Add first cut at "fsck-cache" that validates the SHA1 object store.

It doesn't complain about mine. But it also doesn't yet check for
inter-object reachability etc.

fsck-cache.c

@@ -0,0 +1,106 @@
+#include "cache.h"
+
+#include <sys/types.h>
+#include <dirent.h>
+
+/*
+ * These three functions should build up a graph in memory about
+ * what objects we've referenced, and found, and types..
+ *
+ * Right now we don't do that kind of reachability checking. Yet.
+ */
+static void fsck_tree(void *data, unsigned long size)
+{
+}
+
+static void fsck_commit(void *data, unsigned long size)
+{
+}
+
+static int mark_sha1_seen(unsigned char *sha1, char *tag)
+{
+	return 0;
+}
+
+static int fsck_entry(unsigned char *sha1, char *tag, void *data, unsigned long size)
+{
+	if (!strcmp(tag, "blob")) 
+		/* Nothing to check */;
+	else if (!strcmp(tag, "tree"))
+		fsck_tree(data, size);
+	else if (!strcmp(tag, "commit"))
+		fsck_commit(data, size);
+	else
+		return -1;
+	return mark_sha1_seen(sha1, tag);
+}
+
+static int fsck_name(char *hex)
+{
+	unsigned char sha1[20];
+	if (!get_sha1_hex(hex, sha1)) {
+		unsigned long mapsize;
+		void *map = map_sha1_file(sha1, &mapsize);
+		if (map) {
+			char type[100];
+			unsigned long size;
+			void *buffer = NULL;
+			if (!check_sha1_signature(sha1, map, mapsize))
+				buffer = unpack_sha1_file(map, mapsize, type, &size);
+			munmap(map, mapsize);
+			if (buffer && !fsck_entry(sha1, type, buffer, size))
+				return 0;
+		}
+	}
+	return -1;
+}
+
+static int fsck_dir(int i, char *path)
+{
+	DIR *dir = opendir(path);
+	struct dirent *de;
+
+	if (!dir) {
+		fprintf(stderr, "missing sha1 directory '%s'", path);
+		return -1;
+	}
+
+	while ((de = readdir(dir)) != NULL) {
+		char name[100];
+		int len = strlen(de->d_name);
+
+		switch (len) {
+		case 2:
+			if (de->d_name[1] != '.')
+				break;
+		case 1:
+			if (de->d_name[0] != '.')
+				break;
+			continue;
+		case 38:
+			sprintf(name, "%02x", i);
+			memcpy(name+2, de->d_name, len+1);
+			if (!fsck_name(name))
+				continue;
+		}
+		fprintf(stderr, "bad sha1 file: %s/%s\n", path, de->d_name);
+	}
+	closedir(dir);
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	int i;
+	char *sha1_dir;
+
+	if (argc != 1)
+		usage("fsck-cache");
+	sha1_dir = getenv(DB_ENVIRONMENT) ? : DEFAULT_DB_ENVIRONMENT;
+	for (i = 0; i < 256; i++) {
+		static char dir[4096];
+		sprintf(dir, "%s/%02x", sha1_dir, i);
+		fsck_dir(i, dir);
+	}
+	return 0;
+}