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diff --git a/builtin/fsck.c b/builtin/fsck.c
--- /dev/null
+++ b/builtin/fsck.c
@@ -0,0 +1,684 @@
+#include "builtin.h"
+#include "cache.h"
+#include "commit.h"
+#include "tree.h"
+#include "blob.h"
+#include "tag.h"
+#include "refs.h"
+#include "pack.h"
+#include "cache-tree.h"
+#include "tree-walk.h"
+#include "fsck.h"
+#include "parse-options.h"
+#include "dir.h"
+
+#define REACHABLE 0x0001
+#define SEEN 0x0002
+
+static int show_root;
+static int show_tags;
+static int show_unreachable;
+static int include_reflogs = 1;
+static int check_full = 1;
+static int check_strict;
+static int keep_cache_objects;
+static unsigned char head_sha1[20];
+static const char *head_points_at;
+static int errors_found;
+static int write_lost_and_found;
+static int verbose;
+#define ERROR_OBJECT 01
+#define ERROR_REACHABLE 02
+
+#ifdef NO_D_INO_IN_DIRENT
+#define SORT_DIRENT 0
+#define DIRENT_SORT_HINT(de) 0
+#else
+#define SORT_DIRENT 1
+#define DIRENT_SORT_HINT(de) ((de)->d_ino)
+#endif
+
+static void objreport(struct object *obj, const char *severity,
+ const char *err, va_list params)
+{
+ fprintf(stderr, "%s in %s %s: ",
+ severity, typename(obj->type), sha1_to_hex(obj->sha1));
+ vfprintf(stderr, err, params);
+ fputs("\n", stderr);
+}
+
+__attribute__((format (printf, 2, 3)))
+static int objerror(struct object *obj, const char *err, ...)
+{
+ va_list params;
+ va_start(params, err);
+ errors_found |= ERROR_OBJECT;
+ objreport(obj, "error", err, params);
+ va_end(params);
+ return -1;
+}
+
+__attribute__((format (printf, 3, 4)))
+static int fsck_error_func(struct object *obj, int type, const char *err, ...)
+{
+ va_list params;
+ va_start(params, err);
+ objreport(obj, (type == FSCK_WARN) ? "warning" : "error", err, params);
+ va_end(params);
+ return (type == FSCK_WARN) ? 0 : 1;
+}
+
+static struct object_array pending;
+
+static int mark_object(struct object *obj, int type, void *data)
+{
+ struct object *parent = data;
+
+ if (!obj) {
+ printf("broken link from %7s %s\n",
+ typename(parent->type), sha1_to_hex(parent->sha1));
+ printf("broken link from %7s %s\n",
+ (type == OBJ_ANY ? "unknown" : typename(type)), "unknown");
+ errors_found |= ERROR_REACHABLE;
+ return 1;
+ }
+
+ if (type != OBJ_ANY && obj->type != type)
+ objerror(parent, "wrong object type in link");
+
+ if (obj->flags & REACHABLE)
+ return 0;
+ obj->flags |= REACHABLE;
+ if (!obj->parsed) {
+ if (parent && !has_sha1_file(obj->sha1)) {
+ printf("broken link from %7s %s\n",
+ typename(parent->type), sha1_to_hex(parent->sha1));
+ printf(" to %7s %s\n",
+ typename(obj->type), sha1_to_hex(obj->sha1));
+ errors_found |= ERROR_REACHABLE;
+ }
+ return 1;
+ }
+
+ add_object_array(obj, (void *) parent, &pending);
+ return 0;
+}
+
+static void mark_object_reachable(struct object *obj)
+{
+ mark_object(obj, OBJ_ANY, NULL);
+}
+
+static int traverse_one_object(struct object *obj, struct object *parent)
+{
+ int result;
+ struct tree *tree = NULL;
+
+ if (obj->type == OBJ_TREE) {
+ obj->parsed = 0;
+ tree = (struct tree *)obj;
+ if (parse_tree(tree) < 0)
+ return 1; /* error already displayed */
+ }
+ result = fsck_walk(obj, mark_object, obj);
+ if (tree) {
+ free(tree->buffer);
+ tree->buffer = NULL;
+ }
+ return result;
+}
+
+static int traverse_reachable(void)
+{
+ int result = 0;
+ while (pending.nr) {
+ struct object_array_entry *entry;
+ struct object *obj, *parent;
+
+ entry = pending.objects + --pending.nr;
+ obj = entry->item;
+ parent = (struct object *) entry->name;
+ result |= traverse_one_object(obj, parent);
+ }
+ return !!result;
+}
+
+static int mark_used(struct object *obj, int type, void *data)
+{
+ if (!obj)
+ return 1;
+ obj->used = 1;
+ return 0;
+}
+
+/*
+ * Check a single reachable object
+ */
+static void check_reachable_object(struct object *obj)
+{
+ /*
+ * We obviously want the object to be parsed,
+ * except if it was in a pack-file and we didn't
+ * do a full fsck
+ */
+ if (!obj->parsed) {
+ if (has_sha1_pack(obj->sha1))
+ return; /* it is in pack - forget about it */
+ printf("missing %s %s\n", typename(obj->type), sha1_to_hex(obj->sha1));
+ errors_found |= ERROR_REACHABLE;
+ return;
+ }
+}
+
+/*
+ * Check a single unreachable object
+ */
+static void check_unreachable_object(struct object *obj)
+{
+ /*
+ * Missing unreachable object? Ignore it. It's not like
+ * we miss it (since it can't be reached), nor do we want
+ * to complain about it being unreachable (since it does
+ * not exist).
+ */
+ if (!obj->parsed)
+ return;
+
+ /*
+ * Unreachable object that exists? Show it if asked to,
+ * since this is something that is prunable.
+ */
+ if (show_unreachable) {
+ printf("unreachable %s %s\n", typename(obj->type), sha1_to_hex(obj->sha1));
+ return;
+ }
+
+ /*
+ * "!used" means that nothing at all points to it, including
+ * other unreachable objects. In other words, it's the "tip"
+ * of some set of unreachable objects, usually a commit that
+ * got dropped.
+ *
+ * Such starting points are more interesting than some random
+ * set of unreachable objects, so we show them even if the user
+ * hasn't asked for _all_ unreachable objects. If you have
+ * deleted a branch by mistake, this is a prime candidate to
+ * start looking at, for example.
+ */
+ if (!obj->used) {
+ printf("dangling %s %s\n", typename(obj->type),
+ sha1_to_hex(obj->sha1));
+ if (write_lost_and_found) {
+ char *filename = git_path("lost-found/%s/%s",
+ obj->type == OBJ_COMMIT ? "commit" : "other",
+ sha1_to_hex(obj->sha1));
+ FILE *f;
+
+ if (safe_create_leading_directories(filename)) {
+ error("Could not create lost-found");
+ return;
+ }
+ if (!(f = fopen(filename, "w")))
+ die_errno("Could not open '%s'", filename);
+ if (obj->type == OBJ_BLOB) {
+ enum object_type type;
+ unsigned long size;
+ char *buf = read_sha1_file(obj->sha1,
+ &type, &size);
+ if (buf) {
+ if (fwrite(buf, size, 1, f) != 1)
+ die_errno("Could not write '%s'",
+ filename);
+ free(buf);
+ }
+ } else
+ fprintf(f, "%s\n", sha1_to_hex(obj->sha1));
+ if (fclose(f))
+ die_errno("Could not finish '%s'",
+ filename);
+ }
+ return;
+ }
+
+ /*
+ * Otherwise? It's there, it's unreachable, and some other unreachable
+ * object points to it. Ignore it - it's not interesting, and we showed
+ * all the interesting cases above.
+ */
+}
+
+static void check_object(struct object *obj)
+{
+ if (verbose)
+ fprintf(stderr, "Checking %s\n", sha1_to_hex(obj->sha1));
+
+ if (obj->flags & REACHABLE)
+ check_reachable_object(obj);
+ else
+ check_unreachable_object(obj);
+}
+
+static void check_connectivity(void)
+{
+ int i, max;
+
+ /* Traverse the pending reachable objects */
+ traverse_reachable();
+
+ /* Look up all the requirements, warn about missing objects.. */
+ max = get_max_object_index();
+ if (verbose)
+ fprintf(stderr, "Checking connectivity (%d objects)\n", max);
+
+ for (i = 0; i < max; i++) {
+ struct object *obj = get_indexed_object(i);
+
+ if (obj)
+ check_object(obj);
+ }
+}
+
+static int fsck_sha1(const unsigned char *sha1)
+{
+ struct object *obj = parse_object(sha1);
+ if (!obj) {
+ errors_found |= ERROR_OBJECT;
+ return error("%s: object corrupt or missing",
+ sha1_to_hex(sha1));
+ }
+ if (obj->flags & SEEN)
+ return 0;
+ obj->flags |= SEEN;
+
+ if (verbose)
+ fprintf(stderr, "Checking %s %s\n",
+ typename(obj->type), sha1_to_hex(obj->sha1));
+
+ if (fsck_walk(obj, mark_used, NULL))
+ objerror(obj, "broken links");
+ if (fsck_object(obj, check_strict, fsck_error_func))
+ return -1;
+
+ if (obj->type == OBJ_TREE) {
+ struct tree *item = (struct tree *) obj;
+
+ free(item->buffer);
+ item->buffer = NULL;
+ }
+
+ if (obj->type == OBJ_COMMIT) {
+ struct commit *commit = (struct commit *) obj;
+
+ free(commit->buffer);
+ commit->buffer = NULL;
+
+ if (!commit->parents && show_root)
+ printf("root %s\n", sha1_to_hex(commit->object.sha1));
+ }
+
+ if (obj->type == OBJ_TAG) {
+ struct tag *tag = (struct tag *) obj;
+
+ if (show_tags && tag->tagged) {
+ printf("tagged %s %s", typename(tag->tagged->type), sha1_to_hex(tag->tagged->sha1));
+ printf(" (%s) in %s\n", tag->tag, sha1_to_hex(tag->object.sha1));
+ }
+ }
+
+ return 0;
+}
+
+/*
+ * This is the sorting chunk size: make it reasonably
+ * big so that we can sort well..
+ */
+#define MAX_SHA1_ENTRIES (1024)
+
+struct sha1_entry {
+ unsigned long ino;
+ unsigned char sha1[20];
+};
+
+static struct {
+ unsigned long nr;
+ struct sha1_entry *entry[MAX_SHA1_ENTRIES];
+} sha1_list;
+
+static int ino_compare(const void *_a, const void *_b)
+{
+ const struct sha1_entry *a = _a, *b = _b;
+ unsigned long ino1 = a->ino, ino2 = b->ino;
+ return ino1 < ino2 ? -1 : ino1 > ino2 ? 1 : 0;
+}
+
+static void fsck_sha1_list(void)
+{
+ int i, nr = sha1_list.nr;
+
+ if (SORT_DIRENT)
+ qsort(sha1_list.entry, nr,
+ sizeof(struct sha1_entry *), ino_compare);
+ for (i = 0; i < nr; i++) {
+ struct sha1_entry *entry = sha1_list.entry[i];
+ unsigned char *sha1 = entry->sha1;
+
+ sha1_list.entry[i] = NULL;
+ fsck_sha1(sha1);
+ free(entry);
+ }
+ sha1_list.nr = 0;
+}
+
+static void add_sha1_list(unsigned char *sha1, unsigned long ino)
+{
+ struct sha1_entry *entry = xmalloc(sizeof(*entry));
+ int nr;
+
+ entry->ino = ino;
+ hashcpy(entry->sha1, sha1);
+ nr = sha1_list.nr;
+ if (nr == MAX_SHA1_ENTRIES) {
+ fsck_sha1_list();
+ nr = 0;
+ }
+ sha1_list.entry[nr] = entry;
+ sha1_list.nr = ++nr;
+}
+
+static void fsck_dir(int i, char *path)
+{
+ DIR *dir = opendir(path);
+ struct dirent *de;
+
+ if (!dir)
+ return;
+
+ if (verbose)
+ fprintf(stderr, "Checking directory %s\n", path);
+
+ while ((de = readdir(dir)) != NULL) {
+ char name[100];
+ unsigned char sha1[20];
+
+ if (is_dot_or_dotdot(de->d_name))
+ continue;
+ if (strlen(de->d_name) == 38) {
+ sprintf(name, "%02x", i);
+ memcpy(name+2, de->d_name, 39);
+ if (get_sha1_hex(name, sha1) < 0)
+ break;
+ add_sha1_list(sha1, DIRENT_SORT_HINT(de));
+ continue;
+ }
+ if (!prefixcmp(de->d_name, "tmp_obj_"))
+ continue;
+ fprintf(stderr, "bad sha1 file: %s/%s\n", path, de->d_name);
+ }
+ closedir(dir);
+}
+
+static int default_refs;
+
+static int fsck_handle_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
+ const char *email, unsigned long timestamp, int tz,
+ const char *message, void *cb_data)
+{
+ struct object *obj;
+
+ if (verbose)
+ fprintf(stderr, "Checking reflog %s->%s\n",
+ sha1_to_hex(osha1), sha1_to_hex(nsha1));
+
+ if (!is_null_sha1(osha1)) {
+ obj = lookup_object(osha1);
+ if (obj) {
+ obj->used = 1;
+ mark_object_reachable(obj);
+ }
+ }
+ obj = lookup_object(nsha1);
+ if (obj) {
+ obj->used = 1;
+ mark_object_reachable(obj);
+ }
+ return 0;
+}
+
+static int fsck_handle_reflog(const char *logname, const unsigned char *sha1, int flag, void *cb_data)
+{
+ for_each_reflog_ent(logname, fsck_handle_reflog_ent, NULL);
+ return 0;
+}
+
+static int is_branch(const char *refname)
+{
+ return !strcmp(refname, "HEAD") || !prefixcmp(refname, "refs/heads/");
+}
+
+static int fsck_handle_ref(const char *refname, const unsigned char *sha1, int flag, void *cb_data)
+{
+ struct object *obj;
+
+ obj = parse_object(sha1);
+ if (!obj) {
+ error("%s: invalid sha1 pointer %s", refname, sha1_to_hex(sha1));
+ /* We'll continue with the rest despite the error.. */
+ return 0;
+ }
+ if (obj->type != OBJ_COMMIT && is_branch(refname))
+ error("%s: not a commit", refname);
+ default_refs++;
+ obj->used = 1;
+ mark_object_reachable(obj);
+
+ return 0;
+}
+
+static void get_default_heads(void)
+{
+ if (head_points_at && !is_null_sha1(head_sha1))
+ fsck_handle_ref("HEAD", head_sha1, 0, NULL);
+ for_each_ref(fsck_handle_ref, NULL);
+ if (include_reflogs)
+ for_each_reflog(fsck_handle_reflog, NULL);
+
+ /*
+ * Not having any default heads isn't really fatal, but
+ * it does mean that "--unreachable" no longer makes any
+ * sense (since in this case everything will obviously
+ * be unreachable by definition.
+ *
+ * Showing dangling objects is valid, though (as those
+ * dangling objects are likely lost heads).
+ *
+ * So we just print a warning about it, and clear the
+ * "show_unreachable" flag.
+ */
+ if (!default_refs) {
+ fprintf(stderr, "notice: No default references\n");
+ show_unreachable = 0;
+ }
+}
+
+static void fsck_object_dir(const char *path)
+{
+ int i;
+
+ if (verbose)
+ fprintf(stderr, "Checking object directory\n");
+
+ for (i = 0; i < 256; i++) {
+ static char dir[4096];
+ sprintf(dir, "%s/%02x", path, i);
+ fsck_dir(i, dir);
+ }
+ fsck_sha1_list();
+}
+
+static int fsck_head_link(void)
+{
+ int flag;
+ int null_is_error = 0;
+
+ if (verbose)
+ fprintf(stderr, "Checking HEAD link\n");
+
+ head_points_at = resolve_ref("HEAD", head_sha1, 0, &flag);
+ if (!head_points_at)
+ return error("Invalid HEAD");
+ if (!strcmp(head_points_at, "HEAD"))
+ /* detached HEAD */
+ null_is_error = 1;
+ else if (prefixcmp(head_points_at, "refs/heads/"))
+ return error("HEAD points to something strange (%s)",
+ head_points_at);
+ if (is_null_sha1(head_sha1)) {
+ if (null_is_error)
+ return error("HEAD: detached HEAD points at nothing");
+ fprintf(stderr, "notice: HEAD points to an unborn branch (%s)\n",
+ head_points_at + 11);
+ }
+ return 0;
+}
+
+static int fsck_cache_tree(struct cache_tree *it)
+{
+ int i;
+ int err = 0;
+
+ if (verbose)
+ fprintf(stderr, "Checking cache tree\n");
+
+ if (0 <= it->entry_count) {
+ struct object *obj = parse_object(it->sha1);
+ if (!obj) {
+ error("%s: invalid sha1 pointer in cache-tree",
+ sha1_to_hex(it->sha1));
+ return 1;
+ }
+ mark_object_reachable(obj);
+ obj->used = 1;
+ if (obj->type != OBJ_TREE)
+ err |= objerror(obj, "non-tree in cache-tree");
+ }
+ for (i = 0; i < it->subtree_nr; i++)
+ err |= fsck_cache_tree(it->down[i]->cache_tree);
+ return err;
+}
+
+static char const * const fsck_usage[] = {
+ "git fsck [options] [<object>...]",
+ NULL
+};
+
+static struct option fsck_opts[] = {
+ OPT__VERBOSE(&verbose),
+ OPT_BOOLEAN(0, "unreachable", &show_unreachable, "show unreachable objects"),
+ OPT_BOOLEAN(0, "tags", &show_tags, "report tags"),
+ OPT_BOOLEAN(0, "root", &show_root, "report root nodes"),
+ OPT_BOOLEAN(0, "cache", &keep_cache_objects, "make index objects head nodes"),
+ OPT_BOOLEAN(0, "reflogs", &include_reflogs, "make reflogs head nodes (default)"),
+ OPT_BOOLEAN(0, "full", &check_full, "also consider packs and alternate objects"),
+ OPT_BOOLEAN(0, "strict", &check_strict, "enable more strict checking"),
+ OPT_BOOLEAN(0, "lost-found", &write_lost_and_found,
+ "write dangling objects in .git/lost-found"),
+ OPT_END(),
+};
+
+int cmd_fsck(int argc, const char **argv, const char *prefix)
+{
+ int i, heads;
+ struct alternate_object_database *alt;
+
+ errors_found = 0;
+ read_replace_refs = 0;
+
+ argc = parse_options(argc, argv, prefix, fsck_opts, fsck_usage, 0);
+ if (write_lost_and_found) {
+ check_full = 1;
+ include_reflogs = 0;
+ }
+
+ fsck_head_link();
+ fsck_object_dir(get_object_directory());
+
+ prepare_alt_odb();
+ for (alt = alt_odb_list; alt; alt = alt->next) {
+ char namebuf[PATH_MAX];
+ int namelen = alt->name - alt->base;
+ memcpy(namebuf, alt->base, namelen);
+ namebuf[namelen - 1] = 0;
+ fsck_object_dir(namebuf);
+ }
+
+ if (check_full) {
+ struct packed_git *p;
+
+ prepare_packed_git();
+ for (p = packed_git; p; p = p->next)
+ /* verify gives error messages itself */
+ verify_pack(p);
+
+ for (p = packed_git; p; p = p->next) {
+ uint32_t j, num;
+ if (open_pack_index(p))
+ continue;
+ num = p->num_objects;
+ for (j = 0; j < num; j++)
+ fsck_sha1(nth_packed_object_sha1(p, j));
+ }
+ }
+
+ heads = 0;
+ for (i = 0; i < argc; i++) {
+ const char *arg = argv[i];
+ unsigned char sha1[20];
+ if (!get_sha1(arg, sha1)) {
+ struct object *obj = lookup_object(sha1);
+
+ /* Error is printed by lookup_object(). */
+ if (!obj)
+ continue;
+
+ obj->used = 1;
+ mark_object_reachable(obj);
+ heads++;
+ continue;
+ }
+ error("invalid parameter: expected sha1, got '%s'", arg);
+ }
+
+ /*
+ * If we've not been given any explicit head information, do the
+ * default ones from .git/refs. We also consider the index file
+ * in this case (ie this implies --cache).
+ */
+ if (!heads) {
+ get_default_heads();
+ keep_cache_objects = 1;
+ }
+
+ if (keep_cache_objects) {
+ read_cache();
+ for (i = 0; i < active_nr; i++) {
+ unsigned int mode;
+ struct blob *blob;
+ struct object *obj;
+
+ mode = active_cache[i]->ce_mode;
+ if (S_ISGITLINK(mode))
+ continue;
+ blob = lookup_blob(active_cache[i]->sha1);
+ if (!blob)
+ continue;
+ obj = &blob->object;
+ obj->used = 1;
+ mark_object_reachable(obj);
+ }
+ if (active_cache_tree)
+ fsck_cache_tree(active_cache_tree);
+ }
+
+ check_connectivity();
+ return errors_found;
+}