pack-bitmap: move some initialization to `bitmap_writer_init()`

The pack-bitmap-writer machinery uses a oidmap (backed by khash.h) to
map from commits selected for bitmaps (by OID) to a bitmapped_commit
structure (containing the bitmap itself, among other things like its XOR
offset, etc.)

This map was initialized at the end of `bitmap_writer_build()`. New
entries are added in `pack-bitmap-write.c::store_selected()`, which is
called by the bitmap_builder machinery (which is responsible for
traversing history and generating the actual bitmaps).

Reorganize when this field is initialized and when entries are added to
it so that we can quickly determine whether a commit is a candidate for
pseudo-merge selection, or not (since it was already selected to receive
a bitmap, and thus storing it in a pseudo-merge would be redundant).

The changes are as follows:

  - Introduce a new `bitmap_writer_init()` function which initializes
    the `writer.bitmaps` field (instead of waiting until the end of
    `bitmap_writer_build()`).

  - Add map entries in `push_bitmapped_commit()` (which is called via
    `bitmap_writer_select_commits()`) with OID keys and NULL values to
    track whether or not we *expect* to write a bitmap for some given
    commit.

  - Validate that a NULL entry is found matching the given key when we
    store a selected bitmap.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 6 deletions

diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index 6cae670412..d887015583 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -27,9 +27,12 @@ struct bitmapped_commit {
 	uint32_t commit_pos;
 };
 
-void bitmap_writer_init(struct bitmap_writer *writer)
+void bitmap_writer_init(struct bitmap_writer *writer, struct repository *r)
 {
 	memset(writer, 0, sizeof(struct bitmap_writer));
+	if (writer->bitmaps)
+		BUG("bitmap writer already initialized");
+	writer->bitmaps = kh_init_oid_map();
 }
 
 void bitmap_writer_free(struct bitmap_writer *writer)
@@ -128,11 +131,21 @@ void bitmap_writer_build_type_index(struct bitmap_writer *writer,
 static inline void push_bitmapped_commit(struct bitmap_writer *writer,
 					 struct commit *commit)
 {
+	int hash_ret;
+	khiter_t hash_pos;
+
 	if (writer->selected_nr >= writer->selected_alloc) {
 		writer->selected_alloc = (writer->selected_alloc + 32) * 2;
 		REALLOC_ARRAY(writer->selected, writer->selected_alloc);
 	}
 
+	hash_pos = kh_put_oid_map(writer->bitmaps, commit->object.oid,
+				  &hash_ret);
+	if (!hash_ret)
+		die(_("duplicate entry when writing bitmap index: %s"),
+		    oid_to_hex(&commit->object.oid));
+	kh_value(writer->bitmaps, hash_pos) = NULL;
+
 	writer->selected[writer->selected_nr].commit = commit;
 	writer->selected[writer->selected_nr].bitmap = NULL;
 	writer->selected[writer->selected_nr].write_as = NULL;
@@ -483,14 +496,14 @@ static void store_selected(struct bitmap_writer *writer,
 {
 	struct bitmapped_commit *stored = &writer->selected[ent->idx];
 	khiter_t hash_pos;
-	int hash_ret;
 
 	stored->bitmap = bitmap_to_ewah(ent->bitmap);
 
-	hash_pos = kh_put_oid_map(writer->bitmaps, commit->object.oid, &hash_ret);
-	if (hash_ret == 0)
-		die("Duplicate entry when writing index: %s",
+	hash_pos = kh_get_oid_map(writer->bitmaps, commit->object.oid);
+	if (hash_pos == kh_end(writer->bitmaps))
+		die(_("attempted to store non-selected commit: '%s'"),
 		    oid_to_hex(&commit->object.oid));
+
 	kh_value(writer->bitmaps, hash_pos) = stored;
 }
 
@@ -506,7 +519,6 @@ int bitmap_writer_build(struct bitmap_writer *writer,
 	uint32_t *mapping;
 	int closed = 1; /* until proven otherwise */
 
-	writer->bitmaps = kh_init_oid_map();
 	writer->to_pack = to_pack;
 
 	if (writer->show_progress)