podman: fallback to normal bootstrap

When Podman isn't installed, or the image can not be downloaded, or the
image isn't compatible with host arch - Mock newly falls-back to a
normal 'dnf --installroot' when bootstrap_image_fallback=True (default).

Closes: #1200

mock/docs/site-defaults.cfg

@@ -158,6 +158,13 @@
 # options invalidate the effect of this option.
 #config_opts['bootstrap_image_ready'] = False
 
+# If 'use_bootstrap_image' is True, Mock is instructed download the configured
+# container image from image registry.  This option controls the behavior when
+# the image can not be downloaded.  When set to False, Mock fails hard.  When
+# set to True, Mock falls-back to normal bootstrap chroot installation using
+# package manager (e.g. using dnf --installroot).
+#config_opts['bootstrap_image_fallback'] = True
+
 # anything you specify with 'bootstrap_*' will be copied to bootstrap config
 # e.g. config_opts['bootstrap_system_yum_command'] = '/usr/bin/yum-deprecated' will become
 # config_opts['system_yum_command'] = '/usr/bin/yum-deprecated' for bootstrap config

mock/py/mockbuild/buildroot.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # vim: noai:ts=4:sw=4:expandtab
 
+from contextlib import contextmanager
 import errno
 import fcntl
 import glob
@@ -19,7 +20,7 @@
 from . import uid
 from . import util
 from .exception import (BuildRootLocked, Error, ResultDirNotAccessible,
-                        BadCmdline)
+                        BadCmdline, BootstrapError)
 from .package_manager import package_manager
 from .trace_decorator import getLog, traceLog
 from .podman import Podman
@@ -211,6 +212,39 @@ def _init_locked(self):
         # Detect what package manager to use.
         self.set_package_manager()
 
+    @traceLog()
+    def _load_from_container_image(self):
+        if not self.uses_bootstrap_image or self.chroot_was_initialized:
+            return
+
+        class _FallbackException(Exception):
+            pass
+
+        @contextmanager
+        def _fallback(message):
+            try:
+                yield
+            except BootstrapError as exc:
+                if not self.config["image_fallback"]:
+                    raise
+                raise _FallbackException(
+                    f"{message}, falling back to bootstrap installation: {exc}"
+                ) from exc
+
+        try:
+            with _fallback("Can't work with Podman"):
+                podman = Podman(self, self.bootstrap_image)
+
+            with _fallback("Can't initialize from bootstrap image"):
+                podman.pull_image()
+                podman.cp(self.make_chroot_path(), self.config["tar_binary"])
+                file_util.unlink_if_exists(os.path.join(self.make_chroot_path(),
+                                                        "etc/rpm/macros.image-language-conf"))
+        except _FallbackException as exc:
+            getLog().warning("%s", exc)
+            self.use_bootstrap_image = False
+
+
     @traceLog()
     def _init(self, prebuild):
 
@@ -229,13 +263,7 @@ def _init(self, prebuild):
         self.plugins.call_hooks('preinit')
         # intentionally we do not call bootstrap hook here - it does not have sense
         self.chroot_was_initialized = self.chroot_is_initialized()
-        if self.uses_bootstrap_image and not self.chroot_was_initialized:
-            podman = Podman(self, self.bootstrap_image)
-            podman.pull_image()
-            podman.cp(self.make_chroot_path(), self.config["tar_binary"])
-            file_util.unlink_if_exists(os.path.join(self.make_chroot_path(),
-                                                    "etc/rpm/macros.image-language-conf"))
-
+        self._load_from_container_image()
         self._setup_dirs()
 
         # /dev is later overwritten by systemd-nspawn, but we need this for

mock/py/mockbuild/config.py

@@ -85,6 +85,7 @@ def setup_default_config_opts():
     config_opts['use_bootstrap_image'] = True
     config_opts['bootstrap_image'] = 'fedora:latest'
     config_opts['bootstrap_image_ready'] = False
+    config_opts['bootstrap_image_fallback'] = True
 
     config_opts['internal_dev_setup'] = True
 

mock/py/mockbuild/podman.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # vim: noai:ts=4:sw=4:expandtab
 
+import os
 import logging
 import subprocess
 from contextlib import contextmanager
@@ -10,7 +11,7 @@
 from mockbuild.exception import BootstrapError
 
 
-def podman_check_native_image_architecture(image, logger=None):
+def podman_check_native_image_architecture(image, logger=None, podman_binary=None):
     """
     Return True if image's architecture is "native" for this host.
     Relates:
@@ -19,10 +20,11 @@ def podman_check_native_image_architecture(image, logger=None):
     """
 
     logger = logger or logging.getLogger()
+    podman = podman_binary or "/usr/bin/podman"
     logger.info("Checking that %s image matches host's architecture", image)
-    sys_check_cmd = ["podman", "version", "--format", "{{.OsArch}}"]
-    image_check_cmd = ["podman", "image", "inspect",
-                        "--format", "{{.Os}}/{{.Architecture}}", image]
+    sys_check_cmd = [podman, "version", "--format", "{{.OsArch}}"]
+    image_check_cmd = [podman, "image", "inspect",
+                       "--format", "{{.Os}}/{{.Architecture}}", image]
 
     def _podman_query(cmd):
         return subprocess.check_output(cmd, encoding="utf8").strip()
@@ -46,6 +48,10 @@ class Podman:
 
     @traceLog()
     def __init__(self, buildroot, image):
+        self.podman_binary = "/usr/bin/podman"
+        if not os.path.exists(self.podman_binary):
+            raise BootstrapError(f"'{self.podman_binary}' not installed")
+
         self.buildroot = buildroot
         self.image = image
         self.container_id = None
@@ -56,7 +62,7 @@ def pull_image(self):
         """ pull the latest image """
         logger = getLog()
         logger.info("Pulling image: %s", self.image)
-        cmd = ["podman", "pull", self.image]
+        cmd = [self.podman_binary, "pull", self.image]
         out, exit_status = util.do_with_status(cmd, env=self.buildroot.env,
                                                raiseExc=False, returnOutput=1)
         if exit_status:
@@ -74,8 +80,8 @@ def mounted_image(self):
         bootstrap chroot directory.
         """
         logger = getLog()
-        cmd_mount = ["podman", "image", "mount", self.image]
-        cmd_umount = ["podman", "image", "umount", self.image]
+        cmd_mount = [self.podman_binary, "image", "mount", self.image]
+        cmd_umount = [self.podman_binary, "image", "umount", self.image]
         result = subprocess.run(cmd_mount, capture_output=True, check=False, encoding="utf8")
         if result.returncode:
             message = "Podman mount failed: " + result.stderr