This will enable users to implement bisecting on first parents
which can be useful for when the commits from a feature branch
that we want to merge are not always tested.

Signed-off-by: Tiago Botelho <tiagonbote...@hotmail.com>
---

This patch resolves every issue with the unit tests. 

The bug detected by Junio regarding do_find_bisection() propagating 
a weight of -1 over to the child nodes ended up not being an 
issue since it cannot happen in practice, there will always be at 
least one UNINTERESTING parent which will propagate either a weight 
of 0 or 1 throughout the child nodes.

 bisect.c                   | 43 ++++++++++++++++++++++------------
 bisect.h                   |  1 +
 builtin/rev-list.c         |  3 +++
 revision.c                 |  3 ---
 t/t6002-rev-list-bisect.sh | 58 ++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 90 insertions(+), 18 deletions(-)

diff --git a/bisect.c b/bisect.c
index 4eafc8262..cb80f29c5 100644
--- a/bisect.c
+++ b/bisect.c
@@ -82,15 +82,16 @@ static inline void weight_set(struct commit_list *elem, int 
weight)
        *((int*)(elem->item->util)) = weight;
 }
 
-static int count_interesting_parents(struct commit *commit)
+static int count_interesting_parents(struct commit *commit, unsigned 
bisect_flags)
 {
        struct commit_list *p;
        int count;
 
        for (count = 0, p = commit->parents; p; p = p->next) {
-               if (p->item->object.flags & UNINTERESTING)
-                       continue;
-               count++;
+               if (!(p->item->object.flags & UNINTERESTING))
+                   count++;
+               if (bisect_flags & BISECT_FIRST_PARENT)
+                       break;
        }
        return count;
 }
@@ -117,10 +118,10 @@ static inline int halfway(struct commit_list *p, int nr)
 }
 
 #if !DEBUG_BISECT
-#define show_list(a,b,c,d) do { ; } while (0)
+#define show_list(a,b,c,d,e) do { ; } while (0)
 #else
 static void show_list(const char *debug, int counted, int nr,
-                     struct commit_list *list)
+                     struct commit_list *list, unsigned bisect_flags)
 {
        struct commit_list *p;
 
@@ -146,10 +147,14 @@ static void show_list(const char *debug, int counted, int 
nr,
                else
                        fprintf(stderr, "---");
                fprintf(stderr, " %.*s", 8, oid_to_hex(&commit->object.oid));
-               for (pp = commit->parents; pp; pp = pp->next)
+               for (pp = commit->parents; pp; pp = pp->next) {
                        fprintf(stderr, " %.*s", 8,
                                oid_to_hex(&pp->item->object.oid));
 
+                       if (bisect_flags & BISECT_FIRST_PARENT)
+                               break;
+               }
+
                subject_len = find_commit_subject(buf, &subject_start);
                if (subject_len)
                        fprintf(stderr, " %.*s", subject_len, subject_start);
@@ -267,13 +272,13 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                unsigned flags = commit->object.flags;
 
                p->item->util = &weights[n++];
-               switch (count_interesting_parents(commit)) {
+               switch (count_interesting_parents(commit, bisect_flags)) {
                case 0:
                        if (!(flags & TREESAME)) {
                                weight_set(p, 1);
                                counted++;
                                show_list("bisection 2 count one",
-                                         counted, nr, list);
+                                         counted, nr, list, bisect_flags);
                        }
                        /*
                         * otherwise, it is known not to reach any
@@ -289,7 +294,7 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                }
        }
 
-       show_list("bisection 2 initialize", counted, nr, list);
+       show_list("bisection 2 initialize", counted, nr, list, bisect_flags);
 
        /*
         * If you have only one parent in the resulting set
@@ -319,7 +324,7 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                counted++;
        }
 
-       show_list("bisection 2 count_distance", counted, nr, list);
+       show_list("bisection 2 count_distance", counted, nr, list, 
bisect_flags);
 
        while (counted < nr) {
                for (p = list; p; p = p->next) {
@@ -329,6 +334,11 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                        if (0 <= weight(p))
                                continue;
                        for (q = p->item->parents; q; q = q->next) {
+                               if ((bisect_flags & BISECT_FIRST_PARENT)) {
+                                       if (weight(q) < 0)
+                                               q = NULL;
+                                       break;
+                               }
                                if (q->item->object.flags & UNINTERESTING)
                                        continue;
                                if (0 <= weight(q))
@@ -346,7 +356,7 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                                weight_set(p, weight(q)+1);
                                counted++;
                                show_list("bisection 2 count one",
-                                         counted, nr, list);
+                                         counted, nr, list, bisect_flags);
                        }
                        else
                                weight_set(p, weight(q));
@@ -357,7 +367,7 @@ static struct commit_list *do_find_bisection(struct 
commit_list *list,
                }
        }
 
-       show_list("bisection 2 counted all", counted, nr, list);
+       show_list("bisection 2 counted all", counted, nr, list, bisect_flags);
 
        if (!find_all)
                return best_bisection(list, nr);
@@ -372,7 +382,7 @@ void find_bisection(struct commit_list **commit_list, int 
*reaches,
        struct commit_list *list, *p, *best, *next, *last;
        int *weights;
 
-       show_list("bisection 2 entry", 0, 0, *commit_list);
+       show_list("bisection 2 entry", 0, 0, *commit_list, bisect_flags);
 
        /*
         * Count the number of total and tree-changing items on the
@@ -395,7 +405,7 @@ void find_bisection(struct commit_list **commit_list, int 
*reaches,
                on_list++;
        }
        list = last;
-       show_list("bisection 2 sorted", 0, nr, list);
+       show_list("bisection 2 sorted", 0, nr, list, bisect_flags);
 
        *all = nr;
        weights = xcalloc(on_list, sizeof(*weights));
@@ -962,6 +972,9 @@ int bisect_next_all(const char *prefix, int no_checkout)
        if (skipped_revs.nr)
                bisect_flags |= BISECT_FIND_ALL;
 
+       if (revs.first_parent_only)
+               bisect_flags |= BISECT_FIRST_PARENT;
+
        find_bisection(&revs.commits, &reaches, &all, bisect_flags);
        revs.commits = managed_skipped(revs.commits, &tried);
 
diff --git a/bisect.h b/bisect.h
index 1d40a33ad..e445567c2 100644
--- a/bisect.h
+++ b/bisect.h
@@ -2,6 +2,7 @@
 #define BISECT_H
 
 #define BISECT_FIND_ALL                (1u<<0)
+#define BISECT_FIRST_PARENT            (1u<<1)
 
 /*
  * Find bisection. If something is found, `reaches` will be the number of
diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index 8752f5bbe..66439e1b3 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -538,6 +538,9 @@ int cmd_rev_list(int argc, const char **argv, const char 
*prefix)
        if (bisect_list) {
                int reaches, all;
 
+               if (revs.first_parent_only)
+                       bisect_flags |= BISECT_FIRST_PARENT;
+
                find_bisection(&revs.commits, &reaches, &all, bisect_flags);
 
                if (bisect_show_vars)
diff --git a/revision.c b/revision.c
index 4e0e193e5..b9d063805 100644
--- a/revision.c
+++ b/revision.c
@@ -2474,9 +2474,6 @@ int setup_revisions(int argc, const char **argv, struct 
rev_info *revs, struct s
        if (!revs->reflog_info && revs->grep_filter.use_reflog_filter)
                die("cannot use --grep-reflog without --walk-reflogs");
 
-       if (revs->first_parent_only && revs->bisect)
-               die(_("--first-parent is incompatible with --bisect"));
-
        if (revs->expand_tabs_in_log < 0)
                revs->expand_tabs_in_log = revs->expand_tabs_in_log_default;
 
diff --git a/t/t6002-rev-list-bisect.sh b/t/t6002-rev-list-bisect.sh
index a66140803..c9cd349de 100755
--- a/t/t6002-rev-list-bisect.sh
+++ b/t/t6002-rev-list-bisect.sh
@@ -263,4 +263,62 @@ test_expect_success 'rev-parse --bisect can default to 
good/bad refs' '
        test_cmp expect.sorted actual.sorted
 '
 
+# We generate the following commit graph:
+#
+#   B ------ C
+#  /          \
+# A            FX
+#  \          /
+#   D - CC - EX
+
+test_expect_success 'setup' '
+  test_commit A &&
+  test_commit B &&
+  test_commit C &&
+  git reset --hard A &&
+  test_commit D &&
+  test_commit CC &&
+  test_commit EX &&
+  test_merge FX C
+'
+
+test_output_expect_success "--bisect --first-parent" 'git rev-list --bisect 
--first-parent FX ^A' <<EOF
+$(git rev-parse CC)
+EOF
+
+test_output_expect_success "--first-parent" 'git rev-list --first-parent FX 
^A' <<EOF
+$(git rev-parse FX)
+$(git rev-parse EX)
+$(git rev-parse CC)
+$(git rev-parse D)
+EOF
+
+test_output_expect_success "--bisect-vars --first-parent" 'git rev-list 
--bisect-vars --first-parent FX ^A' <<EOF
+bisect_rev='$(git rev-parse CC)'
+bisect_nr=1
+bisect_good=1
+bisect_bad=1
+bisect_all=4
+bisect_steps=1
+EOF
+
+test_expect_success "--bisect-all --first-parent" '
+cat >expect1 <<EOF &&
+$(git rev-parse CC) (dist=2)
+$(git rev-parse EX) (dist=1)
+$(git rev-parse D) (dist=1)
+$(git rev-parse FX) (dist=0)
+EOF
+
+cat >expect2 <<EOF &&
+$(git rev-parse CC) (dist=2)
+$(git rev-parse D) (dist=1)
+$(git rev-parse EX) (dist=1)
+$(git rev-parse FX) (dist=0)
+EOF
+
+git rev-list --bisect-all --first-parent FX ^A >actual &&
+  ( test_cmp expect1 actual || test_cmp expect2 actual )
+'
+
 test_done
-- 
2.16.3

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