(
+                policy.has_writable_roots_with_cwd(cwd.path()),
+                !policy.get_writable_roots_with_cwd(cwd.path()).is_empty()
+            );
+        }
+    }
+
     #[cfg(windows)]
     #[test]
     fn permission_paths_preserve_native_slash_unc_strings() {