Bug 1720925: Move platform-specific Python upgrade instructions r=ahal

There's a "minimum Python check" that happens when Mach
bootstraps that makes the bootstrap command's check obsolete.

Since we want to use modern Python features throughout Mach, we need to
move Python-out-of-date logic up to the front of the process.

We're duplicating the OS-detection logic a bit, but now we regain
per-platform Python instructions.

Differential Revision: https://phabricator.services.mozilla.com/D120093

build/mach_bootstrap.py

@@ -166,14 +166,47 @@ def mach_sys_path(mozilla_dir):
     ]
 
 
+INSTALL_PYTHON_GUIDANCE_LINUX = """
+See https://firefox-source-docs.mozilla.org/setup/linux_build.html#installingpython
+for guidance on how to install Python on your system.
+""".strip()
+
+INSTALL_PYTHON_GUIDANCE_OSX = """
+See https://firefox-source-docs.mozilla.org/setup/macos_build.html
+for guidance on how to prepare your system to build Firefox. Perhaps
+you need to update Xcode, or install Python using brew?
+""".strip()
+
+INSTALL_PYTHON_GUIDANCE_MOZILLABUILD = """
+Python is provided by MozillaBuild; ensure your MozillaBuild
+installation is up to date.
+See https://firefox-source-docs.mozilla.org/setup/windows_build.html#install-mozillabuild
+for details.
+""".strip()
+
+INSTALL_PYTHON_GUIDANCE_OTHER = """
+We do not have specific instructions for your platform on how to
+install Python. You may find Pyenv (https://github.com/pyenv/pyenv)
+helpful, if your system package manager does not provide a way to
+install a recent enough Python 3.
+""".strip()
+
+
 def bootstrap(topsrcdir):
-    # Ensure we are running Python 2.7 or 3.5+. We put this check here so we
-    # generate a user-friendly error message rather than a cryptic stack trace
-    # on module import.
+    # Ensure we are running Python 3.6+. We run this check as soon as
+    # possible to avoid a cryptic import/usage error.
     major = sys.version_info[:2][0]
     if sys.version_info < (3, 6):
         print("Python 3.6+ is required to run mach.")
         print("You are running Python", platform.python_version())
+        if sys.platform.startswith("linux"):
+            print(INSTALL_PYTHON_GUIDANCE_LINUX)
+        elif sys.platform.startswith("darwin"):
+            print(INSTALL_PYTHON_GUIDANCE_OSX)
+        elif "MOZILLABUILD" in os.environ:
+            print(INSTALL_PYTHON_GUIDANCE_MOZILLABUILD)
+        else:
+            print(INSTALL_PYTHON_GUIDANCE_OTHER)
         sys.exit(1)
 
     # This directory was deleted in bug 1666345, but there may be some ignored

python/mozboot/mozboot/base.py

@@ -6,7 +6,6 @@ from __future__ import absolute_import, print_function, unicode_literals
 
 import hashlib
 import os
-import platform
 import re
 import subprocess
 import sys
@@ -143,7 +142,6 @@ ac_add_options --enable-application=js
 # This should match the OLDEST_NON_LEGACY_VERSION in
 # version-control-tools/hgext/configwizard/__init__.py.
 MODERN_MERCURIAL_VERSION = LooseVersion("4.9")
-MODERN_PYTHON_VERSION = LooseVersion("3.6.0")
 
 # Upgrade rust older than this.
 MODERN_RUST_VERSION = LooseVersion(MINIMUM_RUST_VERSION)
@@ -155,13 +153,6 @@ MODERN_NASM_VERSION = LooseVersion("2.14")
 class BaseBootstrapper(object):
     """Base class for system bootstrappers."""
 
-    INSTALL_PYTHON_GUIDANCE = (
-        "We do not have specific instructions for your platform on how to "
-        "install Python. You may find Pyenv (https://github.com/pyenv/pyenv) "
-        "helpful, if your system package manager does not provide a way to "
-        "install a recent enough Python 3 and 2."
-    )
-
     def __init__(self, no_interactive=False, no_system_changes=False):
         self.package_manager_updated = False
         self.no_interactive = no_interactive
@@ -643,19 +634,6 @@ class BaseBootstrapper(object):
         """
         print(MERCURIAL_UNABLE_UPGRADE % (current, MODERN_MERCURIAL_VERSION))
 
-    def ensure_python_modern(self):
-        version = LooseVersion(platform.python_version())
-        if version >= MODERN_PYTHON_VERSION:
-            print("Your version of Python (%s) is new enough." % version)
-        else:
-            print(
-                "ERROR: Your version of Python (%s) is not new enough. You "
-                "must have Python >= %s to build Firefox."
-                % (version, MODERN_PYTHON_VERSION)
-            )
-            print(self.INSTALL_PYTHON_GUIDANCE)
-            sys.exit(1)
-
     def warn_if_pythonpath_is_set(self):
         if "PYTHONPATH" in os.environ:
             print(

python/mozboot/mozboot/bootstrap.py

@@ -325,10 +325,6 @@ class Bootstrapper(object):
 
         self.instance.warn_if_pythonpath_is_set()
 
-        # This doesn't affect any system state and we'd like to bail out as soon
-        # as possible if this check fails.
-        self.instance.ensure_python_modern()
-
         state_dir = self.create_state_dir()
         self.instance.state_dir = state_dir
 

python/mozboot/mozboot/linux_common.py

@@ -163,12 +163,5 @@ class LinuxBootstrapper(
     StyloInstall,
     WasiSysrootInstall,
 ):
-
-    INSTALL_PYTHON_GUIDANCE = (
-        "See https://firefox-source-docs.mozilla.org/setup/linux_build.html"
-        "#installingpython for guidance on how to install Python on your "
-        "system."
-    )
-
     def __init__(self, **kwargs):
         pass

python/mozboot/mozboot/mozillabuild.py

@@ -109,11 +109,6 @@ def is_windefender_affecting_srcdir(srcdir):
 class MozillaBuildBootstrapper(BaseBootstrapper):
     """Bootstrapper for MozillaBuild to install rustup."""
 
-    INSTALL_PYTHON_GUIDANCE = (
-        "Python is provided by MozillaBuild; ensure your MozillaBuild "
-        "installation is up to date."
-    )
-
     def __init__(self, no_interactive=False, no_system_changes=False):
         BaseBootstrapper.__init__(
             self, no_interactive=no_interactive, no_system_changes=no_system_changes

python/mozboot/mozboot/osx.py

@@ -121,13 +121,6 @@ class OSXBootstrapperLight(BaseBootstrapper):
 
 
 class OSXBootstrapper(BaseBootstrapper):
-
-    INSTALL_PYTHON_GUIDANCE = (
-        "See https://firefox-source-docs.mozilla.org/setup/macos_build.html "
-        "for guidance on how to prepare your system to build Firefox. Perhaps "
-        "you need to update Xcode, or install Python using brew?"
-    )
-
     def __init__(self, version, **kwargs):
         BaseBootstrapper.__init__(self, **kwargs)