Prevent duplicate commits in same project when uploading to refs/changes/n

Under certain circumstances, when pushing to 'refs/changes/n', the same
commit can be pushed onto multiple changes even if they are on the same
branch.

As a result of this, if the commit sha1 is used as change identifier
in the 'review' ssh command, it will fail with the error:

  'fatal: <SHA1> matches multiple patch sets.'

This commit performs the same check as when pushing to 'refs/for/branch'
to prevent this, and will show "commit already exists (in the project)"
error message to reject uploading a commit to an existing change via
`refs/changes/n` if the commit was already successfully pushed to a change
in project scope.

Bug: issue 2374
Change-Id: I1c000b2cc3e155617cdc9c295a46ce2107ec47ca

Documentation/error-commit-already-exists.txt

@@ -1,10 +1,19 @@
 commit already exists
 =====================
 
-With this error message Gerrit rejects to push a commit to an
+With "commit already exists (as current patchset)" or
-existing change via `refs/changes/n` if the commit was already
+"commit already exists (in the change)" error message
-successfully pushed to the change.  In this case there is no
+Gerrit rejects to push a commit to an existing change via
-new commit and consequently there is nothing for Gerrit to do.
+`refs/changes/n` if the commit was already successfully
+pushed to the change.
+
+With "commit already exists (in the project)" error message
+Gerrit rejects to push a commit to an existing change via
+`refs/changes/n` if the commit was already successfully
+pushed to a change in project scope.
+
+In any above case there is no new commit and consequently
+there is nothing for Gerrit to do.
 
 For further information about how to resolve this error, please
 refer to link:error-no-new-changes.html[no new changes].

Documentation/error-no-new-changes.txt

@@ -2,8 +2,9 @@ no new changes
 ==============
 
 With this error message Gerrit rejects to push a commit if the pushed
-commit was already successfully pushed to Gerrit. In this case there
+commit was already successfully pushed to Gerrit in project scope.
-is no new change and consequently there is nothing for Gerrit to do.
+In this case there is no new change and consequently there is nothing
+for Gerrit to do.
 
 If your push is failing with this error message, you normally
 don't have to do anything since the commit was already successfully
@@ -31,7 +32,7 @@ means:
 . you cannot reset a change to an old patch set by pushing the old
   commit for this change again
 . if a commit was pushed to one branch you cannot push this commit
-  to another branch
+  to another branch in project scope.
 . if a commit was pushed directly to a branch (without going through
   code review) you cannot push this commit once again for code
   review (please note that in this case searching by the commit ID
@@ -39,11 +40,11 @@ means:
 
 If you need to re-push a commit you may rewrite this commit by
 link:http://www.kernel.org/pub/software/scm/git/docs/git-commit.html[amending] it or doing an interactive link:http://www.kernel.org/pub/software/scm/git/docs/git-rebase.html[git rebase]. By rewriting the
-commit you actually create a new commit (with a new commit ID) which
+commit you actually create a new commit (with a new commit ID in
-can then be pushed to Gerrit. If the old commit contains a Change-Id
+project scope) which can then be pushed to Gerrit. If the old commit
-in the commit message you also need to replace it with a new
+contains a Change-Id in the commit message you also need to replace
-Change-Id (case 1. and 3. above), otherwise the push will fail with
+it with a new Change-Id (case 1. and 3. above), otherwise the push
-another error message.
+will fail with another error message.
 
 
 GERRIT

gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java

@@ -17,7 +17,6 @@ package com.google.gerrit.server.git;
 import static com.google.gerrit.server.git.MultiProgressMonitor.UNKNOWN;
 import static com.google.gerrit.server.mail.MailUtil.getRecipientsFromApprovals;
 import static com.google.gerrit.server.mail.MailUtil.getRecipientsFromFooters;
-
 import static org.eclipse.jgit.lib.Constants.R_HEADS;
 import static org.eclipse.jgit.lib.RefDatabase.ALL;
 import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
@@ -128,9 +127,9 @@ import org.eclipse.jgit.transport.AdvertiseRefsHook;
 import org.eclipse.jgit.transport.AdvertiseRefsHookChain;
 import org.eclipse.jgit.transport.BaseReceivePack;
 import org.eclipse.jgit.transport.ReceiveCommand;
-import org.eclipse.jgit.transport.RefFilter;
 import org.eclipse.jgit.transport.ReceiveCommand.Result;
 import org.eclipse.jgit.transport.ReceivePack;
+import org.eclipse.jgit.transport.RefFilter;
 import org.eclipse.jgit.transport.ServiceMayNotContinueException;
 import org.eclipse.jgit.transport.UploadPack;
 import org.kohsuke.args4j.CmdLineException;
@@ -1694,6 +1693,7 @@ public class ReceiveCommits {
       if (newCommit == priorCommit) {
         // Ignore requests to make the change its current state.
         skip = true;
+        reject(inputCommand, "commit already exists (as current patchset)");
         return false;
       }
 
@@ -1705,10 +1705,18 @@ public class ReceiveCommits {
         reject(inputCommand, "change " + ontoChange + " closed");
         return false;
       } else if (revisions.containsKey(newCommit)) {
-        reject(inputCommand, "commit already exists");
+        reject(inputCommand, "commit already exists (in the change)");
         return false;
       }
 
+      for (final Ref r : rp.getRepository().getRefDatabase()
+          .getRefs("refs/changes").values()) {
+        if (r.getObjectId().equals(inputCommand.getNewId())) {
+          reject(inputCommand, "commit already exists (in the project)");
+          return false;
+        }
+      }
+
       for (RevCommit prior : revisions.keySet()) {
         // Don't allow a change to directly depend upon itself. This is a
         // very common error due to users making a new commit rather than