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refs: store references hierarchically
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Store references hierarchically in a tree that matches the
pseudo-directory structure of the reference names.  Add a new kind of
ref_entry (with flag REF_DIR) to represent a whole subdirectory of
references.  Sort ref_dirs one subdirectory at a time.

NOTE: the dirs can now be sorted as a side-effect of other function
calls.  Therefore, it would be problematic to do something from a
each_ref_fn callback that could provoke the sorting of a directory
that is currently being iterated over (i.e., the directory containing
the entry that is being processed or any of its parents).

This is a bit far-fetched, because a directory is always sorted just
before being iterated over.  Therefore, read-only accesses cannot
trigger the sorting of a directory whose iteration has already
started.  But if a callback function would add a reference to a parent
directory of the reference in the iteration, then try to resolve a
reference under that directory, a re-sort could be triggered and cause
the iteration to work incorrectly.

Nevertheless...add a comment in refs.h warning against modifications
during iteration.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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@mhagger @gitster
mhagger authored and gitster committed Apr 10, 2012
1 parent 81a79d8 commit 432ad41
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274 changes: 227 additions & 47 deletions refs.c
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Expand Up @@ -120,15 +120,54 @@ struct ref_dir {
struct ref_entry **entries;
};

/* ISSYMREF=0x01, ISPACKED=0x02 and ISBROKEN=0x04 are public interfaces */
#define REF_KNOWS_PEELED 0x10
/* ISSYMREF=0x01, ISPACKED=0x02, and ISBROKEN=0x04 are public interfaces */
#define REF_KNOWS_PEELED 0x08
#define REF_DIR 0x10

/*
* A ref_entry represents either a reference or a "subdirectory" of
* references. Each directory in the reference namespace is
* represented by a ref_entry with (flags & REF_DIR) set and
* containing a subdir member that holds the entries in that
* directory. References are represented by a ref_entry with (flags &
* REF_DIR) unset and a value member that describes the reference's
* value. The flag member is at the ref_entry level, but it is also
* needed to interpret the contents of the value field (in other
* words, a ref_value object is not very much use without the
* enclosing ref_entry).
*
* Reference names cannot end with slash and directories' names are
* always stored with a trailing slash (except for the top-level
* directory, which is always denoted by ""). This has two nice
* consequences: (1) when the entries in each subdir are sorted
* lexicographically by name (as they usually are), the references in
* a whole tree can be generated in lexicographic order by traversing
* the tree in left-to-right, depth-first order; (2) the names of
* references and subdirectories cannot conflict, and therefore the
* presence of an empty subdirectory does not block the creation of a
* similarly-named reference. (The fact that reference names with the
* same leading components can conflict *with each other* is a
* separate issue that is regulated by is_refname_available().)
*
* Please note that the name field contains the fully-qualified
* reference (or subdirectory) name. Space could be saved by only
* storing the relative names. But that would require the full names
* to be generated on the fly when iterating in do_for_each_ref(), and
* would break callback functions, who have always been able to assume
* that the name strings that they are passed will not be freed during
* the iteration.
*/
struct ref_entry {
unsigned char flag; /* ISSYMREF? ISPACKED? */
union {
struct ref_value value;
struct ref_value value; /* if not (flags&REF_DIR) */
struct ref_dir subdir; /* if (flags&REF_DIR) */
} u;
/* The full name of the reference (e.g., "refs/heads/master"): */
/*
* The full name of the reference (e.g., "refs/heads/master")
* or the full name of the directory with a trailing slash
* (e.g., "refs/heads/"):
*/
char name[FLEX_ARRAY];
};

Expand All @@ -151,18 +190,29 @@ static struct ref_entry *create_ref_entry(const char *refname,
return ref;
}

static void clear_ref_dir(struct ref_dir *dir);

static void free_ref_entry(struct ref_entry *entry)
{
if (entry->flag & REF_DIR)
clear_ref_dir(&entry->u.subdir);
free(entry);
}

/* Add a ref_entry to the end of the ref_dir (unsorted). */
static void add_ref(struct ref_dir *refs, struct ref_entry *ref)
/*
* Add a ref_entry to the end of dir (unsorted). Entry is always
* stored directly in dir; no recursion into subdirectories is
* done.
*/
static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry)
{
ALLOC_GROW(refs->entries, refs->nr + 1, refs->alloc);
refs->entries[refs->nr++] = ref;
ALLOC_GROW(dir->entries, dir->nr + 1, dir->alloc);
dir->entries[dir->nr++] = entry;
}

/*
* Clear and free all entries in dir, recursively.
*/
static void clear_ref_dir(struct ref_dir *dir)
{
int i;
Expand All @@ -173,6 +223,21 @@ static void clear_ref_dir(struct ref_dir *dir)
dir->entries = NULL;
}

/*
* Create a struct ref_entry object for the specified dirname.
* dirname is the name of the directory with a trailing slash (e.g.,
* "refs/heads/") or "" for the top-level directory.
*/
static struct ref_entry *create_dir_entry(const char *dirname)
{
struct ref_entry *direntry;
int len = strlen(dirname);
direntry = xcalloc(1, sizeof(struct ref_entry) + len + 1);
memcpy(direntry->name, dirname, len + 1);
direntry->flag = REF_DIR;
return direntry;
}

static int ref_entry_cmp(const void *a, const void *b)
{
struct ref_entry *one = *(struct ref_entry **)a;
Expand All @@ -182,17 +247,21 @@ static int ref_entry_cmp(const void *a, const void *b)

static void sort_ref_dir(struct ref_dir *dir);

/*
* Return the entry with the given refname from the ref_dir
* (non-recursively), sorting dir if necessary. Return NULL if no
* such entry is found.
*/
static struct ref_entry *search_ref_dir(struct ref_dir *dir, const char *refname)
{
struct ref_entry *e, **r;
int len;

if (refname == NULL)
if (refname == NULL || !dir->nr)
return NULL;

if (!dir->nr)
return NULL;
sort_ref_dir(dir);

len = strlen(refname) + 1;
e = xmalloc(sizeof(struct ref_entry) + len);
memcpy(e->name, refname, len);
Expand All @@ -207,28 +276,97 @@ static struct ref_entry *search_ref_dir(struct ref_dir *dir, const char *refname
return *r;
}

/*
* If refname is a reference name, find the ref_dir within the dir
* tree that should hold refname. If refname is a directory name
* (i.e., ends in '/'), then return that ref_dir itself. dir must
* represent the top-level directory. Sort ref_dirs and recurse into
* subdirectories as necessary. If mkdir is set, then create any
* missing directories; otherwise, return NULL if the desired
* directory cannot be found.
*/
static struct ref_dir *find_containing_dir(struct ref_dir *dir,
const char *refname, int mkdir)
{
char *refname_copy = xstrdup(refname);
char *slash;
struct ref_entry *entry;
for (slash = strchr(refname_copy, '/'); slash; slash = strchr(slash + 1, '/')) {
char tmp = slash[1];
slash[1] = '\0';
entry = search_ref_dir(dir, refname_copy);
if (!entry) {
if (!mkdir) {
dir = NULL;
break;
}
entry = create_dir_entry(refname_copy);
add_entry_to_dir(dir, entry);
}
slash[1] = tmp;
assert(entry->flag & REF_DIR);
dir = &entry->u.subdir;
}

free(refname_copy);
return dir;
}

/*
* Find the value entry with the given name in dir, sorting ref_dirs
* and recursing into subdirectories as necessary. If the name is not
* found or it corresponds to a directory entry, return NULL.
*/
static struct ref_entry *find_ref(struct ref_dir *dir, const char *refname)
{
struct ref_entry *entry;
dir = find_containing_dir(dir, refname, 0);
if (!dir)
return NULL;
entry = search_ref_dir(dir, refname);
return (entry && !(entry->flag & REF_DIR)) ? entry : NULL;
}

/*
* Add a ref_entry to the ref_dir (unsorted), recursing into
* subdirectories as necessary. dir must represent the top-level
* directory. Return 0 on success.
*/
static int add_ref(struct ref_dir *dir, struct ref_entry *ref)
{
dir = find_containing_dir(dir, ref->name, 1);
if (!dir)
return -1;
add_entry_to_dir(dir, ref);
return 0;
}

/*
* Emit a warning and return true iff ref1 and ref2 have the same name
* and the same sha1. Die if they have the same name but different
* sha1s.
*/
static int is_dup_ref(const struct ref_entry *ref1, const struct ref_entry *ref2)
{
if (!strcmp(ref1->name, ref2->name)) {
/* Duplicate name; make sure that the SHA1s match: */
if (hashcmp(ref1->u.value.sha1, ref2->u.value.sha1))
die("Duplicated ref, and SHA1s don't match: %s",
ref1->name);
warning("Duplicated ref: %s", ref1->name);
return 1;
} else {
if (strcmp(ref1->name, ref2->name))
return 0;
}

/* Duplicate name; make sure that they don't conflict: */

if ((ref1->flag & REF_DIR) || (ref2->flag & REF_DIR))
/* This is impossible by construction */
die("Reference directory conflict: %s", ref1->name);

if (hashcmp(ref1->u.value.sha1, ref2->u.value.sha1))
die("Duplicated ref, and SHA1s don't match: %s", ref1->name);

warning("Duplicated ref: %s", ref1->name);
return 1;
}

/*
* Sort the entries in dir (if they are not already sorted)
* and remove any duplicate entries.
* Sort the entries in dir non-recursively (if they are not already
* sorted) and remove any duplicate entries.
*/
static void sort_ref_dir(struct ref_dir *dir)
{
Expand Down Expand Up @@ -282,8 +420,9 @@ static int do_one_ref(const char *base, each_ref_fn fn, int trim,

/*
* Call fn for each reference in dir that has index in the range
* offset <= index < dir->nr. This function does not sort the dir;
* sorting should be done by the caller.
* offset <= index < dir->nr. Recurse into subdirectories that are in
* that index range, sorting them before iterating. This function
* does not sort dir itself; it should be sorted beforehand.
*/
static int do_for_each_ref_in_dir(struct ref_dir *dir, int offset,
const char *base,
Expand All @@ -292,7 +431,15 @@ static int do_for_each_ref_in_dir(struct ref_dir *dir, int offset,
int i;
assert(dir->sorted == dir->nr);
for (i = offset; i < dir->nr; i++) {
int retval = do_one_ref(base, fn, trim, flags, cb_data, dir->entries[i]);
struct ref_entry *entry = dir->entries[i];
int retval;
if (entry->flag & REF_DIR) {
sort_ref_dir(&entry->u.subdir);
retval = do_for_each_ref_in_dir(&entry->u.subdir, 0,
base, fn, trim, flags, cb_data);
} else {
retval = do_one_ref(base, fn, trim, flags, cb_data, entry);
}
if (retval)
return retval;
}
Expand All @@ -301,9 +448,10 @@ static int do_for_each_ref_in_dir(struct ref_dir *dir, int offset,

/*
* Call fn for each reference in the union of dir1 and dir2, in order
* by refname. If an entry appears in both dir1 and dir2, then only
* process the version that is in dir2. The input dirs must already
* be sorted.
* by refname. Recurse into subdirectories. If a value entry appears
* in both dir1 and dir2, then only process the version that is in
* dir2. The input dirs must already be sorted, but subdirs will be
* sorted as needed.
*/
static int do_for_each_ref_in_dirs(struct ref_dir *dir1,
struct ref_dir *dir2,
Expand All @@ -315,22 +463,55 @@ static int do_for_each_ref_in_dirs(struct ref_dir *dir1,

assert(dir1->sorted == dir1->nr);
assert(dir2->sorted == dir2->nr);
while (i1 < dir1->nr && i2 < dir2->nr) {
struct ref_entry *e1 = dir1->entries[i1];
struct ref_entry *e2 = dir2->entries[i2];
int cmp = strcmp(e1->name, e2->name);
if (cmp < 0) {
retval = do_one_ref(base, fn, trim, flags, cb_data, e1);
i1++;
while (1) {
struct ref_entry *e1, *e2;
int cmp;
if (i1 == dir1->nr) {
return do_for_each_ref_in_dir(dir2, i2,
base, fn, trim, flags, cb_data);
}
if (i2 == dir2->nr) {
return do_for_each_ref_in_dir(dir1, i1,
base, fn, trim, flags, cb_data);
}
e1 = dir1->entries[i1];
e2 = dir2->entries[i2];
cmp = strcmp(e1->name, e2->name);
if (cmp == 0) {
if ((e1->flag & REF_DIR) && (e2->flag & REF_DIR)) {
/* Both are directories; descend them in parallel. */
sort_ref_dir(&e1->u.subdir);
sort_ref_dir(&e2->u.subdir);
retval = do_for_each_ref_in_dirs(
&e1->u.subdir, &e2->u.subdir,
base, fn, trim, flags, cb_data);
i1++;
i2++;
} else if (!(e1->flag & REF_DIR) && !(e2->flag & REF_DIR)) {
/* Both are references; ignore the one from dir1. */
retval = do_one_ref(base, fn, trim, flags, cb_data, e2);
i1++;
i2++;
} else {
die("conflict between reference and directory: %s",
e1->name);
}
} else {
retval = do_one_ref(base, fn, trim, flags, cb_data, e2);
i2++;
if (cmp == 0) {
/*
* There was a ref in array1 with the
* same name; ignore it.
*/
struct ref_entry *e;
if (cmp < 0) {
e = e1;
i1++;
} else {
e = e2;
i2++;
}
if (e->flag & REF_DIR) {
sort_ref_dir(&e->u.subdir);
retval = do_for_each_ref_in_dir(
&e->u.subdir, 0,
base, fn, trim, flags, cb_data);
} else {
retval = do_one_ref(base, fn, trim, flags, cb_data, e);
}
}
if (retval)
Expand Down Expand Up @@ -655,7 +836,7 @@ static int resolve_gitlink_packed_ref(struct ref_cache *refs,
struct ref_entry *ref;
struct ref_dir *dir = get_packed_refs(refs);

ref = search_ref_dir(dir, refname);
ref = find_ref(dir, refname);
if (ref == NULL)
return -1;

Expand Down Expand Up @@ -727,7 +908,7 @@ int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *sh
static int get_packed_ref(const char *refname, unsigned char *sha1)
{
struct ref_dir *packed = get_packed_refs(get_ref_cache(NULL));
struct ref_entry *entry = search_ref_dir(packed, refname);
struct ref_entry *entry = find_ref(packed, refname);
if (entry) {
hashcpy(sha1, entry->u.value.sha1);
return 0;
Expand Down Expand Up @@ -904,7 +1085,7 @@ int peel_ref(const char *refname, unsigned char *sha1)

if ((flag & REF_ISPACKED)) {
struct ref_dir *dir = get_packed_refs(get_ref_cache(NULL));
struct ref_entry *r = search_ref_dir(dir, refname);
struct ref_entry *r = find_ref(dir, refname);

if (r != NULL && r->flag & REF_KNOWS_PEELED) {
hashcpy(sha1, r->u.value.peeled);
Expand Down Expand Up @@ -1404,8 +1585,7 @@ static int repack_without_ref(const char *refname)
{
struct repack_without_ref_sb data;
struct ref_dir *packed = get_packed_refs(get_ref_cache(NULL));
sort_ref_dir(packed);
if (search_ref_dir(packed, refname) == NULL)
if (find_ref(packed, refname) == NULL)
return 0;
data.refname = refname;
data.fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"), 0);
Expand Down

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