Implement detection for shell run via interpreter

Fix #26, close #27.

Co-Authored-By: granitosaurus <tinarg@protonmail.com>

src/shellingham/posix/__init__.py

@@ -1,4 +1,5 @@
 import os
+import re
 
 from .._core import SHELL_NAMES, ShellDetectionFailure
 from . import proc, ps
@@ -19,20 +20,16 @@ def _get_process_mapping():
     raise ShellDetectionFailure('compatible proc fs or ps utility is required')
 
 
-def _iter_process_command(mapping, pid, max_depth):
-    """Iterator to traverse up the tree, yielding `argv[0]` of each process.
+def _iter_process_args(mapping, pid, max_depth):
+    """Iterator to traverse up the tree, yielding each process's argument list.
     """
     for _ in range(max_depth):
         try:
             proc = mapping[pid]
         except KeyError:    # We've reached the root process. Give up.
             break
-        try:
-            cmd = proc.args[0]
-        except IndexError:  # Process has no name? Whatever, ignore it.
-            pass
-        else:
-            yield cmd
+        if proc.args:       # Persumably the process should always have a name?
+            yield proc.args
         pid = proc.ppid     # Go up one level.
 
 
@@ -47,15 +44,50 @@ def _get_login_shell(proc_cmd):
     return (os.path.basename(proc_cmd).lower(), proc_cmd)
 
 
+_INTERPRETER_SHELL_NAMES = [
+    (re.compile(r'^python(\d+(\.\d+)?)?$'), {'xonsh'}),
+]
+
+
+def _get_interpreter_shell(proc_name, proc_args):
+    """Get shell invoked via an interpreter.
+
+    Some shells are implemented on, and invoked with an interpreter, e.g. xonsh
+    is commonly executed with an executable Python script. This detects what
+    script the interpreter is actually running, and check whether that looks
+    like a shell.
+
+    See sarugaku/shellingham#26 for rational.
+    """
+    for pattern, shell_names in _INTERPRETER_SHELL_NAMES:
+        if not pattern.match(proc_name):
+            continue
+        for arg in proc_args:
+            name = os.path.basename(arg).lower()
+            if os.path.isfile(arg) and name in shell_names:
+                return (name, arg)
+    return None
+
+
+def _get_shell(cmd, *args):
+    if cmd.startswith('-'):     # Login shell! Let's use this.
+        return _get_login_shell(cmd)
+    name = os.path.basename(cmd).lower()
+    if name in SHELL_NAMES:     # Command looks like a shell.
+        return (name, cmd)
+    shell = _get_interpreter_shell(name, args)
+    if shell:
+        return shell
+    return None
+
+
 def get_shell(pid=None, max_depth=6):
     """Get the shell that the supplied pid or os.getpid() is running in.
     """
     pid = str(pid or os.getpid())
     mapping = _get_process_mapping()
-    for proc_cmd in _iter_process_command(mapping, pid, max_depth):
-        if proc_cmd.startswith('-'):    # Login shell! Let's use this.
-            return _get_login_shell(proc_cmd)
-        name = os.path.basename(proc_cmd).lower()
-        if name in SHELL_NAMES:     # The inner-most (non-login) shell.
-            return (name, proc_cmd)
+    for proc_args in _iter_process_args(mapping, pid, max_depth):
+        shell = _get_shell(*proc_args)
+        if shell:
+            return shell
     return None