index: more informative error message

when failed to load an existing index.

src/apk.c

@@ -67,6 +67,8 @@ const char *apk_error_str(int error)
 		return "IO ERROR";
 	case EBADMSG:
 		return "BAD archive";
+	case ENOMSG:
+		return "archive does not contain expected data";
 	default:
 		return strerror(error);
 	}

src/database.c

@@ -1165,12 +1165,14 @@ static int load_apkindex(void *sctx, const struct apk_file_info *fi,
 	apk_db_index_read(ctx->db, bs, ctx->repo);
 	bs->close(bs, NULL);
 
-	return 0;
+	return -ECANCELED;
 }
 
 static int load_index(struct apk_database *db, struct apk_bstream *bs,
 		      int targz, int repo)
 {
+	int r = 0;
+
 	if (targz) {
 		struct apk_istream *is;
 		struct apkindex_ctx ctx;
@@ -1179,17 +1181,19 @@ static int load_index(struct apk_database *db, struct apk_bstream *bs,
 		ctx.repo = repo;
 		apk_sign_ctx_init(&ctx.sctx, APK_SIGN_VERIFY, NULL);
 		is = apk_bstream_gunzip_mpart(bs, apk_sign_ctx_mpart_cb, &ctx.sctx);
-		apk_tar_parse(is, load_apkindex, &ctx);
+		r = apk_tar_parse(is, load_apkindex, &ctx);
 		is->close(is);
 		apk_sign_ctx_free(&ctx.sctx);
-		if (!ctx.sctx.data_verified)
-			return -1;
+		if (r == 0)
+			r = -ENOMSG;
+		else if (r == -ECANCELED)
+			r = 0;
 	} else {
 		bs = apk_bstream_from_istream(apk_bstream_gunzip(bs));
 		apk_db_index_read(db, bs, repo);
 		bs->close(bs, NULL);
 	}
-	return 0;
+	return r;
 }
 
 int apk_db_index_read_file(struct apk_database *db, const char *file, int repo)

src/index.c

@@ -86,7 +86,7 @@ static int index_main(void *ctx, int argc, char **argv)
 	struct counts counts = {0};
 	struct apk_ostream *os;
 	struct apk_file_info fi;
-	int total, i, j, found, newpkgs = 0;
+	int total, r, i, j, found, newpkgs = 0;
 	struct index_ctx *ictx = (struct index_ctx *) ctx;
 
 	if (isatty(STDOUT_FILENO) && ictx->output == NULL &&
@@ -100,10 +100,10 @@ static int index_main(void *ctx, int argc, char **argv)
 		ictx->method = APK_SIGN_GENERATE;
 
 	apk_db_open(&db, NULL, APK_OPENF_READ);
-	if (index_read_file(&db, ictx) < 0) {
+	if ((r = index_read_file(&db, ictx)) < 0) {
 		apk_db_close(&db);
-		apk_error("The index is corrupt, or of unknown format.");
-		return -1;
+		apk_error("%s: %s", ictx->index, apk_error_str(r));
+		return r;
 	}
 
 	for (i = 0; i < argc; i++) {

src/package.c

@@ -723,7 +723,7 @@ int apk_pkg_read(struct apk_database *db, const char *file,
 	if (r < 0 && r != -ECANCELED)
 		goto err;
 	if (ctx.pkg->name == NULL) {
-		r = -EBADMSG;
+		r = -ENOMSG;
 		goto err;
 	}
 	if (sctx->action != APK_SIGN_VERIFY)