returns 400 on validator-faucets api when no validator_ids are sent

apps/scan/src/main/scala/org/lfdecentralizedtrust/splice/scan/admin/http/HttpScanHandler.scala

@@ -2428,16 +2428,23 @@ class HttpScanHandler(
   ): Future[ScanResource.GetValidatorFaucetsByValidatorResponse] = {
     implicit val tc = extracted
     withSpan(s"$workflowId.getValidatorFaucetsByValidator") { _ => _ =>
-      store
-        .getValidatorLicenseByValidator(validators.map(v => PartyId.tryFromProtoPrimitive(v)))
-        .map(licenses =>
-          ScanResource.GetValidatorFaucetsByValidatorResponse.OK(
-            definitions
-              .GetValidatorFaucetsByValidatorResponse(
-                FaucetProcessor.process(licenses)
-              )
-          )
+      if (validators.isEmpty) {
+        Future.successful(
+          ScanResource.GetValidatorFaucetsByValidatorResponse
+            .BadRequest(definitions.ErrorResponse("validator_ids must not be empty"))
         )
+      } else {
+        store
+          .getValidatorLicenseByValidator(validators.map(v => PartyId.tryFromProtoPrimitive(v)))
+          .map(licenses =>
+            ScanResource.GetValidatorFaucetsByValidatorResponse.OK(
+              definitions
+                .GetValidatorFaucetsByValidatorResponse(
+                  FaucetProcessor.process(licenses)
+                )
+            )
+          )
+      }
     }
   }
 

apps/scan/src/test/scala/org/lfdecentralizedtrust/splice/store/db/ScanStoreTest.scala

@@ -1949,6 +1949,22 @@ class DbScanStoreTest
         result should not contain charlesValidatorLicense
       }
     }
+
+    "return an empty result when given no validators" in {
+      val alice = userParty(443)
+      val aliceValidatorLicense = validatorLicense(
+        alice,
+        dsoParty,
+        Some(new FaucetState(new Round(0), new Round(1000L), 0L))
+      )
+      for {
+        store <- mkStore()
+        _ <- dummyDomain.create(aliceValidatorLicense)(store.multiDomainAcsStore)
+        result <- store.getValidatorLicenseByValidator(Vector.empty)
+      } yield {
+        result shouldBe empty
+      }
+    }
   }
   "Changing the acsStoreDescriptorUserVersion" should {
     val alice = userParty(443)

docs/src/release_notes_upcoming.rst

@@ -42,6 +42,9 @@
       wallets can make use of the new batching functionality (see below) on all validator
       nodes.
 
+  - Scan
+    - The ``/v0/validators/validator-faucets`` endpoint now returns HTTP 400 when called with no ``validator_ids`` query parameters, instead of returning an empty result. This makes typos in the query parameter key (e.g. ``validator`` instead of ``validator_ids``) fail loudly rather than silently.
+
   - Daml
 
     - Release the Token Standard V2 APIs (`CIP-112 <https://github.com/canton-foundation/cips/blob/main/cip-0112/cip-0112.md>`__).