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draft ("Better Than Nothing")
2001-08-21 Darin Adler <darin@bentspoon.com> The Nautilus shell, and the file manager inside it, does a lot of I/O. Because of this, there are some special disciplines required when writing Nautilus code. No I/O on the main thread To be able to respond to the user quickly, Nautilus needs to be designed so that the main user input thread does not block. The basic approach is to never do any disk I/O on the main thread. In practice, Nautilus code does assume that some disk I/O is fast, in some cases intentionally and in other cases due to programmer sloppiness. The typical assumption is that reading files from the user's home directory and the installed files in the Nautilus datadir are very fast, effectively instantaneous. So the general approach is to allow I/O for files that have file system paths, assuming that the access to these files is fast, and to prohibit I/O for files that have arbitrary URIs, assuming that access to these could be arbitrarily slow. Although this works pretty well, it is based on an incorrect assumption, because with NFS and other kinds of abstract file systems, there can be arbitrarily slow parts of the file system that have file system paths. For historical reasons, threading in Nautilus is done through the gnome-vfs asynchronous I/O abstraction rather than using threads directly. This means that all the threads are created by gnome-vfs, and Nautilus code runs on the main thread only. Thus, the rule of thumb is that synchronous gnome-vfs operations, like the ones in <libgnomevfs/gnome-vfs-ops.h> are illegal in most Nautilus code. Similarly, it's illegal to ask for a piece of information, say a file size, and then wait until it arrives. The program's main thread must be allowed to get back to the main loop and start asking for user input again. How NautilusFile is used to do this The NautilusFile class presents an API for scheduling this asynchronous I/O, and dealing with the uncertainty of when the information will be available. (It also does a few other things, but that's the main service it provides.) When you want information about a particular file or directory, you get the NautilusFile object for that item, using the nautilus_file_get. This operation, like most NautilusFile operations, is not allowed to do any disk I/O. Once you have a NautilusFile object, you can ask it questions like "What is your file type?" by calling functions like nautilus_file_get_file_type. However, in a newly created NautilusFile object, the answer is almost certainly "I don't know." Each function defines a default, which is the answer given for "I don't know." For example, nautilus_file_get_type will return GNOME_VFS_FILE_TYPE_UNKNOWN if it doesn't yet know the type. It's worth taking a side trip to discuss the nature of the NautilusFile API. Since these classes are a private part of the Nautilus implementation, we make no effort to have the API be "complete" in an abstract sense. Instead we add operations as necessary and give them the semantics that are most handy for our purposes. For example, we could have a nautilus_file_get_size that returns a special distinguishable value to mean "I don't know" or a separate boolean instead of returning 0 for files where the size is unknown. This is entirely motivated by pragmatic concerns. The intent is that we tweak these calls as needed if the semantics aren't good enough. Back to the newly created NautilusFile object. If you actually need to get the type, you need to arrange for that information to be fetched from the file system. There are two ways to make this request. If you are planning to display the type on an ongoing basis, then you want to tell the NautilusFile that you'll be monitoring the type and want to know about changes to it. If you just need one-time information about the type then you'll want to be informed when the type is discovered. The calls used for this are nautilus_file_monitor_add and nautilus_file_call_when_ready respectively. Both of these calls take a list of information needed about a file. If all you need is the file type, for example, you would pass a list containing just NAUTILUS_FILE_ATTRIBUTE_FILE_TYPE (the attributes are defined in nautilus-file-attributes.h). Not every call has a corresponding file attribute type. We add new ones as needed. If you do a nautilus_file_monitor_add, you also typically connect to the NautilusFile object's changed signal. Each time any monitored attribute changes, a changed signal is emitted. The caller typically caches the value of the attribute that was last seen (for example, what's displayed on screen) and does a quick check to see if the attribute it cares about has changed. If you do a nautilus_file_call_when_ready, you don't typically need to connect to the changed signal, because your callback function will be called when and if the requested information is ready. Both a monitor and a call when ready can be cancelled. For ease of use, neither call requires that you store an ID for canceling. Instead, the monitor function uses an arbitrary client pointer, which can be any kind of pointer that's known to not conflict with other monitorers. Usually, this is a pointer to the monitoring object, but it can also be, for example, a pointer to a global variable. The call_when_ready function uses the callback and callback data to identify the particular callback. One advantage of the monitor API is that it also lets the NautilusFile framework know that the file should be monitored for changes made outside Nautilus. This is how we know when to ask FAM to monitor a file for us. Lets review a few of the concepts: 1) Nearly all NautilusFile operations, like nautilus_file_get_type, are not allowed to do any disk I/O. 2) To cause the actual I/O to be done, callers need to use either a monitor or a call when ready. 3) The actual I/O is done by asynchronous gnome-vfs calls, and this is done on another thread. When working with an entire directory of files at once, you work with a NautilusDirectory object. With the NautilusDirectory object you can monitor a whole set of NautilusFile objects at once, and you can connect to a single "files_changed" signal that gets emitted whenever files within the directory are modified. That way you don't have to connect separately to each file you want to monitor. These calls are also the mechanism for finding out which files are in a directory. In most other respects, they are like the NautilusFile calls. Caching, the good and the bad Another feature of the NautilusFile class is the caching. If you keep around a NautilusFile object, it keeps around information about the last known state of that file. Thus, if you call nautilus_file_get_type, you might well get file type of the file found at this location the last time you looked, rather than the information about what the file type is now, or "unknown". There are some problems with this, though. The first problem is that if wrong information is cached, you need some way to "goose" the NautilusFile object and get it to grab new information. This is trickier than it might sound, because we don't want to constantly distrust information we received just moments before. To handle this, we have the nautilus_file_invalidate_attributes and nautilus_file_invalidate_all_attributes calls, as well as the nautilus_directory_force_reload call. If some code in Nautilus makes a change to a file that's known to affect the cached information, it can call one of these to inform the NautilusFile framework. Changes that are made through the framework itself are automatically understood, so usually these calls aren't necessary. The second problem is that it's hard to predict when information will and won't be cached. The current rule that's implemented is that no information is cached if no one retains a reference to the NautilusFile object. This means that someone else holding a NautilusFile object can subtly affect the semantics of whether you have new data or not. Calling nautilus_file_call_when_ready or nautilus_file_monitor_add will not invalidate the cache, but rather will return you the already cached information. These problems are less pronounced when FAM is in use. With FAM, any monitored file is highly likely to have accurate information, because changes to the file will be noticed by FAM, and that in turn will trigger new I/O to determine what the new status of the file is. Operations that change the file You'll note that up until this point, I've only discussed getting information about the file, not making changes to it. NautilusFile also contains some APIs for making changes. There are two kinds of these. The calls that change metadata are an example of the first kind. These calls make changes to the internal state right away, and schedule I/O to write the changes out to the file system. There's no way to detect if the I/O succeeds or fails, and as far as the client code is concerned, the change takes place right away. The calls that make other kinds of file system change are an example of the second kind. These calls take a NautilusFileOperationCallback. They are all cancellable, and they give the callback when the operation completes, whether it succeeds or fails. Icons The current implementation of the Nautilus icon factory uses synchronous I/O to get the icons and ignores these guidelines. The only reason this doesn't ruin the Nautilus user experience is that it also refuses to even try to fetch icons from URIs that don't correspond to file system paths, which for most cases means it limits itself to reading from the high-speed local disk. Don't ask me what the repercussions of this are for NFS; do the research and tell me instead! Slowness caused by asynchronous operations The danger in all this asynchronous I/O is that you might end up doing lots of user interface tasks twice. If you go to display a file right after asking for information about it, you might immediately show an "unknown file type" icon. Then, milliseconds later, you may complete the I/O and discover more information about the file, including the appropriate icon. So you end up drawing everything twice. There are a number of strategies for preventing this problem. One of them is to allow a bit of hysteresis, and wait some fixed amount of time after requesting the I/O before displaying the "unknown" state. [What strategy is used in Nautilus right now?] How to make Nautilus slow If you add I/O to the functions in NautilusFile that are used simply to fetch cached file information, you can make Nautilus incredibly I/O intensive. On the other hand, the NautilusFile API does not provide a way to do arbitrary file reads, for example. So it can be tricky to add features to Nautilus, since you first have to educate NautilusFile about how to do the I/O asynchronously and cache it, then request the information and have some way to deal with the time when it's not yet known. Adding new kinds of I/O usually involves working on the Nautilus I/O state machine in nautilus-directory-async.c. If we changed Nautilus to use threading instead of using gnome-vfs asychronous operations, I'm pretty sure that most of the changes would be here in this file. That's because the external API used for NautilusFile wouldn't really have a reason to change. In either case, you'd want to schedule work to be done, and get called back when the work is complete. [We probably need more about nautilus-directory-async.c here.] That's all for now This is a very rough early draft of this document. Let me know about other topics that would be useful to be covered in here. -- Darin |
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8296d110ee | convert from Mac format to Unix format | ||
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* docs/nautilus-io.txt: Add a new document. |