core: Reject retryThrottling without maxTokens or tokenRatio clearly (grpc#13131)

## Problem

`ServiceConfigUtil.getThrottlePolicy()` reads the two required fields of
`retryThrottling` with `JsonUtil.getNumberAsDouble(...).floatValue()`.
When
either field is missing, `getNumberAsDouble` returns null and the call
fails
with an NPE:

Cannot invoke "java.lang.Double.floatValue()" because the return value
of
"io.grpc.internal.JsonUtil.getNumberAsDouble(java.util.Map, String)" is
null

`ScParser` catches it and the channel reports `UNKNOWN: failed to parse
service config`, but the cause doesn't say which field is missing. This
was
noted in the code as `TODO(dapengzhang0): check if this is null.`

## Fix

Check both fields with `checkNotNull(..., "<field> cannot be empty")`,
the
same message used for the required `retryPolicy` and `hedgingPolicy`
fields
in `ManagedChannelServiceConfig`. The exception type is still the NPE
that
`ScParser` already handles, so the only change is a clearer message.

## Testing

Added `getThrottlePolicy_failOnMissingMaxTokens` and
`getThrottlePolicy_failOnMissingTokenRatio` to `ServiceConfigUtilTest`.
Both
failed before the fix with the NPE above.

Author: JoeCqupt

core/src/main/java/io/grpc/internal/ServiceConfigUtil.java

@@ -112,9 +112,12 @@ static Throttle getThrottlePolicy(@Nullable Map<String, ?> serviceConfig) {
       return null;
     }
 
-    // TODO(dapengzhang0): check if this is null.
-    float maxTokens = JsonUtil.getNumberAsDouble(throttling, "maxTokens").floatValue();
-    float tokenRatio = JsonUtil.getNumberAsDouble(throttling, "tokenRatio").floatValue();
+    float maxTokens = checkNotNull(
+        JsonUtil.getNumberAsDouble(throttling, "maxTokens"),
+        "maxTokens cannot be empty").floatValue();
+    float tokenRatio = checkNotNull(
+        JsonUtil.getNumberAsDouble(throttling, "tokenRatio"),
+        "tokenRatio cannot be empty").floatValue();
     checkState(maxTokens > 0f, "maxToken should be greater than zero");
     checkState(tokenRatio > 0f, "tokenRatio should be greater than zero");
     return new Throttle(maxTokens, tokenRatio);

core/src/test/java/io/grpc/internal/ServiceConfigUtilTest.java

@@ -114,6 +114,28 @@ public void unwrapLoadBalancingConfigList_failOnMalformedConfig() throws Excepti
     }
   }
 
+  @Test
+  public void getThrottlePolicy_failOnMissingMaxTokens() throws Exception {
+    String serviceConfig = "{\"retryThrottling\" : {\"tokenRatio\" : 0.1}}";
+    try {
+      ServiceConfigUtil.getThrottlePolicy(checkObject(JsonParser.parse(serviceConfig)));
+      fail("Should throw");
+    } catch (Exception e) {
+      assertThat(e).hasMessageThat().contains("maxTokens cannot be empty");
+    }
+  }
+
+  @Test
+  public void getThrottlePolicy_failOnMissingTokenRatio() throws Exception {
+    String serviceConfig = "{\"retryThrottling\" : {\"maxTokens\" : 10}}";
+    try {
+      ServiceConfigUtil.getThrottlePolicy(checkObject(JsonParser.parse(serviceConfig)));
+      fail("Should throw");
+    } catch (Exception e) {
+      assertThat(e).hasMessageThat().contains("tokenRatio cannot be empty");
+    }
+  }
+
   @SuppressWarnings("unchecked")
   private static List<Map<String, ?>> checkObjectList(Object o) {
     return (List<Map<String, ?>>) o;