systemd: fix jitter in dbus connection retry backoff

[hakman]

newConnection() retries with exponential backoff when connecting to dbus fails with EAGAIN (#45). The comment says 100ms * 2^attempt + ~12.5% jitter, but the jitter is computed in milliseconds and then converted to time.Duration as is, which makes it nanoseconds:

delay := 100 * time.Millisecond << retry
delay += time.Duration(rand.Int64N(1 + (delay.Milliseconds() >> 3)))

For the first attempt delay.Milliseconds() >> 3 is 12, so the jitter is time.Duration(rand.Int64N(13)), i.e. 0 to 12 nanoseconds. On the last attempt it is at most 800 ns. In effect there is no jitter: clients refused at the same moment retry at the same moment, on every attempt.

This PR makes the jitter 0 to 1/8 of the delay, as the comment intends. rand.N is generic over integer types and takes a time.Duration directly (doc):

delay += rand.N(delay/8 + 1)

Verifying

Maximum jitter per attempt before and after. This is deterministic, so it can be checked by running the following code:

package main

import (
	"fmt"
	"time"
)

func main() {
	for retry := range 7 {
		delay := 100 * time.Millisecond << retry
		// max of time.Duration(rand.Int64N(1 + (delay.Milliseconds() >> 3)))
		before := time.Duration(delay.Milliseconds() >> 3)
		// max of rand.N(delay/8 + 1)
		after := delay / 8                                 
		fmt.Printf("attempt %d: delay %-6v jitter before <= %-6v after <= %v\n", retry, delay, before, after)
	}
}

attempt 0: delay 100ms  jitter before <= 12ns   after <= 12.5ms
attempt 1: delay 200ms  jitter before <= 25ns   after <= 25ms
attempt 2: delay 400ms  jitter before <= 50ns   after <= 50ms
attempt 3: delay 800ms  jitter before <= 100ns  after <= 100ms
attempt 4: delay 1.6s   jitter before <= 200ns  after <= 200ms
attempt 5: delay 3.2s   jitter before <= 400ns  after <= 400ms
attempt 6: delay 6.4s   jitter before <= 800ns  after <= 800ms

Effect under load

Measured with 750 clients connecting at once to a dbus-daemon already at its per-user connection limit (Ubuntu 26.04, dbus-daemon 1.16.2, the 7 attempts from the code):

jitter	clients that exhausted all 7 attempts	wall time for the burst
before (nanoseconds)	76	13.35 s
after (0 to 1/8 of delay)	0	7.90 s

[kolyshkin]

LGTM

Can you add

Fixes: cd71e92 ("systemd: retry when the dbus connection returns EAGAIN")

footer?

[thaJeztah]

LGTM

@kolyshkin didn't merge yet because your comment w.r.t. the commit footer, but good to go otherwise?

[hakman]

Added the footer as requested. Thank you both for the review.

[AkihiroSuda]

GPG signature seems invalid

The email in this signature doesn’t match the committer email.
GPG Key ID: 1F476A1AD1D6829B

[hakman]

GPG signature seems invalid

The email in this signature doesn’t match the committer email.
GPG Key ID: 1F476A1AD1D6829B

Thanks for catching this @AkihiroSuda, fixed now.