Re: [PATCH v2] blame: avoid checking if a file exists on the working tree if a revision is provided

[Jeff King <peff@peff.net>]

On Tue, Nov 17, 2015 at 06:01:25PM -0500, Eric Sunshine wrote:

> On Tue, Nov 17, 2015 at 5:48 PM, Jeff King <peff@peff.net> wrote:
> > On Tue, Nov 17, 2015 at 12:11:25AM -0500, Eric Sunshine wrote:
> >> > blame content even if the path provided does match an existing
> >> > blob on said revision.
> >>
> >> git-blame documentation does not advertise "blame <file> <rev>" as a
> >> valid invocation. It does advertise "blame <rev> -- <file>", and this
> >> case already works correctly even when <file> does not exist in the
> >> worktree.
> >
> > Hmm. Out of curiosity I tried:
> >
> >   git blame v2.4.0 -- t/t6031-merge-recursive.sh
> >
> > and it segfaults. This bisects to Max's recent 1b0d400 (blame: extract
> > find_single_final, 2015-10-30), but I do not see anything obviously
> > wrong with it from a quick glance.
> 
> In the original code, sb->final received was assigned value of obj,
> which may have gone through deref_tag(), however, after 1b0d400,
> sb->final is unconditionally assigned the original value of obj, not
> the (potentially) deref'd value.

Good eye. I fed it a tag, find_single_final knows that points to a
commit, then prepare_final casts the tag object to a commit. Whoops.

The patch below fixes it for me. It probably needs a test, but I have to
run for the moment.

-- >8 --
Subject: [PATCH] blame: fix object casting regression

Commit 1b0d400 refactored the prepare_final() function so
that it could be reused in multiple places. Originally, the
loop had two outputs: a commit to stuff into sb->final, and
the name of the commit from the rev->pending array.

After the refactor, that loop is put in its own function
with a single return value: the object_array_entry from the
rev->pending array. This contains both the name and the object,
but with one important difference: the object is the
_original_ object found by the revision parser, not the
dereferenced commit. If one feeds a tag to "git blame", we
end up casting the tag object to a "struct commit", which
causes a segfault.

Instead, let's return the commit (properly casted) directly
from the function, and take the "name" as an optional
out-parameter. This does the right thing, and actually
simplifies the callers, who no longer need to cast or
dereference the object_array_entry themselves.

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/blame.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/builtin/blame.c b/builtin/blame.c
index ac36738..2184e39 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -2403,10 +2403,12 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	return commit;
 }
 
-static struct object_array_entry *find_single_final(struct rev_info *revs)
+static struct commit *find_single_final(struct rev_info *revs,
+					const char **name_p)
 {
 	int i;
-	struct object_array_entry *found = NULL;
+	struct commit *found = NULL;
+	const char *name = NULL;
 
 	for (i = 0; i < revs->pending.nr; i++) {
 		struct object *obj = revs->pending.objects[i].item;
@@ -2418,22 +2420,20 @@ static struct object_array_entry *find_single_final(struct rev_info *revs)
 			die("Non commit %s?", revs->pending.objects[i].name);
 		if (found)
 			die("More than one commit to dig from %s and %s?",
-			    revs->pending.objects[i].name,
-			    found->name);
-		found = &(revs->pending.objects[i]);
+			    revs->pending.objects[i].name, name);
+		found = (struct commit *)obj;
+		name = revs->pending.objects[i].name;
 	}
+	if (name_p)
+		*name_p = name;
 	return found;
 }
 
 static char *prepare_final(struct scoreboard *sb)
 {
-	struct object_array_entry *found = find_single_final(sb->revs);
-	if (found) {
-		sb->final = (struct commit *) found->item;
-		return xstrdup(found->name);
-	} else {
-		return NULL;
-	}
+	const char *name;
+	sb->final = find_single_final(sb->revs, &name);
+	return xstrdup_or_null(name);
 }
 
 static char *prepare_initial(struct scoreboard *sb)
@@ -2721,11 +2721,9 @@ parse_done:
 		die("Cannot use --contents with final commit object name");
 
 	if (reverse && revs.first_parent_only) {
-		struct object_array_entry *entry = find_single_final(sb.revs);
-		if (!entry)
+		final_commit = find_single_final(sb.revs, NULL);
+		if (!final_commit)
 			die("--reverse and --first-parent together require specified latest commit");
-		else
-			final_commit = (struct commit*) entry->item;
 	}
 
 	/*
-- 
2.6.3.636.g1460207