Fail fast on Flux MPI launches outside an allocation

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:

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':
     #>

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)}