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build_system.md

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The MongoDB Build System

Introduction

System requirements and supported platforms

How to get Help

Where to go

What to bring when you go there (SCons version, server version, SCons command line, versions of relevant tools, config.log, etc.)

Known Issues

Commonly-encountered issues

--disable-warnings-as-errors

Reference to known issues in the ticket system

How to report a problem

For employees

For non-employees

Set up the build environment

Set up the virtualenv and poetry

See Building Python Prerequisites

The Enterprise Module

Getting the module source

Enabling the module

Building the software

Commonly-used build targets

Building a standard “debug” build

--dbg

What goes where?

$BUILD_ROOT/scons and its contents

$BUILD_ROOT/$VARIANT_DIR and its contents

$BUILD_ROOT/install and its contents

DESTDIR and PREFIX

--build-dir

Running core tests to verify the build

Building a standard “release” build

--separate-debug

Installing from the build directory

--install-action

Creating a release archive

Advanced Builds

Compiler and linker options

CC, CXX, CCFLAGS, CFLAGS, CXXFLAGS

CPPDEFINES and CPPPATH

LINKFLAGS

MSVC_VERSION

VERBOSE

Advanced build options

-j

--separate-debug

--link-model

--allocator

--cxx-std

--linker

--variables-files

Cross compiling

HOST_ARCH and TARGET_ARCH

Using Ninja

--ninja

Cached builds

Using the SCons build cache

--cache
--cache-dir

Using ccache

CCACHE

Using Icecream

ICECC, ICECRUN, ICECC_CREATE_ENV

ICECC_VERSION and ICECC_VERSION_ARCH

ICECC_DEBUG

Developer builds

Developer build options

MONGO_{VERSION,GIT_HASH}

By default, the server build system consults the local git repository (assuming one exists) to automatically derive the current version of MongoDB and current git revision that is being built. These values are recorded in the SCons MONGO_VERSION and MONGO_GIT_HASH Environment variables, respectively. The value of MONGO_GIT_HASH is just that: the value of the currently checked out git hash. The value computed for MONGO_VERSION is based on the result of git describe, which looks for tags matching the release numbering scheme. Since git describe relies on tags, it is important to ensure that you periodically synchronize new tags to your local repository with git fetch against the upstream server repository.

While this automated scheme works well for release and CI builds, it has unfortunate consequences for developer builds. Since the git hash changes on every commit (whether locally authored or pulled from an upstream repo), and since by default an abbreviated git hash forms part of the result of git describe, a build after a commit or a pull will see any target that has a direct or indirect dependency on the parts of the codebase that care about MONGO_VERSION and MONGO_GIT_HASH as out of date. Notably, you will at minimum need to relink mongod and other core server binaries.

It is possible to work around this by manually setting values for MONGO_VERSION and MONGO_GIT_HASH on the SCons command line. However, doing so in a way that results in an accurate value for MONGO_VERSION in particular requires writing shell command substitutions into your SCons invocation, which isn't very friendly. The longstanding historical practice of setting MONGO_VERSION=0.0.0 was never well-advised, but because of recent feature compatibility version related work it is no longer safe to do that at all.

To make it easier for developers to manage these variables in a way which avoids useless rebuilds, has better ergonomics, and does not run afoul of FCV management, the server build system provides a variables file to manage these settings automatically: etc/scons/developer_versions.vars . By using this file, you will get an unchanging MONGO_GIT_HASH value of unknown, and a MONGO_VERSION value that is still based on git describe, but with --abbrev=0 affixed, which will eliminate the dependency on the SHA of the current commit. Note that you will still observe rebuilds if you pull a new tag which changes the results of git describe, but this should be a much less frequent event.

You can opt into this variable by adding --variables-files=etc/scons/developer_versions.vars to your SCons command line, either for direct SCons builds, or when generating Ninja.

Support for etc/scons/developer_versioning.vars has been backported as far back as MongoDB v4.0, so you can safely add this to your SCons invocations on almost any branch you are likely to find yourself using.

Using sanitizers

--sanitize
*SAN_OPTIONS

--dbg --opt

--build-tools=[stable|next]

Setting up your development environment

mongo_custom_variables.py

Guidance on what to put in your custom variables
How to suppress use of your custom variables
Useful variables files (e.g. mongodbtoolchain)

Using the Mongo toolchain

Why do we have our own toolchain?
When is it appropriate to use the MongoDB toolchain?
How do I obtain the toolchain?
How do I upgrade the toolchain?
How do I tell the build system to use it?

Creating and using build variants

Using --build-dir to separate variant build artifacts

BUILD_ROOT and BUILD_DIR

VARIANT_DIR

NINJA_PREFIX and NINJA_SUFFIX

Building older versions

Using git-worktree

Speeding up incremental builds

Selecting minimal build targets

Compiler arguments

-gsplit-dwarf and /DEBUG:FASTLINK

Don’t reinstall what you don’t have to (*NIX only)

--install-action=hardlink

Speeding up SCons dependency evaluation

--implicit-cache
--build-fast-and-loose

Using Ninja responsibly

What about ccache?

Making source changes

Adding a new dependency

Linting and Lint Targets

What lint targets are available?

Using clang-format

Testing your changes

How are test test suites defined?

Running test suites

Adding tests to a suite

Running individual tests

Modifying the buid system

What is SCons?

SConstruct and SConscripts

Environments and their Clones

Overriding and altering variables

Targets and Sources

Nodes

File Nodes
Program and Library Nodes

Aliases, Depends and Requires

Builders

Emitters

Scanners

Actions

Configure objects

DAG walk

Reference to SCons documentation

Modules

How modules work

The Enterprise module

The build.py file

Adding a new module

Poetry

What is Poetry

Poetry is a python dependency management system. Poetry tries to find dependencies in pypi (similar to pip). For more details visit the poetry website.

Why use Poetry

Poetry creates a dependency lock file similar to that of a Ruby Gemfile or a Rust Cargo File. This lock file has exact dependencies that will be the same no matter when they are installed. Even if dependencyA has an update available the older pinned dependency will still be installed. The means that there will be less errors that are based on two users having different versions of python dependencies.

Poetry Lock File

In a Poetry project there are two files that determine and resolve the dependencies. The first is pyproject.toml. This file loosely tells poetry what dependencies and needed and the constraints of those dependencies. For example the following are all valid selections.

  1. dependencyA = "1.0.0" # dependencyA can only ever be 1.0.0
  2. dependencyA = "^1.0.0" # dependencyA can be any version greater than or equal to 1.0.0 and less than 2.0.0
  3. dependencyA = "*" # dependencyA can be any version

The poetry.lock file has the exact package versions. This file is generated by poetry by running poetry lock. This file contains a pinned list of all transitive dependencies that satisfy the requirements in pyproject.toml.

LIBDEPS and the LIBDEPS Linter

Why LIBDEPS?

Libdeps is a subsystem within the build, which is centered around the LIBrary DEPendency graph. It tracks and maintains the dependency graph as well as lints, analyzes and provides useful metrics about the graph.

Different LIBDEPS variable types

The LIBDEPS variables are how the library relationships are defined within the build scripts. The primary variables are as follows:

  • LIBDEPS: The 'public' type which propagates lower level dependencies onward automatically.
  • LIBDEPS_PRIVATE: Creates a dependency only between the target and the dependency.
  • LIBDEPS_INTERFACE: Same as LIBDEPS but excludes itself from the propagation onward.
  • LIBDEPS_DEPENDENTS: Creates a reverse LIBDEPS_PRIVATE dependency where the dependency is the one declaring the relationship.
  • PROGDEPS_DEPENDENTS: Same as LIBDEPS_DEPENDENTS but for use with Program builders.

Libraries are added to these variables as lists per each SCons builder instance in the SConscripts depending on what type of relationship is needed. For more detailed information on theses types, refer to The LIBDEPS variables

The LIBDEPS lint rules and tags

The libdeps subsystem is capable of linting and automatically detecting issues. Some of these linting rules are automatically checked during build-time (while the SConscripts are read and the build is performed) while others need to be manually run post-build (after the the generated graph file has been built). Some rules will include exemption tags which can be added to a libraries LIBDEPS_TAGS to override a rule for that library.

The build-time linter also has a print option --libdeps-linting=print which will print all issues without failing the build and ignoring exemption tags. This is useful for getting an idea of what issues are currently outstanding.

For a complete list of build-time lint rules, please refer to Build-time Libdeps Linter

LIBDEPS_TAGS

LIBDEPS_TAGS can also be used to supply flags to the libdeps subsystem to do special handling for certain libraries such as exemptions or inclusions for linting rules and also SCons command line expansion functions.

For a full list of tags refer to LIBDEPS_TAGS

Using the post-build LIBDEPS Linter

To use the post-build tools, you must first build the libdeps dependency graph by building the generate-libdeps-graph target.

You must also install the requirements file:

python3 -m poetry install --no-root --sync -E libdeps

After the graph file is created, it can be used as input into the gacli tool to perform linting and analysis on the complete dependency graph. The gacli tool has options for what types of analysis to perform. A complete list can be found using the --help option. Minimally, you can run the gacli tool by just passing the graph file you wish to analyze:

python3 buildscripts/libdeps/gacli.py --graph-file build/cached/libdeps/libdeps.graphml

Another tool which provides a graphical interface as well as visual representation of the graph is the graph visualizer. Minimally, it requires passing in a directory in which any files with the .graphml extension will be available for analysis. By default it will launch the web interface which is reachable in a web browser at http://localhost:3000.

python3 buildscripts/libdeps/graph_visualizer.py --graphml-dir build/opt/libdeps

For more information about the details of using the post-build linting tools refer to post-build linting and analysis

Debugging build system failures

Using -k and -n

--debug=[explain, time, stacktrace]

--libdeps-debug