path: root/site_scons/site_tools/ninja.py

# Copyright 2019 MongoDB Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Generate build.ninja files from SCons aliases."""

import sys
import os
import importlib
import io
import shutil

from threading import Lock
from glob import glob
from os.path import join as joinpath
from os.path import splitext

import SCons
from SCons.Action import _string_from_cmd_list, get_default_ENV
from SCons.Util import is_String, is_List
from SCons.Script import COMMAND_LINE_TARGETS

NINJA_SYNTAX = "NINJA_SYNTAX"
NINJA_RULES = "__NINJA_CUSTOM_RULES"
NINJA_POOLS = "__NINJA_CUSTOM_POOLS"
NINJA_CUSTOM_HANDLERS = "__NINJA_CUSTOM_HANDLERS"
NINJA_BUILD = "NINJA_BUILD"
NINJA_WHEREIS_MEMO = {}
NINJA_STAT_MEMO = {}
MEMO_LOCK = Lock()

__NINJA_RULE_MAPPING = {}

# These are the types that get_command can do something with
COMMAND_TYPES = (
    SCons.Action.CommandAction,
    SCons.Action.CommandGeneratorAction,
)


def _install_action_function(_env, node):
    """Install files using the install or copy commands"""
    return {
        "outputs": get_outputs(node),
        "rule": "INSTALL",
        "pool": "install_pool",
        "inputs": [get_path(src_file(s)) for s in node.sources],
        "implicit": get_dependencies(node),
    }


def _lib_symlink_action_function(_env, node):
    """Create shared object symlinks if any need to be created"""
    symlinks = getattr(getattr(node, "attributes", None), "shliblinks", None)

    if not symlinks or symlinks is None:
        return None

    outputs = [link.get_dir().rel_path(linktgt) for link, linktgt in symlinks]
    inputs = [link.get_path() for link, _ in symlinks]

    return {
        "outputs": outputs,
        "inputs": inputs,
        "rule": "SYMLINK",
        "implicit": get_dependencies(node),
    }


def is_valid_dependent_node(node):
    """
    Return True if node is not an alias or is an alias that has children

    This prevents us from making phony targets that depend on other
    phony targets that will never have an associated ninja build
    target.

    We also have to specify that it's an alias when doing the builder
    check because some nodes (like src files) won't have builders but
    are valid implicit dependencies.
    """
    return not isinstance(node, SCons.Node.Alias.Alias) or node.children()


def alias_to_ninja_build(node):
    """Convert an Alias node into a Ninja phony target"""
    return {
        "outputs": get_outputs(node),
        "rule": "phony",
        "implicit": [
            get_path(n) for n in node.children() if is_valid_dependent_node(n)
        ],
    }


def get_dependencies(node):
    """Return a list of dependencies for node."""
    return [get_path(src_file(child)) for child in node.children()]


def get_inputs(node):
    """Collect the Ninja inputs for node."""
    executor = node.get_executor()
    if executor is not None:
        inputs = executor.get_all_sources()
    else:
        inputs = node.sources

    inputs = [get_path(src_file(o)) for o in inputs]
    return inputs


def get_outputs(node):
    """Collect the Ninja outputs for node."""
    executor = node.get_executor()
    if executor is not None:
        outputs = executor.get_all_targets()
    else:
        if hasattr(node, "target_peers"):
            outputs = node.target_peers
        else:
            outputs = [node]

    outputs = [get_path(o) for o in outputs]
    return outputs


class SConsToNinjaTranslator:
    """Translates SCons Actions into Ninja build objects."""

    def __init__(self, env):
        self.env = env
        self.func_handlers = {
            # Skip conftest builders
            "_createSource": ninja_noop,
            # SCons has a custom FunctionAction that just makes sure the
            # target isn't static. We let the commands that ninja runs do
            # this check for us.
            "SharedFlagChecker": ninja_noop,
            # The install builder is implemented as a function action.
            "installFunc": _install_action_function,
            "LibSymlinksActionFunction": _lib_symlink_action_function,
        }

        self.func_handlers.update(self.env[NINJA_CUSTOM_HANDLERS])

    # pylint: disable=too-many-return-statements
    def action_to_ninja_build(self, node, action=None):
        """Generate build arguments dictionary for node."""
        # Use False since None is a valid value for this Attribute
        build = getattr(node.attributes, NINJA_BUILD, False)
        if build is not False:
            return build

        if node.builder is None:
            return None

        if action is None:
            action = node.builder.action

        # Ideally this should never happen, and we do try to filter
        # Ninja builders out of being sources of ninja builders but I
        # can't fix every DAG problem so we just skip ninja_builders
        # if we find one
        if node.builder == self.env["BUILDERS"]["Ninja"]:
            return None

        if isinstance(action, SCons.Action.FunctionAction):
            return self.handle_func_action(node, action)

        if isinstance(action, SCons.Action.LazyAction):
            # pylint: disable=protected-access
            action = action._generate_cache(node.env if node.env else self.env)
            return self.action_to_ninja_build(node, action=action)

        if isinstance(action, SCons.Action.ListAction):
            return self.handle_list_action(node, action)

        if isinstance(action, COMMAND_TYPES):
            return get_command(node.env if node.env else self.env, node, action)

        # Return the node to indicate that SCons is required
        return {
            "rule": "SCONS",
            "outputs": get_outputs(node),
            "implicit": get_dependencies(node),
        }

    def handle_func_action(self, node, action):
        """Determine how to handle the function action."""
        name = action.function_name()
        # This is the name given by the Subst/Textfile builders. So return the
        # node to indicate that SCons is required
        if name == "_action":
            return {
                "rule": "TEMPLATE",
                "outputs": get_outputs(node),
                "implicit": get_dependencies(node),
            }

        handler = self.func_handlers.get(name, None)
        if handler is not None:
            return handler(node.env if node.env else self.env, node)

        print(
            "Found unhandled function action {}, "
            " generating scons command to build\n"
            "Note: this is less efficient than Ninja,"
            " you can write your own ninja build generator for"
            " this function using NinjaRegisterFunctionHandler".format(name)
        )

        return {
            "rule": "SCONS",
            "outputs": get_outputs(node),
            "implicit": get_dependencies(node),
        }

    # pylint: disable=too-many-branches
    def handle_list_action(self, node, action):
        """TODO write this comment"""
        results = [
            self.action_to_ninja_build(node, action=act)
            for act in action.list
            if act is not None
        ]
        results = [result for result in results if result is not None]
        if not results:
            return None

        # No need to process the results if we only got a single result
        if len(results) == 1:
            return results[0]

        all_outputs = list({output for build in results for output in build["outputs"]})
        # If we have no outputs we're done
        if not all_outputs:
            return None

        # Used to verify if all rules are the same
        all_one_rule = len(
            [
                r
                for r in results
                if isinstance(r, dict) and r["rule"] == results[0]["rule"]
            ]
        ) == len(results)
        dependencies = get_dependencies(node)

        if not all_one_rule:
            # If they aren't all the same rule use scons to generate these
            # outputs. At this time nothing hits this case.
            return {
                "outputs": all_outputs,
                "rule": "SCONS",
                "implicit": dependencies,
            }

        if results[0]["rule"] == "CMD":
            cmdline = ""
            for cmd in results:

                # Occasionally a command line will expand to a
                # whitespace only string (i.e. '  '). Which is not a
                # valid command but does not trigger the empty command
                # condition if not cmdstr. So here we strip preceding
                # and proceeding whitespace to make strings like the
                # above become empty strings and so will be skipped.
                cmdstr = cmd["variables"]["cmd"].strip()
                if not cmdstr:
                    continue

                # Skip duplicate commands
                if cmdstr in cmdline:
                    continue

                if cmdline:
                    cmdline += " && "

                cmdline += cmdstr

            # Remove all preceding and proceeding whitespace
            cmdline = cmdline.strip()

            # Make sure we didn't generate an empty cmdline
            if cmdline:
                ninja_build = {
                    "outputs": all_outputs,
                    "rule": "CMD",
                    "variables": {"cmd": cmdline},
                    "implicit": dependencies,
                }

                if node.env and node.env.get("NINJA_POOL", None) is not None:
                    ninja_build["pool"] = node.env["pool"]

                return ninja_build

        elif results[0]["rule"] == "phony":
            return {
                "outputs": all_outputs,
                "rule": "phony",
                "implicit": dependencies,
            }

        elif results[0]["rule"] == "INSTALL":
            return {
                "outputs": all_outputs,
                "rule": "INSTALL",
                "pool": "install_pool",
                "inputs": [get_path(src_file(s)) for s in node.sources],
                "implicit": dependencies,
            }

        elif results[0]["rule"] == "SCONS":
            return {
                "outputs": all_outputs,
                "rule": "SCONS",
                "inputs": dependencies,
            }

        raise Exception("Unhandled list action with rule: " + results[0]["rule"])


# pylint: disable=too-many-instance-attributes
class NinjaState:
    """Maintains state of Ninja build system as it's translated from SCons."""

    def __init__(self, env, writer_class):
        self.env = env
        self.writer_class = writer_class
        self.__generated = False
        self.translator = SConsToNinjaTranslator(env)
        self.generated_suffixes = env.get("NINJA_GENERATED_SOURCE_SUFFIXES", [])

        # List of generated builds that will be written at a later stage
        self.builds = list()

        # List of targets for which we have generated a build. This
        # allows us to take multiple Alias nodes as sources and to not
        # fail to build if they have overlapping targets.
        self.built = set()

        # SCons sets this variable to a function which knows how to do
        # shell quoting on whatever platform it's run on. Here we use it
        # to make the SCONS_INVOCATION variable properly quoted for things
        # like CCFLAGS
        escape = env.get("ESCAPE", lambda x: x)

        self.variables = {
            "COPY": "cmd.exe /c copy" if sys.platform == "win32" else "cp",
            "SCONS_INVOCATION": "{} {} __NINJA_NO=1 $out".format(
                sys.executable,
                " ".join(
                    [escape(arg) for arg in sys.argv if arg not in COMMAND_LINE_TARGETS]
                ),
            ),
            "SCONS_INVOCATION_W_TARGETS": "{} {}".format(
                sys.executable, " ".join([escape(arg) for arg in sys.argv])
            ),
            # This must be set to a global default per:
            # https://ninja-build.org/manual.html
            #
            # (The deps section)
            "msvc_deps_prefix": "Note: including file:",
        }

        self.rules = {
            "CMD": {
                "command": "cmd /c $cmd" if sys.platform == "win32" else "$cmd",
                "description": "Building $out",
            },
            # We add the deps processing variables to this below. We
            # don't pipe this through cmd.exe on Windows because we
            # use this to generate a compile_commands.json database
            # which can't use the shell command as it's compile
            # command. This does mean that we assume anything using
            # CMD_W_DEPS is a straight up compile which is true today.
            "CMD_W_DEPS": {"command": "$cmd", "description": "Building $out"},
            "SYMLINK": {
                "command": (
                    "cmd /c mklink $out $in"
                    if sys.platform == "win32"
                    else "ln -s $in $out"
                ),
                "description": "Symlink $in -> $out",
            },
            "INSTALL": {
                "command": "$COPY $in $out",
                "description": "Install $out",
                # On Windows cmd.exe /c copy does not always correctly
                # update the timestamp on the output file. This leads
                # to a stuck constant timestamp in the Ninja database
                # and needless rebuilds.
                #
                # Adding restat here ensures that Ninja always checks
                # the copy updated the timestamp and that Ninja has
                # the correct information.
                "restat": 1,
            },
            "TEMPLATE": {
                "command": "$SCONS_INVOCATION $out",
                "description": "Rendering $out",
                "pool": "scons_pool",
                "restat": 1,
            },
            "SCONS": {
                "command": "$SCONS_INVOCATION $out",
                "description": "SCons $out",
                "pool": "scons_pool",
                # restat
                #    if present, causes Ninja to re-stat the command's outputs
                #    after execution of the command. Each output whose
                #    modification time the command did not change will be
                #    treated as though it had never needed to be built. This
                #    may cause the output's reverse dependencies to be removed
                #    from the list of pending build actions.
                #
                # We use restat any time we execute SCons because
                # SCons calls in Ninja typically create multiple
                # targets. But since SCons is doing it's own up to
                # date-ness checks it may only update say one of
                # them. Restat will find out which of the multiple
                # build targets did actually change then only rebuild
                # those targets which depend specifically on that
                # output.
                "restat": 1,
            },
            "REGENERATE": {
                "command": "$SCONS_INVOCATION_W_TARGETS",
                "description": "Regenerating $out",
                "generator": 1,
                # Console pool restricts to 1 job running at a time,
                # it additionally has some special handling about
                # passing stdin, stdout, etc to process in this pool
                # that we need for SCons to behave correctly when
                # regenerating Ninja
                "pool": "console",
                # Again we restat in case Ninja thought the
                # build.ninja should be regenerated but SCons knew
                # better.
                "restat": 1,
            },
        }

        self.pools = {
            "install_pool": self.env.GetOption("num_jobs") / 2,
            "scons_pool": 1,
        }

        if env["PLATFORM"] == "win32":
            self.rules["CMD_W_DEPS"]["deps"] = "msvc"
        else:
            self.rules["CMD_W_DEPS"]["deps"] = "gcc"
            self.rules["CMD_W_DEPS"]["depfile"] = "$out.d"

        self.rules.update(env.get(NINJA_RULES, {}))
        self.pools.update(env.get(NINJA_POOLS, {}))

    def generate_builds(self, node):
        """Generate a ninja build rule for node and it's children."""
        # Filter out nodes with no builder. They are likely source files
        # and so no work needs to be done, it will be used in the
        # generation for some real target.
        #
        # Note that all nodes have a builder attribute but it is sometimes
        # set to None. So we cannot use a simpler hasattr check here.
        if getattr(node, "builder", None) is None:
            return

        stack = [[node]]
        while stack:
            frame = stack.pop()
            for child in frame:
                outputs = set(get_outputs(child))
                # Check if all the outputs are in self.built, if they
                # are we've already seen this node and it's children.
                if not outputs.isdisjoint(self.built):
                    continue

                self.built = self.built.union(outputs)
                stack.append(child.children())

                if isinstance(child, SCons.Node.Alias.Alias):
                    build = alias_to_ninja_build(child)
                elif node.builder is not None:
                    # Use False since None is a valid value for this attribute
                    build = getattr(child.attributes, NINJA_BUILD, False)
                    if build is False:
                        build = self.translator.action_to_ninja_build(child)
                        setattr(child.attributes, NINJA_BUILD, build)
                else:
                    build = None

                # Some things are unbuild-able or need not be built in Ninja
                if build is None or build == 0:
                    continue

                self.builds.append(build)

    def is_generated_source(self, output):
        """Check if output ends with a known generated suffix."""
        _, suffix = splitext(output)
        return suffix in self.generated_suffixes
        
    def has_generated_sources(self, output):
        """
        Determine if output indicates this is a generated header file.
        """
        for generated in output:
            if self.is_generated_source(generated):
                return True
        return False

    # pylint: disable=too-many-branches,too-many-locals
    def generate(self, ninja_file, fallback_default_target=None):
        """
        Generate the build.ninja.

        This should only be called once for the lifetime of this object.
        """
        if self.__generated:
            return

        content = io.StringIO()
        ninja = self.writer_class(content, width=100)

        ninja.comment("Generated by scons. DO NOT EDIT.")

        for pool_name, size in self.pools.items():
            ninja.pool(pool_name, size)

        for var, val in self.variables.items():
            ninja.variable(var, val)

        for rule, kwargs in self.rules.items():
            ninja.rule(rule, **kwargs)

        generated_source_files = {
            output
            # First find builds which have header files in their outputs.
            for build in self.builds
            if self.has_generated_sources(build["outputs"])
            for output in build["outputs"]
            # Collect only the header files from the builds with them
            # in their output. We do this because is_generated_source
            # returns True if it finds a header in any of the outputs,
            # here we need to filter so we only have the headers and
            # not the other outputs.
            if self.is_generated_source(output)
        }

        if generated_source_files:
            ninja.build(
                outputs="_generated_sources",
                rule="phony",
                implicit=list(generated_source_files),
            )

        template_builders = []

        for build in self.builds:
            if build["rule"] == "TEMPLATE":
                template_builders.append(build)
                continue

            implicit = build.get("implicit", [])
            implicit.append(ninja_file)
            build["implicit"] = implicit

            # Don't make generated sources depend on each other. We
            # have to check that none of the outputs are generated
            # sources and none of the direct implicit dependencies are
            # generated sources or else we will create a dependency
            # cycle.
            if (
                generated_source_files
                and not build["rule"] == "INSTALL"
                and set(build["outputs"]).isdisjoint(generated_source_files)
                and set(implicit).isdisjoint(generated_source_files)
            ):

                # Make all non-generated source targets depend on
                # _generated_sources. We use order_only for generated
                # sources so that we don't rebuild the world if one
                # generated source was rebuilt. We just need to make
                # sure that all of these sources are generated before
                # other builds.
                build["order_only"] = "_generated_sources"

            # When using a depfile Ninja can only have a single output
            # but SCons will usually have emitted an output for every
            # thing a command will create because it's caching is much
            # more complex than Ninja's. This includes things like DWO
            # files. Here we make sure that Ninja only ever sees one
            # target when using a depfile. It will still have a command
            # that will create all of the outputs but most targets don't
            # depend direclty on DWO files and so this assumption is safe
            # to make.
            rule = self.rules.get(build["rule"])

            # Some rules like 'phony' and other builtins we don't have
            # listed in self.rules so verify that we got a result
            # before trying to check if it has a deps key.
            if rule is not None and rule.get("deps"):
                build["outputs"] = build["outputs"][0:1]

            ninja.build(**build)

        template_builds = dict()
        for template_builder in template_builders:

            # Special handling for outputs and implicit since we need to
            # aggregate not replace for each builder.
            for agg_key in ["outputs", "implicit"]:
                new_val = template_builds.get(agg_key, [])

                # Use pop so the key is removed and so the update
                # below will not overwrite our aggregated values.
                cur_val = template_builder.pop(agg_key, [])
                if isinstance(cur_val, list):
                    new_val += cur_val
                else:
                    new_val.append(cur_val)
                template_builds[agg_key] = new_val

            # Collect all other keys
            template_builds.update(template_builder)

        if template_builds.get("outputs", []):
            ninja.build(**template_builds)

        # We have to glob the SCons files here to teach the ninja file
        # how to regenerate itself. We'll never see ourselves in the
        # DAG walk so we can't rely on action_to_ninja_build to
        # generate this rule even though SCons should know we're
        # dependent on SCons files.
        #
        # TODO: We're working on getting an API into SCons that will
        # allow us to query the actual SConscripts used. Right now
        # this glob method has deficiencies like skipping
        # jstests/SConscript and being specific to the MongoDB
        # repository layout.
        ninja.build(
            ninja_file,
            rule="REGENERATE",
            implicit=[
                self.env.File("#SConstruct").get_abspath(),
                os.path.abspath(__file__),
            ]
            + glob("src/**/SConscript", recursive=True),
        )

        ninja.build(
            "scons-invocation",
            rule="CMD",
            pool="console",
            variables={"cmd": "echo $SCONS_INVOCATION_W_TARGETS"},
        )

        # Note the use of CMD_W_DEPS below. CMD_W_DEPS are always
        # compile commands in this generator. If we ever change the
        # name/s of the rules that include compile commands
        # (i.e. something like CC/CXX) we will need to update this
        # build to reflect that complete list.
        ninja.build(
            "compile_commands.json",
            rule="CMD",
            pool="console",
            variables={
                "cmd": "ninja -f {} -t compdb CMD_W_DEPS > compile_commands.json".format(
                    ninja_file
                )
            },
        )

        ninja.build(
            "compiledb", rule="phony", implicit=["compile_commands.json"],
        )

        # Look in SCons's list of DEFAULT_TARGETS, find the ones that
        # we generated a ninja build rule for.
        scons_default_targets = [
            get_path(tgt)
            for tgt in SCons.Script.DEFAULT_TARGETS
            if get_path(tgt) in self.built
        ]

        # If we found an overlap between SCons's list of default
        # targets and the targets we created ninja builds for then use
        # those as ninja's default as well.
        if scons_default_targets:
            ninja.default(" ".join(scons_default_targets))

        # If not then set the default to the fallback_default_target we were given.
        # Otherwise we won't create a default ninja target.
        elif fallback_default_target is not None:
            ninja.default(fallback_default_target)

        with open(ninja_file, "w") as build_ninja:
            build_ninja.write(content.getvalue())

        self.__generated = True


def get_path(node):
    """
    Return a fake path if necessary.

    As an example Aliases use this as their target name in Ninja.
    """
    if hasattr(node, "get_path"):
        return node.get_path()
    return str(node)


def rfile(node):
    """
    Return the repository file for node if it has one. Otherwise return node
    """
    if hasattr(node, "rfile"):
        return node.rfile()
    return node


def src_file(node):
    """Returns the src code file if it exists."""
    if hasattr(node, "srcnode"):
        src = node.srcnode()
        if src.stat() is not None:
            return src
    return get_path(node)


# TODO: Make the Rules smarter. Instead of just using a "cmd" rule
# everywhere we should be smarter about generating CC, CXX, LINK,
# etc. rules
def get_command(env, node, action):  # pylint: disable=too-many-branches
    """Get the command to execute for node."""
    if node.env:
        sub_env = node.env
    else:
        sub_env = env

    executor = node.get_executor()
    if executor is not None:
        tlist = executor.get_all_targets()
        slist = executor.get_all_sources()
    else:
        if hasattr(node, "target_peers"):
            tlist = node.target_peers
        else:
            tlist = [node]
        slist = node.sources

    # Retrieve the repository file for all sources
    slist = [rfile(s) for s in slist]

    # Get the dependencies for all targets
    implicit = list({dep for tgt in tlist for dep in get_dependencies(tgt)})

    # Generate a real CommandAction
    if isinstance(action, SCons.Action.CommandGeneratorAction):
        # pylint: disable=protected-access
        action = action._generate(tlist, slist, sub_env, 1, executor=executor)

    rule = "CMD"

    # Actions like CommandAction have a method called process that is
    # used by SCons to generate the cmd_line they need to run. So
    # check if it's a thing like CommandAction and call it if we can.
    if hasattr(action, "process"):
        cmd_list, _, _ = action.process(tlist, slist, sub_env, executor=executor)

        # Despite being having "list" in it's name this member is not
        # actually a list. It's the pre-subst'd string of the command. We
        # use it to determine if the command we generated needs to use a
        # custom Ninja rule. By default this redirects CC/CXX commands to
        # CMD_W_DEPS but the user can inject custom Ninja rules and tie
        # them to commands by using their pre-subst'd string.
        rule = __NINJA_RULE_MAPPING.get(action.cmd_list, "CMD")

        cmd = _string_from_cmd_list(cmd_list[0])
    else:
        # Anything else works with genstring, this is most commonly hit by
        # ListActions which essentially call process on all of their
        # commands and concatenate it for us.
        genstring = action.genstring(tlist, slist, sub_env)

        # Detect if we have a custom rule for this
        # "ListActionCommandAction" type thing.
        rule = __NINJA_RULE_MAPPING.get(genstring, "CMD")

        if executor is not None:
            cmd = sub_env.subst(genstring, executor=executor)
        else:
            cmd = sub_env.subst(genstring, target=tlist, source=slist)

        # Since we're only enabling Ninja for developer builds right
        # now we skip all Manifest related work on Windows as it's not
        # necessary. We shouldn't have gotten here but on Windows
        # SCons has a ListAction which shows as a
        # CommandGeneratorAction for linking. That ListAction ends
        # with a FunctionAction (embedManifestExeCheck,
        # embedManifestDllCheck) that simply say "does
        # target[0].manifest exist?" if so execute the real command
        # action underlying me, otherwise do nothing.
        #
        # Eventually we'll want to find a way to translate this to
        # Ninja but for now, and partially because the existing Ninja
        # generator does so, we just disable it all together.
        cmd = cmd.replace("\n", " && ").strip()
        if env["PLATFORM"] == "win32" and (
            "embedManifestExeCheck" in cmd or "embedManifestDllCheck" in cmd
        ):
            cmd = " && ".join(cmd.split(" && ")[0:-1])

        if cmd.endswith("&&"):
            cmd = cmd[0:-2].strip()

    outputs = get_outputs(node)
    command_env = ""
    windows = env["PLATFORM"] == "win32"

    # If win32 and rule == CMD_W_DEPS then we don't want to calculate
    # an environment for this command. It's a compile command and
    # compiledb doesn't support shell syntax on Windows. We need the
    # shell syntax to use environment variables on Windows so we just
    # skip this platform / rule combination to keep the compiledb
    # working.
    #
    # On POSIX we can still set environment variables even for compile
    # commands so we do so.
    if not (windows and rule == "CMD_W_DEPS"):

        # Scan the ENV looking for any keys which do not exist in
        # os.environ or differ from it. We assume if it's a new or
        # differing key from the process environment then it's
        # important to pass down to commands in the Ninja file.
        ENV = get_default_ENV(sub_env)
        scons_specified_env = {
            key: value
            for key, value in ENV.items()
            if key not in os.environ or os.environ.get(key, None) != value
        }

        for key, value in scons_specified_env.items():
            # Ensure that the ENV values are all strings:
            if is_List(value):
                # If the value is a list, then we assume it is a
                # path list, because that's a pretty common list-like
                # value to stick in an environment variable:
                value = flatten_sequence(value)
                value = joinpath(map(str, value))
            else:
                # If it isn't a string or a list, then we just coerce
                # it to a string, which is the proper way to handle
                # Dir and File instances and will produce something
                # reasonable for just about everything else:
                value = str(value)

            if windows:
                command_env += "set '{}={}' && ".format(key, value)
            else:
                command_env += "{}={} ".format(key, value)

    variables = {"cmd": command_env + cmd}
    extra_vars = getattr(node.attributes, "NINJA_EXTRA_VARS", {})
    if extra_vars:
        variables.update(extra_vars)

    ninja_build = {
        "outputs": outputs,
        "inputs": get_inputs(node),
        "implicit": implicit,
        "rule": rule,
        "variables": variables,
    }

    # Don't use sub_env here because we require that NINJA_POOL be set
    # on a per-builder call basis to prevent accidental strange
    # behavior like env['NINJA_POOL'] = 'console' and sub_env can be
    # the global Environment object if node.env is None.
    # Example:
    #
    # Allowed:
    #
    #     env.Command("ls", NINJA_POOL="ls_pool")
    #
    # Not allowed and ignored:
    #
    #     env["NINJA_POOL"] = "ls_pool"
    #     env.Command("ls")
    #
    if node.env and node.env.get("NINJA_POOL", None) is not None:
        ninja_build["pool"] = node.env["NINJA_POOL"]

    return ninja_build


def ninja_builder(env, target, source):
    """Generate a build.ninja for source."""
    if not isinstance(source, list):
        source = [source]
    if not isinstance(target, list):
        target = [target]

    # We have no COMSTR equivalent so print that we're generating
    # here.
    print("Generating:", str(target[0]))

    # The environment variable NINJA_SYNTAX points to the
    # ninja_syntax.py module from the ninja sources found here:
    # https://github.com/ninja-build/ninja/blob/master/misc/ninja_syntax.py
    #
    # This should be vendored into the build sources and it's location
    # set in NINJA_SYNTAX. This code block loads the location from
    # that variable, gets the absolute path to the vendored file, gets
    # it's parent directory then uses importlib to import the module
    # dynamically.
    ninja_syntax_file = env[NINJA_SYNTAX]
    if isinstance(ninja_syntax_file, str):
        ninja_syntax_file = env.File(ninja_syntax_file).get_abspath()
    ninja_syntax_mod_dir = os.path.dirname(ninja_syntax_file)
    sys.path.append(ninja_syntax_mod_dir)
    ninja_syntax_mod_name = os.path.basename(ninja_syntax_file)
    ninja_syntax = importlib.import_module(ninja_syntax_mod_name.replace(".py", ""))

    suffix = env.get("NINJA_SUFFIX", "")
    if suffix and not suffix[0] == ".":
        suffix = "." + suffix

    generated_build_ninja = target[0].get_abspath() + suffix
    ninja_state = NinjaState(env, ninja_syntax.Writer)

    for src in source:
        ninja_state.generate_builds(src)

    ninja_state.generate(generated_build_ninja, str(source[0]))

    return 0


# pylint: disable=too-few-public-methods
class AlwaysExecAction(SCons.Action.FunctionAction):
    """Override FunctionAction.__call__ to always execute."""

    def __call__(self, *args, **kwargs):
        kwargs["execute"] = 1
        return super().__call__(*args, **kwargs)


def ninja_print(_cmd, target, _source, env):
    """Tag targets with the commands to build them."""
    if target:
        for tgt in target:
            if (
                tgt.has_builder()
                # Use 'is False' because not would still trigger on
                # None's which we don't want to regenerate
                and getattr(tgt.attributes, NINJA_BUILD, False) is False
                and isinstance(tgt.builder.action, COMMAND_TYPES)
            ):
                ninja_action = get_command(env, tgt, tgt.builder.action)
                setattr(tgt.attributes, NINJA_BUILD, ninja_action)
                # Preload the attributes dependencies while we're still running
                # multithreaded
                get_dependencies(tgt)
    return 0


def register_custom_handler(env, name, handler):
    """Register a custom handler for SCons function actions."""
    env[NINJA_CUSTOM_HANDLERS][name] = handler


def register_custom_rule_mapping(env, pre_subst_string, rule):
    """Register a custom handler for SCons function actions."""
    global __NINJA_RULE_MAPPING
    __NINJA_RULE_MAPPING[pre_subst_string] = rule


def register_custom_rule(env, rule, command, description="", deps=None):
    """Allows specification of Ninja rules from inside SCons files."""
    rule_obj = {
        "command": command,
        "description": description if description else "{} $out".format(rule),
    }

    if deps is not None:
        rule_obj["deps"] = deps

    env[NINJA_RULES][rule] = rule_obj


def register_custom_pool(env, pool, size):
    """Allows the creation of custom Ninja pools"""
    env[NINJA_POOLS][pool] = size


def ninja_csig(original):
    """Return a dummy csig"""

    def wrapper(self):
        name = str(self)
        if "SConscript" in name or "SConstruct" in name:
            return original(self)
        return "dummy_ninja_csig"

    return wrapper


def ninja_contents(original):
    """Return a dummy content without doing IO"""

    def wrapper(self):
        name = str(self)
        if "SConscript" in name or "SConstruct" in name:
            return original(self)
        return bytes("dummy_ninja_contents", encoding="utf-8")

    return wrapper


def ninja_stat(_self, path):
    """
    Eternally memoized stat call.

    SCons is very aggressive about clearing out cached values. For our
    purposes everything should only ever call stat once since we're
    running in a no_exec build the file system state should not
    change. For these reasons we patch SCons.Node.FS.LocalFS.stat to
    use our eternal memoized dictionary.
    
    Since this is happening during the Node walk it's being run while
    threaded, we have to protect adding to the memoized dictionary
    with a threading.Lock otherwise many targets miss the memoization
    due to racing.
    """
    global NINJA_STAT_MEMO

    try:
        return NINJA_STAT_MEMO[path]
    except KeyError:
        try:
            result = os.stat(path)
        except os.error:
            result = None

        with MEMO_LOCK:
            NINJA_STAT_MEMO[path] = result

        return result


def ninja_noop(*_args, **_kwargs):
    """
    A general purpose no-op function.

    There are many things that happen in SCons that we don't need and
    also don't return anything. We use this to disable those functions
    instead of creating multiple definitions of the same thing.
    """
    return None


def ninja_whereis(thing, *_args, **_kwargs):
    """Replace env.WhereIs with a much faster version"""
    global NINJA_WHEREIS_MEMO

    # Optimize for success, this gets called significantly more often
    # when the value is already memoized than when it's not.
    try:
        return NINJA_WHEREIS_MEMO[thing]
    except KeyError:
        # We do not honor any env['ENV'] or env[*] variables in the
        # generated ninja ile. Ninja passes your raw shell environment
        # down to it's subprocess so the only sane option is to do the
        # same during generation. At some point, if and when we try to
        # upstream this, I'm sure a sticking point will be respecting
        # env['ENV'] variables and such but it's actually quite
        # complicated. I have a naive version but making it always work
        # with shell quoting is nigh impossible. So I've decided to
        # cross that bridge when it's absolutely required.
        path = shutil.which(thing)
        NINJA_WHEREIS_MEMO[thing] = path
        return path


def ninja_always_serial(self, num, taskmaster):
    """Replacement for SCons.Job.Jobs constructor which always uses the Serial Job class."""
    # We still set self.num_jobs to num even though it's a lie. The
    # only consumer of this attribute is the Parallel Job class AND
    # the Main.py function which instantiates a Jobs class. It checks
    # if Jobs.num_jobs is equal to options.num_jobs, so if the user
    # provides -j12 but we set self.num_jobs = 1 they get an incorrect
    # warning about this version of Python not supporting parallel
    # builds. So here we lie so the Main.py will not give a false
    # warning to users.
    self.num_jobs = num
    self.job = SCons.Job.Serial(taskmaster)


class NinjaEternalTempFile(SCons.Platform.TempFileMunge):
    """Overwrite the __call__ method of SCons' TempFileMunge to not delete."""

    def __call__(self, target, source, env, for_signature):
        if for_signature:
            return self.cmd

        node = target[0] if SCons.Util.is_List(target) else target
        if node is not None:
            cmdlist = getattr(node.attributes, "tempfile_cmdlist", None)
            if cmdlist is not None:
                return cmdlist

        cmd = super().__call__(target, source, env, for_signature)

        # If TempFileMunge.__call__ returns a string it means that no
        # response file was needed. No processing required so just
        # return the command.
        if isinstance(cmd, str):
            return cmd

        # Strip the removal commands from the command list.
        #
        # SCons' TempFileMunge class has some very strange
        # behavior where it, as part of the command line, tries to
        # delete the response file after executing the link
        # command. We want to keep those response files since
        # Ninja will keep using them over and over. The
        # TempFileMunge class creates a cmdlist to do this, a
        # common SCons convention for executing commands see:
        # https://github.com/SCons/scons/blob/master/src/engine/SCons/Action.py#L949
        #
        # This deletion behavior is not configurable. So we wanted
        # to remove the deletion command from the command list by
        # simply slicing it out here. Unfortunately for some
        # strange reason TempFileMunge doesn't make the "rm"
        # command it's own list element. It appends it to the
        # tempfile argument to cmd[0] (which is CC/CXX) and then
        # adds the tempfile again as it's own element.
        #
        # So we just kind of skip that middle element. Since the
        # tempfile is in the command list on it's own at the end we
        # can cut it out entirely. This is what I would call
        # "likely to break" in future SCons updates. Hopefully it
        # breaks because they start doing the right thing and not
        # weirdly splitting these arguments up. For reference a
        # command list that we get back from the OG TempFileMunge
        # looks like this:
        #
        # [
        #     'g++',
        #     '@/mats/tempfiles/random_string.lnk\nrm',
        #     '/mats/tempfiles/random_string.lnk',
        # ]
        #
        # Note the weird newline and rm command in the middle
        # element and the lack of TEMPFILEPREFIX on the last
        # element.
        prefix = env.subst("$TEMPFILEPREFIX")
        if not prefix:
            prefix = "@"

        new_cmdlist = [cmd[0], prefix + cmd[-1]]
        setattr(node.attributes, "tempfile_cmdlist", new_cmdlist)
        return new_cmdlist

    def _print_cmd_str(*_args, **_kwargs):
        """Disable this method"""
        pass


def exists(env):
    """Enable if called."""

    # This variable disables the tool when storing the SCons command in the
    # generated ninja file to ensure that the ninja tool is not loaded when
    # SCons should do actual work as a subprocess of a ninja build. The ninja
    # tool is very invasive into the internals of SCons and so should never be
    # enabled when SCons needs to build a target.
    if env.get("__NINJA_NO", "0") == "1":
        return False

    return True


def generate(env):
    """Generate the NINJA builders."""
    env[NINJA_SYNTAX] = env.get(NINJA_SYNTAX, "ninja_syntax.py")

    # Add the Ninja builder.
    always_exec_ninja_action = AlwaysExecAction(ninja_builder, {})
    ninja_builder_obj = SCons.Builder.Builder(action=always_exec_ninja_action)
    env.Append(BUILDERS={"Ninja": ninja_builder_obj})

    # This adds the required flags such that the generated compile
    # commands will create depfiles as appropriate in the Ninja file.
    if env["PLATFORM"] == "win32":
        env.Append(CCFLAGS=["/showIncludes"])
    else:
        env.Append(CCFLAGS=["-MMD", "-MF", "${TARGET}.d"])

    # Provides a way for users to handle custom FunctionActions they
    # want to translate to Ninja.
    env[NINJA_CUSTOM_HANDLERS] = {}
    env.AddMethod(register_custom_handler, "NinjaRegisterFunctionHandler")

    # Provides a mechanism for inject custom Ninja rules which can
    # then be mapped using NinjaRuleMapping.
    env[NINJA_RULES] = {}
    env.AddMethod(register_custom_rule, "NinjaRule")

    # Provides a mechanism for inject custom Ninja pools which can
    # be used by providing the NINJA_POOL="name" as an
    # OverrideEnvironment variable in a builder call.
    env[NINJA_POOLS] = {}
    env.AddMethod(register_custom_pool, "NinjaPool")

    # Add the ability to register custom NinjaRuleMappings for Command
    # builders. We don't store this dictionary in the env to prevent
    # accidental deletion of the CC/XXCOM mappings. You can still
    # overwrite them if you really want to but you have to explicit
    # about it this way. The reason is that if they were accidentally
    # deleted you would get a very subtly incorrect Ninja file and
    # might not catch it.
    env.AddMethod(register_custom_rule_mapping, "NinjaRuleMapping")
    env.NinjaRuleMapping("${CCCOM}", "CMD_W_DEPS")
    env.NinjaRuleMapping("${CXXCOM}", "CMD_W_DEPS")

    # Make SCons node walk faster by preventing unnecessary work
    env.Decider("timestamp-match")

    # Used to determine if a build generates a source file. Ninja
    # requires that all generated sources are added as order_only
    # dependencies to any builds that *might* use them.
    env["NINJA_GENERATED_SOURCE_SUFFIXES"] = [".h", ".hpp"]

    # This is the point of no return, anything after this comment
    # makes changes to SCons that are irreversible and incompatible
    # with a normal SCons build. We return early if __NINJA_NO=1 has
    # been given on the command line (i.e. by us in the generated
    # ninja file) here to prevent these modifications from happening
    # when we want SCons to do work. Everything before this was
    # necessary to setup the builder and other functions so that the
    # tool can be unconditionally used in the users's SCons files.

    if not exists(env):
        return

    # Set a known variable that other tools can query so they can
    # behave correctly during ninja generation.
    env["GENERATING_NINJA"] = True

    # These methods are no-op'd because they do not work during ninja
    # generation, expected to do no work, or simply fail. All of which
    # are slow in SCons. So we overwrite them with no logic.
    SCons.Node.FS.File.make_ready = ninja_noop
    SCons.Node.FS.File.prepare = ninja_noop
    SCons.Node.FS.File.push_to_cache = ninja_noop
    SCons.Executor.Executor.prepare = ninja_noop
    SCons.Node.FS.File.built = ninja_noop

    # We make lstat a no-op because it is only used for SONAME
    # symlinks which we're not producing.
    SCons.Node.FS.LocalFS.lstat = ninja_noop

    # This is a slow method that isn't memoized. We make it a noop
    # since during our generation we will never use the results of
    # this or change the results.
    SCons.Node.FS.is_up_to_date = ninja_noop

    # We overwrite stat and WhereIs with eternally memoized
    # implementations. See the docstring of ninja_stat and
    # ninja_whereis for detailed explanations.
    SCons.Node.FS.LocalFS.stat = ninja_stat
    SCons.Util.WhereIs = ninja_whereis

    # Monkey patch get_csig and get_contents for some classes. It
    # slows down the build significantly and we don't need contents or
    # content signatures calculated when generating a ninja file since
    # we're not doing any SCons caching or building.
    SCons.Executor.Executor.get_contents = ninja_contents(
        SCons.Executor.Executor.get_contents
    )
    SCons.Node.Alias.Alias.get_contents = ninja_contents(
        SCons.Node.Alias.Alias.get_contents
    )
    SCons.Node.FS.File.get_contents = ninja_contents(SCons.Node.FS.File.get_contents)
    SCons.Node.FS.File.get_csig = ninja_csig(SCons.Node.FS.File.get_csig)
    SCons.Node.FS.Dir.get_csig = ninja_csig(SCons.Node.FS.Dir.get_csig)
    SCons.Node.Alias.Alias.get_csig = ninja_csig(SCons.Node.Alias.Alias.get_csig)

    # Replace false Compiling* messages with a more accurate output
    #
    # We also use this to tag all Nodes with Builders using
    # CommandActions with the final command that was used to compile
    # it for passing to Ninja. If we don't inject this behavior at
    # this stage in the build too much state is lost to generate the
    # command at the actual ninja_builder execution time for most
    # commands.
    #
    # We do attempt command generation again in ninja_builder if it
    # hasn't been tagged and it seems to work for anything that
    # doesn't represent as a non-FunctionAction during the print_func
    # call.
    env["PRINT_CMD_LINE_FUNC"] = ninja_print

    # This reduces unnecessary subst_list calls to add the compiler to
    # the implicit dependencies of targets. Since we encode full paths
    # in our generated commands we do not need these slow subst calls
    # as executing the command will fail if the file is not found
    # where we expect it.
    env["IMPLICIT_COMMAND_DEPENDENCIES"] = False

    # Set build to no_exec, our sublcass of FunctionAction will force
    # an execution for ninja_builder so this simply effects all other
    # Builders.
    env.SetOption("no_exec", True)

    # This makes SCons more aggressively cache MD5 signatures in the
    # SConsign file.
    env.SetOption("max_drift", 1)

    # The Serial job class is SIGNIFICANTLY (almost twice as) faster
    # than the Parallel job class for generating Ninja files. So we
    # monkey the Jobs constructor to only use the Serial Job class.
    SCons.Job.Jobs.__init__ = ninja_always_serial

    # We will eventually need to overwrite TempFileMunge to make it
    # handle persistent tempfiles or get an upstreamed change to add
    # some configurability to it's behavior in regards to tempfiles.
    #
    # Set all three environment variables that Python's
    # tempfile.mkstemp looks at as it behaves differently on different
    # platforms and versions of Python.
    os.environ["TMPDIR"] = env.Dir("$BUILD_DIR/response_files").get_abspath()
    os.environ["TEMP"] = os.environ["TMPDIR"]
    os.environ["TMP"] = os.environ["TMPDIR"]
    if not os.path.isdir(os.environ["TMPDIR"]):
        env.Execute(SCons.Defaults.Mkdir(os.environ["TMPDIR"]))

    env["TEMPFILE"] = NinjaEternalTempFile

    # Force the SConsign to be written, we benefit from SCons caching of
    # implicit dependencies and conftests. Unfortunately, we have to do this
    # using an atexit handler because SCons will not write the file when in a
    # no_exec build.
    import atexit

    atexit.register(SCons.SConsign.write)