test: test transactions with cycles

Provides coverage for systemd#26872.

With systemd#26875 reverted:

[16444.287652] testsuite-03.sh[71]: + for i in {0..19}
[16444.287652] testsuite-03.sh[71]: + systemctl start transaction-cycle0.service
[16444.359503] systemd[1]: =================================================================
[16444.360321] systemd[1]: ==1==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6180002e578e at pc 0x7f73b25ec7a6 bp 0x7ffc5531c6f0 sp 0x7ffc5531be68
[16444.360798] systemd[1]:
[16444.361044] systemd[1]: READ of size 783 at 0x6180002e578e thread T0 (systemd)
[16444.391684] systemd[1]:     #0 0x7f73b25ec7a5  (/lib64/libasan.so.5+0x557a5)
[16444.392167] systemd[1]:     systemd#1 0x7f73b260a1d5 in __interceptor_vasprintf (/lib64/libasan.so.5+0x731d5)
[16444.392442] systemd[1]:     systemd#2 0x7f73afa1d1e1 in log_format_iovec ../src/basic/log.c:996
[16444.392750] systemd[1]:     systemd#3 0x7f73afa1e7b6 in log_struct_internal ../src/basic/log.c:1058
[16444.393101] systemd[1]:     systemd#4 0x7f73b1979136 in transaction_verify_order_one ../src/core/transaction.c:392
[16444.393540] systemd[1]:     systemd#5 0x7f73b197ac82 in transaction_verify_order_one ../src/core/transaction.c:463
[16444.393946] systemd[1]:     systemd#6 0x7f73b197ac82 in transaction_verify_order_one ../src/core/transaction.c:463
[16444.394262] systemd[1]:     systemd#7 0x7f73b197ac82 in transaction_verify_order_one ../src/core/transaction.c:463
[16444.394532] systemd[1]:     systemd#8 0x7f73b197ac82 in transaction_verify_order_one ../src/core/transaction.c:463
[16444.394812] systemd[1]:     systemd#9 0x7f73b197ac82 in transaction_verify_order_one ../src/core/transaction.c:463
...

test/units/testsuite-03.sh

@@ -110,4 +110,21 @@ ELAPSED=$((END_SEC-START_SEC))
 [[ "$ELAPSED" -ge 3 ]] && [[ "$ELAPSED" -le 5 ]] || exit 1
 [[ "$RESULT" -ne 0 ]] || exit 1
 
+# Test transactions with cycles
+# Provides coverage for issues like https://github.com/systemd/systemd/issues/26872
+for i in {0..19}; do
+    cat >"/run/systemd/system/transaction-cycle$i.service" <<EOF
+[Unit]
+After=transaction-cycle$(((i + 1) % 20)).service
+Requires=transaction-cycle$(((i + 1) % 20)).service
+
+[Service]
+ExecStart=true
+EOF
+done
+systemctl daemon-reload
+for i in {0..19}; do
+    systemctl start "transaction-cycle$i.service"
+done
+
 touch /testok