From 274e7e957ceba37f7e6026c104acd4f027796001 Mon Sep 17 00:00:00 2001
From: Spencer Bryngelson <sbryngelson@gmail.com>
Date: Fri, 9 Oct 2026 07:14:59 -0700
Subject: [PATCH] Fail fast on Flux MPI launches outside an allocation

---
 toolchain/templates/default.mako         | 7 +++++++
 toolchain/templates/include/helpers.mako | 7 +++++++
 toolchain/templates/tuo.mako             | 4 ++++
 3 files changed, 18 insertions(+)

diff --git a/toolchain/templates/default.mako b/toolchain/templates/default.mako
index 2d05d3ab3c..5d9c546e3f 100644
--- a/toolchain/templates/default.mako
+++ b/toolchain/templates/default.mako
@@ -36,6 +36,13 @@ if engine == 'batch':
         else
             ok ":) Selected MPI launcher $MAGENTA$binary$COLOR_RESET. Use$MAGENTA --binary$COLOR_RESET to override."
         fi
+
+        % if engine == 'interactive':
+            # On Flux systems, srun is a wrapper that submits a batch job outside an allocation.
+            if [ "$binary" == "srun" ] && command -v flux > /dev/null; then
+                ${helpers.require_flux_allocation()}
+            fi
+        % endif
     % endif
 
     % for target in targets:
diff --git a/toolchain/templates/include/helpers.mako b/toolchain/templates/include/helpers.mako
index 58e34a3666..3fd21baa98 100644
--- a/toolchain/templates/include/helpers.mako
+++ b/toolchain/templates/include/helpers.mako
@@ -45,6 +45,13 @@ END
 % endif
 </%def>
 
+<%def name="require_flux_allocation()">
+    if [ -z "$FLUX_ENCLOSING_ID" ]; then
+        error ":( Not inside a Flux allocation: each launch would be submitted as its own batch job. Run inside 'flux alloc ...', use '-e batch', or pass --no-mpi.\n"
+        exit 1
+    fi
+</%def>
+
 <%def name="template_epilogue()">
 % if os.name != 'nt':
     #>
diff --git a/toolchain/templates/tuo.mako b/toolchain/templates/tuo.mako
index 74c6dc4fc5..47a1cc9fa6 100644
--- a/toolchain/templates/tuo.mako
+++ b/toolchain/templates/tuo.mako
@@ -37,6 +37,10 @@ echo
 
 export HSA_XNACK=0
 
+% if mpi and engine == 'interactive':
+    ${helpers.require_flux_allocation()}
+% endif
+
 % for target in targets:
     ${helpers.run_prologue(target)}