pack-objects: avoid delta chains that are too long.

This tries to rework the solution for the excess delta chain
problem. An earlier commit worked it around ``cheaply'', but
repeated repacking risks unbound growth of delta chains.

This version counts the length of delta chain we are reusing
from the existing pack, and makes sure a base object that has
sufficiently long delta chain does not get deltified.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/pack-objects.c b/pack-objects.c
index 38e1c99..0c9f4c9 100644 (file)
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -10,16 +10,22 @@ static const char pack_usage[] = "git-pack-objects [-q] [--no-reuse-delta] [--no
 struct object_entry {
        unsigned char sha1[20];
        unsigned long size;     /* uncompressed size */
-       unsigned long offset;   /* offset into the final pack file (nonzero if already written) */
+       unsigned long offset;   /* offset into the final pack file;
+                                * nonzero if already written.
+                                */
        unsigned int depth;     /* delta depth */
+       unsigned int delta_limit;       /* base adjustment for in-pack delta */
        unsigned int hash;      /* name hint hash */
        enum object_type type;
-       unsigned char edge;     /* reused delta chain points at this entry. */
        enum object_type in_pack_type;  /* could be delta */
        unsigned long delta_size;       /* delta data size (uncompressed) */
        struct object_entry *delta;     /* delta base object */
        struct packed_git *in_pack;     /* already in pack */
        unsigned int in_pack_offset;
+       struct object_entry *delta_child; /* delitified objects who bases me */
+       struct object_entry *delta_sibling; /* other deltified objects who
+                                            * uses the same base as me
+                                            */
 };
 
 /*
@@ -470,7 +476,8 @@ static void check_object(struct object_entry *entry)
                        entry->delta = base_entry;
                        entry->type = OBJ_DELTA;
 
-                       base_entry->edge = 1;
+                       entry->delta_sibling = base_entry->delta_child;
+                       base_entry->delta_child = entry;
 
                        return;
                }
@@ -513,15 +520,32 @@ static void hash_objects(void)
        }
 }
 
+static unsigned int check_delta_limit(struct object_entry *me, unsigned int n)
+{
+       struct object_entry *child = me->delta_child;
+       unsigned int m = n;
+       while (child) {
+               unsigned int c = check_delta_limit(child, n + 1);
+               if (m < c)
+                       m = c;
+               child = child->delta_sibling;
+       }
+       return m;
+}
+
 static void get_object_details(void)
 {
        int i;
-       struct object_entry *entry = objects;
+       struct object_entry *entry;
 
        hash_objects();
        prepare_pack_ix();
-       for (i = 0; i < nr_objects; i++)
-               check_object(entry++);
+       for (i = 0, entry = objects; i < nr_objects; i++, entry++)
+               check_object(entry);
+       for (i = 0, entry = objects; i < nr_objects; i++, entry++)
+               if (!entry->delta && entry->delta_child)
+                       entry->delta_limit =
+                               check_delta_limit(entry, 1);
 }
 
 typedef int (*entry_sort_t)(const struct object_entry *, const struct object_entry *);
@@ -598,8 +622,11 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de
         * that depend on the current object into account -- otherwise
         * they would become too deep.
         */
-       if (cur_entry->edge)
-               max_depth /= 4;
+       if (cur_entry->delta_child) {
+               if (max_depth <= cur_entry->delta_limit)
+                       return 0;
+               max_depth -= cur_entry->delta_limit;
+       }
 
        size = cur_entry->size;
        if (size < 50)