feat(pkg/sshd/circuit*): add a circuit primitive

it will be used in the health probe, so the builder isn’t considered up
until the SSH server is actually running

Author: Aaron Schlesinger

pkg/sshd/circuit.go

@@ -0,0 +1,56 @@
+package sshd
+
+import (
+	"fmt"
+	"sync/atomic"
+)
+
+type CircuitState uint32
+
+const (
+	OpenState   CircuitState = 0
+	ClosedState CircuitState = 1
+)
+
+func (c CircuitState) String() string {
+	if c == OpenState {
+		return "OPEN"
+	} else if c == ClosedState {
+		return "CLOSED"
+	} else {
+		return fmt.Sprintf("Unknown (%d)", c.toUint32())
+	}
+}
+
+func (s CircuitState) toUint32() uint32 {
+	return uint32(s)
+}
+
+// Circuit is a concurrency-safe data structure that can take one of two states at any point in time:
+//
+// - OpenState - non functional
+// - ClosedState - functional
+//
+//
+type Circuit struct {
+	bit uint32
+}
+
+func NewCircuit() *Circuit {
+	return &Circuit{bit: OpenState.toUint32()}
+}
+
+// State returns the current state of the circuit. Note that concurrent modifications may be happening, so the state may be different than what's returned
+func (c *Circuit) State() CircuitState {
+	return CircuitState(atomic.LoadUint32(&c.bit))
+}
+
+// Close closes the circuit if it wasn't already closed. Returns true if it had to be closed, false if it was already closed
+func (c *Circuit) Close() bool {
+	return atomic.CompareAndSwapUint32(&c.bit, OpenState.toUint32(), ClosedState.toUint32())
+}
+
+// Open opens the circuit if it wasn't already closed. Returns true if it had to be opened, false if it was already open
+func (c *Circuit) Open() bool {
+	return atomic.CompareAndSwapUint32(&c.bit, ClosedState.toUint32(), OpenState.toUint32())
+}

pkg/sshd/circuit_test.go

@@ -0,0 +1,28 @@
+package sshd
+
+import (
+	"testing"
+)
+
+func TestOpenCloseSerial(t *testing.T) {
+	c := NewCircuit()
+	if c.State() != OpenState {
+		t.Fatalf("unexpected initial circuit state. want %s, got %s", OpenState, c.State())
+	}
+	if b := c.Close(); !b {
+		t.Fatalf("tried to close the circuit but it said it was already closed")
+	}
+	if c.State() != ClosedState {
+		t.Fatalf("unexpected circuit state. want %s, got %s", ClosedState, c.State())
+	}
+	if b := c.Open(); !b {
+		t.Fatalf("tried to open the circuit but it said it was already opened")
+	}
+	if c.State() != OpenState {
+		t.Fatalf("unexpected circuit state. want %s, got %s", OpenState, c.State())
+	}
+}
+
+func TestOpenCloseConcurrent(t *testing.T) {
+
+}