From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ACA75C77B78 for ; Tue, 18 Apr 2023 19:20:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230479AbjDRTUE (ORCPT ); Tue, 18 Apr 2023 15:20:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37148 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229813AbjDRTUB (ORCPT ); Tue, 18 Apr 2023 15:20:01 -0400 Received: from mail-yb1-xb31.google.com (mail-yb1-xb31.google.com [IPv6:2607:f8b0:4864:20::b31]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6213D1FC3 for ; Tue, 18 Apr 2023 12:19:36 -0700 (PDT) Received: by mail-yb1-xb31.google.com with SMTP id v9so9653989ybm.0 for ; Tue, 18 Apr 2023 12:19:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ttaylorr-com.20221208.gappssmtp.com; s=20221208; t=1681845528; x=1684437528; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=cEz+IGEOZrlMZAJLqUNyPK7jLzwh9XjxUPuK8lSgZng=; b=dgAd320Vm2WSnWH1b0zcTdCX+PF+AVeeXgRt2uaMbynHjQRJjAJuYWQnvN9Szv5AOJ 41NnAv7GQdgghBpXXxAhAGtEFkKcLrPsP58+NOqliK15j1SIbtNS+4gLLTGPv3K6wKky 722tNgcyl01YA8Nx2Xo+1E5sVfLMyMfpJxnh4Y9UdRzrNU1doKPmbVqCTbxEOyo3SVAO nqolBQ6bmZR3DVXsdiIY+kBim46HVtXZ5IoUoI0PlkAZo+kWAgqXXYYkfl7+c7gD8CjS P5sahixRH2zt59B6GHD/nyeKASoKNAiL7jbe4H13V0bbThTLETP4lQU7Jyrme9t/6pfJ fUcQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1681845528; x=1684437528; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=cEz+IGEOZrlMZAJLqUNyPK7jLzwh9XjxUPuK8lSgZng=; b=ItXuyQ/BKjcgSBw0biZlU8YFN8I/YGzMq3aP+g8bUjWs4KPPL/sbWBRbnjxmW0ydAb MIO54oriAQa2O+fHIqQZOWHjNUov/L6VO+Dnma5wxGbt4Wu6IOge+N6jxbIWKlMuC6I9 s4Nwy+2z8r0DCx8S2jVjE69DHXsloxbPPaelkkUIge+7j1i0sfVg8B6uFSQxsXLrU5hS 6ZvR2zJCcwzo3FGEQdJ6hJX5qNHFC/sOOu6Iq1N2Juh1oI88R2ycUHxgh/fq5RaysVAt c7MxpklzWRWWEdoKTkSMwORurM1UfGSUNsH4er0K5r4ZXnzc06xICPVCS02AbK/M/tDQ aq1A== X-Gm-Message-State: AAQBX9eiIgshJGI3GZ6CaJ80gldZ1GhshvuqpdWSbPVEEknMFSlwOcIt kRNq/XEYuYicv4oSVmjKb8eQaut30urrXVeEab2xLw== X-Google-Smtp-Source: AKy350aTjYd9gp2pxx+eXPc6CTRgaN6TpJCr/SnIf8DkWW4AWo+TuBWjIGvU4aIqfIp/owEUlhT64w== X-Received: by 2002:a05:6902:707:b0:b8f:3685:c12a with SMTP id k7-20020a056902070700b00b8f3685c12amr22691367ybt.39.1681845527952; Tue, 18 Apr 2023 12:18:47 -0700 (PDT) Received: from localhost (104-178-186-189.lightspeed.milwwi.sbcglobal.net. [104.178.186.189]) by smtp.gmail.com with ESMTPSA id 126-20020a250a84000000b00b8f54571fc0sm2326961ybk.5.2023.04.18.12.18.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Apr 2023 12:18:47 -0700 (PDT) Date: Tue, 18 Apr 2023 15:18:46 -0400 From: Taylor Blau To: git@vger.kernel.org Cc: Jeff King , Chris Torek , Junio C Hamano Subject: [PATCH v2 2/6] string-list: introduce `string_list_setlen()` Message-ID: <2a20ad8bc5513aae912c53a294092ee5087e1873.1681845518.git.me@ttaylorr.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org It is sometimes useful to reduce the size of a `string_list`'s list of items without having to re-allocate them. For example, doing the following: struct strbuf buf = STRBUF_INIT; struct string_list parts = STRING_LIST_INIT_NO_DUP; while (strbuf_getline(&buf, stdin) != EOF) { parts.nr = 0; string_list_split_in_place(&parts, buf.buf, ":", -1); /* ... */ } string_list_clear(&parts, 0); is preferable over calling `string_list_clear()` on every iteration of the loop. This is because `string_list_clear()` causes us free our existing `items` array. This means that every time we call `string_list_split_in_place()`, the string-list internals re-allocate the same size array. Since in the above example we do not care about the individual parts after processing each line, it is much more efficient to pretend that there aren't any elements in the `string_list` by setting `list->nr` to 0 while leaving the list of elements allocated as-is. This allows `string_list_split_in_place()` to overwrite any existing entries without needing to free and re-allocate them. However, setting `list->nr` manually is not safe in all instances. There are a couple of cases worth worrying about: - If the `string_list` is initialized with `strdup_strings`, truncating the list can lead to overwriting strings which are allocated elsewhere. If there aren't any other pointers to those strings other than the ones inside of the `items` array, they will become unreachable and leak. (We could ourselves free the truncated items between string_list->items[nr] and `list->nr`, but no present or future callers would benefit from this additional complexity). - If the given `nr` is larger than the current value of `list->nr`, we'll trick the `string_list` into a state where it thinks there are more items allocated than there actually are, which can lead to undefined behavior if we try to read or write those entries. Guard against both of these by introducing a helper function which guards assignment of `list->nr` against each of the above conditions. Co-authored-by: Jeff King Signed-off-by: Jeff King Signed-off-by: Taylor Blau --- string-list.c | 9 +++++++++ string-list.h | 10 ++++++++++ 2 files changed, 19 insertions(+) diff --git a/string-list.c b/string-list.c index b27a53f2e1..f0b3cdae94 100644 --- a/string-list.c +++ b/string-list.c @@ -203,6 +203,15 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c list->nr = list->alloc = 0; } +void string_list_setlen(struct string_list *list, size_t nr) +{ + if (list->strdup_strings) + BUG("cannot setlen a string_list which owns its entries"); + if (nr > list->nr) + BUG("cannot grow a string_list with setlen"); + list->nr = nr; +} + struct string_list_item *string_list_append_nodup(struct string_list *list, char *string) { diff --git a/string-list.h b/string-list.h index f01bbb0bb6..b41ecda6f4 100644 --- a/string-list.h +++ b/string-list.h @@ -134,6 +134,16 @@ typedef void (*string_list_clear_func_t)(void *p, const char *str); /** Call a custom clear function on each util pointer */ void string_list_clear_func(struct string_list *list, string_list_clear_func_t clearfunc); +/* + * Set the length of a string_list to `nr`, provided that (a) `list` + * does not own its own storage, and (b) that `nr` is no larger than + * `list->nr`. + * + * Useful when "shrinking" `list` to write over existing entries that + * are no longer used without reallocating. + */ +void string_list_setlen(struct string_list *list, size_t nr); + /** * Apply `func` to each item. If `func` returns nonzero, the * iteration aborts and the return value is propagated. -- 2.40.0.358.g56d2318a6d