log: use true parents for diff even when rewriting

When using pathspec filtering in combination with diff-based log
output, parent simplification happens before the diff is computed.
The diff is therefore against the *simplified* parents.

This works okay, arguably by accident, in the normal case:
simplification reduces to one parent as long as the commit is TREESAME
to it.  So the simplified parent of any given commit must have the
same tree contents on the filtered paths as its true (unfiltered)
parent.

However, --full-diff breaks this guarantee, and indeed gives pretty
spectacular results when comparing the output of

  git log --graph --stat ...
  git log --graph --full-diff --stat ...

(--graph internally kicks in parent simplification, much like
--parents).

To fix it, store a copy of the parent list before simplification (in a
slab) whenever --full-diff is in effect.  Then use the stored parents
instead of the simplified ones in the commit display code paths.  The
latter do not actually check for --full-diff to avoid duplicated code;
they just grab the original parents if save_parents() has not been
called for this revision walk.

For ordinary commits it should be obvious that this is the right thing
to do.

Merge commits are a bit subtle.  Observe that with default
simplification, merge simplification is an all-or-nothing decision:
either the merge is TREESAME to one parent and disappears, or it is
different from all parents and the parent list remains intact.
Redundant parents are not pruned, so the existing code also shows them
as a merge.

So if we do show a merge commit, the parent list just consists of the
rewrite result on each parent.  Running, e.g., --cc on this in
--full-diff mode is not very useful: if any commits were skipped, some
hunks will disagree with all sides of the merge (with one side,
because commits were skipped; with the others, because they didn't
have those changes in the first place).  This triggers --cc showing
these hunks spuriously.

Therefore I believe that even for merge commits it is better to show
the diffs wrt. the original parents.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/revision.h b/revision.h
index 95859ba119..e7f1d211bf 100644
--- a/revision.h
+++ b/revision.h
@@ -25,6 +25,7 @@
 struct rev_info;
 struct log_info;
 struct string_list;
+struct saved_parents;
 
 struct rev_cmdline_info {
 	unsigned int nr;
@@ -187,6 +188,9 @@ struct rev_info {
 
 	/* line level range that we are chasing */
 	struct decoration line_log_data;
+
+	/* copies of the parent lists, for --full-diff display */
+	struct saved_parents *saved_parents_slab;
 };
 
 #define REV_TREE_SAME		0
@@ -273,4 +277,20 @@ typedef enum rewrite_result (*rewrite_parent_fn_t)(struct rev_info *revs, struct
 
 extern int rewrite_parents(struct rev_info *revs, struct commit *commit,
 	rewrite_parent_fn_t rewrite_parent);
+
+/*
+ * Save a copy of the parent list, and return the saved copy.  This is
+ * used by the log machinery to retrieve the original parents when
+ * commit->parents has been modified by history simpification.
+ *
+ * You may only call save_parents() once per commit (this is checked
+ * for non-root commits).
+ *
+ * get_saved_parents() will transparently return commit->parents if
+ * history simplification is off.
+ */
+extern void save_parents(struct rev_info *revs, struct commit *commit);
+extern struct commit_list *get_saved_parents(struct rev_info *revs, const struct commit *commit);
+extern void free_saved_parents(struct rev_info *revs);
+
 #endif