---
myst:
  html_meta:
    "description": "Build an eOn develop checkout on EESSI 2026.06 with EESSI-extend, CapnProto 1.4.0, a run-path list, and calculator groups."
    "keywords": "eOn, EESSI, EasyBuild, Meson, develop, CapnProto"
---

# Build develop on EESSI

This page builds a `develop` checkout on the European Environment for Scientific Software Installations (EESSI), release 2026.06. The check is `meson test --suite eon`. The fail count is 0.

EESSI 2026.06 is the tree under `/cvmfs/software.eessi.io/versions/2026.06`. A clone with no branch name follows the repository default. This build uses `develop`.

```{code-block} bash
git clone -b develop https://github.com/TheochemUI/eOn.git
cd eOn
```

The first `meson setup` downloads the wraps in `subprojects/`. That step needs a network.

## Install the modules

`eOn-devel/2026.06-GCCcore-15.2.0` is one EasyBuild bundle. It loads Meson, Ninja, CMake, pkgconf, Eigen, Python, and Rust. Cap'n Proto is a second module. OpenMPI comes from `foss/2026.1`.

Set the install prefix before the module load. If `EESSI_USER_INSTALL` is unset, the prefix is `$HOME/eessi`. If you set it, the directory must already exist. The load stops when that path is missing.

```{code-block} bash
source /cvmfs/software.eessi.io/versions/2026.06/init/bash
mkdir -p "${EESSI_USER_INSTALL:-$HOME/eessi}"
module load EESSI-extend
```

A user prefix sets `EASYBUILD_UMASK` to `077`. Lmod, running as root, then reports the new module as unknown, because the module directory is not searchable by others. Pass `--umask=022` on every `eb` command.

A shell with nounset (`set -u`) aborts during this load. `EESSI-extend` reads `LD_LIBRARY_PATH`, and the stack leaves that variable unset. Load the module with nounset off.

```{code-block} bash
eb --umask=022 eessi/eOn-devel-2026.06-GCCcore-15.2.0.eb
```

The direct in-process rgpot arm builds rgpot's remote procedure call stack. That stack needs the `capnp` program and the `capnp-rpc` pkg-config file. EESSI 2026.06 has no CapnProto 1.4.0 module for GCCcore 15.2.0. EasyBuild fetches that CapnProto easyconfig from pull request [26480](https://github.com/easybuilders/easybuild-easyconfigs/pull/26480). Pass the CapnProto filename. The other recipes in that pull request then stay unused.

```{code-block} bash
eb --umask=022 --from-pr 26480 CapnProto-1.4.0-GCCcore-15.2.0.eb
```

A commit id works in place of the pull request number. The id is the full 40-character string. This one is the pull request head. `--from-pr` follows later updates. `--from-commit` stays on this id, and it avoids the GitHub rate limit on pull requests.

```{code-block} bash
eb --umask=022 --from-commit 66fa89934f0476cd4f9ff14154ee4c87ae5c5d82 CapnProto-1.4.0-GCCcore-15.2.0.eb
```

That commit can still be rejected. EasyBuild rejects toolchain GCCcore 15.2.0 when the pull request build is unsupported in EESSI 2026.06. Read the supported list in that message. When the list is empty, the message prints an export. Set that variable and run `eb` again.

```{code-block} bash
export EESSI_SITE_TOP_LEVEL_TOOLCHAINS_2026_06='[{"name": "GCCcore", "version": "15.2.0"}]'
```

`capnp --version` should print `Cap'n Proto version 1.4.0`. `pkg-config --exists capnp-rpc` should return 0.

## Configure and build

Open a new shell. Load the compiler stack, the bundle, and CapnProto.

```{code-block} bash
source /cvmfs/software.eessi.io/versions/2026.06/init/bash
module load EESSI-extend
module load foss/2026.1 eOn-devel/2026.06-GCCcore-15.2.0 CapnProto/1.4.0-GCCcore-15.2.0
hash -r
command -v mpicc
command -v capnp
```

`foss/2026.1` provides `mpicc`, `mpicxx`, and `mpif90`. Export those three as `CC`, `CXX`, and `FC`. Meson then stores the Message Passing Interface (MPI) compiler wrappers.

Loaded modules do not set `LD_LIBRARY_PATH`. Leave it unset. An empty value is not a search path. The bundle writes one run path into `LDFLAGS`, the GCCcore 15.2.0 `lib64` directory. `buildenv/default-foss-2026.1` is part of this release, and its `LDFLAGS` are `-L` link-search directories. Loading the bundle after that module replaces `LDFLAGS` with the single GCCcore run path. The loader still needs one `-Wl,-rpath` for every loaded module library.

Walk each `EBROOT` variable. Append `lib` and `lib64` when the directory exists.

```{code-block} bash
EON_LDFLAGS=""
for v in $(env | grep -o '^EBROOT[A-Z0-9_]*'); do
  for l in lib lib64; do
    if [ -d "${!v}/$l" ]; then
      EON_LDFLAGS="$EON_LDFLAGS -Wl,-rpath,${!v}/$l"
    fi
  done
done
export LDFLAGS="$EON_LDFLAGS"
export CC=mpicc CXX=mpicxx FC=mpif90
export CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=gcc
```

Point `CARGO_HOME` at an empty directory outside the checkout. Cargo then skips a host cargo configuration in its home. Cargo also reads `.cargo/config.toml` from the checkout and from every parent directory. A tree under `$HOME` still reads `$HOME/.cargo/config.toml`. Move the checkout in that case.

Clear host linker flags and compiler wrappers before configure.

```{code-block} bash
unset RUSTC_WRAPPER CARGO_BUILD_RUSTC_WRAPPER RUSTC_WORKSPACE_WRAPPER
unset RUSTFLAGS CARGO_ENCODED_RUSTFLAGS
export CARGO_HOME="$PWD/../eon-cargo-home"
mkdir -p "$CARGO_HOME"
```

A git `insteadOf` rule that rewrites `https://github.com/` to SSH also rewrites Meson wrap downloads. Drop that rule for the configure, or the wrap fetch fails.

That configure has two Message Passing Interface (MPI) switches. `with_mpi` in `meson_options.txt` is the eOn client switch, and this build leaves it off. Calculator groups are the rgpot subproject switch, `-Drgpot:with_mpi=enabled`. `ranks_per_image` greater than 0 throws when that switch is off.

```{code-block} bash
meson setup build-eessi \
  --prefix "$PWD/../eon-prefix" \
  --libdir lib \
  --buildtype release \
  -Dwith_tests=true \
  -Dpython.install_env=prefix \
  -Drgpot:with_mpi=enabled
meson compile -C build-eessi
```

## Run the suite

Give the ranks a writable directory, and put that full path in `TMPDIR`. Keep `OMP_NUM_THREADS` at 1 so OpenMP does not multiply the ranks. When the host has fewer free cores than ranks, set `OMPI_MCA_rmaps_base_oversubscribe` to 1.

Those ranks, on a host whose user id is 0, need two further OpenMPI variables. OpenMPI refuses to start without them. A rootless namespace is that case. `PMIX_MCA_psec=none` skips the PMIx security handshake, which that namespace does not provide. A normal account can leave the two `OMPI_ALLOW_RUN_AS_ROOT` variables unset.

```{code-block} bash
mkdir -p "$PWD/../eon-tmp"
export TMPDIR="$PWD/../eon-tmp"
export OMP_NUM_THREADS=1
export OMPI_MCA_rmaps_base_oversubscribe=1
export PMIX_MCA_psec=none
export OMPI_ALLOW_RUN_AS_ROOT=1
export OMPI_ALLOW_RUN_AS_ROOT_CONFIRM=1
meson test -C build-eessi --suite eon
```

Those ranks, on 2026-09-30, for `develop` commit `13b3b8a5`, gave a suite of 48 passed and a fail count of 0. Later commits on `develop` add tests, so a newer checkout can report a higher pass count. The fail count on that run was 0.