health_check: fix divide-by-zero in retry interval calculations. (#4737)

A fuzz failure indicated a divide-by-zero in HealthCheckerImplBase::interval(). Beyond this, on
inspection, it seems we allow zero retry intervals, which can live lock the main thread. This PR
fixes these issues.

Fixes oss-fuzz issue https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10786.

Risk Level: Low
Testing: Additional unit test and a corpus entry added.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ b61b28fe7ae9927452477a0f4e337c53830a7070

envoy/api/v2/core/health_check.proto

@@ -39,11 +39,11 @@ message HealthCheck {
   ];
 
   // An optional jitter amount in millseconds. If specified, during every
-  // interval Envoy will add 0 to interval_jitter to the wait time.
+  // interval Envoy will add interval_jitter to the wait time.
   google.protobuf.Duration interval_jitter = 3;
 
   // An optional jitter amount as a percentage of interval_ms. If specified,
-  // during every interval Envoy will add 0 to interval_ms *
+  // during every interval Envoy will add interval_ms *
   // interval_jitter_percent / 100 to the wait time.
   //
   // If interval_jitter_ms and interval_jitter_percent are both set, both of
@@ -185,14 +185,14 @@ message HealthCheck {
   // any other.
   //
   // The default value for "no traffic interval" is 60 seconds.
-  google.protobuf.Duration no_traffic_interval = 12;
+  google.protobuf.Duration no_traffic_interval = 12 [(validate.rules).duration.gt = {}];
 
   // The "unhealthy interval" is a health check interval that is used for hosts that are marked as
   // unhealthy. As soon as the host is marked as healthy, Envoy will shift back to using the
   // standard health check interval that is defined.
   //
   // The default value for "unhealthy interval" is the same as "interval".
-  google.protobuf.Duration unhealthy_interval = 14;
+  google.protobuf.Duration unhealthy_interval = 14 [(validate.rules).duration.gt = {}];
 
   // The "unhealthy edge interval" is a special health check interval that is used for the first
   // health check right after a host is marked as unhealthy. For subsequent health checks
@@ -200,14 +200,14 @@ message HealthCheck {
   // check interval that is defined.
   //
   // The default value for "unhealthy edge interval" is the same as "unhealthy interval".
-  google.protobuf.Duration unhealthy_edge_interval = 15;
+  google.protobuf.Duration unhealthy_edge_interval = 15 [(validate.rules).duration.gt = {}];
 
   // The "healthy edge interval" is a special health check interval that is used for the first
   // health check right after a host is marked as healthy. For subsequent health checks
   // Envoy will shift back to using the standard health check interval that is defined.
   //
   // The default value for "healthy edge interval" is the same as the default interval.
-  google.protobuf.Duration healthy_edge_interval = 16;
+  google.protobuf.Duration healthy_edge_interval = 16 [(validate.rules).duration.gt = {}];
 
   // Specifies the path to the :ref:`health check event log <arch_overview_health_check_logging>`.
   // If empty, no event log will be written.