From: Zooko O'Whielacronx Date: Wed, 15 Aug 2007 19:28:04 +0000 (-0700) Subject: webapi.txt: shorter and hopefully clearer description of names vs. identifiers X-Git-Url: https://git.rkrishnan.org/vdrive/$sch_link?a=commitdiff_plain;h=4f2244bfdd488e9bf70f4875ee3b332c3514e49e;p=tahoe-lafs%2Ftahoe-lafs.git webapi.txt: shorter and hopefully clearer description of names vs. identifiers Brian (and anyone who has an interest in the API and documentation): please review. --- diff --git a/docs/webapi.txt b/docs/webapi.txt index 6135ec99..b849fdfc 100644 --- a/docs/webapi.txt +++ b/docs/webapi.txt @@ -249,53 +249,46 @@ allmydata.org uri format to relieve the user of this requirement. described above. -== Time-Of-Check-To-Time-Of-Use ("TOCTTOU") bugs == - -Note that since directories are mutable you can get surprises if you query -the vdrive, e.g. "GET $URL?t=json", examine the resulting JSON-encoded -information, and then fetch from or update the vdrive using a name-based URL. -This is because the actual state of the vdrive could have changed after you -did the "GET $URL?t=json" query and before you did the subsequent fetch or -update. - -For example, suppose you query to find out that "vdrive/private/somedir/foo" -is a file which has a certain number of bytes, and then you issue a "GET -vdrive/private/somedir/foo" to fetch the file. The file that you get might -have a different number of bytes than the one that you chose to fetch, -because the "foo" entry in the "somedir" directory may have been changed to -point to a different file between your query and your fetch, or because the -"somedir" entry in the private vdrive might have been changed to point to a -different directory. - -Potentially more damaging, suppose that the "foo" entry was changed to point -to a directory instead of a file. Then instead of receiving the expected -file, you receive a file containing an HTML page describing the directory -contents! - -These are examples of TOCTTOU bugs ( http://en.wikipedia.org/wiki/TOCTTOU ). - -A good way to avoid these bugs is to issue your second request, not with a -URL based on the sequence of names that lead to the object, but instead with -the URI of the object. For example, in the case that you query a directory -listing (with "GET vdrive/private/somedir?t=json"), find a file named "foo" -therein that you want to download, and then download the file, if you -download it with its URI ("GET uri/$URI") instead of its URL ("GET -vdrive/private/somedir/foo") then you will get the file that was in the -"somedir/" directory under the name "foo" when you queried that directory, -even if the "somedir/" directory has since been changed so that its "foo" -child now points to a different file or to a directory. +== names versus identifiers == + +The vdrive provides a mutable filesystem, but the ways that the filesystem +can change are limited. The only thing that can change is that the mapping +from child names to child objects that each directory contains can be changed +by adding a new child name pointing to an object, removing an existing child +name, or changing an existing child name to point to a different object. + +Obviously if you query tahoe for information about the filesystem and then +act upon the filesystem (such as by getting a listing of the contents of a +directory and then adding a file to the directory), then the filesystem might +have been changed after you queried it and before you acted upon it. +However, if you use the URI instead of the pathname of an object when you act +upon the object, then the only change that can happen is when the object is a +directory then the set of child names it has might be different. If, on the +other hand, you act upon the object using its pathname, then a different +object might be in that place, which can result in more kinds of surprises. + +For example, suppose you are writing code which recursively downloads the +contents of a directory. The first thing your code does is fetch the listing +of the contents of the directory. For each child that it fetched, if that +child is a file then it downloads the file, and if that child is a directory +then it recurses into that directory. Now, if the download and the recurse +actions are performed using the child's name, then the results might be +wrong, because for example a child name that pointed to a sub-directory when +you listed the directory might have been changed to point to a file, in which +case your attempt to recurse into it would result in an error and the file +would be skipped, or a child name that pointed to a file when you listed the +directory might now point to a sub-directory, in which case your attempt to +download the child would result in a file containing HTML text describing the +sub-directory! + +If your recursive algorithm uses the URI of the child instead of the name of +the child, then those kinds of mistakes just can't happen. Note that both the +child's name and the child's URI are included in the results of listing the +parent directory, so it isn't harder to use the URI for this purpose. In general, use names if you want "whatever object (whether file or -directory) is found by following this sequence of names when my request -reaches the server". Use URIs if you want "this particular object". - -If you are basing your decision to fetch from or update the vdrive on -filesystem information that was returned by an earlier query, then you -usually intend to fetch or update the particular object that was in that -location when you first queried it, rather than whatever object is going to -be in that location when your subsequent fetch request finally reaches the -server. - +directory) is found by following this name (or sequence of names) when my +request reaches the server". Use URIs if you want "this particular object". == POST forms ==