[PATCH stalld 01/52] stalld: Reject --force_fifo in single-threaded mode

When --force_fifo (-F) was combined with single-threaded mode (-O),
stalld silently fell back to adaptive mode instead of honoring the
requested configuration. This violates the principle of least
surprise since the user explicitly requested single-threaded mode
but gets a different threading model without warning.

Replace the silent fallback with a usage() error that exits
immediately, consistent with the other argument validation checks
in parse_args(). Also fix the corresponding test which was missing
the -O flag and therefore tested adaptive mode instead of
single-threaded mode.

Signed-off-by: Wander Lairson Costa <[email protected]>
Assisted-by: Claude Code:claude-opus-4-6[1m] [PAL]
---
 src/utils.c                         |  9 ++-------
 tests/functional/test_force_fifo.sh | 18 ++----------------
 2 files changed, 4 insertions(+), 23 deletions(-)

diff --git a/src/utils.c b/src/utils.c
index 8f1fe92..3dddb0f 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -1182,13 +1182,8 @@ int parse_args(int argc, char **argv)
 	if (config_boost_duration > config_starving_threshold)
 		usage("the boost duration cannot be longer than the starving threshold ");
 
-	if (config_force_fifo && config_single_threaded) {
-		log_msg("-F/--force_fifo does not work in single-threaded mode\n");
-		log_msg("falling back to the adaptive mode\n");
-		config_adaptive_multi_threaded = 1;
-		config_single_threaded = 0;
-		config_aggressive = 0;
-	}
+	if (config_force_fifo && config_single_threaded)
+		usage("--force_fifo does not work in single-threaded mode");
 
 	if (config_reservation && (config_aggressive || config_adaptive_multi_threaded))
 		usage("-R/--reservation only works in the single-threaded mode");
diff --git a/tests/functional/test_force_fifo.sh b/tests/functional/test_force_fifo.sh
index 239bc37..4fdb13b 100755
--- a/tests/functional/test_force_fifo.sh
+++ b/tests/functional/test_force_fifo.sh
@@ -207,22 +207,8 @@ log "=========================================="
 log "Test 5: Single-threaded mode with FIFO (should fail)"
 log "=========================================="
 
-# Try to run stalld with -F but without -A (single-threaded mode)
-# According to CLAUDE.md, this should die/exit
-FIFO_SINGLE_LOG="/tmp/stalld_test_force_fifo_single_$$.log"
-CLEANUP_FILES+=("${FIFO_SINGLE_LOG}")
-
-log "Testing single-threaded mode with -F (should exit)"
-timeout 5 ${TEST_ROOT}/../stalld -f -v -c "${TEST_CPU}" -t ${threshold} -F > "${FIFO_SINGLE_LOG}" 2>&1
-ret=$?
-
-if [ $ret -ne 0 ] && [ $ret -ne 124 ]; then
-    pass "single-threaded mode rejected FIFO"
-elif grep -qiE "single.*thread|falling back|adaptive" "${FIFO_SINGLE_LOG}"; then
-    pass "stalld detected incompatibility and fell back to adaptive mode"
-else
-    fail "stalld silently accepted FIFO in single-threaded mode"
-fi
+log "Testing single-threaded mode (-O) with -F (should exit)"
+assert_stalld_rejects "Single-threaded mode rejected FIFO" -f -v -c "${TEST_CPU}" -t ${threshold} -F -O
 
 #=============================================================================
 # Test 6: Compare effectiveness (informational)
-- 
2.54.0