healthcheck: monitor access to chain backend

Add a new health check package which will periodically poll health
check functions and shutdown if we do not succeed after our set number
of attempts. The first check that we add is one for our chain backend,
to ensure that we are connected to a bitcoin node.

lncfg/healthcheck.go

@@ -0,0 +1,65 @@
+package lncfg
+
+import (
+	"fmt"
+	"time"
+)
+
+var (
+	// MinHealthCheckInterval is the minimum interval we allow between
+	// health checks.
+	MinHealthCheckInterval = time.Minute
+
+	// MinHealthCheckTimeout is the minimum timeout we allow for health
+	// check calls.
+	MinHealthCheckTimeout = time.Second
+
+	// MinHealthCheckBackoff is the minimum back off we allow between health
+	// check retries.
+	MinHealthCheckBackoff = time.Second
+)
+
+// HealthCheckConfig contains the configuration for the different health checks
+// the lnd runs.
+type HealthCheckConfig struct {
+	ChainCheck *CheckConfig `group:"chainbackend" namespace:"chainbackend"`
+}
+
+// Validate checks the values configured for our health checks.
+func (h *HealthCheckConfig) Validate() error {
+	return h.ChainCheck.validate("chain backend")
+}
+
+type CheckConfig struct {
+	Interval time.Duration `long:"interval" description:"How often to run a health check."`
+
+	Attempts int `long:"attempts" description:"The number of calls we will make for the check before failing. Set this value to 0 to disable a check."`
+
+	Timeout time.Duration `long:"timeout" description:"The amount of time we allow the health check to take before failing due to timeout."`
+
+	Backoff time.Duration `long:"backoff" description:"The amount of time to back-off between failed health checks."`
+}
+
+// validate checks the values in a health check config entry if it is enabled.
+func (c *CheckConfig) validate(name string) error {
+	if c.Attempts == 0 {
+		return nil
+	}
+
+	if c.Backoff < MinHealthCheckBackoff {
+		return fmt.Errorf("%v backoff: %v below minimum: %v", name,
+			c.Backoff, MinHealthCheckBackoff)
+	}
+
+	if c.Timeout < MinHealthCheckTimeout {
+		return fmt.Errorf("%v timeout: %v below minimum: %v", name,
+			c.Timeout, MinHealthCheckTimeout)
+	}
+
+	if c.Interval < MinHealthCheckInterval {
+		return fmt.Errorf("%v interval: %v below minimum: %v", name,
+			c.Interval, MinHealthCheckInterval)
+	}
+
+	return nil
+}