last-modified: fix bug when some paths remain unhandled

The recently introduced new subcommand git-last-modified(1) runs into an
error in some scenarios. It then would exit with the message:

    BUG: paths remaining beyond boundary in last-modified

This seems to happens for example when criss-cross merges are involved.
In that scenario, the function diff_tree_combined() gets called.

The function diff_tree_combined() copies the `struct diff_options` from
the input `struct rev_info` to override some flags. One flag is
`recursive`, which is always set to 1. This has been the case since the
inception of this function in af3feef (diff-tree -c: show a merge
commit a bit more sensibly., 2006-01-24).

This behavior is incompatible with git-last-modified(1), when called
non-recursive (which is the default).

The last-modified machinery uses a hashmap for all the paths it wants to
get the last-modified commit for. Through log_tree_commit() the callback
mark_path() is called. The diff machinery uses diff_tree_combined()
internally, and due to it's recursive behavior the callback receives
entries inside subtrees, but not the subtree entries themselves. So a
directory is never expelled from the hashmap, and the BUG() statement
gets hit.

Because there are many callers calling into diff_tree_combined(), both
directly and indirectly, we cannot simply change it's behavior.

Instead, add a flag `no_recursive_diff_tree_combined` which supresses
the behavior of diff_tree_combined() to override `recursive` and set
this flag in builtin/last-modified.c.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: To1ne

builtin/last-modified.c

@@ -265,6 +265,7 @@ static int last_modified_init(struct last_modified *lm, struct repository *r,
 	lm->rev.boundary = 1;
 	lm->rev.no_commit_id = 1;
 	lm->rev.diff = 1;
+	lm->rev.diffopt.flags.no_recursive_diff_tree_combined = 1;
 	lm->rev.diffopt.flags.recursive = lm->recursive;
 	lm->rev.diffopt.flags.tree_in_recursive = lm->show_trees;
 

combine-diff.c

@@ -1515,8 +1515,9 @@ void diff_tree_combined(const struct object_id *oid,
 
 	diffopts = *opt;
 	copy_pathspec(&diffopts.pathspec, &opt->pathspec);
-	diffopts.flags.recursive = 1;
 	diffopts.flags.allow_external = 0;
+	if (!opt->flags.no_recursive_diff_tree_combined)
+		diffopts.flags.recursive = 1;
 
 	/* find set of paths that everybody touches
 	 *

diff.h

@@ -126,6 +126,13 @@ struct diff_flags {
 	unsigned recursive;
 	unsigned tree_in_recursive;
 
+	/*
+	 * Historically diff_tree_combined() overrides recursive to 1. To
+	 * suppress this behavior, set the flag below.
+	 * It has no effect if recursive is already set to 1.
+	 */
+	unsigned no_recursive_diff_tree_combined;
+
 	/* Affects the way how a file that is seemingly binary is treated. */
 	unsigned binary;
 	unsigned text;

t/t8020-last-modified.sh

@@ -128,6 +128,22 @@ test_expect_success 'only last-modified files in the current tree' '
 	EOF
 '
 
+test_expect_success 'last-modified with subdir and criss-cross merge' '
+	git checkout -b branch-k1 1 &&
+	mkdir -p a k &&
+	test_commit k1 a/file2 &&
+	git checkout -b branch-k2 &&
+	test_commit k2 k/file2 &&
+	git checkout branch-k1 &&
+	test_merge km2 branch-k2 &&
+	test_merge km3 3 &&
+	check_last_modified <<-\EOF
+	km3 a
+	k2 k
+	1 file
+	EOF
+'
+
 test_expect_success 'cross merge boundaries in blaming' '
 	git checkout HEAD^0 &&
 	git rm -rf . &&