Add --with-be to allow using pkg pieces from alternative BE

Fixes #506

Author: citrus-it

src/client.py

@@ -44,6 +44,59 @@
 # PKG_SUCCESS_ON_NOP - when an operation completes with nothing to do, exit with
 #                      the success code (0) instead of the NOP one (4).
 
+# Support re-execing using pieces from an alternate mounted boot environment to
+# aid recovery.
+#
+# For example:
+#
+# beadm mount 20250902
+# Mounted successfully on: '/tmp/tmp.miaO0W'
+# pkg --with-be /tmp/tmp.miaO0W update
+#
+# Deliberately use minimal dependencies as we may be in a somewhat broken
+# python environment. We hope at least that these core modules are intact.
+import os, sys, sysconfig
+
+
+def altbe():
+    argv = sys.argv[1:]
+
+    be = None
+    rest = None
+
+    if argv:
+        if argv[0] == "--with-be" and len(argv) >= 2:
+            be = argv[1]
+            rest = argv[2:]
+        elif argv[0].startswith("--with-be="):
+            be = argv[0].split("=", 1)[1]
+            rest = argv[1:]
+
+    if be is None:
+        return
+
+    exe = sysconfig.get_config_var("EXENAME")
+    pyver = sysconfig.get_config_var("py_version_short")
+    libdir = sysconfig.get_config_var("LIBDIR")
+    destlib = sysconfig.get_config_var("DESTLIB")
+    pkglib = f"/usr/lib/pkg/python{pyver}"
+
+    env = os.environ.copy()
+    env["LD_LIBRARY_PATH"] = f"{be}{libdir}"
+    env["PYTHONPATH"] = f"{be}{pkglib}"
+    env["PYTHONPATH"] += f":{be}{destlib}/vendor-packages"
+
+    try:
+        os.execvpe(
+            f"{be}/{exe}", [exe, f"{be}/usr/bin/pkg"] + (rest or []), env
+        )
+    except OSError as e:
+        sys.stderr.write(f"error: failed to exec alt be pkg from '{be}': {e}\n")
+    sys.exit(127)
+
+
+altbe()
+
 try:
     import pkg.site_paths
 except ImportError:
@@ -72,10 +125,8 @@
     import itertools
     import locale
     import logging
-    import os
     import re
     import socket
-    import sys
     import tempfile
     import textwrap
     import time

src/man/pkg.1

@@ -1,7 +1,7 @@
 .\" Copyright (c) 2007, 2016, Oracle and/or its affiliates. All rights reserved.
 .\" Copyright 2024 Oxide Computer Company
 .\" Copyright 2025 OmniOS Community Edition (OmniOSce) Association.
-.Dd June 25, 2025
+.Dd September 8, 2025
 .Dt PKG 1
 .Os
 .Sh NAME
@@ -650,6 +650,12 @@ is
 See the
 .Sx Environment Variables
 section for more information.
+.It Fl \&-with-be Ar path
+Run using python components from a boot environment mounted at
+.Ar path .
+This is useful to recover from a situation where
+.Nm
+does not run due to a problem with python modules.
 .El
 .\"
 .Sh SUB-COMMANDS
@@ -4297,6 +4303,12 @@ and display verbose result.
 .Bd -literal
     $ pkg verify -v -p /usr/bin/ls
 .Ed
+.It Sy Example 38 No Update using python pieces in a different BE
+.Pp
+.Bd -literal
+    $ beadm mount <old be> /be
+    $ pkg --with-be /be update
+.Ed
 .El
 .Sh ENVIRONMENT VARIABLES
 .Bl -tag -width Ds