Do not fail with 500 ISE if base of merge change cannot be computed due to NoMergeBaseException

If the repository content doesn't allow us to compute an auto-merge
commit for the base of a merge change because JGit cannot find a merge
base (e.g. due to CONFLICTS_DURING_MERGE_BASE_CALCULATION), we now
return '409 Conflict' for the ListFiles and change-edit-related REST
endpoints. If the files were supposed to be included into a ChangeInfo /
RevisionInfo we ignore the failure and just do not populate the file
list (but we log a warning). Letting the request succeed in this case is
better, as the caller can at least get the rest of the change and
revision information.

Treating NoMergeBaseException as '409 Conflict' is consistent with how
we treat this exception in other places (e.g. in the CreateChange REST
endpoint).

Change-Id: I78b4636c23635048eedf2a1e4b1075eb87ecb262

java/com/google/gerrit/server/change/FileInfoJson.java

@@ -21,6 +21,7 @@ import com.google.gerrit.entities.PatchSet;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.extensions.client.DiffPreferencesInfo.Whitespace;
 import com.google.gerrit.extensions.common.FileInfo;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.server.patch.PatchList;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchListEntry;
@@ -30,6 +31,8 @@ import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import java.util.Map;
 import java.util.TreeMap;
+import java.util.concurrent.ExecutionException;
+import org.eclipse.jgit.errors.NoMergeBaseException;
 import org.eclipse.jgit.lib.ObjectId;
 
 @Singleton
@@ -42,31 +45,44 @@ public class FileInfoJson {
   }
 
   public Map<String, FileInfo> toFileInfoMap(Change change, PatchSet patchSet)
-      throws PatchListNotAvailableException {
+      throws ResourceConflictException, PatchListNotAvailableException {
     return toFileInfoMap(change, patchSet.commitId(), null);
   }
 
   public Map<String, FileInfo> toFileInfoMap(
       Change change, ObjectId objectId, @Nullable PatchSet base)
-      throws PatchListNotAvailableException {
+      throws ResourceConflictException, PatchListNotAvailableException {
     ObjectId a = base != null ? base.commitId() : null;
     return toFileInfoMap(change, PatchListKey.againstCommit(a, objectId, Whitespace.IGNORE_NONE));
   }
 
   public Map<String, FileInfo> toFileInfoMap(Change change, ObjectId objectId, int parent)
-      throws PatchListNotAvailableException {
+      throws ResourceConflictException, PatchListNotAvailableException {
     return toFileInfoMap(
         change, PatchListKey.againstParentNum(parent + 1, objectId, Whitespace.IGNORE_NONE));
   }
 
   private Map<String, FileInfo> toFileInfoMap(Change change, PatchListKey key)
-      throws PatchListNotAvailableException {
+      throws ResourceConflictException, PatchListNotAvailableException {
     return toFileInfoMap(change.getProject(), key);
   }
 
   public Map<String, FileInfo> toFileInfoMap(Project.NameKey project, PatchListKey key)
-      throws PatchListNotAvailableException {
+      throws ResourceConflictException, PatchListNotAvailableException {
-    PatchList list = patchListCache.get(key, project);
+    PatchList list;
+    try {
+      list = patchListCache.get(key, project);
+    } catch (PatchListNotAvailableException e) {
+      Throwable cause = e.getCause();
+      if (cause instanceof ExecutionException) {
+        cause = cause.getCause();
+      }
+      if (cause instanceof NoMergeBaseException) {
+        throw new ResourceConflictException(
+            String.format("Cannot create auto merge commit: %s", e.getMessage()), e);
+      }
+      throw e;
+    }
 
     Map<String, FileInfo> files = new TreeMap<>();
     for (PatchListEntry e : list.getPatches()) {

java/com/google/gerrit/server/change/RevisionJson.java

@@ -49,6 +49,7 @@ import com.google.gerrit.extensions.config.DownloadScheme;
 import com.google.gerrit.extensions.registration.DynamicMap;
 import com.google.gerrit.extensions.registration.Extension;
 import com.google.gerrit.extensions.restapi.AuthException;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.server.AnonymousUser;
 import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.GpgException;
@@ -311,9 +312,13 @@ public class RevisionJson {
     }
 
     if (has(ALL_FILES) || (out.isCurrent && has(CURRENT_FILES))) {
-      out.files = fileInfoJson.toFileInfoMap(c, in);
+      try {
-      out.files.remove(Patch.COMMIT_MSG);
+        out.files = fileInfoJson.toFileInfoMap(c, in);
-      out.files.remove(Patch.MERGE_LIST);
+        out.files.remove(Patch.COMMIT_MSG);
+        out.files.remove(Patch.MERGE_LIST);
+      } catch (ResourceConflictException e) {
+        logger.atWarning().withCause(e).log("creating file list failed");
+      }
     }
 
     if (out.isCurrent && has(CURRENT_ACTIONS) && userProvider.get().isIdentifiedUser()) {

java/com/google/gerrit/server/restapi/change/ChangeEdits.java

@@ -184,7 +184,8 @@ public class ChangeEdits implements ChildCollection<ChangeResource, ChangeEditRe
 
     @Override
     public Response<EditInfo> apply(ChangeResource rsrc)
-        throws AuthException, IOException, ResourceNotFoundException, PermissionBackendException {
+        throws AuthException, IOException, ResourceNotFoundException, ResourceConflictException,
+            PermissionBackendException {
       Optional<ChangeEdit> edit = editUtil.byChange(rsrc.getNotes(), rsrc.getUser());
       if (!edit.isPresent()) {
         return Response.none();

java/com/google/gerrit/server/restapi/project/FilesInCommitCollection.java

@@ -20,6 +20,7 @@ import com.google.gerrit.extensions.common.FileInfo;
 import com.google.gerrit.extensions.registration.DynamicMap;
 import com.google.gerrit.extensions.restapi.ChildCollection;
 import com.google.gerrit.extensions.restapi.IdString;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.RestReadView;
@@ -86,7 +87,7 @@ public class FilesInCommitCollection implements ChildCollection<CommitResource,
 
     @Override
     public Response<Map<String, FileInfo>> apply(CommitResource resource)
-        throws PatchListNotAvailableException {
+        throws ResourceConflictException, PatchListNotAvailableException {
       RevCommit commit = resource.getCommit();
       PatchListKey key;