We currently have seven callers of `reduce_heads(foo)`. Six of them do
not use the original list `foo` again, and actually, all six of those
end up leaking it.

Introduce and use `reduce_heads_replace(&foo)` as a leak-free version of
`foo = reduce_heads(foo)` to fix several of these. Fix the remaining
leaks using `free_commit_list()`.

While we're here, document `reduce_heads()` and mark it as `extern`.

Signed-off-by: Martin Ågren <martin.ag...@gmail.com>
---
 builtin/commit.c        |  2 +-
 builtin/fmt-merge-msg.c |  2 +-
 builtin/merge-base.c    |  6 +++++-
 builtin/merge.c         |  1 +
 builtin/pull.c          |  4 +++-
 commit.c                |  7 +++++++
 commit.h                | 18 +++++++++++++++++-
 7 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index d75b3805e..11c474018 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1728,7 +1728,7 @@ int cmd_commit(int argc, const char **argv, const char 
*prefix)
                                allow_fast_forward = 0;
                }
                if (allow_fast_forward)
-                       parents = reduce_heads(parents);
+                       reduce_heads_replace(&parents);
        } else {
                if (!reflog_msg)
                        reflog_msg = (whence == FROM_CHERRY_PICK)
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index e99b5ddbf..27a2361e9 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -571,7 +571,7 @@ static void find_merge_parents(struct merge_parents *result,
        head_commit = lookup_commit(head);
        if (head_commit)
                commit_list_insert(head_commit, &parents);
-       parents = reduce_heads(parents);
+       reduce_heads_replace(&parents);
 
        while (parents) {
                struct commit *cmit = pop_commit(&parents);
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 6dbd167d3..b1b7590c4 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -59,6 +59,8 @@ static int handle_independent(int count, const char **args)
                commit_list_insert(get_commit_reference(args[i]), &revs);
 
        result = reduce_heads(revs);
+       free_commit_list(revs);
+
        if (!result)
                return 1;
 
@@ -78,7 +80,9 @@ static int handle_octopus(int count, const char **args, int 
show_all)
        for (i = count - 1; i >= 0; i--)
                commit_list_insert(get_commit_reference(args[i]), &revs);
 
-       result = reduce_heads(get_octopus_merge_bases(revs));
+       result = get_octopus_merge_bases(revs);
+       free_commit_list(revs);
+       reduce_heads_replace(&result);
 
        if (!result)
                return 1;
diff --git a/builtin/merge.c b/builtin/merge.c
index ab5ffe85e..fbbf2a9e5 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -999,6 +999,7 @@ static struct commit_list *reduce_parents(struct commit 
*head_commit,
 
        /* Find what parents to record by checking independent ones. */
        parents = reduce_heads(remoteheads);
+       free_commit_list(remoteheads);
 
        remoteheads = NULL;
        remotes = &remoteheads;
diff --git a/builtin/pull.c b/builtin/pull.c
index 6f772e8a2..5eeaa8c68 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -745,8 +745,10 @@ static int get_octopus_merge_base(struct object_id 
*merge_base,
        if (!is_null_oid(fork_point))
                commit_list_insert(lookup_commit_reference(fork_point), &revs);
 
-       result = reduce_heads(get_octopus_merge_bases(revs));
+       result = get_octopus_merge_bases(revs);
        free_commit_list(revs);
+       reduce_heads_replace(&result);
+
        if (!result)
                return 1;
 
diff --git a/commit.c b/commit.c
index 1e0e63379..cab8d4455 100644
--- a/commit.c
+++ b/commit.c
@@ -1090,6 +1090,13 @@ struct commit_list *reduce_heads(struct commit_list 
*heads)
        return result;
 }
 
+void reduce_heads_replace(struct commit_list **heads)
+{
+       struct commit_list *result = reduce_heads(*heads);
+       free_commit_list(*heads);
+       *heads = result;
+}
+
 static const char gpg_sig_header[] = "gpgsig";
 static const int gpg_sig_header_len = sizeof(gpg_sig_header) - 1;
 
diff --git a/commit.h b/commit.h
index 6d769590f..99a3fea68 100644
--- a/commit.h
+++ b/commit.h
@@ -313,7 +313,23 @@ extern int interactive_add(int argc, const char **argv, 
const char *prefix, int
 extern int run_add_interactive(const char *revision, const char *patch_mode,
                               const struct pathspec *pathspec);
 
-struct commit_list *reduce_heads(struct commit_list *heads);
+/*
+ * Takes a list of commits and returns a new list where those
+ * have been removed that can be reached from other commits in
+ * the list. It is useful for, e.g., reducing the commits
+ * randomly thrown at the git-merge command and removing
+ * redundant commits that the user shouldn't have given to it.
+ *
+ * This function destroys the STALE bit of the commit objects'
+ * flags.
+ */
+extern struct commit_list *reduce_heads(struct commit_list *heads);
+
+/*
+ * Like `reduce_heads()`, except it replaces the list. Use this
+ * instead of `foo = reduce_heads(foo);` to avoid memory leaks.
+ */
+extern void reduce_heads_replace(struct commit_list **heads);
 
 struct commit_extra_header {
        struct commit_extra_header *next;
-- 
2.15.0.415.gac1375d7e

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