Hi Alban,
On Wed, 17 Mar 2021, Alban Gruin wrote:
This rewrites `git merge-octopus' from shell to C. As for the two last
conversions, this port removes calls to external processes to avoid
reading and writing the index over and over again.
- Calls to `read-tree -u -m (--aggressive)?' are replaced by calls to
unpack_trees().
- The call to `write-tree' is replaced by a call to
write_index_as_tree().
- The call to `diff-index ...' is replaced by a call to
repo_index_has_changes().
- The call to `merge-index', needed to invoke `git merge-one-file', is
replaced by a call to merge_all_index().
The index is read in cmd_merge_octopus(), and is wrote back by
s/wrote/written/
merge_strategies_octopus().
I wonder why, though. Maybe the commit message could clarify that?
Here to, merge_strategies_octopus() takes two commit lists and a string
s/to,/too,/
to reduce frictions when try_merge_strategies() will be modified to call
s/frictions/friction/
quoted hunk ↗ jump to hunk
it directly.
Signed-off-by: Alban Gruin <redacted>
---
[...]
diff --git a/builtin/merge-octopus.c b/builtin/merge-octopus.c
new file mode 100644
index 0000000000..9b9939b6b2
--- /dev/null
+++ b/builtin/merge-octopus.c
@@ -0,0 +1,70 @@
+/*
+ * Builtin "git merge-octopus"
+ *
+ * Copyright (c) 2020 Alban Gruin
+ *
+ * Based on git-merge-octopus.sh, written by Junio C Hamano.
+ *
+ * Resolve two or more trees.
+ */
+
+#include "cache.h"
+#include "builtin.h"
+#include "commit.h"
+#include "merge-strategies.h"
+
+static const char builtin_merge_octopus_usage[] =
+ "git merge-octopus [<bases>...] -- <head> <remote1> <remote2> [<remotes>...]";
+
+int cmd_merge_octopus(int argc, const char **argv, const char *prefix)
+{
+ int i, sep_seen = 0;
+ struct commit_list *bases = NULL, *remotes = NULL;
+ struct commit_list **next_base = &bases, **next_remote = &remotes;
+ const char *head_arg = NULL;
+ struct repository *r = the_repository;
+
+ if (argc < 5)
+ usage(builtin_merge_octopus_usage);
+
+ setup_work_tree();
+ if (repo_read_index(r) < 0)
+ die("invalid index");
+
+ /*
+ * The first parameters up to -- are merge bases; the rest are
+ * heads.
+ */
+ for (i = 1; i < argc; i++) {
+ if (strcmp(argv[i], "--") == 0)
+ sep_seen = 1;
+ else if (strcmp(argv[i], "-h") == 0)
+ usage(builtin_merge_octopus_usage);
+ else if (sep_seen && !head_arg)
+ head_arg = argv[i];
+ else {
+ struct object_id oid;
+ struct commit *commit;
+
+ if (get_oid(argv[i], &oid))
+ die("object %s not found.", argv[i]);
+
+ commit = oideq(&oid, r->hash_algo->empty_tree) ?
+ NULL : lookup_commit_or_die(&oid, argv[i]);
+
+ if (sep_seen)
+ next_remote = commit_list_append(commit, next_remote);
+ else
+ next_base = commit_list_append(commit, next_base);
+ }
+ }
+
+ /*
+ * Reject if this is not an octopus -- resolve should be used
+ * instead.
+ */
+ if (commit_list_count(remotes) < 2)
+ return 2;
As with `merge-resolve`, I would suggest to:
- move this input validation down to `merge_strategies_octopus()`, and
- change that function's signature to return an `enum`, and then
- make sure that that `enum` uses easy-to-understand labels.
quoted hunk ↗ jump to hunk
+
+ return merge_strategies_octopus(r, bases, head_arg, remotes);
+}
[...]
diff --git a/merge-strategies.c b/merge-strategies.c
index a51700dae5..ebc0d0b1e2 100644
--- a/merge-strategies.c
+++ b/merge-strategies.c
@@ -367,3 +368,177 @@ int merge_strategies_resolve(struct repository *r,
return 0;
}
+
+static int write_tree(struct repository *r, struct tree **reference_tree)
+{
+ struct object_id oid;
+ int ret;
+
+ if (!(ret = write_index_as_tree(&oid, r->index, r->index_file,
+ WRITE_TREE_SILENT, NULL)))
+ *reference_tree = lookup_tree(r, &oid);
+
+ return ret;
+}
+
+static int octopus_fast_forward(struct repository *r, const char *branch_name,
+ struct tree *tree_head, struct tree *current_tree,
+ struct tree **reference_tree)
While I objected to the name of the `fast_forward()` function, I think the
`octopus_fast_forward()` function is named aptly.
quoted hunk ↗ jump to hunk
+{
+ /*
+ * The first head being merged was a fast-forward. Advance the
+ * reference commit to the head being merged, and use that tree
+ * as the intermediate result of the merge. We still need to
+ * count this as part of the parent set.
+ */
+ struct tree_desc t[2];
+
+ printf(_("Fast-forwarding to: %s\n"), branch_name);
+
+ init_tree_desc(t, tree_head->buffer, tree_head->size);
+ if (add_tree(current_tree, t + 1))
+ return -1;
+ if (fast_forward(r, t, 2, 0))
+ return -1;
+ if (write_tree(r, reference_tree))
+ return -1;
+
+ return 0;
+}
+
+static int octopus_do_merge(struct repository *r, const char *branch_name,
+ struct commit_list *common, struct tree *current_tree,
+ struct tree **reference_tree)
+{
+ struct tree_desc t[MAX_UNPACK_TREES];
+ struct commit_list *i;
+ int nr = 0, ret = 0;
+
+ printf(_("Trying simple merge with %s\n"), branch_name);
+
+ for (i = common; i; i = i->next) {
+ struct tree *tree = repo_get_commit_tree(r, i->item);
+ if (add_tree(tree, t + (nr++)))
+ return -1;
+ }
+
+ if (add_tree(*reference_tree, t + (nr++)))
+ return -1;
+ if (add_tree(current_tree, t + (nr++)))
+ return -1;
+ if (fast_forward(r, t, nr, 1))
+ return 2;
+
+ if (write_tree(r, reference_tree)) {
+ struct lock_file lock = LOCK_INIT;
+
+ puts(_("Simple merge did not work, trying automatic merge."));
+ repo_hold_locked_index(r, &lock, LOCK_DIE_ON_ERROR);
It is a bit funny to see this as the only time in this patch where the
index is locked, and it is immediately released thereafter.
I would have expected the lock to be taken first thing in
`merge_strategies_octopus()` and then being committed only on success, or
on failure to merge.
quoted hunk ↗ jump to hunk
+ ret = !!merge_all_index(r->index, 0, 0, merge_one_file_func, NULL);
+ write_locked_index(r->index, &lock, COMMIT_LOCK);
+
+ write_tree(r, reference_tree);
+ }
+
+ return ret;
+}
+
+int merge_strategies_octopus(struct repository *r,
+ struct commit_list *bases, const char *head_arg,
+ struct commit_list *remotes)
+{
+ int ff_merge = 1, ret = 0, nr_references = 1;
+ struct commit **reference_commits, *head_commit;
+ struct tree *reference_tree, *head_tree;
+ struct commit_list *i;
+ struct object_id head;
+ struct strbuf sb = STRBUF_INIT;
+
+ get_oid(head_arg, &head);
+ head_commit = lookup_commit_reference(r, &head);
+ head_tree = repo_get_commit_tree(r, head_commit);
+
+ if (parse_tree(head_tree))
+ return 2;
+
+ if (repo_index_has_changes(r, head_tree, &sb)) {
+ error(_("Your local changes to the following files "
+ "would be overwritten by merge:\n %s"),
+ sb.buf);
+ strbuf_release(&sb);
+ return 2;
+ }
+
+ CALLOC_ARRAY(reference_commits, commit_list_count(remotes) + 1);
+ reference_commits[0] = head_commit;
+ reference_tree = head_tree;
+
+ for (i = remotes; i && i->item; i = i->next) {
+ struct commit *c = i->item;
+ struct object_id *oid = &c->object.oid;
+ struct tree *current_tree = repo_get_commit_tree(r, c);
+ struct commit_list *common, *j;
+ char *branch_name = merge_get_better_branch_name(oid_to_hex(oid));
+ int up_to_date = 0;
+
+ common = repo_get_merge_bases_many(r, c, nr_references, reference_commits);
+ if (!common) {
+ error(_("Unable to find common commit with %s"), branch_name);
+
+ free(branch_name);
+ free_commit_list(common);
+ free(reference_commits);
+
+ return 2;
+ }
+
+ for (j = common; j && !up_to_date && ff_merge; j = j->next) {
+ up_to_date |= oideq(&j->item->object.oid, oid);
Semantically, I would argue that this is an `||=`, not `|=`: we want a
Boolean "or", not a bit-wise one.
quoted hunk ↗ jump to hunk
+
+ if (!j->next &&
+ !oideq(&j->item->object.oid,
+ &reference_commits[nr_references - 1]->object.oid))
+ ff_merge = 0;
+ }
Hmm. This is combining two things into the same loop, with a combined loop
condition. The two things are:
case "$LF$common$LF" in
*"$LF$SHA1$LF"*)
eval_gettextln "Already up to date with \$pretty_name"
continue
;;
esac
if test "$common,$NON_FF_MERGE" = "$MRC,0"
then
# The first head being merged was a fast-forward.
# Advance MRC to the head being merged, and use that
# tree as the intermediate result of the merge.
# We still need to count this as part of the parent set.
eval_gettextln "Fast-forwarding to: \$pretty_name"
git read-tree -u -m $head $SHA1 || exit
MRC=$SHA1 MRT=$(git write-tree)
continue
fi
NON_FF_MERGE=1
The first one tries to verify that the `common` list contains `oid`. The C
code does this, too, using the intuitive variable name `up_to_date`, which
is good.
Now, big question: is there a way for the loop to exit before we had a
chance to see the common commit that is identical to `oid`? And I think
there is: `ff_merge` is not reset between the outer loop (the one
iterating over `remotes`). If that is the case, then we would miss that
we're already up to date.
Next thing is that `if test "$common,$NON_FF_MERGE" = "$MRC,0"` thing.
This is turned into that `if (!j->next && ...)` thing, and I _think_ that
it does the wrong thing. Rather than verifying that the `common` list
is identical to "MRC" (= the merge reference list), it would only ever
compare the last entries of `common` and MRC.
I have a hard time convincing myself that this is idempotent to the shell
script version.
Instead, I think it should read somewhat like this:
for (j = common, k = 0; j && (!up_to_date || ff_merge); j = j->next) {
up_to_date ||= oideq(&j->item->object.oid, oid);
if (ff_merge &&
(k >= nr_references ||
!oideq(&j->item->object.oid,
&reference_commits[k++]->object.oid))
ff_merge = 0;
}
But quite honestly, this still looks "too clever" and too fragile to me.
For something as rare as an octopus merge, I'd _much_ rather have simpler
code that is easy to reason about and does the job reliably (if somewhat
slower than a hyper-optimized version):
/*
* If `oid` is reachable from `HEAD`, we're already up to
* date.
*/
for (j = common; j; j = j->next)
if (oideq(&j->item->object.oid, oid)) {
up_to_date = 1;
break;
}
if (up_to_date) {
printf(_("Already up to date with %s\n"), branch_name);
free(branch_name);
free_commit_list(common);
continue;
}
for (j = common, k = 0; ff_merge && j; j = j->next)
if (k >= nr_references ||
!oideq(&j->item->object.oid,
&reference_commits[k++]->object.oid))
ff_merge = 0;
if (k != nr_references)
ff_merge = 0;
But the more I stare at the shell script code, the more I start to believe
that this `MRC` business is just a very convoluted way to essentially
verify that the `HEAD` is the _single_ merge base.
I say that because I cannot fail to notice that `$common` separates the
merge bases by newlines, while `$MRC` separates its entries by spaces.
Therefore,
test "$common,$NON_FF_MERGE" = "$MRC,0"
can only ever evaluate to `true` if both `$common` and `$MRC` contains
exactly one and the same oid, namely the one of the revision to which we
just fast-forwarded in the previous iteration.
Therefore, the logic does not even need a loop. It would be as trivial as:
/*
* If we could fast-forward so far and `HEAD` is the
* single merge base with the current `remote` revision,
* keep fast-forwarding.
*/
if (ff_merge && common && !common->next && nr_references == 1 &&
oideq(common->item->object.oid,
reference_commit[0]->object.oid)) {
ret = octopus_fast_forward(r, branch_name, head_tree,
current_tree, &reference_tree);
nr_references = 0;
} else {
ff_merge = 0;
ret = octopus_do_merge(r, branch_name, common,
current_tree, &reference_tree);
}
quoted hunk ↗ jump to hunk
+
+ if (up_to_date) {
+ printf(_("Already up to date with %s\n"), branch_name);
+
+ free(branch_name);
+ free_commit_list(common);
+ continue;
+ }
+
+ if (ff_merge) {
+ ret = octopus_fast_forward(r, branch_name, head_tree,
+ current_tree, &reference_tree);
+ nr_references = 0;
+ } else {
+ ret = octopus_do_merge(r, branch_name, common,
+ current_tree, &reference_tree);
+ }
+
+ free(branch_name);
+ free_commit_list(common);
+
+ if (ret == -1 || ret == 2)
+ break;
+ else if (ret && i->next) {
+ /*
+ * We allow only last one to have a
+ * hand-resolvable conflicts. Last round failed
+ * and we still had a head to merge.
+ */
+ puts(_("Automated merge did not work."));
+ puts(_("Should not be doing an octopus."));
+
+ free(reference_commits);
+ return 2;
I see that you moved this block from the beginning of the loop to the end
(in the script, it was at the start of the loop). This is a good change.
I wonder, though, whether it wouldn't make more sense to replace the last
two lines with this:
ret = 2;
break;
That way, we need not worry about releasing resources in multiple places
in the future: it will all be done at the end of the function.
Phew. What a lot to unpack.
Please let me express my gratitude for working on this. My many comments
may seem as if I am unhappy with the progress, but nothing could be
further from the truth. I am impressed by your tenacity, and I hope that I
could do my little bit to make this patch series as good as we can.
Thanks,
Dscho
quoted hunk ↗ jump to hunk
+ }
+
+ reference_commits[nr_references++] = c;
+ }
+
+ free(reference_commits);
+ return ret;
+}
diff --git a/merge-strategies.h b/merge-strategies.h
index bba4bf999c..8de2249ee6 100644
--- a/merge-strategies.h
+++ b/merge-strategies.h
@@ -32,5 +32,8 @@ int merge_all_index(struct index_state *istate, int oneshot, int quiet,
int merge_strategies_resolve(struct repository *r,
struct commit_list *bases, const char *head_arg,
struct commit_list *remote);
+int merge_strategies_octopus(struct repository *r,
+ struct commit_list *bases, const char *head_arg,
+ struct commit_list *remote);
#endif /* MERGE_STRATEGIES_H */
--
2.31.0