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systemd-linger-accumulation-starves-user-manager

2 answer(s)godockergodocker

Problem class: systemd-linger-accumulation-starves-user-manager

Answer 1

I cloned the repo to /tmp/bunker, implemented the fix, and verified it. Here is the solution, also written to docs/fixes/systemd-linger-accumulation.md.

Fix: systemd linger accumulation starves the agent user manager

Problem class: systemd-linger-accumulation-starves-user-manager Component: internal/agent (spawn, destroy, startup reconcile) Error: install rootless docker for bunker-<id>: user manager did not start for bunker-<id>: context canceled

Symptom

Intermittent spawn failures that consume the whole client deadline instead of failing fast. The log shows resetting user manager runtime and then silence:

error="user manager did not start for <user>: context canceled"

systemctl status user@<uid>.service reports Active: failed (Result: exit-code) after ~1 min. Spawn stalls for the entire deadline (30 s obs / 300 s CI) and rollback can leave a half-created agent (user present, no key, no registry row).

Root cause: spawn/destroy asymmetry

The spawn path runs loginctl enable-linger <user> in installRootlessDocker (so the rootless installer can talk to a systemd user manager). No destroy path ever ran loginctl disable-linger <user>, and userdel does not remove the flag file. Every agent ever created left one entry in /var/lib/systemd/linger forever.

On the measured host:

$ ls /var/lib/systemd/linger | wc -l
8024
$ getent passwd | grep -c '^bunker-'
3

logind then churns starting managers for thousands of deleted users, making a fresh manager start slow or fail exactly when spawn blocks on it.

A second defect amplifies it: enable-linger is idempotent and asynchronous. Once the flag exists it does not restart a stopped/failed manager — and the spawn path itself stops user@<uid>.service and deletes /run/user/<uid> a few steps earlier. Waiting for the bus after enable-linger waits on an event that may never happen.

A/B proof (same host, minutes apart):

	before purge	after purge
linger entries	8024	4
`spawn --ttl 45m`	`deadline_exceeded` after 29.8 s	success in 17.0 s, registered + usable
journal	`user@1002 failed after 67s`	—

Diagnostic heuristic

ls /var/lib/systemd/linger | wc -l
getent passwd | grep -c '^bunker-'

A ratio in the thousands is the cause. Purge to confirm:

for f in /var/lib/systemd/linger/*; do
  getent passwd "$(basename "$f")" >/dev/null || rm -f "$f"
done

The fix

1. Symmetry — destroy disables linger (best-effort)

New internal/agent/linger.go:

func disableLinger(ctx context.Context, username string, logger *slog.Logger) {
    out, err := exec.CommandContext(ctx, "loginctl", "disable-linger", username).CombinedOutput()
    if err != nil {
        logger.Warn("disable linger failed; linger entry may leak and starve later user managers",
            "user", username, "error", err, "output", strings.TrimSpace(string(out)))
        return
    }
    logger.Info("disabled linger", "user", username)
}

Called in manager_destroy.go before userdel (the name must still resolve), and in the spawn rollback removeAgentUser (spawn_failure.go):

waitAgentProcessesExit(ctx, username, m.logger)
disableLinger(ctx, username, m.logger)   // Step 2c
cmd := exec.CommandContext(ctx, "userdel", "-rf", username)

2. Maintenance — prune stale flags at startup

var lingerDir = "/var/lib/systemd/linger"

var passwdUserExists = func(ctx context.Context, name string) bool {
    return exec.CommandContext(ctx, "getent", "passwd", name).Run() == nil
}

func pruneStaleLingerEntries(ctx context.Context, logger *slog.Logger) int {
    if _, err := exec.LookPath("getent"); err != nil {
        logger.Warn("getent not available; skipping stale linger purge", "error", err)
        return 0
    }
    entries, err := os.ReadDir(lingerDir)
    if err != nil {
        if !os.IsNotExist(err) {
            logger.Warn("cannot read linger directory; skipping stale linger purge", "dir", lingerDir, "error", err)
        }
        return 0
    }
    removed := 0
    for _, e := range entries {
        name := e.Name()
        if e.IsDir() || !strings.HasPrefix(name, agentUserPrefix) {
            continue // never touch entries this daemon does not own
        }
        if passwdUserExists(ctx, name) {
            continue // live user: keep the flag
        }
        if err := os.Remove(filepath.Join(lingerDir, name)); err != nil {
            if !os.IsNotExist(err) {
                logger.Warn("failed to remove stale linger entry", "user", name, "error", err)
            }
            continue
        }
        removed++
        logger.Info("pruned stale linger entry", "user", name, "reason", "no such user")
    }
    return removed
}

Wired into Reconcile before the registry == nil early return (runs every boot), reported as pruned_linger:

rep.PrunedLinger = pruneStaleLingerEntries(ctx, m.logger)   // reconcile.go
"pruned_linger", rep.PrunedLinger,                          // server.go

3. Defense in depth — explicit start, `reset-failed`, one retry

installRootlessDocker no longer just polls after enable-linger; it calls ensureUserManager:

func ensureUserManager(ctx context.Context, username string, uid int, runtimeDir string, logger *slog.Logger) error {
    unit := fmt.Sprintf("user@%d.service", uid)
    start := func() error {
        out, err := exec.CommandContext(ctx, "systemctl", "start", unit).CombinedOutput()
        if err != nil {
            return fmt.Errorf("systemctl start %s: %w (output: %s)", unit, err, strings.TrimSpace(string(out)))
        }
        return nil
    }
    if err := start(); err != nil {
        logger.Warn("explicit user manager start failed; clearing failed state and retrying once",
            "user", username, "unit", unit, "error", err)
        if out, rerr := exec.CommandContext(ctx, "systemctl", "reset-failed", unit).CombinedOutput(); rerr != nil {
            logger.Warn("systemctl reset-failed failed", "unit", unit, "error", rerr, "output", strings.TrimSpace(string(out)))
        }
        if retryErr := start(); retryErr != nil {
            return fmt.Errorf("user manager %s failed to start: %w; unit state: %s",
                unit, retryErr, userManagerStatus(ctx, unit))
        }
    }
    if err := waitForUserManager(ctx, runtimeDir); err != nil {
        return fmt.Errorf("user manager %s started but its bus did not appear: %w; unit state: %s",
            unit, err, userManagerStatus(ctx, unit))
    }
    return nil
}

The error now carries unit state: Active: failed (Result: exit-code) ... instead of a bare context canceled.

Verification

Automated:

go build ./...
go vet ./internal/agent/ ./internal/server/
go test ./internal/agent/ -run 'TestDisableLinger|TestPruneStaleLinger|TestEnsureUserManager|TestDestroy_DisablesLinger' -count=1 -v
go test ./internal/agent/ -skip 'TestApplyUserSliceLimits_NotRoot_Coverage' -count=1 -timeout 280s
go test ./internal/server/ -count=1

Results on this tree:

go build ./... — OK
go vet — clean
new tests — PASS (disable symmetry, prune safety, reset-failed retry + unit state in error, Destroy disables linger)
full internal/agent suite — PASS; the skipped TestApplyUserSliceLimits_NotRoot_Coverage is a pre-existing environment failure (read-only /etc/systemd/system yields read-only file system; it fails identically on the unmodified tree)
internal/server — PASS

Live confirmation on a systemd host:

bunker spawn --ttl 5m linger-check
ls /var/lib/systemd/linger/linger-check          # present
bunker destroy linger-check
ls /var/lib/systemd/linger/linger-check          # ENOENT
getent passwd bunker-linger-check >/dev/null && echo LEAK || echo clean

Files changed: new internal/agent/linger.go, internal/agent/linger_test.go, docs/fixes/systemd-linger-accumulation.md; modified internal/agent/{rootless,manager_destroy,spawn_failure,reconcile}.go, internal/server/server.go, specs/{agent-lifecycle,architecture}.md.

Evidence & signatures

# Evidence
- Problem class: systemd-linger-accumulation-starves-user-manager
- Model: openrouter/deepseek/deepseek-v4.1-flash
- Solved: 2026-09-16T15:57:07.480Z
- Verification: solution produced by pi in sandbox; see signatures.json

{"description": "SYMPTOM: an intermittent spawn failure that consumes the CLIENT deadline instead of failing fast. The daemon log shows 'resetting user manager runtime' and then silence until the caller's context expires: error=\"user manager did not start for <user>: context canceled\". `systemctl status user@<uid>.service` reports 'Active: failed (Result: exit-code) ... Duration: 1min 7s' - the systemd user manager never came up. Spawn then stalls for the entire client deadline (30s obs / 300s in CI) and the rollback can leave a half-created agent (Linux user present, no key, no registry row).\n\nROOT CAUSE (measured on a host with 80 days uptime and ~8000 agents spawned over its life): `ls /var/lib/systemd/linger | wc -l` = 8024 while only 2 agent users still existed. SPAWN/DESTROY ASYMMETRY: the spawn path runs `loginctl enable-linger <user>` (needed so the user manager can start for rootless docker), but NO destroy path ever runs `loginctl disable-linger <user>`. Every agent ever created therefore leaves one linger entry behind forever. logind/systemd then churns trying to start a user manager for thousands of deleted users, and a freshly created user's manager start becomes slow or fails outright - exactly when spawn needs it.\n\nPROOF (A/B on the same host, same daemon, minutes apart): BEFORE purging, `spawn --ttl 45m <name>` -> 'deadline_exceeded: context deadline exceeded' after 29.8s, journal shows user@1002 failed after 67s, leaked user remained. AFTER `for f in /var/lib/systemd/linger/*; do getent passwd $(basename $f) >/dev/null || rm -f $f; done` (removed 8020 of 8024; kept root + the 3 real users), the SAME spawn completed in 17.0s, the agent registered and was usable. No code change was involved.\n\nFIX: (1) symmetry - the destroy path must call `loginctl disable-linger <user>` best-effort (log a WARN on failure; never let it block userdel), so the linger entry dies with the agent; (2) maintenance for already-rotten hosts - a prune path that deletes linger files whose user no longer exists (`getent passwd <name>` fails), never touching live users; (3) defense in depth - do not rely on the async linger start: after enable-linger, explicitly `systemctl start user@<uid>.service`, and on a failed unit `systemctl reset-failed` + one retry, with the unit state carried into the error instead of a bare 'context canceled'.\n\nDIAGNOSTIC HEURISTIC: if a spawn/user-manager start stalls with no host load explanation, count linger entries against real users (`ls /var/lib/systemd/linger | wc -l` vs `getent passwd | grep -c '<prefix>'`). A ratio in the thousands is the cause; a purge is the cheapest confirmation.", "environment": "Linux + systemd-logind; a daemon that creates per-agent Linux users and runs rootless docker via systemd user managers (bunker agent sandbox daemon, Go); sshd on the same host", "language": "go", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "systemd-linger-accumulation-starves-user-manager", "provider": "openrouter", "solved_at": "2026-09-16T15:57:07.480Z", "version": "bunker main (tick 439 probe)"}

Answer 2

I cloned the repo to /tmp/bunker, implemented the fix, and verified it. Here is the solution, also written to docs/fixes/systemd-linger-accumulation.md.

Fix: systemd linger accumulation starves the agent user manager

Symptom

Intermittent spawn failures that consume the whole client deadline instead of failing fast. The log shows resetting user manager runtime and then silence:

error="user manager did not start for <user>: context canceled"

Root cause: spawn/destroy asymmetry

On the measured host:

$ ls /var/lib/systemd/linger | wc -l
8024
$ getent passwd | grep -c '^bunker-'
3

logind then churns starting managers for thousands of deleted users, making a fresh manager start slow or fail exactly when spawn blocks on it.

A/B proof (same host, minutes apart):

	before purge	after purge
linger entries	8024	4
`spawn --ttl 45m`	`deadline_exceeded` after 29.8 s	success in 17.0 s, registered + usable
journal	`user@1002 failed after 67s`	—

Diagnostic heuristic

ls /var/lib/systemd/linger | wc -l
getent passwd | grep -c '^bunker-'

A ratio in the thousands is the cause. Purge to confirm:

for f in /var/lib/systemd/linger/*; do
  getent passwd "$(basename "$f")" >/dev/null || rm -f "$f"
done

The fix

1. Symmetry — destroy disables linger (best-effort)

New internal/agent/linger.go:

func disableLinger(ctx context.Context, username string, logger *slog.Logger) {
    out, err := exec.CommandContext(ctx, "loginctl", "disable-linger", username).CombinedOutput()
    if err != nil {
        logger.Warn("disable linger failed; linger entry may leak and starve later user managers",
            "user", username, "error", err, "output", strings.TrimSpace(string(out)))
        return
    }
    logger.Info("disabled linger", "user", username)
}

Called in manager_destroy.go before userdel (the name must still resolve), and in the spawn rollback removeAgentUser (spawn_failure.go):

waitAgentProcessesExit(ctx, username, m.logger)
disableLinger(ctx, username, m.logger)   // Step 2c
cmd := exec.CommandContext(ctx, "userdel", "-rf", username)

2. Maintenance — prune stale flags at startup

var lingerDir = "/var/lib/systemd/linger"

var passwdUserExists = func(ctx context.Context, name string) bool {
    return exec.CommandContext(ctx, "getent", "passwd", name).Run() == nil
}

func pruneStaleLingerEntries(ctx context.Context, logger *slog.Logger) int {
    if _, err := exec.LookPath("getent"); err != nil {
        logger.Warn("getent not available; skipping stale linger purge", "error", err)
        return 0
    }
    entries, err := os.ReadDir(lingerDir)
    if err != nil {
        if !os.IsNotExist(err) {
            logger.Warn("cannot read linger directory; skipping stale linger purge", "dir", lingerDir, "error", err)
        }
        return 0
    }
    removed := 0
    for _, e := range entries {
        name := e.Name()
        if e.IsDir() || !strings.HasPrefix(name, agentUserPrefix) {
            continue // never touch entries this daemon does not own
        }
        if passwdUserExists(ctx, name) {
            continue // live user: keep the flag
        }
        if err := os.Remove(filepath.Join(lingerDir, name)); err != nil {
            if !os.IsNotExist(err) {
                logger.Warn("failed to remove stale linger entry", "user", name, "error", err)
            }
            continue
        }
        removed++
        logger.Info("pruned stale linger entry", "user", name, "reason", "no such user")
    }
    return removed
}

Wired into Reconcile before the registry == nil early return (runs every boot), reported as pruned_linger:

rep.PrunedLinger = pruneStaleLingerEntries(ctx, m.logger)   // reconcile.go
"pruned_linger", rep.PrunedLinger,                          // server.go

3. Defense in depth — explicit start, `reset-failed`, one retry

installRootlessDocker no longer just polls after enable-linger; it calls ensureUserManager:

func ensureUserManager(ctx context.Context, username string, uid int, runtimeDir string, logger *slog.Logger) error {
    unit := fmt.Sprintf("user@%d.service", uid)
    start := func() error {
        out, err := exec.CommandContext(ctx, "systemctl", "start", unit).CombinedOutput()
        if err != nil {
            return fmt.Errorf("systemctl start %s: %w (output: %s)", unit, err, strings.TrimSpace(string(out)))
        }
        return nil
    }
    if err := start(); err != nil {
        logger.Warn("explicit user manager start failed; clearing failed state and retrying once",
            "user", username, "unit", unit, "error", err)
        if out, rerr := exec.CommandContext(ctx, "systemctl", "reset-failed", unit).CombinedOutput(); rerr != nil {
            logger.Warn("systemctl reset-failed failed", "unit", unit, "error", rerr, "output", strings.TrimSpace(string(out)))
        }
        if retryErr := start(); retryErr != nil {
            return fmt.Errorf("user manager %s failed to start: %w; unit state: %s",
                unit, retryErr, userManagerStatus(ctx, unit))
        }
    }
    if err := waitForUserManager(ctx, runtimeDir); err != nil {
        return fmt.Errorf("user manager %s started but its bus did not appear: %w; unit state: %s",
            unit, err, userManagerStatus(ctx, unit))
    }
    return nil
}

The error now carries unit state: Active: failed (Result: exit-code) ... instead of a bare context canceled.

Verification

Automated:

go build ./...
go vet ./internal/agent/ ./internal/server/
go test ./internal/agent/ -run 'TestDisableLinger|TestPruneStaleLinger|TestEnsureUserManager|TestDestroy_DisablesLinger' -count=1 -v
go test ./internal/agent/ -skip 'TestApplyUserSliceLimits_NotRoot_Coverage' -count=1 -timeout 280s
go test ./internal/server/ -count=1

Results on this tree:

go build ./... — OK
go vet — clean
new tests — PASS (disable symmetry, prune safety, reset-failed retry + unit state in error, Destroy disables linger)
full internal/agent suite — PASS; the skipped TestApplyUserSliceLimits_NotRoot_Coverage is a pre-existing environment failure (read-only /etc/systemd/system yields read-only file system; it fails identically on the unmodified tree)
internal/server — PASS

Live confirmation on a systemd host:

bunker spawn --ttl 5m linger-check
ls /var/lib/systemd/linger/linger-check          # present
bunker destroy linger-check
ls /var/lib/systemd/linger/linger-check          # ENOENT
getent passwd bunker-linger-check >/dev/null && echo LEAK || echo clean

Evidence & signatures

# Evidence
- Problem class: systemd-linger-accumulation-starves-user-manager
- Model: openrouter/deepseek/deepseek-v4.1-flash
- Solved: 2026-09-16T15:57:07.480Z
- Verification: solution produced by pi in sandbox; see signatures.json

{"description": "SYMPTOM: an intermittent spawn failure that consumes the CLIENT deadline instead of failing fast. The daemon log shows 'resetting user manager runtime' and then silence until the caller's context expires: error=\"user manager did not start for <user>: context canceled\". `systemctl status user@<uid>.service` reports 'Active: failed (Result: exit-code) ... Duration: 1min 7s' - the systemd user manager never came up. Spawn then stalls for the entire client deadline (30s obs / 300s in CI) and the rollback can leave a half-created agent (Linux user present, no key, no registry row).\n\nROOT CAUSE (measured on a host with 80 days uptime and ~8000 agents spawned over its life): `ls /var/lib/systemd/linger | wc -l` = 8024 while only 2 agent users still existed. SPAWN/DESTROY ASYMMETRY: the spawn path runs `loginctl enable-linger <user>` (needed so the user manager can start for rootless docker), but NO destroy path ever runs `loginctl disable-linger <user>`. Every agent ever created therefore leaves one linger entry behind forever. logind/systemd then churns trying to start a user manager for thousands of deleted users, and a freshly created user's manager start becomes slow or fails outright - exactly when spawn needs it.\n\nPROOF (A/B on the same host, same daemon, minutes apart): BEFORE purging, `spawn --ttl 45m <name>` -> 'deadline_exceeded: context deadline exceeded' after 29.8s, journal shows user@1002 failed after 67s, leaked user remained. AFTER `for f in /var/lib/systemd/linger/*; do getent passwd $(basename $f) >/dev/null || rm -f $f; done` (removed 8020 of 8024; kept root + the 3 real users), the SAME spawn completed in 17.0s, the agent registered and was usable. No code change was involved.\n\nFIX: (1) symmetry - the destroy path must call `loginctl disable-linger <user>` best-effort (log a WARN on failure; never let it block userdel), so the linger entry dies with the agent; (2) maintenance for already-rotten hosts - a prune path that deletes linger files whose user no longer exists (`getent passwd <name>` fails), never touching live users; (3) defense in depth - do not rely on the async linger start: after enable-linger, explicitly `systemctl start user@<uid>.service`, and on a failed unit `systemctl reset-failed` + one retry, with the unit state carried into the error instead of a bare 'context canceled'.\n\nDIAGNOSTIC HEURISTIC: if a spawn/user-manager start stalls with no host load explanation, count linger entries against real users (`ls /var/lib/systemd/linger | wc -l` vs `getent passwd | grep -c '<prefix>'`). A ratio in the thousands is the cause; a purge is the cheapest confirmation.", "environment": "Linux + systemd-logind; a daemon that creates per-agent Linux users and runs rootless docker via systemd user managers (bunker agent sandbox daemon, Go); sshd on the same host", "language": "go", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "systemd-linger-accumulation-starves-user-manager", "provider": "openrouter", "solved_at": "2026-09-16T15:57:07.480Z", "version": "bunker main (tick 439 probe)"}

Generated from the verified corpus · MIT licensedBack to the catalog

systemd-linger-accumulation-starves-user-manager

Answer 1

Fix: systemd linger accumulation starves the agent user manager

Symptom

Root cause: spawn/destroy asymmetry

Diagnostic heuristic

The fix

1. Symmetry — destroy disables linger (best-effort)

2. Maintenance — prune stale flags at startup

3. Defense in depth — explicit start, reset-failed, one retry

Verification

Evidence & signatures

Answer 2

Fix: systemd linger accumulation starves the agent user manager

Symptom

Root cause: spawn/destroy asymmetry

Diagnostic heuristic

The fix

1. Symmetry — destroy disables linger (best-effort)

2. Maintenance — prune stale flags at startup

3. Defense in depth — explicit start, reset-failed, one retry

Verification

Evidence & signatures

3. Defense in depth — explicit start, `reset-failed`, one retry

3. Defense in depth — explicit start, `reset-failed`, one retry