1 files changed, 20 insertions, 0 deletions
diff --git a/src/fs.zig b/src/fs.zig
index 965eb47c..e5980fe9 100644
--- a/src/fs.zig
+++ b/src/fs.zig
@@ -685,6 +685,26 @@ pub fn makeDirs(dir: []const u8) void {
     _ = libc.mkdir(z[0..dir.len :0], 0o755);
 }
 
+/// Whether `dir` is not there to make because one above it refuses: its
+/// nearest ancestor that is there may not be searched or written (EACCES:
+/// `noperm/a/b`, `/proc/1/root/x`).
+pub fn deniedAbove(dir: []const u8) bool {
+    if (comptime !platform_has_fs) return false;
+    var at: ?[]const u8 = dir;
+    while (at) |d| : (at = std.fs.path.dirname(d)) {
+        var z: [4096]u8 = undefined;
+        const dz = std.fmt.bufPrintSentinel(&z, "{s}", .{d}, 0) catch return false;
+        if (libc.access(dz.ptr, 0) != 0) continue; // F_OK
+        return libc.access(dz.ptr, 2 | 1) != 0; // W_OK | X_OK
+    }
+    return false;
+}
+
+test "a directory under one that may not be searched or written is denied, one merely missing is not" {
+    try std.testing.expect(deniedAbove("/proc/1/root/x/y"));
+    try std.testing.expect(!deniedAbove("/tmp/pardes-surely-missing-dir/a/b"));
+}
+
 pub fn localPath(path: []const u8) ?[]const u8 {
     if (std.mem.eql(u8, path, "/n/os")) return "/";
     if (std.mem.startsWith(u8, path, "/n/os/")) return path[5..];