Let the F-Stack tools find a vdev / file_prefix stack by itself (#419)

[alberthaolihai]

Problem

Fixes the tools side of #419. With a [vdevN] config (e.g. F-Stack on top of OVS-DPDK vhost-user) the stack runs with --file-prefix=container, and with [dpdk] file_prefix=<v> with --file-prefix=container-<v>. Unless graceful_reload=1, the tools (ifconfig, sysctl, netstat, top, ...) attach as a DPDK secondary under the default prefix rte, so they fail with:

EAL: failed to send to (/var/run/dpdk/rte/mp_socket) due to Connection refused
VDEV_BUS: vdev_scan(): Failed to request vdev from primary

or, when another DPDK primary (such as OVS) owns rte, attach to that process instead. The only way round it was to know the naming rule and set FF_FILE_PREFIX by hand.

Change

• lib/ff_dpdk_if.c: after rte_eal_init(), a primary writes /var/run/ff_primary.conf (pid, start_time, runtime_dir), in the same format as the resident slim primary's record. Written through a tmp file + rename(); best effort, a failure never stops the stack.
• tools/compat/ff_ipc.c: the prefix lookup is now FF_FILE_PREFIX > /var/run/ff_slim_primary.conf > /var/run/ff_primary.conf > rte. The new record goes through the existing ff_ipc_record_prefix() checks: the pid is alive with the same start time, and the directory is a direct child of /var/run/dpdk/.
• tools/README.md: documents the lookup order.

The file_prefix / container naming is unchanged, so existing deployments, scripts and hugepage file names stay the same.

Testing

TencentOS 4.4, kernel 6.6, DPDK 24.11.6, 2 MB hugepages. dpdk-testpmd --file-prefix=ovs419 --vdev net_vhost0,iface=/tmp/vhost-419.sock stood in for OVS, and helloworld used [vdev0] path=/tmp/vhost-419.sock with no file_prefix, so the stack ran under container.

Case	Before	After
ifconfig, vdev and no file_prefix	Connection refused (reproduces #419)	lists f-stack-0, inet 192.168.1.2
sysctl, netstat -an	—	OK
FF_FILE_PREFIX=rte / =container	—	the env var still takes precedence
stack stopped, record left behind	—	record rejected, falls back to the default, no wrong attach
file_prefix=t419 (container-t419)	fails	OK

lib (-Werror), tools and example build cleanly. Not covered: tests/unit (no cmocka on the host), the data path over vhost-user, several stacks on one host, and graceful_reload.

Notes

• With several independent stacks on one host the record names the one started last, so FF_FILE_PREFIX is still needed to pick another one (documented in tools/README.md).

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