Added support of NixOS core tools in predict_threadable (#5440)

### Motivation

Closes #5003

### The case

Core utils in Nix are symlinks to one binary file that contains all
utils:

```xsh
docker run --rm -it nixos/nix bash

which echo
# /nix/store/k6h0vjh342kqlkq69sxjj8i5y50l6jfr-coreutils-9.3/bin/echo

ls -la /nix/store/k6h0vjh342kqlkq69sxjj8i5y50l6jfr-coreutils-9.3/bin/
# b2sum -> coreutils
# base32 -> coreutils
# ...
# All tools are symlinks to one binary file - `coreutils`.
```

When
[`default_predictor_readbin`](https://github.com/xonsh/xonsh/blob/61bda708c992a300eab212f877718cf0050a5e3a/xonsh/commands_cache.py#L392)
read `coreutils` it catches `(b'isatty', b'tcgetattr', b'tcsetattr')`
and return `threadable=False` forever.

The list of Nix coreutils tools are exactly the same as in [brew
coreutils](https://formulae.brew.sh/formula/coreutils). So I can check
the real predicts on distinct binaries and see that only 2 tools among
106 are unthreadable and they already covered by
`default_threadable_predictors` (If it's wrong please add unthreadable
tools to the
[default_threadable_predictors](https://github.com/xonsh/xonsh/blob/61bda708c992a300eab212f877718cf0050a5e3a/xonsh/commands_cache.py#L518)):

```xsh
brew install coreutils

ls /opt/homebrew/opt/coreutils/libexec/gnubin/ | wc -l
# 106

for t in p`/opt/homebrew/opt/coreutils/libexec/gnubin/.*`:
    if not (th := __xonsh__.commands_cache.predict_threadable([t.name])):
        print($(which @(t.name)), th)
# /opt/homebrew/opt/coreutils/libexec/gnubin/cat False
# /opt/homebrew/opt/coreutils/libexec/gnubin/yes False

defaults = __import__('xonsh').commands_cache.default_threadable_predictors().keys()
defaults['cat']
# <function xonsh.commands_cache.predict_false>
defaults['yes']
# <function xonsh.commands_cache.predict_false>
```

So the rest of we need is to check the symlink and apply default
prediction if the tools is the symlink to coreutils. This implements
this PR. Test included.

## For community
⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍
comment**

---------

Co-authored-by: a <1@1.1>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

news/fix_nix.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* Added support of NixOS core tools in ``predict_threadable``.
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

tests/test_commands_cache.py

@@ -202,3 +202,31 @@ def test_update_cache(xession, tmp_path):
     cached = cache.update_cache()
 
     assert file2.samefile(cached[basename][0])
+
+
+@skip_if_on_windows
+def test_nixos_coreutils(tmp_path):
+    """On NixOS the core tools are the symlinks to one universal ``coreutils`` binary file."""
+    path = tmp_path / "core"
+    coreutils = path / "coreutils"
+    echo = path / "echo"
+    echo2 = path / "echo2"
+    echo3 = path / "echo3"
+    cat = path / "cat"
+
+    path.mkdir()
+    coreutils.write_bytes(b"Binary with isatty, tcgetattr, tcsetattr.")
+    echo.symlink_to(echo2)
+    echo2.symlink_to(echo3)
+    echo3.symlink_to(coreutils)
+    cat.symlink_to(coreutils)
+
+    for toolpath in [coreutils, echo, echo2, echo3, cat]:
+        # chmod a+x toolpath
+        current_permissions = toolpath.stat().st_mode
+        toolpath.chmod(current_permissions | 0o111)
+
+    cache = CommandsCache({"PATH": [path]})
+
+    assert cache.predict_threadable(["echo", "1"]) is True
+    assert cache.predict_threadable(["cat", "file"]) is False

xonsh/commands_cache.py

@@ -132,6 +132,27 @@ def all_commands(self):
         self.update_cache()
         return self._cmds_cache
 
+    def resolve_symlink(self, path):
+        visited = set()
+        current_path = path
+        while os.path.islink(current_path):
+            if current_path in visited:
+                # Detected a loop while resolving symlink
+                return None
+            visited.add(current_path)
+            try:
+                current_path = os.readlink(current_path)
+            except Exception:
+                return None
+            if not os.path.isabs(current_path):
+                current_path = os.path.join(os.path.dirname(path), current_path)
+                current_path = os.path.normpath(current_path)
+
+        if current_path == path:
+            return None
+
+        return current_path
+
     def update_cache(self):
         env = self.env
         # iterate backwards so that entries at the front of PATH overwrite
@@ -383,6 +404,12 @@ def default_predictor_readbin(self, name, cmd0, timeout, failure):
             return failure
         if not os.path.isfile(fname):
             return failure
+        if (link := self.resolve_symlink(fname)) and link.endswith("coreutils"):
+            """
+            On NixOS the core tools are the symlinks to one universal ``coreutils`` binary file.
+            Detect it and use the default mode.
+            """
+            return failure
 
         try:
             fd = os.open(fname, os.O_RDONLY | os.O_NONBLOCK)