Reindex-from-remote: Fail on manual slicing param (#137275)

When the `_reindex` API is used with a remote source (`source.remote`
is set in the request body), neither manual slicing (via
`source.slice` in the request body) nor automatic slicing (via the
`slices` URL parameter) are supported.

Prior to this change, on sending a request with a remote source and
manual slicing enabled, the manual slicing specification would be
ignored, and the complete source would be reindexed without
slicing. After this change, the API will return a response with an
HTTP code 400 (Bad Request) instead.

The new behaviour more correctly indicates to the user that the
requested functionality is not available, rather than silently
ignoring part of their request.

Note that:

- Prior to this change, sending a request with a remote source and
*automatic* slicing enabled already resulted in the correct 400
response.

- The
  [docs](https://www.elastic.co/docs/reference/elasticsearch/rest-apis/reindex-indices#docs-reindex-slice)
  already correctly stated "Reindexing from remote clusters does not
  support manual or automatic slicing."

- As a drive-by, this change also adds a unit test for the validation
  with the `slices` parameter set to `auto`. (Previously, only the
  case where it was set to a number greater than 1 was tested.)

Closes #136269

Author: PeteGillinElastic

docs/changelog/137275.yaml

@@ -0,0 +1,6 @@
+pr: 137275
+summary: "Reindex-from-remote: Fail on manual slicing param"
+area: Indices APIs
+type: bug
+issues:
+ - 136269

server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java

@@ -116,6 +116,12 @@ public ActionRequestValidationException validate() {
             if (getSlices() == AbstractBulkByScrollRequest.AUTO_SLICES || getSlices() > 1) {
                 e = addValidationError("reindex from remote sources doesn't support slices > 1 but was [" + getSlices() + "]", e);
             }
+            if (getSearchRequest().source().slice() != null) {
+                e = addValidationError(
+                    "reindex from remote sources doesn't support source.slice but was [" + getSearchRequest().source().slice() + "]",
+                    e
+                );
+            }
             if (getRemoteInfo().getUsername() != null && getRemoteInfo().getPassword() == null) {
                 e = addValidationError("reindex from remote source included username but not password", e);
             }

server/src/test/java/org/elasticsearch/index/reindex/ReindexRequestTests.java

@@ -162,7 +162,7 @@ public void testReindexFromRemoteDoesNotSupportSearchQuery() {
         );
     }
 
-    public void testReindexFromRemoteDoesNotSupportSlices() {
+    public void testReindexFromRemoteDoesNotSupportSlicesParameterGreaterThan1() {
         ReindexRequest reindex = newRequest();
         reindex.setRemoteInfo(
             new RemoteInfo(
@@ -178,6 +178,7 @@ public void testReindexFromRemoteDoesNotSupportSlices() {
                 RemoteInfo.DEFAULT_CONNECT_TIMEOUT
             )
         );
+        // Enable automatic slicing with a random number of slices greater than 1 (like setting the slices URL parameter):
         reindex.setSlices(between(2, Integer.MAX_VALUE));
         ActionRequestValidationException e = reindex.validate();
         assertEquals(
@@ -186,6 +187,60 @@ public void testReindexFromRemoteDoesNotSupportSlices() {
         );
     }
 
+    public void testReindexFromRemoteDoesNotSupportSlicesParameterSetToAuto() {
+        ReindexRequest reindex = newRequest();
+        reindex.setRemoteInfo(
+            new RemoteInfo(
+                randomAlphaOfLength(5),
+                randomAlphaOfLength(5),
+                between(1, Integer.MAX_VALUE),
+                null,
+                matchAll,
+                null,
+                null,
+                emptyMap(),
+                RemoteInfo.DEFAULT_SOCKET_TIMEOUT,
+                RemoteInfo.DEFAULT_CONNECT_TIMEOUT
+            )
+        );
+        // Enable automatic slicing with an automatically chosen number of slices (like setting the slices URL parameter to "auto"):
+        reindex.setSlices(AbstractBulkByScrollRequest.AUTO_SLICES);
+        ActionRequestValidationException e = reindex.validate();
+        assertEquals(
+            "Validation Failed: 1: reindex from remote sources doesn't support slices > 1 but was [" + reindex.getSlices() + "];",
+            e.getMessage()
+        );
+    }
+
+    public void testReindexFromRemoteDoesNotSupportSlicesSourceField() {
+        ReindexRequest reindex = newRequest();
+        reindex.setRemoteInfo(
+            new RemoteInfo(
+                randomAlphaOfLength(5),
+                randomAlphaOfLength(5),
+                between(1, Integer.MAX_VALUE),
+                null,
+                matchAll,
+                null,
+                null,
+                emptyMap(),
+                RemoteInfo.DEFAULT_SOCKET_TIMEOUT,
+                RemoteInfo.DEFAULT_CONNECT_TIMEOUT
+            )
+        );
+        // Enable manual slicing (like setting source.slice.max and source.slice.id in the request body):
+        int numSlices = randomIntBetween(2, Integer.MAX_VALUE);
+        int sliceId = randomIntBetween(0, numSlices - 1);
+        reindex.getSearchRequest().source().slice(new SliceBuilder(sliceId, numSlices));
+        ActionRequestValidationException e = reindex.validate();
+        assertEquals(
+            "Validation Failed: 1: reindex from remote sources doesn't support source.slice but was ["
+                + reindex.getSearchRequest().source().slice()
+                + "];",
+            e.getMessage()
+        );
+    }
+
     public void testReindexFromRemoteRejectsUsernameWithNoPassword() {
         ReindexRequest reindex = newRequest();
         reindex.setRemoteInfo(