Release session resources on partial-setup and error paths

Two leaks in the syncer session lifecycle:

• newSession opened the source transport (and started the memory-measurement ticker goroutine) before listing refs and setting up the target. If any of those steps failed it returned nil, err, so the caller had no session to finish() — leaking the source conn (an SSH transport spawns a process), a half-open target conn, and the ticker goroutine.
• finish() never stopped the measurement ticker; it was only stopped when a Result was built, so any early error return from runSync et al. leaked the goroutine even on a fully-constructed session.

Add a success-guarded deferred cleanup in newSession that calls finish() unless the session is handed back, hand the target conn to the session as soon as it is opened so cleanup can close it, and have finish() also invoke the sync.Once-guarded measurementDone so the ticker stops on every path.

Sessions

Changes

2

• internal/syncer

• Ameasurement_test.go+46

• Msyncer.go+30/-13

package syncer

import (
    "runtime"
    "testing"
    "time"
)

// The measurement closure must stop its ticker goroutine when invoked, and be
// safe to invoke more than once — finish() now calls it on every path
// (including error paths) in addition to the happy-path Result builder, so a
// double-call must neither panic (double close) nor change the reported value.
func TestStartMeasurementStopsGoroutineAndIsIdempotent(t *testing.T) {
    before := runtime.NumGoroutine()

done := startMeasurement(true)

m1 := done()
    if !m1.Enabled {
        t.Fatalf("expected an enabled measurement, got %+v", m1)
    }
m2 := done() // second call (the finish() path) must be a safe no-op
    if m1 != m2 {
        t.Fatalf("measurement changed across calls: %+v vs %+v", m1, m2)
    }

// The ticker goroutine must have exited; poll to avoid races with the
    // scheduler tearing it down.
    deadline := time.Now().Add(2 * time.Second)
    for runtime.NumGoroutine() > before {
        if time.Now().After(deadline) {
            t.Fatalf("measurement goroutine leaked: %d goroutines, baseline %d",
                runtime.NumGoroutine(), before)
        }
        time.Sleep(10 * time.Millisecond)
    }
}

// When disabled, no goroutine is started and the closure is still safe to call.
func TestStartMeasurementDisabledIsInert(t *testing.T) {
    done := startMeasurement(false)
    if m := done(); m.Enabled {
        t.Fatalf("disabled measurement should not be enabled: %+v", m)
    }
    _ = done() // idempotent
}

Internal/syncer/measurement_test.go

// other code ...

Internal/syncer/syncer.go

// other code ...