This teaches the deepest part of the callchain for "git push" (and
"git send-pack") to enforce "the old value of the ref must be this,
otherwise fail this push" (aka "compare-and-swap" / "--lockref").

Signed-off-by: Junio C Hamano <gits...@pobox.com>
---
 builtin/send-pack.c |  5 +++++
 remote.c            | 49 ++++++++++++++++++++++++++++++++++++-------------
 remote.h            |  1 +
 send-pack.c         |  1 +
 transport-helper.c  |  6 ++++++
 transport.c         |  5 +++++
 6 files changed, 54 insertions(+), 13 deletions(-)

diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 6027ead..41dc512 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -55,6 +55,11 @@ static void print_helper_status(struct ref *ref)
                        msg = "needs force";
                        break;
 
+               case REF_STATUS_REJECT_STALE:
+                       res = "error";
+                       msg = "stale info";
+                       break;
+
                case REF_STATUS_REJECT_ALREADY_EXISTS:
                        res = "error";
                        msg = "already exists";
diff --git a/remote.c b/remote.c
index 93e5b65..0109487 100644
--- a/remote.c
+++ b/remote.c
@@ -1396,12 +1396,13 @@ int match_push_refs(struct ref *src, struct ref **dst,
 }
 
 void set_ref_status_for_push(struct ref *remote_refs, int send_mirror,
-       int force_update)
+                            int force_update)
 {
        struct ref *ref;
 
        for (ref = remote_refs; ref; ref = ref->next) {
                int force_ref_update = ref->force || force_update;
+               int reject_reason = 0;
 
                if (ref->peer_ref)
                        hashcpy(ref->new_sha1, ref->peer_ref->new_sha1);
@@ -1416,6 +1417,26 @@ void set_ref_status_for_push(struct ref *remote_refs, 
int send_mirror,
                }
 
                /*
+                * Bypass the usual "must fast-forward" check but
+                * replace it with a weaker "the old value must be
+                * this value we observed".  If the remote ref has
+                * moved and is now different from what we expect,
+                * reject any push.
+                *
+                * It also is an error if the user told us to check
+                * with the remote-tracking branch to find the value
+                * to expect, but we did not have such a tracking
+                * branch.
+                */
+               if (ref->expect_old_sha1) {
+                       if (ref->expect_old_no_trackback ||
+                           hashcmp(ref->old_sha1, ref->old_sha1_expect))
+                               reject_reason = REF_STATUS_REJECT_STALE;
+               }
+
+               /*
+                * The usual "must fast-forward" rules.
+                *
                 * Decide whether an individual refspec A:B can be
                 * pushed.  The push will succeed if any of the
                 * following are true:
@@ -1433,24 +1454,26 @@ void set_ref_status_for_push(struct ref *remote_refs, 
int send_mirror,
                 *     passing the --force argument
                 */
 
-               if (!ref->deletion && !is_null_sha1(ref->old_sha1)) {
-                       int why = 0; /* why would this push require --force? */
-
+               else if (!ref->deletion && !is_null_sha1(ref->old_sha1)) {
                        if (!prefixcmp(ref->name, "refs/tags/"))
-                               why = REF_STATUS_REJECT_ALREADY_EXISTS;
+                               reject_reason = 
REF_STATUS_REJECT_ALREADY_EXISTS;
                        else if (!has_sha1_file(ref->old_sha1))
-                               why = REF_STATUS_REJECT_FETCH_FIRST;
+                               reject_reason = REF_STATUS_REJECT_FETCH_FIRST;
                        else if (!lookup_commit_reference_gently(ref->old_sha1, 
1) ||
                                 !lookup_commit_reference_gently(ref->new_sha1, 
1))
-                               why = REF_STATUS_REJECT_NEEDS_FORCE;
+                               reject_reason = REF_STATUS_REJECT_NEEDS_FORCE;
                        else if (!ref_newer(ref->new_sha1, ref->old_sha1))
-                               why = REF_STATUS_REJECT_NONFASTFORWARD;
-
-                       if (!force_ref_update)
-                               ref->status = why;
-                       else if (why)
-                               ref->forced_update = 1;
+                               reject_reason = 
REF_STATUS_REJECT_NONFASTFORWARD;
                }
+
+               /*
+                * "--force" will defeat any rejection implemented
+                * by the rules above.
+                */
+               if (!force_ref_update)
+                       ref->status = reject_reason;
+               else if (reject_reason)
+                       ref->forced_update = 1;
        }
 }
 
diff --git a/remote.h b/remote.h
index 4c564c5..baa1c68 100644
--- a/remote.h
+++ b/remote.h
@@ -107,6 +107,7 @@ struct ref {
                REF_STATUS_REJECT_NODELETE,
                REF_STATUS_REJECT_FETCH_FIRST,
                REF_STATUS_REJECT_NEEDS_FORCE,
+               REF_STATUS_REJECT_STALE,
                REF_STATUS_UPTODATE,
                REF_STATUS_REMOTE_REJECT,
                REF_STATUS_EXPECTING_REPORT
diff --git a/send-pack.c b/send-pack.c
index 9a9908c..b228d65 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -227,6 +227,7 @@ int send_pack(struct send_pack_args *args,
                case REF_STATUS_REJECT_ALREADY_EXISTS:
                case REF_STATUS_REJECT_FETCH_FIRST:
                case REF_STATUS_REJECT_NEEDS_FORCE:
+               case REF_STATUS_REJECT_STALE:
                case REF_STATUS_UPTODATE:
                        continue;
                default:
diff --git a/transport-helper.c b/transport-helper.c
index db9bd18..95d22f8 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -683,6 +683,11 @@ static int push_update_ref_status(struct strbuf *buf,
                        free(msg);
                        msg = NULL;
                }
+               else if (!strcmp(msg, "stale info")) {
+                       status = REF_STATUS_REJECT_STALE;
+                       free(msg);
+                       msg = NULL;
+               }
        }
 
        if (*ref)
@@ -756,6 +761,7 @@ static int push_refs_with_push(struct transport *transport,
                /* Check for statuses set by set_ref_status_for_push() */
                switch (ref->status) {
                case REF_STATUS_REJECT_NONFASTFORWARD:
+               case REF_STATUS_REJECT_STALE:
                case REF_STATUS_REJECT_ALREADY_EXISTS:
                case REF_STATUS_UPTODATE:
                        continue;
diff --git a/transport.c b/transport.c
index 5dd92b7..b321d6a 100644
--- a/transport.c
+++ b/transport.c
@@ -709,6 +709,10 @@ static int print_one_push_status(struct ref *ref, const 
char *dest, int count, i
                print_ref_status('!', "[rejected]", ref, ref->peer_ref,
                                                 "needs force", porcelain);
                break;
+       case REF_STATUS_REJECT_STALE:
+               print_ref_status('!', "[rejected]", ref, ref->peer_ref,
+                                                "stale info", porcelain);
+               break;
        case REF_STATUS_REMOTE_REJECT:
                print_ref_status('!', "[remote rejected]", ref,
                                                 ref->deletion ? NULL : 
ref->peer_ref,
@@ -1078,6 +1082,7 @@ static int run_pre_push_hook(struct transport *transport,
        for (r = remote_refs; r; r = r->next) {
                if (!r->peer_ref) continue;
                if (r->status == REF_STATUS_REJECT_NONFASTFORWARD) continue;
+               if (r->status == REF_STATUS_REJECT_STALE) continue;
                if (r->status == REF_STATUS_UPTODATE) continue;
 
                strbuf_reset(&buf);
-- 
1.8.3.2-912-g65cf5cf

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