remote.c: add command line option parser for "--force-with-lease"

Update "git push" and "git send-pack" to parse this commnd line
option.

The intended sematics is:

 * "--force-with-lease" alone, without specifying the details, will
   protect _all_ remote refs that are going to be updated by
   requiring their current value to be the same as some reasonable
   default, unless otherwise specified;

 * "--force-with-lease=refname", without specifying the expected
   value, will protect that refname, if it is going to be updated,
   by requiring its current value to be the same as some reasonable
   default.

 * "--force-with-lease=refname:value" will protect that refname, if
   it is going to be updated, by requiring its current value to be
   the same as the specified value; and

 * "--no-force-with-lease" will cancel all the previous --force-with-lease on the
   command line.

For now, "some reasonable default" is tentatively defined as "the
value of the remote-tracking branch we have for the ref of the
remote being updated", and it is an error if we do not have such a
remote-tracking branch.  But this is known to be fragile, its use is
not yet recommended, and hopefully we will find more reasonable
default as we gain experience with this feature.  The manual marks
the feature as experimental unless the expected value is specified
explicitly for this reason.

Because the command line options are parsed _before_ we know which
remote we are pushing to, there needs further processing to the
parsed data after we instantiate the transport object to:

 * expand "refname" given by the user to a full refname to be
   matched with the list of "struct ref" used in match_push_refs()
   and set_ref_status_for_push(); and

 * learning the actual local ref that is the remote-tracking branch
   for the specified remote ref.

Further, some processing need to be deferred until we find the set
of remote refs and match_push_refs() returns in order to find the
ones that need to be checked after explicit ones have been processed
for "--force-with-lease" (no specific details).

These post-processing will be the topic of the next patch.

This option was originally called "cas" (for "compare and swap"),
the name which nobody liked because it was too technical.  The
second attempt called it "lockref" (because it is conceptually like
pushing after taking a lock) but the word "lock" was hated because
it implied that it may reject push by others, which is not the way
this option works.  This round calls it "force-with-lease".  You
assume you took the lease on the ref when you fetched to decide what
the rebased history should be, and you can push back only if the
lease has not been broken.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

5 files changed, 168 insertions, 11 deletions

diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index f7dfe48d28..e2992f17a0 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -11,6 +11,7 @@ SYNOPSIS
 [verse]
 'git push' [--all | --mirror | --tags] [--follow-tags] [-n | --dry-run] [--receive-pack=<git-receive-pack>]
 	   [--repo=<repository>] [-f | --force] [--prune] [-v | --verbose] [-u | --set-upstream]
+	   [--force-with-lease[=<refname>[:<expect>]]]
 	   [--no-verify] [<repository> [<refspec>...]]
 
 DESCRIPTION
@@ -130,21 +131,75 @@ already exists on the remote side.
 	repository over ssh, and you do not have the program in
 	a directory on the default $PATH.
 
+--[no-]force-with-lease::
+--force-with-lease=<refname>::
+--force-with-lease=<refname>:<expect>::
+	Usually, "git push" refuses to update a remote ref that is
+	not an ancestor of the local ref used to overwrite it.
++
+This option bypasses the check, but instead requires that the
+current value of the ref to be the expected value.  "git push"
+fails otherwise.
++
+Imagine that you have to rebase what you have already published.
+You will have to bypass the "must fast-forward" rule in order to
+replace the history you originally published with the rebased history.
+If somebody else built on top of your original history while you are
+rebasing, the tip of the branch at the remote may advance with her
+commit, and blindly pushing with `--force` will lose her work.
++
+This option allows you to say that you expect the history you are
+updating is what you rebased and want to replace. If the remote ref
+still points at the commit you specified, you can be sure that no
+other people did anything to the ref (it is like taking a "lease" on
+the ref without explicitly locking it, and you update the ref while
+making sure that your earlier "lease" is still valid).
++
+`--force-with-lease` alone, without specifying the details, will protect
+all remote refs that are going to be updated by requiring their
+current value to be the same as the remote-tracking branch we have
+for them, unless specified with a `--force-with-lease=<refname>:<expect>`
+option that explicitly states what the expected value is.
++
+`--force-with-lease=<refname>`, without specifying the expected value, will
+protect the named ref (alone), if it is going to be updated, by
+requiring its current value to be the same as the remote-tracking
+branch we have for it.
++
+`--force-with-lease=<refname>:<expect>` will protect the named ref (alone),
+if it is going to be updated, by requiring its current value to be
+the same as the specified value <expect> (which is allowed to be
+different from the remote-tracking branch we have for the refname,
+or we do not even have to have such a remote-tracking branch when
+this form is used).
++
+Note that all forms other than `--force-with-lease=<refname>:<expect>`
+that specifies the expected current value of the ref explicitly are
+still experimental and their semantics may change as we gain experience
+with this feature.
++
+"--no-force-with-lease" will cancel all the previous --force-with-lease on the
+command line.
+
 -f::
 --force::
 	Usually, the command refuses to update a remote ref that is
 	not an ancestor of the local ref used to overwrite it.
-	This flag disables the check.  This can cause the
-	remote repository to lose commits; use it with care.
-	Note that `--force` applies to all the refs that are pushed,
-	hence using it with `push.default` set to `matching` or with
-	multiple push destinations configured with `remote.*.push`
-	may overwrite refs other than the current branch (including
-	local refs that are strictly behind their remote counterpart).
-	To force a push to only one branch, use a `+` in front of the
-	refspec to push	(e.g `git push origin +master` to force a push
-	to the `master`	branch). See the `<refspec>...` section above
-	for details.
+	Also, when `--force-with-lease` option is used, the command refuses
+	to update a remote ref whose current value does not match
+	what is expected.
++
+This flag disables these checks, and can cause the remote repository
+to lose commits; use it with care.
++
+Note that `--force` applies to all the refs that are pushed, hence
+using it with `push.default` set to `matching` or with multiple push
+destinations configured with `remote.*.push` may overwrite refs
+other than the current branch (including local refs that are
+strictly behind their remote counterpart).  To force a push to only
+one branch, use a `+` in front of the refspec to push (e.g `git push
+origin +master` to force a push to the `master` branch). See the
+`<refspec>...` section above for details.
 
 --repo=<repository>::
 	This option is only relevant if no <repository> argument is
diff --git a/builtin/push.c b/builtin/push.c
index 342d792f93..31a5ba085d 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -21,6 +21,8 @@ static const char *receivepack;
 static int verbosity;
 static int progress = -1;
 
+static struct push_cas_option cas;
+
 static const char **refspec;
 static int refspec_nr;
 static int refspec_alloc;
@@ -432,6 +434,10 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_BIT('n' , "dry-run", &flags, N_("dry run"), TRANSPORT_PUSH_DRY_RUN),
 		OPT_BIT( 0,  "porcelain", &flags, N_("machine-readable output"), TRANSPORT_PUSH_PORCELAIN),
 		OPT_BIT('f', "force", &flags, N_("force updates"), TRANSPORT_PUSH_FORCE),
+		{ OPTION_CALLBACK,
+		  0, CAS_OPT_NAME, &cas, N_("refname>:<expect"),
+		  N_("require old value of ref to be at this value"),
+		  PARSE_OPT_OPTARG, parseopt_push_cas_option },
 		{ OPTION_CALLBACK, 0, "recurse-submodules", &flags, N_("check"),
 			N_("control recursive pushing of submodules"),
 			PARSE_OPT_OPTARG, option_parse_recurse_submodules },
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index e86d3b5d67..a23b26db17 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -103,6 +103,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	int flags;
 	unsigned int reject_reasons;
 	int progress = -1;
+	struct push_cas_option cas = {0};
 
 	argv++;
 	for (i = 1; i < argc; i++, argv++) {
@@ -165,6 +166,22 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 				helper_status = 1;
 				continue;
 			}
+			if (!strcmp(arg, "--" CAS_OPT_NAME)) {
+				if (parse_push_cas_option(&cas, NULL, 0) < 0)
+					exit(1);
+				continue;
+			}
+			if (!strcmp(arg, "--no-" CAS_OPT_NAME)) {
+				if (parse_push_cas_option(&cas, NULL, 1) < 0)
+					exit(1);
+				continue;
+			}
+			if (!prefixcmp(arg, "--" CAS_OPT_NAME "=")) {
+				if (parse_push_cas_option(&cas,
+							  strchr(arg, '=') + 1, 1) < 0)
+					exit(1);
+				continue;
+			}
 			usage(send_pack_usage);
 		}
 		if (!dest) {
diff --git a/remote.c b/remote.c
index b1ff7a208f..0d383533d4 100644
--- a/remote.c
+++ b/remote.c
@@ -1921,3 +1921,60 @@ struct ref *get_stale_heads(struct refspec *refs, int ref_count, struct ref *fet
 	string_list_clear(&ref_names, 0);
 	return stale_refs;
 }
+
+/*
+ * Compare-and-swap
+ */
+void clear_cas_option(struct push_cas_option *cas)
+{
+	int i;
+
+	for (i = 0; i < cas->nr; i++)
+		free(cas->entry[i].refname);
+	free(cas->entry);
+	memset(cas, 0, sizeof(*cas));
+}
+
+static struct push_cas *add_cas_entry(struct push_cas_option *cas,
+				      const char *refname,
+				      size_t refnamelen)
+{
+	struct push_cas *entry;
+	ALLOC_GROW(cas->entry, cas->nr + 1, cas->alloc);
+	entry = &cas->entry[cas->nr++];
+	memset(entry, 0, sizeof(*entry));
+	entry->refname = xmemdupz(refname, refnamelen);
+	return entry;
+}
+
+int parse_push_cas_option(struct push_cas_option *cas, const char *arg, int unset)
+{
+	const char *colon;
+	struct push_cas *entry;
+
+	if (unset) {
+		/* "--no-<option>" */
+		clear_cas_option(cas);
+		return 0;
+	}
+
+	if (!arg) {
+		/* just "--<option>" */
+		cas->use_tracking_for_rest = 1;
+		return 0;
+	}
+
+	/* "--<option>=refname" or "--<option>=refname:value" */
+	colon = strchrnul(arg, ':');
+	entry = add_cas_entry(cas, arg, colon - arg);
+	if (!*colon)
+		entry->use_tracking = 1;
+	else if (get_sha1(colon + 1, entry->expect))
+		return error("cannot parse expected object name '%s'", colon + 1);
+	return 0;
+}
+
+int parseopt_push_cas_option(const struct option *opt, const char *arg, int unset)
+{
+	return parse_push_cas_option(opt->value, arg, unset);
+}
diff --git a/remote.h b/remote.h
index a850059804..843c3cef45 100644
--- a/remote.h
+++ b/remote.h
@@ -1,6 +1,8 @@
 #ifndef REMOTE_H
 #define REMOTE_H
 
+#include "parse-options.h"
+
 enum {
 	REMOTE_CONFIG,
 	REMOTE_REMOTES,
@@ -226,4 +228,24 @@ struct ref *guess_remote_head(const struct ref *head,
 /* Return refs which no longer exist on remote */
 struct ref *get_stale_heads(struct refspec *refs, int ref_count, struct ref *fetch_map);
 
+/*
+ * Compare-and-swap
+ */
+#define CAS_OPT_NAME "force-with-lease"
+
+struct push_cas_option {
+	unsigned use_tracking_for_rest:1;
+	struct push_cas {
+		unsigned char expect[20];
+		unsigned use_tracking:1;
+		char *refname;
+	} *entry;
+	int nr;
+	int alloc;
+};
+
+extern int parseopt_push_cas_option(const struct option *, const char *arg, int unset);
+extern int parse_push_cas_option(struct push_cas_option *, const char *arg, int unset);
+extern void clear_cas_option(struct push_cas_option *);
+
 #endif