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Re: [PATCH v2] push: point to 'git pull' and 'git push --force' in case of non-fast forward

From
Nanako Shiraishi <nanako3@lavabit.com>
Date
Aug 11, 2009, 03:03 UTC
Message-ID
<20090811120313.6117@nanako3.lavabit.com>
In-Reply-To
<7vy6pujmsc.fsf@alter.siamese.dyndns.org>
From: Matthieu Moy <Matthieu.Moy@imag.fr>
Date: Sat,  8 Aug 2009 09:51:08 +0200
Subject: [PATCH] push: point to 'git pull' and 'git push --force' in case of non-fast forward

'git push' failing because of non-fast forward is a very common situation, and a beginner does not necessarily understand "fast forward" immediately.

Add a new section to the git-push documentation and refer them to it.
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
---
  Quoting Junio C Hamano <gitster@pobox.com> writes:
  > So how about phrasing it like this?
  >
  >     Non-fast forward pushes were rejected because you would discard remote
  >     changes you have not seen.  Integrate them with your changes and then
  >     push again. See 'non-fast forward' section of 'git push --help'.
  >
  > I think you can throw in a discussion on --force to the manual page, too.
  Here is my attempt to coagulate the improvements discussed on the thread so far. I added a paragraph that mentions --force in the new section of the manual, reworded the explanatory message on the UI, and forged two signatures.
 Documentation/git-push.txt |   86 ++++++++++++++++++++++++++++++++++++++++++++
 builtin-push.c             |    9 ++++-
 transport.c                |   10 ++++--
 transport.h                |    3 +-
 4 files changed, 103 insertions(+), 5 deletions(-)
diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index 2653388..58d2bd5 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -195,6 +195,92 @@ reason::
 	refs, no explanation is needed. For a failed ref, the reason for
 	failure is described.
 
+Note about fast-forwards
+------------------------
+
+When an update changes a branch (or more in general, a ref) that used to
+point at commit A to point at another commit B, it is called a
+fast-forward update if and only if B is a descendant of A.
+
+In a fast-forward update from A to B, the set of commits that the original
+commit A built on top of is a subset of the commits the new commit B
+builds on top of.  Hence, it does not lose any history.
+
+In contrast, a non-fast-forward update will lose history.  For example,
+suppose you and somebody else started at the same commit X, and you built
+a history leading to commit B while the other person built a history
+leading to commit A.  The history looks like this:
+
+----------------
+
+      B
+     /
+ ---X---A
+
+----------------
+
+Further suppose that the other person already pushed changes leading to A
+back to the original repository you two obtained the original commit X.
+
+The push done by the other person updated the branch that used to point at
+commit X to point at commit A.  It is a fast-forward.
+
+But if you try to push, you will attempt to update the branch (that
+now points at A) with commit B.  This does _not_ fast-forward.  If you did
+so, the changes introduced by commit A will be lost, because everybody
+will now start building on top of B.
+
+The command by default does not allow an update that is not a fast-forward
+to prevent such loss of history.
+
+If you do not want to lose your work (history from X to B) nor the work by
+the other person (history from X to A), you would need to first fetch the
+history from the repository, create a history that contains changes done
+by both parties, and push the result back.
+
+You can perform "git pull", resolve potential conflicts, and "git push"
+the result.  A "git pull" will create a merge commit C between commits A
+and B.
+
+----------------
+
+      B---C
+     /   /
+ ---X---A
+
+----------------
+
+Updating A with the resulting merge commit will fast-forward and your
+push will be accepted.
+
+Alternatively, you can rebase your change between X and B on top of A,
+with "git pull --rebase", and push the result back.  The rebase will
+create a new commit D that builds the change between X and B on top of
+A.
+
+----------------
+
+      B   D
+     /   /
+ ---X---A
+
+----------------
+
+Again, updating A with this commit will fast-forward and your push will be
+accepted.
+
+There is another common situation where you may encounter non-fast-forward
+rejection when you try to push, and it is possible even when you are
+pushing into a repository nobody else pushes into. After you push commit
+A yourself (in the first picture in this section), replace it with "git
+commit --amend" to produce commit B, and you try to push it out, because
+forgot that you have pushed A out already. In such a case, and only if
+you are certain that nobody in the meantime fetched your earlier commit A
+(and started building on top of it), you can run "git push --force" to
+overwrite it. In other words, "git push --force" is a method reserved for
+a case where you do mean to lose history.
+
+
 Examples
 --------
 
diff --git a/builtin-push.c b/builtin-push.c
index 1d92e22..50328f4 100644
--- a/builtin-push.c
+++ b/builtin-push.c
@@ -140,6 +140,7 @@ static int do_push(const char *repo, int flags)
 		struct transport *transport =
 			transport_get(remote, url[i]);
 		int err;
+		int nonfastforward;
 		if (receivepack)
 			transport_set_option(transport,
 					     TRANS_OPT_RECEIVEPACK, receivepack);
@@ -148,13 +149,19 @@ static int do_push(const char *repo, int flags)
 
 		if (flags & TRANSPORT_PUSH_VERBOSE)
 			fprintf(stderr, "Pushing to %s\n", url[i]);
-		err = transport_push(transport, refspec_nr, refspec, flags);
+		err = transport_push(transport, refspec_nr, refspec, flags,
+				     &nonfastforward);
 		err |= transport_disconnect(transport);
 
 		if (!err)
 			continue;
 
 		error("failed to push some refs to '%s'", url[i]);
+		if (nonfastforward) {
+			printf("To prevent you from losing history, non-fast-forward updates were rejected.\n"
+			       "Merge the remote changes before pushing again.\n"
+			       "See 'non-fast forward' section of 'git push --help' for details.\n");
+		}
 		errs++;
 	}
 	return !!errs;
diff --git a/transport.c b/transport.c
index de0d587..f231b35 100644
--- a/transport.c
+++ b/transport.c
@@ -820,7 +820,7 @@ static int print_one_push_status(struct ref *ref, const char *dest, int count, i
 }
 
 static void print_push_status(const char *dest, struct ref *refs,
-							  int verbose, int porcelain)
+			      int verbose, int porcelain, int * nonfastforward)
 {
 	struct ref *ref;
 	int n = 0;
@@ -835,11 +835,14 @@ static void print_push_status(const char *dest, struct ref *refs,
 		if (ref->status == REF_STATUS_OK)
 			n += print_one_push_status(ref, dest, n, porcelain);
 
+	*nonfastforward = 0;
 	for (ref = refs; ref; ref = ref->next) {
 		if (ref->status != REF_STATUS_NONE &&
 		    ref->status != REF_STATUS_UPTODATE &&
 		    ref->status != REF_STATUS_OK)
 			n += print_one_push_status(ref, dest, n, porcelain);
+		if (ref->status == REF_STATUS_REJECT_NONFASTFORWARD)
+			*nonfastforward = 1;
 	}
 }
 
@@ -997,7 +1000,8 @@ int transport_set_option(struct transport *transport,
 }
 
 int transport_push(struct transport *transport,
-		   int refspec_nr, const char **refspec, int flags)
+		   int refspec_nr, const char **refspec, int flags,
+		   int * nonfastforward)
 {
 	verify_remote_names(refspec_nr, refspec);
 
@@ -1024,7 +1028,7 @@ int transport_push(struct transport *transport,
 
 		ret = transport->push_refs(transport, remote_refs, flags);
 
-		print_push_status(transport->url, remote_refs, verbose | porcelain, porcelain);
+		print_push_status(transport->url, remote_refs, verbose | porcelain, porcelain, nonfastforward);
 
 		if (!(flags & TRANSPORT_PUSH_DRY_RUN)) {
 			struct ref *ref;
diff --git a/transport.h b/transport.h
index 51b5397..639f13d 100644
--- a/transport.h
+++ b/transport.h
@@ -68,7 +68,8 @@ int transport_set_option(struct transport *transport, const char *name,
 			 const char *value);
 
 int transport_push(struct transport *connection,
-		   int refspec_nr, const char **refspec, int flags);
+		   int refspec_nr, const char **refspec, int flags,
+		   int * nonfastforward);
 
 const struct ref *transport_get_remote_refs(struct transport *transport);
 

-- 
1.6.2.GIT

-- 
Nanako Shiraishi
http://ivory.ap.teacup.com/nanako3/
Previous: Matthieu MoyNext: Jeff King
Message 21 of 48 in “push: point to 'git pull' and 'git push --force' in case of non-fast forward”
  1. push: point to 'git pull' and 'git push --force' in case of non-fast forwardMatthieu Moy, Aug 6, 2009
  2. Michael J GruberAug 6, 2009
  3. Matthieu MoyAug 7, 2009
  4. Michael J GruberAug 7, 2009
  5. Junio C HamanoAug 6, 2009
  6. Re: push: point to 'git pull' and 'git push --force' in case of non-fast forwardNicolas Sebrecht, Aug 6, 2009
  7. Junio C HamanoAug 6, 2009
  8. Matthieu MoyAug 7, 2009
  9. Junio C HamanoAug 7, 2009
  10. Matthieu MoyAug 7, 2009
  11. push: point to 'git pull' and 'git push --force' in case of non-fast forwardMatthieu Moy, Aug 8, 2009
  12. Teemu LikonenAug 8, 2009
  13. Matthieu MoyAug 8, 2009
  14. Junio C HamanoAug 8, 2009
  15. Re: push: point to 'git pull' and 'git push --force' in case of non-fast forwardNicolas Sebrecht, Aug 8, 2009
  16. Matthieu MoyAug 9, 2009
  17. Junio C HamanoAug 9, 2009
  18. Matthieu MoyAug 10, 2009
  19. Junio C HamanoAug 10, 2009
  20. Matthieu MoyAug 10, 2009
  21. Nanako ShiraishiAug 11, 2009
  22. 0/4 make helpful messages optionalJeff King, Sep 6, 2009
  23. 1/4 push: fix english in non-fast-forward messageJeff King, Sep 6, 2009
  24. 2/4 push: re-flow non-fast-forward messageJeff King, Sep 6, 2009
  25. 3/4 push: make non-fast-forward help message configurableJeff King, Sep 6, 2009
  26. Junio C HamanoSep 6, 2009
  27. Jeff KingSep 6, 2009
  28. Junio C HamanoSep 6, 2009
  29. Jeff KingSep 6, 2009
  30. Junio C HamanoSep 6, 2009
  31. Sverre RabbelierSep 6, 2009
  32. Nanako ShiraishiSep 7, 2009
  33. Johannes SixtSep 7, 2009
  34. Mike HommeySep 7, 2009
  35. Jeff KingSep 7, 2009
  36. Matthieu MoySep 7, 2009
  37. Jeff KingSep 7, 2009
  38. Matthieu MoySep 7, 2009
  39. Uri OkrentSep 8, 2009
  40. Jeff KingSep 9, 2009
  41. 0/2 configurable advice messagesJeff King, Sep 9, 2009
  42. 1/2 push: make non-fast-forward help message configurableJeff King, Sep 9, 2009
  43. Junio C HamanoSep 9, 2009
  44. Jeff KingSep 9, 2009
  45. Junio C HamanoSep 9, 2009
  46. 2/2 status: make "how to stage" messages optionalJeff King, Sep 9, 2009
  47. 4/4 status: make "how to stage" messages optionalJeff King, Sep 6, 2009
  48. Matthieu MoySep 6, 2009

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