From patchwork Mon Apr 24 22:20:14 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Taylor Blau X-Patchwork-Id: 13222638 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 E9445C7618E for ; Mon, 24 Apr 2023 22:20:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233113AbjDXWUW (ORCPT ); Mon, 24 Apr 2023 18:20:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38914 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233080AbjDXWUU (ORCPT ); Mon, 24 Apr 2023 18:20:20 -0400 Received: from mail-yb1-xb33.google.com (mail-yb1-xb33.google.com [IPv6:2607:f8b0:4864:20::b33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F3BA96A59 for ; Mon, 24 Apr 2023 15:20:16 -0700 (PDT) Received: by mail-yb1-xb33.google.com with SMTP id 3f1490d57ef6-b8f549d36e8so8938580276.3 for ; Mon, 24 Apr 2023 15:20:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ttaylorr-com.20221208.gappssmtp.com; s=20221208; t=1682374816; x=1684966816; 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=bFI2BL7bcCkF07NR9MDI1AUzu6vynoeiliNLGS3V+eE=; b=tO0GK+ZgnrvtZHy21lC9g52JCH5TIcfZkFaF1QRHVjtAgP8kYMRu+YwKIBNz6pQtXw mdb4QiQ0T22lAydmDA16ndZ2O3uwNsxc0OfGZa4xa3uuCOscHt75urbh0iBxC5VStTc6 sNCUn9K6Zwc6xEAT2XF+qHk0alvdjSgWOOittn5hQ7kE9mStHfW1Mh3ainAy/kb9/WoD V3GsVNl1dzTgQbMI+iwDK+eO5HJf+XYwSM6kpP1alBotr8ELGpA0Mr5qM9nAjjG1t+CK yp4h5x3r1v9Q4N0T0zbpBof3rV7EoV30+APWo4Q6ZFQsIYMGb5IEvtsMHSiv31jZdUlT YWjg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1682374816; x=1684966816; 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=bFI2BL7bcCkF07NR9MDI1AUzu6vynoeiliNLGS3V+eE=; b=d9fzKfHw+IPpsDkoFmDuevU2JycUdWp+Nbgj56y2NXUc3SUe+udjOZjb+4yZjvetvk F7OjFxxIUigenXGjsVUa2+/Olk3GZYeGfDIZeuGaz6dZrUPBmB+OXuDUn08mPLeDAoiz FwTI1L8ZPNZRk+h4nVlXvY5kvLrRD0CTp/PcNDuTgJYYqVzSwX08BtIPDzzvRGkSYc06 1KtRUlFRhIHWoqDmJnXW0gT95LHOmUB860X9sjUWhlEoSJBPk3sv/fyoZeZxdlBrv84p dQCXYCbD92LdJr+nEoLReziKuDO2DCrHlhuTODwNiGqogne7WMJcJkzApJUyECZWSs9l 93GA== X-Gm-Message-State: AAQBX9ehD9wKInF2NcgFwvUg0lIpFJgfUlCKeVxTLsR0RFb1oldbtpSt VWoCkUADkAil+coE08KQXdbInh3QtAnA0Sy9NU8e3g== X-Google-Smtp-Source: AKy350abYgRNkSsC97cztM/xJNlK/+LA8ZnGT5/d4sYY0zEilhaaF7ZuvG4I1DW2IeJ573etFN/Uog== X-Received: by 2002:a25:a188:0:b0:b99:4af6:185d with SMTP id a8-20020a25a188000000b00b994af6185dmr8714602ybi.6.1682374815766; Mon, 24 Apr 2023 15:20:15 -0700 (PDT) Received: from localhost (104-178-186-189.lightspeed.milwwi.sbcglobal.net. [104.178.186.189]) by smtp.gmail.com with ESMTPSA id s14-20020a5b074e000000b00b993995abe4sm2395874ybq.60.2023.04.24.15.20.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Apr 2023 15:20:15 -0700 (PDT) Date: Mon, 24 Apr 2023 18:20:14 -0400 From: Taylor Blau To: git@vger.kernel.org Cc: Jeff King , Chris Torek , Junio C Hamano , Jeff Hostetler , =?utf-8?b?UmVuw6k=?= Scharfe Subject: [PATCH v3 2/6] string-list: introduce `string_list_setlen()` Message-ID: References: MIME-Version: 1.0 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 5f5b60fe1c..0f8ac117fd 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 77854840f7..122b318641 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.