From 8174acb3c040336f59cac8bdc9e259a53b91cfce Mon Sep 17 00:00:00 2001
From: Mohan <86064887+binarybaron@users.noreply.github.com>
Date: Mon, 22 Jun 2026 13:40:50 +0200
Subject: [PATCH] monero-sys: make Monero C++ build parallelism configurable
 via MONERO_BUILD_JOBS

The Monero C++ compile in build.rs was hardcoded to -j1 in CI/Docker
(and all cores locally) with no way to tune it. Monero's translation
units are RAM-hungry, so serial is a safe default on memory-constrained
runners, but builders with enough memory have no escape hatch and pay
for a fully serial compile.

Add an optional MONERO_BUILD_JOBS env var that overrides the default
job count, and plumb it through swap-asb/Dockerfile as a build arg so
the image build can be tuned with `--build-arg MONERO_BUILD_JOBS=N`.
Behavior is unchanged when the variable is unset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 monero-sys/build.rs | 35 +++++++++++++++++++++++++++--------
 swap-asb/Dockerfile |  6 ++++++
 2 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/monero-sys/build.rs b/monero-sys/build.rs
index 12ca0153e7..1d857c8b13 100644
--- a/monero-sys/build.rs
+++ b/monero-sys/build.rs
@@ -110,6 +110,29 @@ fn find_workspace_target_dir() -> std::path::PathBuf {
     panic!("Could not find target directory from OUT_DIR: {out_dir}");
 }
 
+/// Number of parallel jobs (`-j`) for the Monero C++ build.
+///
+/// Monero's C++ compilation is very RAM-hungry, so we default to serial (`-j1`)
+/// in CI/Docker where memory is constrained, and to all logical cores locally.
+/// An explicit `MONERO_BUILD_JOBS` overrides both, allowing the Docker/CI build
+/// to be tuned without editing this file (mind the peak memory: each Monero
+/// translation unit can take hundreds of MB, so `-j2`/`-j4` is the safe range).
+fn monero_build_jobs(is_github_actions: bool, is_docker_build: bool) -> usize {
+    if let Some(jobs) = std::env::var("MONERO_BUILD_JOBS")
+        .ok()
+        .and_then(|v| v.parse::<usize>().ok())
+        .filter(|&jobs| jobs > 0)
+    {
+        return jobs;
+    }
+
+    if is_github_actions || is_docker_build {
+        1
+    } else {
+        num_cpus::get()
+    }
+}
+
 fn main() {
     let is_github_actions: bool = std::env::var("GITHUB_ACTIONS").is_ok();
     let is_docker_build: bool = std::env::var("DOCKER_BUILD").is_ok();
@@ -124,6 +147,9 @@ fn main() {
     // Rerun if the patches directory or any patch files change
     println!("cargo:rerun-if-changed=patches");
 
+    // Rerun if the Monero build parallelism override changes
+    println!("cargo:rerun-if-env-changed=MONERO_BUILD_JOBS");
+
     // Apply embedded patches before building
     apply_patches().expect("Failed to apply our patches");
 
@@ -196,14 +222,7 @@ fn main() {
                 .to_string(),
         ) // This is needed for libsodium.a to be found on mingw-w64
         .build_arg("-Wno-dev") // Disable warnings we can't fix anyway
-        .build_arg(format!(
-            "-j{}",
-            if is_github_actions || is_docker_build {
-                1
-            } else {
-                num_cpus::get()
-            }
-        ))
+        .build_arg(format!("-j{}", monero_build_jobs(is_github_actions, is_docker_build)))
         .build_arg("-I.")
         .build();
 
diff --git a/swap-asb/Dockerfile b/swap-asb/Dockerfile
index 3d7b3cd61b..10a91a7980 100644
--- a/swap-asb/Dockerfile
+++ b/swap-asb/Dockerfile
@@ -88,6 +88,12 @@ RUN --mount=type=cache,target=/root/.cargo/registry,sharing=locked \
 # Act as if we are in a GitHub Actions environment
 ENV DOCKER_BUILD=true
 
+# Number of parallel jobs for the Monero C++ build (monero-sys/build.rs).
+# Defaults to serial (-j1) when unset because Monero's compilation is RAM-hungry.
+# Override with `--build-arg MONERO_BUILD_JOBS=N` on builders with enough memory.
+ARG MONERO_BUILD_JOBS
+ENV MONERO_BUILD_JOBS=${MONERO_BUILD_JOBS}
+
 # Now we build our binaries
 COPY . .