Mutate request instances based on existing request

Author: Rosie Ellis

SingularityService/src/main/java/com/hubspot/singularity/config/SingularityConfiguration.java

@@ -453,6 +453,8 @@ public class SingularityConfiguration extends Configuration {
 
   private boolean skipPersistingTooLongTaskIds = false;
 
+  private boolean allowSettingRequestInstances = false;
+
   public long getAskDriverToKillTasksAgainAfterMillis() {
     return askDriverToKillTasksAgainAfterMillis;
   }
@@ -2119,4 +2121,12 @@ public boolean skipPersistingTooLongTaskIds() {
   public void setSkipPersistingTooLongTaskIds(boolean skipPersistingTooLongTaskIds) {
     this.skipPersistingTooLongTaskIds = skipPersistingTooLongTaskIds;
   }
+
+  public boolean allowSettingRequestInstances() {
+    return allowSettingRequestInstances;
+  }
+
+  public void setAllowSettingRequestInstances(boolean allowSettingRequestInstances) {
+    this.allowSettingRequestInstances = allowSettingRequestInstances;
+  }
 }

SingularityService/src/main/java/com/hubspot/singularity/data/SingularityValidator.java

@@ -230,10 +230,30 @@ public SingularityRequest checkSingularityRequest(
       request.getId().length(),
       request.getId()
     );
-    checkBadRequest(
-      !request.getInstances().isPresent() || request.getInstances().get() > 0,
-      "Instances must be greater than 0"
-    );
+
+    if (singularityConfiguration.allowSettingRequestInstances()) {
+      checkBadRequest(
+        (
+          !existingRequest.flatMap(SingularityRequest::getInstances).isPresent() &&
+          !request.getInstances().isPresent()
+        ) ||
+        request.getInstances().get() > 0,
+        "Instances must be greater than 0"
+      );
+
+      if (
+        existingRequest.flatMap(SingularityRequest::getInstances).isPresent() &&
+        !request.getInstances().isPresent()
+      ) {
+        request =
+          request.toBuilder().setInstances(existingRequest.get().getInstances()).build();
+      }
+    } else {
+      checkBadRequest(
+        !request.getInstances().isPresent() || request.getInstances().get() > 0,
+        "Instances must be greater than 0"
+      );
+    }
 
     checkBadRequest(
       request.getInstancesSafe() <= maxInstancesPerRequest,