snapd shouldn't start if API port is already in use

[andrzej-k]

How to reproduce:

1. Start snapd few times:

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# ./snapd -t 0 &
[1] 10097
root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# INFO[0000] Starting snapd (version: v0.9.0-beta-128-g05a376c)
INFO[0000] setting GOMAXPROCS to: 1 core(s)
INFO[0000] control started                               _block=start _module=control
INFO[0000] module started                                _module=snapd block=main snap-module=control
INFO[0000] scheduler started                             _block=start-scheduler _module=scheduler
INFO[0000] module started                                _module=snapd block=main snap-module=scheduler
INFO[0000] setting plugin trust level to: disabled
INFO[0000] auto discover path is disabled
INFO[0000] Configuring REST API with HTTPS set to: false  _module=_mgmt-rest
INFO[0000] Starting REST API on :8181                    _module=_mgmt-rest
INFO[0000] Rest API is enabled
INFO[0000] snapd started                                 _module=snapd block=main
INFO[0000] setting log level to: warning

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# ./snapd -t 0 &
[2] 10104
root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# INFO[0000] Starting snapd (version: v0.9.0-beta-128-g05a376c)
INFO[0000] setting GOMAXPROCS to: 1 core(s)
INFO[0000] control started                               _block=start _module=control
INFO[0000] module started                                _module=snapd block=main snap-module=control
INFO[0000] scheduler started                             _block=start-scheduler _module=scheduler
INFO[0000] module started                                _module=snapd block=main snap-module=scheduler
INFO[0000] setting plugin trust level to: disabled
INFO[0000] auto discover path is disabled
INFO[0000] Configuring REST API with HTTPS set to: false  _module=_mgmt-rest
INFO[0000] Starting REST API on :8181                    _module=_mgmt-rest
INFO[0000] Rest API is enabled
INFO[0000] snapd started                                 _module=snapd block=main
INFO[0000] setting log level to: warning

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# ./snapd -t 0 &
[3] 10110
root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# INFO[0000] Starting snapd (version: v0.9.0-beta-128-g05a376c)
INFO[0000] setting GOMAXPROCS to: 1 core(s)
INFO[0000] control started                               _block=start _module=control
INFO[0000] module started                                _module=snapd block=main snap-module=control
INFO[0000] scheduler started                             _block=start-scheduler _module=scheduler
INFO[0000] module started                                _module=snapd block=main snap-module=scheduler
INFO[0000] setting plugin trust level to: disabled
INFO[0000] auto discover path is disabled
INFO[0000] Configuring REST API with HTTPS set to: false  _module=_mgmt-rest
INFO[0000] Starting REST API on :8181                    _module=_mgmt-rest
INFO[0000] Rest API is enabled
INFO[0000] snapd started                                 _module=snapd block=main
INFO[0000] setting log level to: warning

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# ./snapd -t 0 &
[4] 10116
root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# INFO[0000] Starting snapd (version: v0.9.0-beta-128-g05a376c)
INFO[0000] setting GOMAXPROCS to: 1 core(s)
INFO[0000] control started                               _block=start _module=control
INFO[0000] module started                                _module=snapd block=main snap-module=control
INFO[0000] scheduler started                             _block=start-scheduler _module=scheduler
INFO[0000] module started                                _module=snapd block=main snap-module=scheduler
INFO[0000] setting plugin trust level to: disabled
INFO[0000] auto discover path is disabled
INFO[0000] Configuring REST API with HTTPS set to: false  _module=_mgmt-rest
INFO[0000] Starting REST API on :8181                    _module=_mgmt-rest
INFO[0000] Rest API is enabled
INFO[0000] snapd started                                 _module=snapd block=main
INFO[0000] setting log level to: warning

1. Verify that all snap daemons are running:

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# ps -fae | grep snapd
root     10097  4039  0 07:15 pts/8    00:00:00 ./snapd -t 0
root     10104  4039  0 07:15 pts/8    00:00:00 ./snapd -t 0
root     10110  4039  0 07:15 pts/8    00:00:00 ./snapd -t 0
root     10116  4039  0 07:15 pts/8    00:00:00 ./snapd -t 0

1. But only the first one is listening on 8181 - pid 10097 in this case:

root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin# netstat -tulnp | grep snapd
tcp6       0      0 :::8181                 :::*                    LISTEN      10097/snapd
root@gklab-126-077:~/go/src/github.com/intelsdi-x/snap/build/bin#

[geauxvirtual]

This was first reported in #416 and fixed with PR #430.

PR #532 went in that changed the behavior of the REST API startup. Will investigate why the new changes are not throwing an error if the port is already in use.