set limit on number of files the API will process

src/main/resources/reference.conf

@@ -94,3 +94,7 @@ googlecloud {
                             "153601": 0.045
                            }
 }
+
+firecloud {
+  max-filematching-bucket-files = 25000
+}

src/main/scala/org/broadinstitute/dsde/firecloud/FireCloudConfig.scala

@@ -153,6 +153,7 @@ object FireCloudConfig {
     lazy val supportDomain = firecloud.getString("supportDomain")
     lazy val supportPrefix = firecloud.getString("supportPrefix")
     lazy val userAdminAccount = firecloud.getString("userAdminAccount")
+    lazy val maxFileMatchingFileCount = firecloud.getInt("max-filematching-bucket-files")
   }
 
   object Shibboleth {

src/main/scala/org/broadinstitute/dsde/firecloud/service/ExportEntitiesByTypeActor.scala

@@ -99,6 +99,9 @@ class ExportEntitiesByTypeActor(rawlsDAO: RawlsDAO,
     case None => ModelSchemaRegistry.getModelForSchemaType(SchemaTypes.FIRECLOUD)
   }
 
+  // maximum allowed count of files in a bucket for the file-matching API
+  private val maxFileMatchingFileCount = FireCloudConfig.FireCloud.maxFileMatchingFileCount
+
   def ExportEntities = streamEntities()
 
   /**
@@ -412,6 +415,12 @@ class ExportEntitiesByTypeActor(rawlsDAO: RawlsDAO,
       val fileList: List[GcsObjectName] =
         googleServicesDao.listBucket(workspaceBucket, Option(matchingOptions.prefix), recursive)
 
+      // sanity check
+      if (fileList.length > maxFileMatchingFileCount) {
+        throw new FireCloudExceptionWithErrorReport(errorReport =
+          ErrorReport(StatusCodes.BadRequest, s"Too many files in bucket (${fileList.length}); cannot continue.")
+        )
+      }
       logger.info(s"found ${fileList.length} files")
 
       // transform the list of GcsObjectName to a list of java.nio.Path

src/test/resources/reference.conf

@@ -101,3 +101,7 @@ spray.can.host-connector {
 notification {
   fullyQualifiedNotificationTopic = "dummy"
 }
+
+firecloud {
+  max-filematching-bucket-files = 25000
+}

src/test/scala/org/broadinstitute/dsde/firecloud/service/ExportEntitiesByTypeServiceSpec.scala

@@ -11,12 +11,15 @@ import akka.http.scaladsl.testkit.RouteTestTimeout
 import akka.http.scaladsl.unmarshalling.Unmarshal
 import better.files.File
 import org.apache.commons.lang3.StringUtils
+import org.broadinstitute.dsde.firecloud.FireCloudConfig
 import org.broadinstitute.dsde.firecloud.dataaccess.MockRawlsDAO
 import org.broadinstitute.dsde.firecloud.filematch.FileMatchingOptions
 import org.broadinstitute.dsde.firecloud.filematch.FileMatchingOptionsFormat.fileMatchingOptionsFormat
 import org.broadinstitute.dsde.firecloud.mock.MockGoogleServicesDAO
 import org.broadinstitute.dsde.firecloud.model._
 import org.broadinstitute.dsde.firecloud.webservice.{CookieAuthedApiService, ExportEntitiesApiService}
+import org.broadinstitute.dsde.rawls.model.ErrorReport
+import org.broadinstitute.dsde.rawls.model.WorkspaceJsonSupport.ErrorReportFormat
 import org.broadinstitute.dsde.workbench.model.google.{GcsBucketName, GcsObjectName}
 import org.mockito.ArgumentMatchers
 import org.mockito.ArgumentMatchers.any
@@ -403,6 +406,27 @@ class ExportEntitiesByTypeServiceSpec
         responseAs[String] shouldBe expected
       }
     }
+
+    s"should throw an error if the bucket contains too many files" in {
+      val configuredMax = FireCloudConfig.FireCloud.maxFileMatchingFileCount
+      val bucketListResponse: List[GcsObjectName] =
+        Range.apply(0, configuredMax + 1).map(idx => GcsObjectName(s"file$idx")).toList
+
+      when(mockitoGoogleServicesDao.listBucket(any(), any(), any())).thenReturn(bucketListResponse)
+      val fileMatchingOptions = FileMatchingOptions("prefix")
+      Post("/api/workspaces/broad-dsde-dev/valid/entities/my-entity-type/paired-tsv",
+           fileMatchingOptions
+      ) ~> dummyUserIdHeaders("1234") ~> sealRoute(
+        exportEntitiesRoutes
+      ) ~> check {
+        handled should be(true)
+        status should be(BadRequest)
+        contentType shouldEqual ContentTypes.`application/json`
+        val actualError = responseAs[ErrorReport]
+        actualError.message should startWith("Too many files in bucket")
+      }
+    }
+
   }
 
   val validCookieFireCloudEntitiesLargeSampleTSVPath =