# Bootstrappable Bitcoin Core Builds

This directory contains the files necessary to perform bootstrappable Bitcoin
Core builds.

[Bootstrappability][b17e] furthers our binary security guarantees by allowing us
to _audit and reproduce_ our toolchain instead of blindly _trusting_ binary
downloads.

We achieve bootstrappability by using Guix as a functional package manager.

# Requirements

Conservatively, you will need:

- 16GB of free disk space on the partition that /gnu/store will reside in
- 8GB of free disk space **per platform triple** you're planning on building
  (see the `HOSTS` [environment variable description][env-vars-list])

# Installation and Setup

If you don't have Guix installed and set up, please follow the instructions in
[INSTALL.md](./INSTALL.md)

# Usage

If you haven't considered your security model yet, please read [the relevant
section](#choosing-your-security-model) before proceeding to perform a build.

## Making the Xcode SDK available for macOS cross-compilation

In order to perform a build for macOS (which is included in the default set of
platform triples to build), you'll need to extract the macOS SDK tarball using
tools found in the [`macdeploy` directory](../macdeploy/README.md#sdk-extraction).

You can then either point to the SDK using the `SDK_PATH` environment variable:

```sh
# Extract the SDK tarball to /path/to/parent/dir/of/extracted/SDK/Xcode-<foo>-<bar>-extracted-SDK-with-libcxx-headers
tar -C /path/to/parent/dir/of/extracted/SDK -xaf /path/to/Xcode-<foo>-<bar>-extracted-SDK-with-libcxx-headers.tar

# Indicate where to locate the SDK tarball
export SDK_PATH=/path/to/parent/dir/of/extracted/SDK
```

or extract it into `depends/SDKs`:

```sh
mkdir -p depends/SDKs
tar -C depends/SDKs -xaf /path/to/SDK/tarball
```

## Building

*The author highly recommends at least reading over the [common usage patterns
and examples](#common-guix-build-invocation-patterns-and-examples) section below
before starting a build. For a full list of customization options, see the
[recognized environment variables][env-vars-list] section.*

To build Bitcoin Core reproducibly with all default options, invoke the
following from the top of a clean repository:

```sh
./contrib/guix/guix-build
```

## Codesigning build outputs

The `guix-codesign` command attaches codesignatures (produced by codesigners) to
existing non-codesigned outputs. Please see the [release process
documentation](/doc/release-process.md#codesigning) for more context.

It respects many of the same environment variable flags as `guix-build`, with 2
crucial differences:

1. Since only Windows and macOS build outputs require codesigning, the `HOSTS`
   environment variable will have a sane default value of `x86_64-w64-mingw32
   x86_64-apple-darwin arm64-apple-darwin` instead of all the platforms.
2. The `guix-codesign` command ***requires*** a `DETACHED_SIGS_REPO` flag.
    * _**DETACHED_SIGS_REPO**_

      Set the directory where detached codesignatures can be found for the current
      Bitcoin Core version being built.

      _REQUIRED environment variable_

An invocation with all default options would look like:

```
env DETACHED_SIGS_REPO=<path/to/bitcoin-detached-sigs> ./contrib/guix/guix-codesign
```

## Cleaning intermediate work directories

By default, `guix-build` leaves all intermediate files or "work directories"
(e.g. `depends/work`, `guix-build-*/distsrc-*`) intact at the end of a build so
that they are available to the user (to aid in debugging, etc.). However, these
directories usually take up a large amount of disk space. Therefore, a
`guix-clean` convenience script is provided which cleans the current `git`
worktree to save disk space:

```
./contrib/guix/guix-clean
```

## Gathering shasums of build outputs

After a successful build, the shasums of the build outputs are gathered
into files named `SHA256SUMS`. These files are located in each of the
architecture-specific output directories.

To gather all shasums and output them together to console, for e.g.
inclusion in a Guix pull request comment, run:

``` sh
source contrib/shell/git-utils.bash && uname -m && find guix-build-$(git_head_version)/output/ -type f -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
```

## Attesting to build outputs

Much like how Gitian build outputs are attested to in a `gitian.sigs`
repository, Guix build outputs are attested to in the [`guix.sigs`
repository](https://github.com/bitcoin-core/guix.sigs).

After you've cloned the `guix.sigs` repository, to attest to the current
worktree's commit/tag:

```
env GUIX_SIGS_REPO=<path/to/guix.sigs> SIGNER=<gpg-key-name> ./contrib/guix/guix-attest
```

See `./contrib/guix/guix-attest --help` for more information on the various ways
`guix-attest` can be invoked.

## Verifying build output attestations

After at least one other signer has uploaded their signatures to the `guix.sigs`
repository:

```
git -C <path/to/guix.sigs> pull
env GUIX_SIGS_REPO=<path/to/guix.sigs> ./contrib/guix/guix-verify
```


## Common `guix-build` invocation patterns and examples

### Keeping caches and SDKs outside of the worktree

If you perform a lot of builds and have a bunch of worktrees, you may find it
more efficient to keep the depends tree's download cache, build cache, and SDKs
outside of the worktrees to avoid duplicate downloads and unnecessary builds. To
help with this situation, the `guix-build` script honours the `SOURCES_PATH`,
`BASE_CACHE`, and `SDK_PATH` environment variables and will pass them on to the
depends tree so that you can do something like:

```sh
env SOURCES_PATH="$HOME/depends-SOURCES_PATH" BASE_CACHE="$HOME/depends-BASE_CACHE" SDK_PATH="$HOME/macOS-SDKs" ./contrib/guix/guix-build
```

Note that the paths that these environment variables point to **must be
directories**, and **NOT symlinks to directories**.

See the [recognized environment variables][env-vars-list] section for more
details.

### Building a subset of platform triples

Sometimes you only want to build a subset of the supported platform triples, in
which case you can override the default list by setting the space-separated
`HOSTS` environment variable:

```sh
env HOSTS='x86_64-w64-mingw32 x86_64-apple-darwin' ./contrib/guix/guix-build
```

See the [recognized environment variables][env-vars-list] section for more
details.

### Controlling the number of threads used by `guix` build commands

Depending on your system's RAM capacity, you may want to decrease the number of
threads used to decrease RAM usage or vice versa.

By default, the scripts under `./contrib/guix` will invoke all `guix` build
commands with `--cores="$JOBS"`. Note that `$JOBS` defaults to `$(nproc)` if not
specified. However, astute manual readers will also notice that `guix` build
commands also accept a `--max-jobs=` flag (which defaults to 1 if unspecified).

Here is the difference between `--cores=` and `--max-jobs=`:

> Note: When I say "derivation," think "package"

`--cores=`

  - controls the number of CPU cores to build each derivation. This is the value
    passed to `make`'s `--jobs=` flag.

`--max-jobs=`

  - controls how many derivations can be built in parallel
  - defaults to 1

Therefore, the default is for `guix` build commands to build one derivation at a
time, utilizing `$JOBS` threads.

Specifying the `$JOBS` environment variable will only modify `--cores=`, but you
can also modify the value for `--max-jobs=` by specifying
`$ADDITIONAL_GUIX_COMMON_FLAGS`. For example, if you have a LOT of memory, you
may want to set:

```sh
export ADDITIONAL_GUIX_COMMON_FLAGS='--max-jobs=8'
```

Which allows for a maximum of 8 derivations to be built at the same time, each
utilizing `$JOBS` threads.

Or, if you'd like to avoid spurious build failures caused by issues with
parallelism within a single package, but would still like to build multiple
packages when the dependency graph allows for it, you may want to try:

```sh
export JOBS=1 ADDITIONAL_GUIX_COMMON_FLAGS='--max-jobs=8'
```

See the [recognized environment variables][env-vars-list] section for more
details.

## Recognized environment variables

* _**HOSTS**_

  Override the space-separated list of platform triples for which to perform a
  bootstrappable build.

  _(defaults to "x86\_64-linux-gnu arm-linux-gnueabihf aarch64-linux-gnu
  riscv64-linux-gnu powerpc64-linux-gnu powerpc64le-linux-gnu
  x86\_64-w64-mingw32 x86\_64-apple-darwin arm64-apple-darwin")_

* _**SOURCES_PATH**_

  Set the depends tree download cache for sources. This is passed through to the
  depends tree. Setting this to the same directory across multiple builds of the
  depends tree can eliminate unnecessary redownloading of package sources.

  The path that this environment variable points to **must be a directory**, and
  **NOT a symlink to a directory**.

* _**BASE_CACHE**_

  Set the depends tree cache for built packages. This is passed through to the
  depends tree. Setting this to the same directory across multiple builds of the
  depends tree can eliminate unnecessary building of packages.

  The path that this environment variable points to **must be a directory**, and
  **NOT a symlink to a directory**.

* _**SDK_PATH**_

  Set the path where _extracted_ SDKs can be found. This is passed through to
  the depends tree. Note that this should be set to the _parent_ directory of
  the actual SDK (e.g. `SDK_PATH=$HOME/Downloads/macOS-SDKs` instead of
  `$HOME/Downloads/macOS-SDKs/Xcode-26.1.1-17B100-extracted-SDK-with-libcxx-headers`).

  The path that this environment variable points to **must be a directory**, and
  **NOT a symlink to a directory**.

* _**JOBS**_

  Override the number of jobs to run simultaneously, you might want to do so on
  a memory-limited machine. This may be passed to:

  - `guix` build commands as in `guix shell --cores="$JOBS"`
  - `make` as in `make --jobs="$JOBS"`
  - `cmake` as in `cmake --build build -j "$JOBS"`
  - `xargs` as in `xargs -P"$JOBS"`

  See [here](#controlling-the-number-of-threads-used-by-guix-build-commands) for
  more details.

  _(defaults to the value of `nproc` outside the container)_

* _**SOURCE_DATE_EPOCH**_

  Override the reference UNIX timestamp used for bit-for-bit reproducibility,
  the variable name conforms to [standard][r12e/source-date-epoch].

  _(defaults to the output of `$(git log --format=%at -1)`)_

* _**V**_

  If non-empty, will pass `V=1` to all `make` invocations, making `make` output
  verbose.

  Note that any given value is ignored. The variable is only checked for
  emptiness. More concretely, this means that `V=` (setting `V` to the empty
  string) is interpreted the same way as not setting `V` at all, and that `V=0`
  has the same effect as `V=1`.

* _**SUBSTITUTE_URLS**_

  A whitespace-delimited list of URLs from which to download pre-built packages.
  A URL is only used if its signing key is authorized (refer to the [substitute
  servers section](#option-1-building-with-substitutes) for more details).

* _**ADDITIONAL_GUIX_COMMON_FLAGS**_

  Additional flags to be passed to all `guix` commands.

* _**ADDITIONAL_GUIX_TIMEMACHINE_FLAGS**_

  Additional flags to be passed to `guix time-machine`.

* _**ADDITIONAL_GUIX_ENVIRONMENT_FLAGS**_

  Additional flags to be passed to the invocation of `guix shell` inside
  `guix time-machine`.

# Choosing your security model

No matter how you installed Guix, you need to decide on your security model for
building packages with Guix.

Guix allows us to achieve better binary security by using our CPU time to build
everything from scratch. However, it doesn't sacrifice user choice in pursuit of
this: users can decide whether or not to use **substitutes** (pre-built
packages).

## Option 1: Building with substitutes

### Step 1: Authorize the signing keys

Depending on the installation procedure you followed, you may have already
authorized the Guix build farm key. In particular, the official shell installer
script asks you if you want the key installed, and the debian distribution
package authorized the key during installation.

You can check the current list of authorized keys at `/etc/guix/acl`.

At the time of writing, a `/etc/guix/acl` with just the Guix build farm key
authorized looks something like:

```lisp
(acl
 (entry
  (public-key
   (ecc
    (curve Ed25519)
    (q #8D156F295D24B0D9A86FA5741A840FF2D24F60F7B6C4134814AD55625971B394#)
    )
   )
  (tag
   (guix import)
   )
  )
 )
```

If you've determined that the official Guix build farm key hasn't been
authorized, and you would like to authorize it, run the following as root:

```
guix archive --authorize < /var/guix/profiles/per-user/root/current-guix/share/guix/ci.guix.gnu.org.pub
```

If
`/var/guix/profiles/per-user/root/current-guix/share/guix/ci.guix.gnu.org.pub`
doesn't exist, try:

```sh
guix archive --authorize < <PREFIX>/share/guix/ci.guix.gnu.org.pub
```

Where `<PREFIX>` is likely:
- `/usr` if you installed from a distribution package
- `/usr/local` if you installed Guix from source and didn't supply any
  prefix-modifying flags to Guix's `./configure`

#### Removing authorized keys

To remove previously authorized keys, simply edit `/etc/guix/acl` and remove the
`(entry (public-key ...))` entry.

### Step 2: Specify the substitute servers

Once its key is authorized, the official Guix build farm at
https://ci.guix.gnu.org is automatically used unless the `--no-substitutes` flag
is supplied. This default list of substitute servers is overridable both on a
`guix-daemon` level and when you invoke `guix` commands. See examples below for
the various ways of adding a substitute server after having [authorized
its signing key](#step-1-authorize-the-signing-keys).

Change the **default list** of substitute servers by starting `guix-daemon` with
the `--substitute-urls` option (you will likely need to edit your init script):

```sh
guix-daemon <cmd> --substitute-urls='https://bordeaux.guix.gnu.org https://ci.guix.gnu.org'
```

Override the default list of substitute servers by passing the
`--substitute-urls` option for invocations of `guix` commands:

```sh
guix <cmd> --substitute-urls='https://bordeaux.guix.gnu.org https://ci.guix.gnu.org'
```

For scripts under `./contrib/guix`, set the `SUBSTITUTE_URLS` environment
variable:

```sh
export SUBSTITUTE_URLS='https://bordeaux.guix.gnu.org https://ci.guix.gnu.org'
```

## Option 2: Disabling substitutes on an ad-hoc basis

If you prefer not to use any substitutes, make sure to supply `--no-substitutes`
like in the following snippet. The first build will take a while, but the
resulting packages will be cached for future builds.

For direct invocations of `guix`:
```sh
guix <cmd> --no-substitutes
```

For the scripts under `./contrib/guix/`:
```sh
export ADDITIONAL_GUIX_COMMON_FLAGS='--no-substitutes'
```

## Option 3: Disabling substitutes by default

`guix-daemon` accepts a `--no-substitutes` flag, which will make sure that,
unless otherwise overridden by a command line invocation, no substitutes will be
used.

If you start `guix-daemon` using an init script, you can edit said script to
supply this flag.

[b17e]: https://bootstrappable.org/
[r12e/source-date-epoch]: https://reproducible-builds.org/docs/source-date-epoch/
[env-vars-list]: #recognized-environment-variables
