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authorLubomir Rintel <lubo.rintel@gooddata.com>2013-08-01 15:13:01 +0200
committerVijay Bellur <vbellur@redhat.com>2013-08-04 07:39:56 -0700
commita7564731e30312047f0300760c248f86c1080d84 (patch)
tree54ca3ed61b14041b5db03876ad9adf8abcf66b46 /doc
parentd5a4e34f02415cfcc7637efaa1e135305a7da230 (diff)
doc: Import Jeff Darcy's translator tutorial
It's way better than what we have currently. It's the original text from Jeff's blog [1][2][3][4], unedited aside from the title, so that the content can be properly attributed. [1] http://hekafs.org/index.php/2011/11/translator-101-class-1-setting-the-stage/ [2] http://hekafs.org/index.php/2011/11/translator-101-lesson-2-init-fini-and-private-context/ [3] http://hekafs.org/index.php/2011/11/translator-101-lesson-3-this-time-for-real/ [4] http://hekafs.org/index.php/2011/11/translator-101-lesson-4-debugging-a-translator/ Original-Author: Jeff Darcy <jdarcy@redhat.com> Necessary editing will follow: * Nicer endings/openings of the subsections, as they are no longer separate articles * Wrap lines in terminal outputs so that they render nicely in PDF * Remove HeksFS references * Remove first person fear of developer wrath for manual creation of a volfile Change-Id: Ie9bb537b5817e8fa575ec0c66a58c48e6584698e Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com> Reviewed-on: http://review.gluster.org/5480 Reviewed-by: Vijay Bellur <vbellur@redhat.com> Tested-by: Vijay Bellur <vbellur@redhat.com>
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+Translator development
+======================
+
+Setting the Stage
+-----------------
+
+This is the first post in a series that will explain some of the details of
+writing a GlusterFS translator, using some actual code to illustrate.
+
+Before we begin, a word about environments. GlusterFS is over 300K lines of
+code spread across a few hundred files. That's no Linux kernel or anything, but
+ you're still going to be navigating through a lot of code in every
+code-editing session, so some kind of cross-referencing is *essential*. I use
+cscope with the vim bindings, and if I couldn't do Crtl+G and such to jump
+between definitions all the time my productivity would be cut in half. You may
+prefer different tools, but as I go through these examples you'll need
+something functionally similar to follow on. OK, on with the show.
+
+The first thing you need to know is that translators are not just bags of
+functions and variables. They need to have a very definite internal structure
+so that the translator-loading code can figure out where all the pieces are.
+The way it does this is to use dlsym to look for specific names within your
+shared-object file, as follow (from `xlator.c`):
+
+```
+if (!(xl->fops = dlsym (handle, "fops"))) {
+ gf_log ("xlator", GF_LOG_WARNING, "dlsym(fops) on %s",
+ dlerror ());
+ goto out;
+}
+
+if (!(xl->cbks = dlsym (handle, "cbks"))) {
+ gf_log ("xlator", GF_LOG_WARNING, "dlsym(cbks) on %s",
+ dlerror ());
+ goto out;
+}
+
+if (!(xl->init = dlsym (handle, "init"))) {
+ gf_log ("xlator", GF_LOG_WARNING, "dlsym(init) on %s",
+ dlerror ());
+ goto out;
+}
+
+if (!(xl->fini = dlsym (handle, "fini"))) {
+ gf_log ("xlator", GF_LOG_WARNING, "dlsym(fini) on %s",
+ dlerror ());
+ goto out;
+}
+```
+
+In this example, `xl` is a pointer to the in-memory object for the translator
+we're loading. As you can see, it's looking up various symbols *by name* in the
+ shared object it just loaded, and storing pointers to those symbols. Some of
+them (e.g. init are functions, while others e.g. fops are dispatch tables
+containing pointers to many functions. Together, these make up the translator's
+ public interface.
+
+Most of this glue or boilerplate can easily be found at the bottom of one of
+the source files that make up each translator. We're going to use the `rot-13`
+translator just for fun, so in this case you'd look in `rot-13.c` to see this:
+
+```
+struct xlator_fops fops = {
+ .readv = rot13_readv,
+ .writev = rot13_writev
+};
+
+struct xlator_cbks cbks = {
+};
+
+struct volume_options options[] = {
+{ .key = {"encrypt-write"},
+ .type = GF_OPTION_TYPE_BOOL
+},
+{ .key = {"decrypt-read"},
+ .type = GF_OPTION_TYPE_BOOL
+},
+{ .key = {NULL} },
+};
+```
+
+The `fops` table, defined in `xlator.h`, is one of the most important pieces.
+This table contains a pointer to each of the filesystem functions that your
+translator might implement -- `open`, `read`, `stat`, `chmod`, and so on. There
+are 82 such functions in all, but don't worry; any that you don't specify here
+will be see as null and filled with defaults from `defaults.c` when your
+translator is loaded. In this particular example, since `rot-13` is an
+exceptionally simple translator, we only fill in two entries for `readv` and
+`writev`.
+
+There are actually two other tables, also required to have predefined names,
+that are also used to find translator functions: `cbks` (which is empty in this
+ snippet) and `dumpops` (which is missing entirely). The first of these specify
+ entry points for when inodes are forgotten or file descriptors are released.
+In other words, they're destructors for objects in which your translator might
+ have an interest. Mostly you can ignore them, because the default behavior
+handles even the simpler cases of translator-specific inode/fd context
+automatically. However, if the context you attach is a complex structure
+requiring complex cleanup, you'll need to supply these functions. As for
+dumpops, that's just used if you want to provide functions to pretty-print
+various structures in logs. I've never used it myself, though I probably
+should. What's noteworthy here is that we don't even define dumpops. That's
+because all of the functions that might use these dispatch functions will check
+ for `xl->dumpops` being `NULL` before calling through it. This is in sharp
+contrast to the behavior for `fops` and `cbks1`, which *must* be present. If
+they're not, translator loading will fail because these pointers are not
+checked every time and if they're `NULL` then we'll segfault. That's why we
+provide an empty definition for cbks; it's OK for the individual function
+pointers to be NULL, but not for the whole table to be absent.
+
+The last piece I'll cover today is options. As you can see, this is a table of
+translator-specific option names and some information about their types.
+GlusterFS actually provides a pretty rich set of types (`volume_option_type_t`
+in `options.`h) which includes paths, translator names, percentages, and times
+in addition to the obvious integers and strings. Also, the `volume_option_t`
+structure can include information about alternate names, min/max/default
+values, enumerated string values, and descriptions. We don't see any of these
+here, so let's take a quick look at some more complex examples from afr.c and
+then come back to `rot-13`.
+
+```
+{ .key = {"data-self-heal-algorithm"},
+ .type = GF_OPTION_TYPE_STR,
+ .default_value = "",
+ .description = "Select between \"full\", \"diff\". The "
+ "\"full\" algorithm copies the entire file from "
+ "source to sink. The \"diff\" algorithm copies to "
+ "sink only those blocks whose checksums don't match "
+ "with those of source.",
+ .value = { "diff", "full", "" }
+},
+{ .key = {"data-self-heal-window-size"},
+ .type = GF_OPTION_TYPE_INT,
+ .min = 1,
+ .max = 1024,
+ .default_value = "1",
+ .description = "Maximum number blocks per file for which self-heal "
+ "process would be applied simultaneously."
+},
+```
+
+When your translator is loaded, all of this information is used to parse the
+options actually provided in the volfile, and then the result is turned into a
+dictionary and stored as `xl->options`. This dictionary is then processed by
+your init function, which you can see being looked up in the first code
+fragment above. We're only going to look at a small part of the `rot-13`'s
+init for now.
+
+```
+priv->decrypt_read = 1;
+priv->encrypt_write = 1;
+
+data = dict_get (this->options, "encrypt-write");
+if (data) {
+ if (gf_string2boolean (data->data, &priv->encrypt_write) == -1) {
+ gf_log (this->name, GF_LOG_ERROR,
+ "encrypt-write takes only boolean options");
+ return -1;
+ }
+}
+```
+
+What we can see here is that we're setting some defaults in our priv structure,
+then looking to see if an `encrypt-write` option was actually provided. If so,
+we convert and store it. This is a pretty classic use of dict_get to fetch a
+field from a dictionary, and of using one of many conversion functions in
+`common-utils.c` to convert `data->data` into something we can use.
+
+So far we've covered the basic of how a translator gets loaded, how we find its
+various parts, and how we process its options. In my next Translator 101 post,
+we'll go a little deeper into other things that init and its companion fini
+might do, and how some other fields in our `xlator_t` structure (commonly
+referred to as this) are commonly used.
+
+`init`, `fini`, and private context
+-----------------------------------
+
+In the previous Translator 101 post, we looked at some of the dispatch tables
+and options processing in a translator. This time we're going to cover the rest
+ of the "shell" of a translator -- i.e. the other global parts not specific to
+handling a particular request.
+
+Let's start by looking at the relationship between a translator and its shared
+library. At a first approximation, this is the relationship between an object
+and a class in just about any object-oriented programming language. The class
+defines behaviors, but has to be instantiated as an object to have any kind of
+existence. In our case the object is an `xlator_t`. Several of these might be
+created within the same daemon, sharing all of the same code through init/fini
+and dispatch tables, but sharing *no data*. You could implement shared data (as
+ static variables in your shared libraries) but that's strongly discouraged.
+Every function in your shared library will get an `xlator_t` as an argument,
+and should use it. This lack of class-level data is one of the points where
+the analogy to common OOP systems starts to break down. Another place is the
+complete lack of inheritance. Translators inherit behavior (code) from exactly
+one shared library -- looked up and loaded using the `type` field in a volfile
+`volume ... end-volume` block -- and that's it -- not even single inheritance,
+no subclasses or superclasses, no mixins or prototypes, just the relationship
+between an object and its class. With that in mind, let's turn to the init
+function that we just barely touched on last time.
+
+```
+int32_t
+init (xlator_t *this)
+{
+ data_t *data = NULL;
+ rot_13_private_t *priv = NULL;
+
+ if (!this->children || this->children->next) {
+ gf_log ("rot13", GF_LOG_ERROR,
+ "FATAL: rot13 should have exactly one child");
+ return -1;
+ }
+
+ if (!this->parents) {
+ gf_log (this->name, GF_LOG_WARNING,
+ "dangling volume. check volfile ");
+ }
+
+ priv = GF_CALLOC (sizeof (rot_13_private_t), 1, 0);
+ if (!priv)
+ return -1;
+```
+
+At the very top, we see the function signature -- we get a pointer to the
+`xlator_t` object that we're initializing, and we return an `int32_t` status.
+As with most functions in the translator API, this should be zero to indicate
+success. In this case it's safe to return -1 for failure, but watch out: in
+dispatch-table functions, the return value means the status of the *function
+call* rather than the *request*. A request error should be reflected as a
+callback with a non-zero `op_re`t value, but the dispatch function itself
+should still return zero. In fact, the handling of a non-zero return from a
+dispatch function is not all that robust (we recently had a bug report in
+HekaFS related to this) so it's something you should probably avoid
+altogether. This only underscores the difference between dispatch functions
+and `init`/`fini` functions, where non-zero returns *are* expected and handled
+logically by aborting the translator setup. We can see that down at the
+bottom, where we return -1 to indicate that we couldn't allocate our
+private-data area (more about that later).
+
+The first thing this init function does is check that the translator is being
+set up in the right kind of environment. Translators are called by parents and
+in turn call children. Some translators are "initial" translators that inject
+requests into the system from elsewhere -- e.g. mount/fuse injecting requests
+from the kernel, protocol/server injecting requests from the network. Those
+translators don't need parents, but `rot-13` does and so we check for that.
+Similarly, some translators are "final" translators that (from the perspective
+of the current process) terminate requests instead of passing them on -- e.g.
+`protocol/client` passing them to another node, `storage/posix` passing them to
+a local filesystem. Other translators "multiplex" between multiple children --
+ passing each parent request on to one (`cluster/dht`), some
+(`cluster/stripe`), or all (`cluster/afr`) of those children. `rot-13` fits
+into none of those categories either, so it checks that it has *exactly one*
+child. It might be more convenient or robust if translator shared libraries
+had standard variables describing these requirements, to be checked in a
+consistent way by the translator-loading infrastructure itself instead of by
+each separate init function, but this is the way translators work today.
+
+The last thing we see in this fragment is allocating our private data area.
+This can literally be anything we want; the infrastructure just provides the
+priv pointer as a convenience but takes no responsibility for how it's used. In
+ this case we're using `GF_CALLOC` to allocate our own `rot_13_private_t`
+structure. This gets us all the benefits of GlusterFS's memory-leak detection
+infrastructure, but the way we're calling it is not quite ideal. For one thing,
+ the first two arguments -- from `calloc(3)` -- are kind of reversed. For
+another, notice how the last argument is zero. That can actually be an
+enumerated value, to tell the GlusterFS allocator *what* type we're
+allocating. This can be very useful information for memory profiling and leak
+detection, so it's recommended that you follow the example of any
+x`xx-mem-types.h` file elsewhere in the source tree instead of just passing
+zero here (even though that works).
+
+To finish our tour of standard initialization/termination, let's look at the
+end of `init` and the beginning of `fini`:
+
+```
+ this->private = priv;
+ gf_log ("rot13", GF_LOG_DEBUG, "rot13 xlator loaded");
+ return 0;
+}
+
+void
+fini (xlator_t *this)
+{
+ rot_13_private_t *priv = this->private;
+
+ if (!priv)
+ return;
+ this->private = NULL;
+ GF_FREE (priv);
+```
+
+At the end of init we're just storing our private-data pointer in the `priv`
+field of our `xlator_t`, then returning zero to indicate that initialization
+succeeded. As is usually the case, our fini is even simpler. All it really has
+to do is `GF_FREE` our private-data pointer, which we do in a slightly
+roundabout way here. Notice how we don't even have a return value here, since
+there's nothing obvious and useful that the infrastructure could do if `fini`
+failed.
+
+That's practically everything we need to know to get our translator through
+loading, initialization, options processing, and termination. If we had defined
+ no dispatch functions, we could actually configure a daemon to use our
+translator and it would work as a basic pass-through from its parent to a
+single child. In the next post I'll cover how to build the translator and
+configure a daemon to use it, so that we can actually step through it in a
+debugger and see how it all fits together before we actually start adding
+functionality.
+
+This Time For Real
+------------------
+
+In the first two parts of this series, we learned how to write a basic
+translator skeleton that can get through loading, initialization, and option
+processing. This time we'll cover how to build that translator, configure a
+volume to use it, and run the glusterfs daemon in debug mode.
+
+Unfortunately, there's not much direct support for writing new translators. You
+can check out a GlusterFS tree and splice in your own translator directory, but
+ that's a bit painful because you'll have to update multiple makefiles plus a
+bunch of autoconf garbage. As part of the HekaFS project, I basically reverse
+engineered the truly necessary parts of the translator-building process and
+then pestered one of the Fedora glusterfs package maintainers (thanks
+daMaestro!) to add a `glusterfs-devel` package with the required headers. Since
+ then the complexity level in the HekaFS tree has crept back up a bit, but I
+still remember the simple method and still consider it the easiest way to get
+started on a new translator. For the sake of those not using Fedora, I'm going
+to describe a method that doesn't depend on that header package. What it does
+depend on is a GlusterFS source tree, much as you might have cloned from GitHub
+ or the Gluster review site. This tree doesn't have to be fully built, but you
+do need to run `autogen.sh` and configure in it. Then you can take the
+following simple makefile and put it in a directory with your actual source.
+
+```
+# Change these to match your source code.
+TARGET = rot-13.so
+OBJECTS = rot-13.o
+
+# Change these to match your environment.
+GLFS_SRC = /play/glusterfs
+GLFS_LIB = /opt/glusterfs/3git/lib64
+HOST_OS = GF_LINUX_HOST_OS
+
+# You shouldn't need to change anything below here.
+
+CFLAGS = -fPIC -Wall -O0 -g \
+ -DHAVE_CONFIG_H -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE -D$(HOST_OS) \
+ -I$(GLFS_SRC) -I$(GLFS_SRC)/libglusterfs/src \
+ -I$(GLFS_SRC)/contrib/uuid
+LDFLAGS = -shared -nostartfiles -L$(GLFS_LIB) -lglusterfs -lpthread
+
+$(TARGET): $(OBJECTS)
+ $(CC) $(OBJECTS) $(LDFLAGS) -o $(TARGET)
+```
+
+Yes, it's still Linux-specific. Mea culpa. As you can see, we're sticking with
+the `rot-13` example, so you can just copy the files from
+`xlators/encryption/rot-13/src` in your GlusterFS tree to follow on. Type
+`make` and you should be rewarded with a nice little `.so` file.
+
+```
+[jeff@gfs-i8c-01 xlator_example]$ ls -l rot-13.so
+-rwxr-xr-x. 1 jeff jeff 40784 Nov 16 16:41 rot-13.so
+```
+
+Notice that we've built with optimization level zero and debugging symbols
+included, which would not typically be the case for a packaged version of
+GlusterFS. Let's put our version of `rot-13.so` into a slightly different file
+on our system, so that it doesn't stomp on the installed version (not that
+you'd ever want to use that anyway).
+
+```
+[root@gfs-i8c-01 xlator_example]# ls /opt/glusterfs/3git/lib64/glusterfs/3git/xlator/encryption/
+crypt.so crypt.so.0 crypt.so.0.0.0 rot-13.so rot-13.so.0 rot-13.so.0.0.0
+[root@gfs-i8c-01 xlator_example]# cp rot-13.so /opt/glusterfs/3git/lib64/glusterfs/3git/xlator/encryption/my-rot-13.so
+```
+
+These paths represent the current Gluster filesystem layout, which is likely to
+be deprecated in favor of the Fedora layout; your paths may vary. At this point
+ we're ready to configure a volume using our new translator. To do that, I'm
+going to suggest something that's strongly discouraged except during
+development (the Gluster guys are going to hate me for this): write our own
+volfile. Here's just about the simplest volfile you'll ever see.
+
+```
+volume my-posix
+ type storage/posix
+ option directory /play/export
+end-volume
+
+volume my-rot13
+ type encryption/my-rot-13
+ subvolumes my-posix
+end-volume
+```
+
+All we have here is a basic brick using `/play/export` for its data, and then
+an instance of our translator layered on top -- no client or server is
+necessary for what we're doing, and the system will automatically push a
+mount/fuse translator on top if there's no server translator. To try this out,
+all we need is the following command (assuming the directories involved already
+ exist).
+
+```
+[jeff@gfs-i8c-01 xlator_example]$ glusterfs --debug -f my.vol /play/import
+```
+
+You should be rewarded with a whole lot of log output, including the text of
+the volfile (this is very useful for debugging problems in the field). If you
+go to another window on the same machine, you can see that you have a new
+filesystem mounted.
+
+```
+[jeff@gfs-i8c-01 ~]$ df /play/import
+Filesystem 1K-blocks Used Available Use% Mounted on
+/play/xlator_example/my.vol
+ 114506240 2706176 105983488 3% /play/import
+```
+
+Just for fun, write something into a file in `/play/import`, then look at the
+corresponding file in `/play/export` to see it all `rot-13`'ed for you.
+
+```
+[jeff@gfs-i8c-01 ~]$ echo hello > /play/import/a_file
+[jeff@gfs-i8c-01 ~]$ cat /play/export/a_file
+uryyb
+```
+
+There you have it -- functionality you control, implemented easily, layered on
+top of local storage. Now you could start adding functionality -- real
+encryption, perhaps -- and inevitably having to debug it. You could do that the
+ old-school way, with `gf_log` (preferred) or even plain old `printf`, or you
+could run daemons under `gdb` instead. Alternatively, you could wait for the
+next Translator 101 post, where we'll be doing exactly that.
+
+Debugging a Translator
+----------------------
+
+Now that we've learned what a translator looks like and how to build one, it's
+time to run one and actually watch it work. The best way to do this is good
+old-fashioned `gdb`, as follows (using some of the examples from last time).
+
+```
+[root@gfs-i8c-01 xlator_example]# gdb glusterfs
+GNU gdb (GDB) Red Hat Enterprise Linux (7.2-50.el6)
+...
+(gdb) r --debug -f my.vol /play/import
+Starting program: /usr/sbin/glusterfs --debug -f my.vol /play/import
+...
+[2011-11-23 11:23:16.495516] I [fuse-bridge.c:2971:fuse_init] 0-glusterfs-fuse: FUSE inited with protocol versions: glusterfs 7.13 kernel 7.13
+```
+
+If you get to this point, your glusterfs client process is already running. You
+can go to another window to see the mountpoint, do file operations, etc.
+
+```
+[root@gfs-i8c-01 ~]# df /play/import
+Filesystem 1K-blocks Used Available Use% Mounted on
+/root/xlator_example/my.vol
+ 114506240 2643968 106045568 3% /play/import
+[root@gfs-i8c-01 ~]# ls /play/import
+a_file
+[root@gfs-i8c-01 ~]# cat /play/import/a_file
+hello
+```
+
+Now let's interrupt the process and see where we are.
+
+```
+^C
+Program received signal SIGINT, Interrupt.
+0x0000003a0060b3dc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+(gdb) info threads
+ 5 Thread 0x7fffeffff700 (LWP 27206) 0x0000003a002dd8c7 in readv ()
+ from /lib64/libc.so.6
+ 4 Thread 0x7ffff50e3700 (LWP 27205) 0x0000003a0060b75b in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+ 3 Thread 0x7ffff5f02700 (LWP 27204) 0x0000003a0060b3dc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+ 2 Thread 0x7ffff6903700 (LWP 27203) 0x0000003a0060f245 in sigwait ()
+ from /lib64/libpthread.so.0
+* 1 Thread 0x7ffff7957700 (LWP 27196) 0x0000003a0060b3dc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+```
+
+Like any non-toy server, this one has multiple threads. What are they all
+doing? Honestly, even I don't know. Thread 1 turns out to be in
+`event_dispatch_epoll`, which means it's the one handling all of our network
+I/O. Note that with socket multi-threading patch this will change, with one
+thread in `socket_poller` per connection. Thread 2 is in `glusterfs_sigwaiter`
+which means signals will be isolated to that thread. Thread 3 is in
+`syncenv_task`, so it's a worker process for synchronous requests such as
+those used by the rebalance and repair code. Thread 4 is in
+`janitor_get_next_fd`, so it's waiting for a chance to close no-longer-needed
+file descriptors on the local filesystem. (I admit I had to look that one up,
+BTW.) Lastly, thread 5 is in `fuse_thread_proc`, so it's the one fetching
+requests from our FUSE interface. You'll often see many more threads than
+this, but it's a pretty good basic set. Now, let's set a breakpoint so we can
+actually watch a request.
+
+```
+(gdb) b rot13_writev
+Breakpoint 1 at 0x7ffff50e4f0b: file rot-13.c, line 119.
+(gdb) c
+Continuing.
+```
+
+At this point we go into our other window and do something that will involve a write.
+
+```
+[root@gfs-i8c-01 ~]# echo goodbye > /play/import/another_file
+(back to the first window)
+[Switching to Thread 0x7fffeffff700 (LWP 27206)]
+
+Breakpoint 1, rot13_writev (frame=0x7ffff6e4402c, this=0x638440, fd=0x7ffff409802c,
+ vector=0x7fffe8000cd8, count=1, offset=0, iobref=0x7fffe8001070) at rot-13.c:119
+119 rot_13_private_t *priv = (rot_13_private_t *)this->private;
+```
+
+Remember how we built with debugging symbols enabled and no optimization? That
+will be pretty important for the next few steps. As you can see, we're in
+`rot13_writev`, with several parameters.
+
+* `frame` is our always-present frame pointer for this request. Also,
+ `frame->local` will point to any local data we created and attached to the
+ request ourselves.
+* `this` is a pointer to our instance of the `rot-13` translator. You can examine
+ it if you like to see the name, type, options, parent/children, inode table,
+ and other stuff associated with it.
+* `fd` is a pointer to a file-descriptor *object* (`fd_t`, not just a
+ file-descriptor index which is what most people use "fd" for). This in turn
+ points to an inode object (`inode_t`) and we can associate our own
+ `rot-13`-specific data with either of these.
+* `vector` and `count` together describe the data buffers for this write, which
+ we'll get to in a moment.
+* `offset` is the offset into the file at which we're writing.
+* `iobref` is a buffer-reference object, which is used to track the life cycle
+ of buffers containing read/write data. If you look closely, you'll notice that
+ `vector[0].iov_base` points to the same address as `iobref->iobrefs[0].ptr`, which
+ should give you some idea of the inter-relationships between vector and iobref.
+
+OK, now what about that `vector`? We can use it to examine the data being
+written, like this.
+
+```
+(gdb) p vector[0]
+$2 = {iov_base = 0x7ffff7936000, iov_len = 8}
+(gdb) x/s 0x7ffff7936000
+0x7ffff7936000: "goodbye\n"
+```
+
+It's not always safe to view this data as a string, because it might just as
+well be binary data, but since we're generating the write this time it's safe
+and convenient. With that knowledge, let's step through things a bit.
+
+```
+(gdb) s
+120 if (priv->encrypt_write)
+(gdb)
+121 rot13_iovec (vector, count);
+(gdb)
+rot13_iovec (vector=0x7fffe8000cd8, count=1) at rot-13.c:57
+57 for (i = 0; i < count; i++) {
+(gdb)
+58 rot13 (vector[i].iov_base, vector[i].iov_len);
+(gdb)
+rot13 (buf=0x7ffff7936000 "goodbye\n", len=8) at rot-13.c:45
+45 for (i = 0; i < len; i++) {
+(gdb)
+46 if (buf[i] >= 'a' && buf[i] <= 'z')
+(gdb)
+47 buf[i] = 'a' + ((buf[i] - 'a' + 13) % 26);
+```
+
+Here we've stepped into `rot13_iovec`, which iterates through our vector
+calling `rot13`, which in turn iterates through the characters in that chunk
+doing the `rot-13` operation if/as appropriate. This is pretty straightforward
+stuff, so let's skip to the next interesting bit.
+
+```
+(gdb) fin
+Run till exit from #0 rot13 (buf=0x7ffff7936000 "goodbye\n", len=8) at rot-13.c:47
+rot13_iovec (vector=0x7fffe8000cd8, count=1) at rot-13.c:57
+57 for (i = 0; i < count; i++) {
+(gdb) fin
+Run till exit from #0 rot13_iovec (vector=0x7fffe8000cd8, count=1) at rot-13.c:57
+rot13_writev (frame=0x7ffff6e4402c, this=0x638440, fd=0x7ffff409802c,
+ vector=0x7fffe8000cd8, count=1, offset=0, iobref=0x7fffe8001070) at rot-13.c:123
+123 STACK_WIND (frame,
+(gdb) b 129
+Breakpoint 2 at 0x7ffff50e4f35: file rot-13.c, line 129.
+(gdb) b rot13_writev_cbk
+Breakpoint 3 at 0x7ffff50e4db3: file rot-13.c, line 106.
+(gdb) c
+```
+
+So we've set breakpoints on both the callback and the statement following the
+`STACK_WIND`. Which one will we hit first?
+
+```
+Breakpoint 3, rot13_writev_cbk (frame=0x7ffff6e4402c, cookie=0x7ffff6e440d8,
+ this=0x638440, op_ret=8, op_errno=0, prebuf=0x7fffefffeca0,
+ postbuf=0x7fffefffec30) at rot-13.c:106
+106 STACK_UNWIND_STRICT (writev, frame, op_ret, op_errno, prebuf, postbuf);
+(gdb) bt
+#0 rot13_writev_cbk (frame=0x7ffff6e4402c, cookie=0x7ffff6e440d8, this=0x638440,
+ op_ret=8, op_errno=0, prebuf=0x7fffefffeca0, postbuf=0x7fffefffec30)
+ at rot-13.c:106
+#1 0x00007ffff52f1b37 in posix_writev (frame=0x7ffff6e440d8,
+ this=<value optimized out>, fd=<value optimized out>,
+ vector=<value optimized out>, count=1, offset=<value optimized out>,
+ iobref=0x7fffe8001070) at posix.c:2217
+#2 0x00007ffff50e513e in rot13_writev (frame=0x7ffff6e4402c, this=0x638440,
+ fd=0x7ffff409802c, vector=0x7fffe8000cd8, count=1, offset=0,
+ iobref=0x7fffe8001070) at rot-13.c:123
+```
+
+Surprise! We're in `rot13_writev_cbk` now, called (indirectly) while we're
+still in `rot13_writev` before `STACK_WIND` returns (still at rot-13.c:123). If
+ you did any request cleanup here, then you need to be careful about what you
+do in the remainder of `rot13_writev` because data may have been freed etc.
+It's tempting to say you should just do the cleanup in `rot13_writev` after
+the `STACK_WIND,` but that's not valid because it's also possible that some
+other translator returned without calling `STACK_UNWIND` -- i.e. before
+`rot13_writev` is called, so then it would be the one getting null-pointer
+errors instead. To put it another way, the callback and the return from
+`STACK_WIND` can occur in either order or even simultaneously on different
+threads. Even if you were to use reference counts, you'd have to make sure to
+use locking or atomic operations to avoid races, and it's not worth it. Unless
+you *really* understand the possible flows of control and know what you're
+doing, it's better to do cleanup in the callback and nothing after
+`STACK_WIND.`
+
+At this point all that's left is a `STACK_UNWIND` and a return. The
+`STACK_UNWIND` invokes our parent's completion callback, and in this case our
+parent is FUSE so at that point the VFS layer is notified of the write being
+complete. Finally, we return through several levels of normal function calls
+until we come back to fuse_thread_proc, which waits for the next request.
+
+So that's it. For extra fun, you might want to repeat this exercise by stepping
+through some other call -- stat or setxattr might be good choices -- but you'll
+ have to use a translator that actually implements those calls to see much
+that's interesting. Then you'll pretty much know everything I knew when I
+started writing my first for-real translators, and probably even a bit more. I
+hope you've enjoyed this series, or at least found it useful, and if you have
+any suggestions for other topics I should cover please let me know (via
+comments or email, IRC or Twitter).