setup: document prefix
[gitweb.git] / notes.c
diff --git a/notes.c b/notes.c
index 023adce982c668f39b01652e525b54fd512d5603..e425e198278bfb5c6a039dc88825568f1518e875 100644 (file)
--- a/notes.c
+++ b/notes.c
@@ -1,10 +1,12 @@
 #include "cache.h"
-#include "commit.h"
 #include "notes.h"
-#include "refs.h"
+#include "blob.h"
+#include "tree.h"
 #include "utf8.h"
 #include "strbuf.h"
 #include "tree-walk.h"
+#include "string-list.h"
+#include "refs.h"
 
 /*
  * Use a non-balancing simple 16-tree structure with struct int_node as
@@ -25,10 +27,10 @@ struct int_node {
 /*
  * Leaf nodes come in two variants, note entries and subtree entries,
  * distinguished by the LSb of the leaf node pointer (see above).
- * As a note entry, the key is the SHA1 of the referenced commit, and the
+ * As a note entry, the key is the SHA1 of the referenced object, and the
  * value is the SHA1 of the note object.
  * As a subtree entry, the key is the prefix SHA1 (w/trailing NULs) of the
- * referenced commit, using the last byte of the key to store the length of
+ * referenced object, using the last byte of the key to store the length of
  * the prefix. The value is the SHA1 of the tree object containing the notes
  * subtree.
  */
@@ -37,6 +39,21 @@ struct leaf_node {
        unsigned char val_sha1[20];
 };
 
+/*
+ * A notes tree may contain entries that are not notes, and that do not follow
+ * the naming conventions of notes. There are typically none/few of these, but
+ * we still need to keep track of them. Keep a simple linked list sorted alpha-
+ * betically on the non-note path. The list is populated when parsing tree
+ * objects in load_subtree(), and the non-notes are correctly written back into
+ * the tree objects produced by write_notes_tree().
+ */
+struct non_note {
+       struct non_note *next; /* grounded (last->next == NULL) */
+       char *path;
+       unsigned int mode;
+       unsigned char sha1[20];
+};
+
 #define PTR_TYPE_NULL     0
 #define PTR_TYPE_INTERNAL 1
 #define PTR_TYPE_NOTE     2
@@ -46,17 +63,18 @@ struct leaf_node {
 #define CLR_PTR_TYPE(ptr)       ((void *) ((uintptr_t) (ptr) & ~3))
 #define SET_PTR_TYPE(ptr, type) ((void *) ((uintptr_t) (ptr) | (type)))
 
-#define GET_NIBBLE(n, sha1) (((sha1[n >> 1]) >> ((~n & 0x01) << 2)) & 0x0f)
+#define GET_NIBBLE(n, sha1) (((sha1[(n) >> 1]) >> ((~(n) & 0x01) << 2)) & 0x0f)
 
 #define SUBTREE_SHA1_PREFIXCMP(key_sha1, subtree_sha1) \
        (memcmp(key_sha1, subtree_sha1, subtree_sha1[19]))
 
-static struct int_node root_node;
+struct notes_tree default_notes_tree;
 
-static int initialized;
+static struct string_list display_notes_refs;
+static struct notes_tree **display_notes_trees;
 
-static void load_subtree(struct leaf_node *subtree, struct int_node *node,
-               unsigned int n);
+static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
+               struct int_node *node, unsigned int n);
 
 /*
  * Search the tree until the appropriate location for the given key is found:
@@ -73,7 +91,7 @@ static void load_subtree(struct leaf_node *subtree, struct int_node *node,
  *      - an unused leaf node (NULL)
  *      In any case, set *tree and *n, and return pointer to the tree location.
  */
-static void **note_tree_search(struct int_node **tree,
+static void **note_tree_search(struct notes_tree *t, struct int_node **tree,
                unsigned char *n, const unsigned char *key_sha1)
 {
        struct leaf_node *l;
@@ -85,27 +103,27 @@ static void **note_tree_search(struct int_node **tree,
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_sha1)) {
                        /* unpack tree and resume search */
                        (*tree)->a[0] = NULL;
-                       load_subtree(l, *tree, *n);
+                       load_subtree(t, l, *tree, *n);
                        free(l);
-                       return note_tree_search(tree, n, key_sha1);
+                       return note_tree_search(t, tree, n, key_sha1);
                }
        }
 
        i = GET_NIBBLE(*n, key_sha1);
        p = (*tree)->a[i];
-       switch(GET_PTR_TYPE(p)) {
+       switch (GET_PTR_TYPE(p)) {
        case PTR_TYPE_INTERNAL:
                *tree = CLR_PTR_TYPE(p);
                (*n)++;
-               return note_tree_search(tree, n, key_sha1);
+               return note_tree_search(t, tree, n, key_sha1);
        case PTR_TYPE_SUBTREE:
                l = (struct leaf_node *) CLR_PTR_TYPE(p);
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_sha1)) {
                        /* unpack tree and resume search */
                        (*tree)->a[i] = NULL;
-                       load_subtree(l, *tree, *n);
+                       load_subtree(t, l, *tree, *n);
                        free(l);
-                       return note_tree_search(tree, n, key_sha1);
+                       return note_tree_search(t, tree, n, key_sha1);
                }
                /* fall through */
        default:
@@ -118,10 +136,11 @@ static void **note_tree_search(struct int_node **tree,
  * Search to the tree location appropriate for the given key:
  * If a note entry with matching key, return the note entry, else return NULL.
  */
-static struct leaf_node *note_tree_find(struct int_node *tree, unsigned char n,
+static struct leaf_node *note_tree_find(struct notes_tree *t,
+               struct int_node *tree, unsigned char n,
                const unsigned char *key_sha1)
 {
-       void **p = note_tree_search(&tree, &n, key_sha1);
+       void **p = note_tree_search(t, &tree, &n, key_sha1);
        if (GET_PTR_TYPE(*p) == PTR_TYPE_NOTE) {
                struct leaf_node *l = (struct leaf_node *) CLR_PTR_TYPE(*p);
                if (!hashcmp(key_sha1, l->key_sha1))
@@ -130,55 +149,12 @@ static struct leaf_node *note_tree_find(struct int_node *tree, unsigned char n,
        return NULL;
 }
 
-/* Create a new blob object by concatenating the two given blob objects */
-static int concatenate_notes(unsigned char *cur_sha1,
-               const unsigned char *new_sha1)
-{
-       char *cur_msg, *new_msg, *buf;
-       unsigned long cur_len, new_len, buf_len;
-       enum object_type cur_type, new_type;
-       int ret;
-
-       /* read in both note blob objects */
-       new_msg = read_sha1_file(new_sha1, &new_type, &new_len);
-       if (!new_msg || !new_len || new_type != OBJ_BLOB) {
-               free(new_msg);
-               return 0;
-       }
-       cur_msg = read_sha1_file(cur_sha1, &cur_type, &cur_len);
-       if (!cur_msg || !cur_len || cur_type != OBJ_BLOB) {
-               free(cur_msg);
-               free(new_msg);
-               hashcpy(cur_sha1, new_sha1);
-               return 0;
-       }
-
-       /* we will separate the notes by a newline anyway */
-       if (cur_msg[cur_len - 1] == '\n')
-               cur_len--;
-
-       /* concatenate cur_msg and new_msg into buf */
-       buf_len = cur_len + 1 + new_len;
-       buf = (char *) xmalloc(buf_len);
-       memcpy(buf, cur_msg, cur_len);
-       buf[cur_len] = '\n';
-       memcpy(buf + cur_len + 1, new_msg, new_len);
-
-       free(cur_msg);
-       free(new_msg);
-
-       /* create a new blob object from buf */
-       ret = write_sha1_file(buf, buf_len, "blob", cur_sha1);
-       free(buf);
-       return ret;
-}
-
 /*
  * To insert a leaf_node:
  * Search to the tree location appropriate for the given leaf_node's key:
  * - If location is unused (NULL), store the tweaked pointer directly there
  * - If location holds a note entry that matches the note-to-be-inserted, then
- *   concatenate the two notes.
+ *   combine the two notes (by calling the given combine_notes function).
  * - If location holds a note entry that matches the subtree-to-be-inserted,
  *   then unpack the subtree-to-be-inserted into the location.
  * - If location holds a matching subtree entry, unpack the subtree at that
@@ -186,16 +162,17 @@ static int concatenate_notes(unsigned char *cur_sha1,
  * - Else, create a new int_node, holding both the node-at-location and the
  *   node-to-be-inserted, and store the new int_node into the location.
  */
-static void note_tree_insert(struct int_node *tree, unsigned char n,
-               struct leaf_node *entry, unsigned char type)
+static void note_tree_insert(struct notes_tree *t, struct int_node *tree,
+               unsigned char n, struct leaf_node *entry, unsigned char type,
+               combine_notes_fn combine_notes)
 {
        struct int_node *new_node;
        struct leaf_node *l;
-       void **p = note_tree_search(&tree, &n, entry->key_sha1);
+       void **p = note_tree_search(t, &tree, &n, entry->key_sha1);
 
        assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
        l = (struct leaf_node *) CLR_PTR_TYPE(*p);
-       switch(GET_PTR_TYPE(*p)) {
+       switch (GET_PTR_TYPE(*p)) {
        case PTR_TYPE_NULL:
                assert(!*p);
                *p = SET_PTR_TYPE(entry, type);
@@ -208,12 +185,11 @@ static void note_tree_insert(struct int_node *tree, unsigned char n,
                                if (!hashcmp(l->val_sha1, entry->val_sha1))
                                        return;
 
-                               if (concatenate_notes(l->val_sha1,
-                                               entry->val_sha1))
-                                       die("failed to concatenate note %s "
-                                           "into note %s for commit %s",
-                                           sha1_to_hex(entry->val_sha1),
+                               if (combine_notes(l->val_sha1, entry->val_sha1))
+                                       die("failed to combine notes %s and %s"
+                                           " for object %s",
                                            sha1_to_hex(l->val_sha1),
+                                           sha1_to_hex(entry->val_sha1),
                                            sha1_to_hex(l->key_sha1));
                                free(entry);
                                return;
@@ -223,7 +199,7 @@ static void note_tree_insert(struct int_node *tree, unsigned char n,
                        if (!SUBTREE_SHA1_PREFIXCMP(l->key_sha1,
                                                    entry->key_sha1)) {
                                /* unpack 'entry' */
-                               load_subtree(entry, tree, n);
+                               load_subtree(t, entry, tree, n);
                                free(entry);
                                return;
                        }
@@ -234,9 +210,10 @@ static void note_tree_insert(struct int_node *tree, unsigned char n,
                if (!SUBTREE_SHA1_PREFIXCMP(entry->key_sha1, l->key_sha1)) {
                        /* unpack 'l' and restart insert */
                        *p = NULL;
-                       load_subtree(l, tree, n);
+                       load_subtree(t, l, tree, n);
                        free(l);
-                       note_tree_insert(tree, n, entry, type);
+                       note_tree_insert(t, tree, n, entry, type,
+                                        combine_notes);
                        return;
                }
                break;
@@ -246,9 +223,83 @@ static void note_tree_insert(struct int_node *tree, unsigned char n,
        assert(GET_PTR_TYPE(*p) == PTR_TYPE_NOTE ||
               GET_PTR_TYPE(*p) == PTR_TYPE_SUBTREE);
        new_node = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
-       note_tree_insert(new_node, n + 1, l, GET_PTR_TYPE(*p));
+       note_tree_insert(t, new_node, n + 1, l, GET_PTR_TYPE(*p),
+                        combine_notes);
        *p = SET_PTR_TYPE(new_node, PTR_TYPE_INTERNAL);
-       note_tree_insert(new_node, n + 1, entry, type);
+       note_tree_insert(t, new_node, n + 1, entry, type, combine_notes);
+}
+
+/*
+ * How to consolidate an int_node:
+ * If there are > 1 non-NULL entries, give up and return non-zero.
+ * Otherwise replace the int_node at the given index in the given parent node
+ * with the only entry (or a NULL entry if no entries) from the given tree,
+ * and return 0.
+ */
+static int note_tree_consolidate(struct int_node *tree,
+       struct int_node *parent, unsigned char index)
+{
+       unsigned int i;
+       void *p = NULL;
+
+       assert(tree && parent);
+       assert(CLR_PTR_TYPE(parent->a[index]) == tree);
+
+       for (i = 0; i < 16; i++) {
+               if (GET_PTR_TYPE(tree->a[i]) != PTR_TYPE_NULL) {
+                       if (p) /* more than one entry */
+                               return -2;
+                       p = tree->a[i];
+               }
+       }
+
+       /* replace tree with p in parent[index] */
+       parent->a[index] = p;
+       free(tree);
+       return 0;
+}
+
+/*
+ * To remove a leaf_node:
+ * Search to the tree location appropriate for the given leaf_node's key:
+ * - If location does not hold a matching entry, abort and do nothing.
+ * - Replace the matching leaf_node with a NULL entry (and free the leaf_node).
+ * - Consolidate int_nodes repeatedly, while walking up the tree towards root.
+ */
+static void note_tree_remove(struct notes_tree *t, struct int_node *tree,
+               unsigned char n, struct leaf_node *entry)
+{
+       struct leaf_node *l;
+       struct int_node *parent_stack[20];
+       unsigned char i, j;
+       void **p = note_tree_search(t, &tree, &n, entry->key_sha1);
+
+       assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
+       if (GET_PTR_TYPE(*p) != PTR_TYPE_NOTE)
+               return; /* type mismatch, nothing to remove */
+       l = (struct leaf_node *) CLR_PTR_TYPE(*p);
+       if (hashcmp(l->key_sha1, entry->key_sha1))
+               return; /* key mismatch, nothing to remove */
+
+       /* we have found a matching entry */
+       free(l);
+       *p = SET_PTR_TYPE(NULL, PTR_TYPE_NULL);
+
+       /* consolidate this tree level, and parent levels, if possible */
+       if (!n)
+               return; /* cannot consolidate top level */
+       /* first, build stack of ancestors between root and current node */
+       parent_stack[0] = t->root;
+       for (i = 0; i < n; i++) {
+               j = GET_NIBBLE(i, entry->key_sha1);
+               parent_stack[i + 1] = CLR_PTR_TYPE(parent_stack[i]->a[j]);
+       }
+       assert(i == n && parent_stack[i] == tree);
+       /* next, unwind stack until note_tree_consolidate() is done */
+       while (i > 0 &&
+              !note_tree_consolidate(parent_stack[i], parent_stack[i - 1],
+                                     GET_NIBBLE(i - 1, entry->key_sha1)))
+               i--;
 }
 
 /* Free the entire notes data contained in the given tree */
@@ -257,7 +308,7 @@ static void note_tree_free(struct int_node *tree)
        unsigned int i;
        for (i = 0; i < 16; i++) {
                void *p = tree->a[i];
-               switch(GET_PTR_TYPE(p)) {
+               switch (GET_PTR_TYPE(p)) {
                case PTR_TYPE_INTERNAL:
                        note_tree_free(CLR_PTR_TYPE(p));
                        /* fall through */
@@ -274,7 +325,7 @@ static void note_tree_free(struct int_node *tree)
  * - hex_len  - Length of above segment. Must be multiple of 2 between 0 and 40
  * - sha1     - Partial SHA1 value is written here
  * - sha1_len - Max #bytes to store in sha1, Must be >= hex_len / 2, and < 20
- * Returns -1 on error (invalid arguments or invalid SHA1 (not in hex format).
+ * Returns -1 on error (invalid arguments or invalid SHA1 (not in hex format)).
  * Otherwise, returns number of bytes written to sha1 (i.e. hex_len / 2).
  * Pads sha1 with NULs up to sha1_len (not included in returned length).
  */
@@ -296,14 +347,67 @@ static int get_sha1_hex_segment(const char *hex, unsigned int hex_len,
        return len;
 }
 
-static void load_subtree(struct leaf_node *subtree, struct int_node *node,
-               unsigned int n)
+static int non_note_cmp(const struct non_note *a, const struct non_note *b)
+{
+       return strcmp(a->path, b->path);
+}
+
+static void add_non_note(struct notes_tree *t, const char *path,
+               unsigned int mode, const unsigned char *sha1)
+{
+       struct non_note *p = t->prev_non_note, *n;
+       n = (struct non_note *) xmalloc(sizeof(struct non_note));
+       n->next = NULL;
+       n->path = xstrdup(path);
+       n->mode = mode;
+       hashcpy(n->sha1, sha1);
+       t->prev_non_note = n;
+
+       if (!t->first_non_note) {
+               t->first_non_note = n;
+               return;
+       }
+
+       if (non_note_cmp(p, n) < 0)
+               ; /* do nothing  */
+       else if (non_note_cmp(t->first_non_note, n) <= 0)
+               p = t->first_non_note;
+       else {
+               /* n sorts before t->first_non_note */
+               n->next = t->first_non_note;
+               t->first_non_note = n;
+               return;
+       }
+
+       /* n sorts equal or after p */
+       while (p->next && non_note_cmp(p->next, n) <= 0)
+               p = p->next;
+
+       if (non_note_cmp(p, n) == 0) { /* n ~= p; overwrite p with n */
+               assert(strcmp(p->path, n->path) == 0);
+               p->mode = n->mode;
+               hashcpy(p->sha1, n->sha1);
+               free(n);
+               t->prev_non_note = p;
+               return;
+       }
+
+       /* n sorts between p and p->next */
+       n->next = p->next;
+       p->next = n;
+}
+
+static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
+               struct int_node *node, unsigned int n)
 {
-       unsigned char commit_sha1[20];
+       unsigned char object_sha1[20];
        unsigned int prefix_len;
        void *buf;
        struct tree_desc desc;
        struct name_entry entry;
+       int len, path_len;
+       unsigned char type;
+       struct leaf_node *l;
 
        buf = fill_tree_descriptor(&desc, subtree->val_sha1);
        if (!buf)
@@ -312,86 +416,721 @@ static void load_subtree(struct leaf_node *subtree, struct int_node *node,
 
        prefix_len = subtree->key_sha1[19];
        assert(prefix_len * 2 >= n);
-       memcpy(commit_sha1, subtree->key_sha1, prefix_len);
+       memcpy(object_sha1, subtree->key_sha1, prefix_len);
        while (tree_entry(&desc, &entry)) {
-               int len = get_sha1_hex_segment(entry.path, strlen(entry.path),
-                               commit_sha1 + prefix_len, 20 - prefix_len);
+               path_len = strlen(entry.path);
+               len = get_sha1_hex_segment(entry.path, path_len,
+                               object_sha1 + prefix_len, 20 - prefix_len);
                if (len < 0)
-                       continue; /* entry.path is not a SHA1 sum. Skip */
+                       goto handle_non_note; /* entry.path is not a SHA1 */
                len += prefix_len;
 
                /*
-                * If commit SHA1 is complete (len == 20), assume note object
-                * If commit SHA1 is incomplete (len < 20), assume note subtree
+                * If object SHA1 is complete (len == 20), assume note object
+                * If object SHA1 is incomplete (len < 20), and current
+                * component consists of 2 hex chars, assume note subtree
                 */
                if (len <= 20) {
-                       unsigned char type = PTR_TYPE_NOTE;
-                       struct leaf_node *l = (struct leaf_node *)
+                       type = PTR_TYPE_NOTE;
+                       l = (struct leaf_node *)
                                xcalloc(sizeof(struct leaf_node), 1);
-                       hashcpy(l->key_sha1, commit_sha1);
+                       hashcpy(l->key_sha1, object_sha1);
                        hashcpy(l->val_sha1, entry.sha1);
                        if (len < 20) {
-                               if (!S_ISDIR(entry.mode))
-                                       continue; /* entry cannot be subtree */
+                               if (!S_ISDIR(entry.mode) || path_len != 2)
+                                       goto handle_non_note; /* not subtree */
                                l->key_sha1[19] = (unsigned char) len;
                                type = PTR_TYPE_SUBTREE;
                        }
-                       note_tree_insert(node, n, l, type);
+                       note_tree_insert(t, node, n, l, type,
+                                        combine_notes_concatenate);
+               }
+               continue;
+
+handle_non_note:
+               /*
+                * Determine full path for this non-note entry:
+                * The filename is already found in entry.path, but the
+                * directory part of the path must be deduced from the subtree
+                * containing this entry. We assume here that the overall notes
+                * tree follows a strict byte-based progressive fanout
+                * structure (i.e. using 2/38, 2/2/36, etc. fanouts, and not
+                * e.g. 4/36 fanout). This means that if a non-note is found at
+                * path "dead/beef", the following code will register it as
+                * being found on "de/ad/beef".
+                * On the other hand, if you use such non-obvious non-note
+                * paths in the middle of a notes tree, you deserve what's
+                * coming to you ;). Note that for non-notes that are not
+                * SHA1-like at the top level, there will be no problems.
+                *
+                * To conclude, it is strongly advised to make sure non-notes
+                * have at least one non-hex character in the top-level path
+                * component.
+                */
+               {
+                       char non_note_path[PATH_MAX];
+                       char *p = non_note_path;
+                       const char *q = sha1_to_hex(subtree->key_sha1);
+                       int i;
+                       for (i = 0; i < prefix_len; i++) {
+                               *p++ = *q++;
+                               *p++ = *q++;
+                               *p++ = '/';
+                       }
+                       strcpy(p, entry.path);
+                       add_non_note(t, non_note_path, entry.mode, entry.sha1);
+               }
+       }
+       free(buf);
+}
+
+/*
+ * Determine optimal on-disk fanout for this part of the notes tree
+ *
+ * Given a (sub)tree and the level in the internal tree structure, determine
+ * whether or not the given existing fanout should be expanded for this
+ * (sub)tree.
+ *
+ * Values of the 'fanout' variable:
+ * - 0: No fanout (all notes are stored directly in the root notes tree)
+ * - 1: 2/38 fanout
+ * - 2: 2/2/36 fanout
+ * - 3: 2/2/2/34 fanout
+ * etc.
+ */
+static unsigned char determine_fanout(struct int_node *tree, unsigned char n,
+               unsigned char fanout)
+{
+       /*
+        * The following is a simple heuristic that works well in practice:
+        * For each even-numbered 16-tree level (remember that each on-disk
+        * fanout level corresponds to _two_ 16-tree levels), peek at all 16
+        * entries at that tree level. If all of them are either int_nodes or
+        * subtree entries, then there are likely plenty of notes below this
+        * level, so we return an incremented fanout.
+        */
+       unsigned int i;
+       if ((n % 2) || (n > 2 * fanout))
+               return fanout;
+       for (i = 0; i < 16; i++) {
+               switch (GET_PTR_TYPE(tree->a[i])) {
+               case PTR_TYPE_SUBTREE:
+               case PTR_TYPE_INTERNAL:
+                       continue;
+               default:
+                       return fanout;
+               }
+       }
+       return fanout + 1;
+}
+
+static void construct_path_with_fanout(const unsigned char *sha1,
+               unsigned char fanout, char *path)
+{
+       unsigned int i = 0, j = 0;
+       const char *hex_sha1 = sha1_to_hex(sha1);
+       assert(fanout < 20);
+       while (fanout) {
+               path[i++] = hex_sha1[j++];
+               path[i++] = hex_sha1[j++];
+               path[i++] = '/';
+               fanout--;
+       }
+       strcpy(path + i, hex_sha1 + j);
+}
+
+static int for_each_note_helper(struct notes_tree *t, struct int_node *tree,
+               unsigned char n, unsigned char fanout, int flags,
+               each_note_fn fn, void *cb_data)
+{
+       unsigned int i;
+       void *p;
+       int ret = 0;
+       struct leaf_node *l;
+       static char path[40 + 19 + 1];  /* hex SHA1 + 19 * '/' + NUL */
+
+       fanout = determine_fanout(tree, n, fanout);
+       for (i = 0; i < 16; i++) {
+redo:
+               p = tree->a[i];
+               switch (GET_PTR_TYPE(p)) {
+               case PTR_TYPE_INTERNAL:
+                       /* recurse into int_node */
+                       ret = for_each_note_helper(t, CLR_PTR_TYPE(p), n + 1,
+                               fanout, flags, fn, cb_data);
+                       break;
+               case PTR_TYPE_SUBTREE:
+                       l = (struct leaf_node *) CLR_PTR_TYPE(p);
+                       /*
+                        * Subtree entries in the note tree represent parts of
+                        * the note tree that have not yet been explored. There
+                        * is a direct relationship between subtree entries at
+                        * level 'n' in the tree, and the 'fanout' variable:
+                        * Subtree entries at level 'n <= 2 * fanout' should be
+                        * preserved, since they correspond exactly to a fanout
+                        * directory in the on-disk structure. However, subtree
+                        * entries at level 'n > 2 * fanout' should NOT be
+                        * preserved, but rather consolidated into the above
+                        * notes tree level. We achieve this by unconditionally
+                        * unpacking subtree entries that exist below the
+                        * threshold level at 'n = 2 * fanout'.
+                        */
+                       if (n <= 2 * fanout &&
+                           flags & FOR_EACH_NOTE_YIELD_SUBTREES) {
+                               /* invoke callback with subtree */
+                               unsigned int path_len =
+                                       l->key_sha1[19] * 2 + fanout;
+                               assert(path_len < 40 + 19);
+                               construct_path_with_fanout(l->key_sha1, fanout,
+                                                          path);
+                               /* Create trailing slash, if needed */
+                               if (path[path_len - 1] != '/')
+                                       path[path_len++] = '/';
+                               path[path_len] = '\0';
+                               ret = fn(l->key_sha1, l->val_sha1, path,
+                                        cb_data);
+                       }
+                       if (n > fanout * 2 ||
+                           !(flags & FOR_EACH_NOTE_DONT_UNPACK_SUBTREES)) {
+                               /* unpack subtree and resume traversal */
+                               tree->a[i] = NULL;
+                               load_subtree(t, l, tree, n);
+                               free(l);
+                               goto redo;
+                       }
+                       break;
+               case PTR_TYPE_NOTE:
+                       l = (struct leaf_node *) CLR_PTR_TYPE(p);
+                       construct_path_with_fanout(l->key_sha1, fanout, path);
+                       ret = fn(l->key_sha1, l->val_sha1, path, cb_data);
+                       break;
+               }
+               if (ret)
+                       return ret;
+       }
+       return 0;
+}
+
+struct tree_write_stack {
+       struct tree_write_stack *next;
+       struct strbuf buf;
+       char path[2]; /* path to subtree in next, if any */
+};
+
+static inline int matches_tree_write_stack(struct tree_write_stack *tws,
+               const char *full_path)
+{
+       return  full_path[0] == tws->path[0] &&
+               full_path[1] == tws->path[1] &&
+               full_path[2] == '/';
+}
+
+static void write_tree_entry(struct strbuf *buf, unsigned int mode,
+               const char *path, unsigned int path_len, const
+               unsigned char *sha1)
+{
+       strbuf_addf(buf, "%o %.*s%c", mode, path_len, path, '\0');
+       strbuf_add(buf, sha1, 20);
+}
+
+static void tree_write_stack_init_subtree(struct tree_write_stack *tws,
+               const char *path)
+{
+       struct tree_write_stack *n;
+       assert(!tws->next);
+       assert(tws->path[0] == '\0' && tws->path[1] == '\0');
+       n = (struct tree_write_stack *)
+               xmalloc(sizeof(struct tree_write_stack));
+       n->next = NULL;
+       strbuf_init(&n->buf, 256 * (32 + 40)); /* assume 256 entries per tree */
+       n->path[0] = n->path[1] = '\0';
+       tws->next = n;
+       tws->path[0] = path[0];
+       tws->path[1] = path[1];
+}
+
+static int tree_write_stack_finish_subtree(struct tree_write_stack *tws)
+{
+       int ret;
+       struct tree_write_stack *n = tws->next;
+       unsigned char s[20];
+       if (n) {
+               ret = tree_write_stack_finish_subtree(n);
+               if (ret)
+                       return ret;
+               ret = write_sha1_file(n->buf.buf, n->buf.len, tree_type, s);
+               if (ret)
+                       return ret;
+               strbuf_release(&n->buf);
+               free(n);
+               tws->next = NULL;
+               write_tree_entry(&tws->buf, 040000, tws->path, 2, s);
+               tws->path[0] = tws->path[1] = '\0';
+       }
+       return 0;
+}
+
+static int write_each_note_helper(struct tree_write_stack *tws,
+               const char *path, unsigned int mode,
+               const unsigned char *sha1)
+{
+       size_t path_len = strlen(path);
+       unsigned int n = 0;
+       int ret;
+
+       /* Determine common part of tree write stack */
+       while (tws && 3 * n < path_len &&
+              matches_tree_write_stack(tws, path + 3 * n)) {
+               n++;
+               tws = tws->next;
+       }
+
+       /* tws point to last matching tree_write_stack entry */
+       ret = tree_write_stack_finish_subtree(tws);
+       if (ret)
+               return ret;
+
+       /* Start subtrees needed to satisfy path */
+       while (3 * n + 2 < path_len && path[3 * n + 2] == '/') {
+               tree_write_stack_init_subtree(tws, path + 3 * n);
+               n++;
+               tws = tws->next;
+       }
+
+       /* There should be no more directory components in the given path */
+       assert(memchr(path + 3 * n, '/', path_len - (3 * n)) == NULL);
+
+       /* Finally add given entry to the current tree object */
+       write_tree_entry(&tws->buf, mode, path + 3 * n, path_len - (3 * n),
+                        sha1);
+
+       return 0;
+}
+
+struct write_each_note_data {
+       struct tree_write_stack *root;
+       struct non_note *next_non_note;
+};
+
+static int write_each_non_note_until(const char *note_path,
+               struct write_each_note_data *d)
+{
+       struct non_note *n = d->next_non_note;
+       int cmp, ret;
+       while (n && (!note_path || (cmp = strcmp(n->path, note_path)) <= 0)) {
+               if (note_path && cmp == 0)
+                       ; /* do nothing, prefer note to non-note */
+               else {
+                       ret = write_each_note_helper(d->root, n->path, n->mode,
+                                                    n->sha1);
+                       if (ret)
+                               return ret;
                }
+               n = n->next;
+       }
+       d->next_non_note = n;
+       return 0;
+}
+
+static int write_each_note(const unsigned char *object_sha1,
+               const unsigned char *note_sha1, char *note_path,
+               void *cb_data)
+{
+       struct write_each_note_data *d =
+               (struct write_each_note_data *) cb_data;
+       size_t note_path_len = strlen(note_path);
+       unsigned int mode = 0100644;
+
+       if (note_path[note_path_len - 1] == '/') {
+               /* subtree entry */
+               note_path_len--;
+               note_path[note_path_len] = '\0';
+               mode = 040000;
        }
+       assert(note_path_len <= 40 + 19);
+
+       /* Weave non-note entries into note entries */
+       return  write_each_non_note_until(note_path, d) ||
+               write_each_note_helper(d->root, note_path, mode, note_sha1);
+}
+
+struct note_delete_list {
+       struct note_delete_list *next;
+       const unsigned char *sha1;
+};
+
+static int prune_notes_helper(const unsigned char *object_sha1,
+               const unsigned char *note_sha1, char *note_path,
+               void *cb_data)
+{
+       struct note_delete_list **l = (struct note_delete_list **) cb_data;
+       struct note_delete_list *n;
+
+       if (has_sha1_file(object_sha1))
+               return 0; /* nothing to do for this note */
+
+       /* failed to find object => prune this note */
+       n = (struct note_delete_list *) xmalloc(sizeof(*n));
+       n->next = *l;
+       n->sha1 = object_sha1;
+       *l = n;
+       return 0;
+}
+
+int combine_notes_concatenate(unsigned char *cur_sha1,
+               const unsigned char *new_sha1)
+{
+       char *cur_msg = NULL, *new_msg = NULL, *buf;
+       unsigned long cur_len, new_len, buf_len;
+       enum object_type cur_type, new_type;
+       int ret;
+
+       /* read in both note blob objects */
+       if (!is_null_sha1(new_sha1))
+               new_msg = read_sha1_file(new_sha1, &new_type, &new_len);
+       if (!new_msg || !new_len || new_type != OBJ_BLOB) {
+               free(new_msg);
+               return 0;
+       }
+       if (!is_null_sha1(cur_sha1))
+               cur_msg = read_sha1_file(cur_sha1, &cur_type, &cur_len);
+       if (!cur_msg || !cur_len || cur_type != OBJ_BLOB) {
+               free(cur_msg);
+               free(new_msg);
+               hashcpy(cur_sha1, new_sha1);
+               return 0;
+       }
+
+       /* we will separate the notes by a newline anyway */
+       if (cur_msg[cur_len - 1] == '\n')
+               cur_len--;
+
+       /* concatenate cur_msg and new_msg into buf */
+       buf_len = cur_len + 1 + new_len;
+       buf = (char *) xmalloc(buf_len);
+       memcpy(buf, cur_msg, cur_len);
+       buf[cur_len] = '\n';
+       memcpy(buf + cur_len + 1, new_msg, new_len);
+       free(cur_msg);
+       free(new_msg);
+
+       /* create a new blob object from buf */
+       ret = write_sha1_file(buf, buf_len, blob_type, cur_sha1);
        free(buf);
+       return ret;
+}
+
+int combine_notes_overwrite(unsigned char *cur_sha1,
+               const unsigned char *new_sha1)
+{
+       hashcpy(cur_sha1, new_sha1);
+       return 0;
+}
+
+int combine_notes_ignore(unsigned char *cur_sha1,
+               const unsigned char *new_sha1)
+{
+       return 0;
+}
+
+static int string_list_add_one_ref(const char *path, const unsigned char *sha1,
+                                  int flag, void *cb)
+{
+       struct string_list *refs = cb;
+       if (!unsorted_string_list_has_string(refs, path))
+               string_list_append(path, refs);
+       return 0;
+}
+
+void string_list_add_refs_by_glob(struct string_list *list, const char *glob)
+{
+       if (has_glob_specials(glob)) {
+               for_each_glob_ref(string_list_add_one_ref, glob, list);
+       } else {
+               unsigned char sha1[20];
+               if (get_sha1(glob, sha1))
+                       warning("notes ref %s is invalid", glob);
+               if (!unsorted_string_list_has_string(list, glob))
+                       string_list_append(glob, list);
+       }
+}
+
+void string_list_add_refs_from_colon_sep(struct string_list *list,
+                                        const char *globs)
+{
+       struct strbuf globbuf = STRBUF_INIT;
+       struct strbuf **split;
+       int i;
+
+       strbuf_addstr(&globbuf, globs);
+       split = strbuf_split(&globbuf, ':');
+
+       for (i = 0; split[i]; i++) {
+               if (!split[i]->len)
+                       continue;
+               if (split[i]->buf[split[i]->len-1] == ':')
+                       strbuf_setlen(split[i], split[i]->len-1);
+               string_list_add_refs_by_glob(list, split[i]->buf);
+       }
+
+       strbuf_list_free(split);
+       strbuf_release(&globbuf);
+}
+
+static int string_list_add_refs_from_list(struct string_list_item *item,
+                                         void *cb)
+{
+       struct string_list *list = cb;
+       string_list_add_refs_by_glob(list, item->string);
+       return 0;
+}
+
+static int notes_display_config(const char *k, const char *v, void *cb)
+{
+       int *load_refs = cb;
+
+       if (*load_refs && !strcmp(k, "notes.displayref")) {
+               if (!v)
+                       config_error_nonbool(k);
+               string_list_add_refs_by_glob(&display_notes_refs, v);
+       }
+
+       return 0;
 }
 
-static void initialize_notes(const char *notes_ref_name)
+static const char *default_notes_ref(void)
 {
-       unsigned char sha1[20], commit_sha1[20];
+       const char *notes_ref = NULL;
+       if (!notes_ref)
+               notes_ref = getenv(GIT_NOTES_REF_ENVIRONMENT);
+       if (!notes_ref)
+               notes_ref = notes_ref_name; /* value of core.notesRef config */
+       if (!notes_ref)
+               notes_ref = GIT_NOTES_DEFAULT_REF;
+       return notes_ref;
+}
+
+void init_notes(struct notes_tree *t, const char *notes_ref,
+               combine_notes_fn combine_notes, int flags)
+{
+       unsigned char sha1[20], object_sha1[20];
        unsigned mode;
        struct leaf_node root_tree;
 
-       if (!notes_ref_name || read_ref(notes_ref_name, commit_sha1) ||
-           get_tree_entry(commit_sha1, "", sha1, &mode))
+       if (!t)
+               t = &default_notes_tree;
+       assert(!t->initialized);
+
+       if (!notes_ref)
+               notes_ref = default_notes_ref();
+
+       if (!combine_notes)
+               combine_notes = combine_notes_concatenate;
+
+       t->root = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
+       t->first_non_note = NULL;
+       t->prev_non_note = NULL;
+       t->ref = notes_ref ? xstrdup(notes_ref) : NULL;
+       t->combine_notes = combine_notes;
+       t->initialized = 1;
+       t->dirty = 0;
+
+       if (flags & NOTES_INIT_EMPTY || !notes_ref ||
+           read_ref(notes_ref, object_sha1))
                return;
+       if (get_tree_entry(object_sha1, "", sha1, &mode))
+               die("Failed to read notes tree referenced by %s (%s)",
+                   notes_ref, object_sha1);
 
        hashclr(root_tree.key_sha1);
        hashcpy(root_tree.val_sha1, sha1);
-       load_subtree(&root_tree, &root_node, 0);
+       load_subtree(t, &root_tree, t->root, 0);
 }
 
-static unsigned char *lookup_notes(const unsigned char *commit_sha1)
+struct load_notes_cb_data {
+       int counter;
+       struct notes_tree **trees;
+};
+
+static int load_one_display_note_ref(struct string_list_item *item,
+                                    void *cb_data)
 {
-       struct leaf_node *found = note_tree_find(&root_node, 0, commit_sha1);
-       if (found)
-               return found->val_sha1;
-       return NULL;
+       struct load_notes_cb_data *c = cb_data;
+       struct notes_tree *t = xcalloc(1, sizeof(struct notes_tree));
+       init_notes(t, item->string, combine_notes_ignore, 0);
+       c->trees[c->counter++] = t;
+       return 0;
 }
 
-void free_notes(void)
+struct notes_tree **load_notes_trees(struct string_list *refs)
 {
-       note_tree_free(&root_node);
-       memset(&root_node, 0, sizeof(struct int_node));
-       initialized = 0;
+       struct notes_tree **trees;
+       struct load_notes_cb_data cb_data;
+       trees = xmalloc((refs->nr+1) * sizeof(struct notes_tree *));
+       cb_data.counter = 0;
+       cb_data.trees = trees;
+       for_each_string_list(load_one_display_note_ref, refs, &cb_data);
+       trees[cb_data.counter] = NULL;
+       return trees;
+}
+
+void init_display_notes(struct display_notes_opt *opt)
+{
+       char *display_ref_env;
+       int load_config_refs = 0;
+       display_notes_refs.strdup_strings = 1;
+
+       assert(!display_notes_trees);
+
+       if (!opt || !opt->suppress_default_notes) {
+               string_list_append(default_notes_ref(), &display_notes_refs);
+               display_ref_env = getenv(GIT_NOTES_DISPLAY_REF_ENVIRONMENT);
+               if (display_ref_env) {
+                       string_list_add_refs_from_colon_sep(&display_notes_refs,
+                                                           display_ref_env);
+                       load_config_refs = 0;
+               } else
+                       load_config_refs = 1;
+       }
+
+       git_config(notes_display_config, &load_config_refs);
+
+       if (opt && opt->extra_notes_refs)
+               for_each_string_list(string_list_add_refs_from_list,
+                                    opt->extra_notes_refs,
+                                    &display_notes_refs);
+
+       display_notes_trees = load_notes_trees(&display_notes_refs);
+       string_list_clear(&display_notes_refs, 0);
+}
+
+void add_note(struct notes_tree *t, const unsigned char *object_sha1,
+               const unsigned char *note_sha1, combine_notes_fn combine_notes)
+{
+       struct leaf_node *l;
+
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+       t->dirty = 1;
+       if (!combine_notes)
+               combine_notes = t->combine_notes;
+       l = (struct leaf_node *) xmalloc(sizeof(struct leaf_node));
+       hashcpy(l->key_sha1, object_sha1);
+       hashcpy(l->val_sha1, note_sha1);
+       note_tree_insert(t, t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
+}
+
+void remove_note(struct notes_tree *t, const unsigned char *object_sha1)
+{
+       struct leaf_node l;
+
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+       t->dirty = 1;
+       hashcpy(l.key_sha1, object_sha1);
+       hashclr(l.val_sha1);
+       note_tree_remove(t, t->root, 0, &l);
 }
 
-void get_commit_notes(const struct commit *commit, struct strbuf *sb,
-               const char *output_encoding, int flags)
+const unsigned char *get_note(struct notes_tree *t,
+               const unsigned char *object_sha1)
+{
+       struct leaf_node *found;
+
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+       found = note_tree_find(t, t->root, 0, object_sha1);
+       return found ? found->val_sha1 : NULL;
+}
+
+int for_each_note(struct notes_tree *t, int flags, each_note_fn fn,
+               void *cb_data)
+{
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+       return for_each_note_helper(t, t->root, 0, 0, flags, fn, cb_data);
+}
+
+int write_notes_tree(struct notes_tree *t, unsigned char *result)
+{
+       struct tree_write_stack root;
+       struct write_each_note_data cb_data;
+       int ret;
+
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+
+       /* Prepare for traversal of current notes tree */
+       root.next = NULL; /* last forward entry in list is grounded */
+       strbuf_init(&root.buf, 256 * (32 + 40)); /* assume 256 entries */
+       root.path[0] = root.path[1] = '\0';
+       cb_data.root = &root;
+       cb_data.next_non_note = t->first_non_note;
+
+       /* Write tree objects representing current notes tree */
+       ret = for_each_note(t, FOR_EACH_NOTE_DONT_UNPACK_SUBTREES |
+                               FOR_EACH_NOTE_YIELD_SUBTREES,
+                       write_each_note, &cb_data) ||
+               write_each_non_note_until(NULL, &cb_data) ||
+               tree_write_stack_finish_subtree(&root) ||
+               write_sha1_file(root.buf.buf, root.buf.len, tree_type, result);
+       strbuf_release(&root.buf);
+       return ret;
+}
+
+void prune_notes(struct notes_tree *t)
+{
+       struct note_delete_list *l = NULL;
+
+       if (!t)
+               t = &default_notes_tree;
+       assert(t->initialized);
+
+       for_each_note(t, 0, prune_notes_helper, &l);
+
+       while (l) {
+               remove_note(t, l->sha1);
+               l = l->next;
+       }
+}
+
+void free_notes(struct notes_tree *t)
+{
+       if (!t)
+               t = &default_notes_tree;
+       if (t->root)
+               note_tree_free(t->root);
+       free(t->root);
+       while (t->first_non_note) {
+               t->prev_non_note = t->first_non_note->next;
+               free(t->first_non_note->path);
+               free(t->first_non_note);
+               t->first_non_note = t->prev_non_note;
+       }
+       free(t->ref);
+       memset(t, 0, sizeof(struct notes_tree));
+}
+
+void format_note(struct notes_tree *t, const unsigned char *object_sha1,
+               struct strbuf *sb, const char *output_encoding, int flags)
 {
        static const char utf8[] = "utf-8";
-       unsigned char *sha1;
+       const unsigned char *sha1;
        char *msg, *msg_p;
        unsigned long linelen, msglen;
        enum object_type type;
 
-       if (!initialized) {
-               const char *env = getenv(GIT_NOTES_REF_ENVIRONMENT);
-               if (env)
-                       notes_ref_name = getenv(GIT_NOTES_REF_ENVIRONMENT);
-               else if (!notes_ref_name)
-                       notes_ref_name = GIT_NOTES_DEFAULT_REF;
-               initialize_notes(notes_ref_name);
-               initialized = 1;
-       }
+       if (!t)
+               t = &default_notes_tree;
+       if (!t->initialized)
+               init_notes(t, NULL, NULL, 0);
 
-       sha1 = lookup_notes(commit->object.sha1);
+       sha1 = get_note(t, object_sha1);
        if (!sha1)
                return;
 
@@ -415,8 +1154,18 @@ void get_commit_notes(const struct commit *commit, struct strbuf *sb,
        if (msglen && msg[msglen - 1] == '\n')
                msglen--;
 
-       if (flags & NOTES_SHOW_HEADER)
-               strbuf_addstr(sb, "\nNotes:\n");
+       if (flags & NOTES_SHOW_HEADER) {
+               const char *ref = t->ref;
+               if (!ref || !strcmp(ref, GIT_NOTES_DEFAULT_REF)) {
+                       strbuf_addstr(sb, "\nNotes:\n");
+               } else {
+                       if (!prefixcmp(ref, "refs/"))
+                               ref += 5;
+                       if (!prefixcmp(ref, "notes/"))
+                               ref += 6;
+                       strbuf_addf(sb, "\nNotes (%s):\n", ref);
+               }
+       }
 
        for (msg_p = msg; msg_p < msg + msglen; msg_p += linelen + 1) {
                linelen = strchrnul(msg_p, '\n') - msg_p;
@@ -429,3 +1178,31 @@ void get_commit_notes(const struct commit *commit, struct strbuf *sb,
 
        free(msg);
 }
+
+void format_display_notes(const unsigned char *object_sha1,
+                         struct strbuf *sb, const char *output_encoding, int flags)
+{
+       int i;
+       assert(display_notes_trees);
+       for (i = 0; display_notes_trees[i]; i++)
+               format_note(display_notes_trees[i], object_sha1, sb,
+                           output_encoding, flags);
+}
+
+int copy_note(struct notes_tree *t,
+             const unsigned char *from_obj, const unsigned char *to_obj,
+             int force, combine_notes_fn combine_fn)
+{
+       const unsigned char *note = get_note(t, from_obj);
+       const unsigned char *existing_note = get_note(t, to_obj);
+
+       if (!force && existing_note)
+               return 1;
+
+       if (note)
+               add_note(t, to_obj, note, combine_fn);
+       else if (existing_note)
+               add_note(t, to_obj, null_sha1, combine_fn);
+
+       return 0;
+}