systemd

This package provides an access to systemd via D-Bus to list services (think systemctl list-units) with a low overhead for a caller. If you find the API too limiting or missing some of functionality, perhaps https://github.com/coreos/go-systemd might suit you better.

c, err := systemd.New()
if err != nil {
    log.Fatal(err)
}
defer c.Close()

err = c.ListUnits(systemd.IsService, func(u *systemd.Unit) {
    fmt.Printf("%s %s\n", u.Name, u.ActiveState)
})
if err != nil {
    log.Fatal(err)
}
// Output:
// dirmngr.service inactive
// dbus.service active
// snapd.session-agent.service inactive
// gpg-agent.service inactive
// pk-debconf-helper.service inactive

Check out units program to see how to get PIDs of services.

$ go run ./cmd/units -svc
0 dirmngr.service inactive
2375 dbus.service active
0 snapd.session-agent.service inactive
0 gpg-agent.service inactive
0 pk-debconf-helper.service inactive

You can get the same results with dbus-send.

$ dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnits
$ dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1/unit/dbus_2eservice org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Service' string:'MainPID'

Testing

Run tests and linters.

$ go test -v -count=1 .
$ golangci-lint run

Benchmarks help to spot performance changes and troubleshoot performance issues. For example, you can see where and how much memory gets allocated when a 35KB D-Bus ListUnits reply is decoded into 157 Unit structs.

$ go test -run=^$ -bench=^BenchmarkDecodeListUnits$ -benchmem -memprofile list_units.allocs
$ go tool pprof list_units.allocs

It is recommended to run benchmarks multiple times and check how stable they are using Benchstat tool.

$ go test -timeout 20m -bench=. -benchmem -count=50 . | tee bench-new.txt
$ benchstat bench-new.txt

The old and new stats are compared as follows.

$ benchstat bench-old.txt bench-new.txt

name               old time/op    new time/op    delta
AuthExternal-2        475ns ± 2%     467ns ± 2%    -1.72%  (p=0.000 n=46+45)
DecodeString-2       54.3ns ± 3%    53.4ns ± 4%    -1.67%  (p=0.000 n=49+50)
EscapeBusLabel-2     43.8ns ± 2%    43.2ns ± 2%    -1.35%  (p=0.000 n=48+50)
DecodeHeader-2        340ns ± 4%     340ns ±12%    -0.17%  (p=0.017 n=48+45)
EncodeHeader-2        185ns ± 1%     187ns ± 3%    +1.28%  (p=0.000 n=48+49)
EncodeHello-2         224ns ± 2%     221ns ± 3%    -1.03%  (p=0.000 n=46+50)
DecodeHello-2         122ns ± 4%      84ns ±11%   -31.21%  (p=0.000 n=49+49)
EncodeMainPID-2       383ns ± 4%     383ns ± 3%      ~     (p=0.548 n=43+48)
DecodeMainPID-2       140ns ± 2%      96ns ± 3%   -31.63%  (p=0.000 n=48+50)
EncodeListUnits-2     234ns ± 2%     236ns ± 2%    +0.61%  (p=0.001 n=41+47)
DecodeListUnits-2    92.9µs ± 2%    92.5µs ± 5%    -0.39%  (p=0.028 n=48+45)

name               old alloc/op   new alloc/op   delta
AuthExternal-2        80.0B ± 0%     80.0B ± 0%      ~     (all equal)
DecodeString-2        0.00B          0.00B           ~     (all equal)
EscapeBusLabel-2      0.00B          0.00B           ~     (all equal)
DecodeHeader-2        15.0B ± 0%     15.0B ± 0%      ~     (all equal)
EncodeHeader-2        0.00B          0.00B           ~     (all equal)
EncodeHello-2         0.00B          0.00B           ~     (all equal)
DecodeHello-2         29.0B ± 0%      5.0B ± 0%   -82.76%  (p=0.000 n=50+50)
EncodeMainPID-2       45.0B ± 0%     45.0B ± 0%      ~     (all equal)
DecodeMainPID-2       24.0B ± 0%      0.0B       -100.00%  (p=0.000 n=50+50)
EncodeListUnits-2     0.00B          0.00B           ~     (all equal)
DecodeListUnits-2    21.0kB ± 0%    20.9kB ± 0%    -0.11%  (p=0.000 n=50+50)

name               old allocs/op  new allocs/op  delta
AuthExternal-2         3.00 ± 0%      3.00 ± 0%      ~     (all equal)
DecodeString-2         0.00           0.00           ~     (all equal)
EscapeBusLabel-2       0.00           0.00           ~     (all equal)
DecodeHeader-2         0.00           0.00           ~     (all equal)
EncodeHeader-2         0.00           0.00           ~     (all equal)
EncodeHello-2          0.00           0.00           ~     (all equal)
DecodeHello-2          1.00 ± 0%      0.00       -100.00%  (p=0.000 n=50+50)
EncodeMainPID-2        0.00           0.00           ~     (all equal)
DecodeMainPID-2        1.00 ± 0%      0.00       -100.00%  (p=0.000 n=50+50)
EncodeListUnits-2      0.00           0.00           ~     (all equal)
DecodeListUnits-2      6.00 ± 0%      5.00 ± 0%   -16.67%  (p=0.000 n=50+50)

When there is a statistically significant improvement, please update bench-old.txt and the benchmark results above.

$ cp bench-new.txt bench-old.txt