Thunderbird and testing

Thunderbird has come a long way in its automated test suite since I started working on it 5 years ago. Back then, much of our code was untestable and it was rare that a patch added tests. Now, our code coverage results look like this. It has almost unthinkable to have a patch that doesn't have a test, and there are only a few places in our code where testing is impossible. Now I'm going to propose how to fill in these gaps.

LDAP

Ah, LDAP. The big red part of comm-central whenever I make my coverage treemaps. The problem here could be solved if we had an LDAP fakeserver; having written both IMAP and NNTP servers, this shouldn't be hard? Except that LDAP is not built off of a textual-based layer that you can emulate with telnet but an over-engineered protocol called ASN.1 and more specifically one of its binary encodings. The underlying fakeserver technology is built with the assumption that I'm dealing with a CRLF-based protocol, but it turns out that, with some of my patches, it's actually easy to just pass through the binary data (yay for layering).

The full LDAP specification is actually quite complicated and relies on a lot of pieces, but the underlying model for an LDAP fakeserver could rather easily be controlled by just an LDIF file with perhaps a simplified schema model. At the very least, it's a usable start, and considering that the IMAP fakeserver still isn't RFC 3501-compliant 4 years later, it's good enough for testing.

Here, a big issue arises: the actual protocol decoding. I started by looking for a nice library I could use for ASN.1 decoding so I don't have to do it myself. I first played with using the LDAP lber routines myself via ctypes, but I found myself dissatisfied with how much work it took just to parse the login of the LDAP serve. I then looked into NSS's structured ASN.1 decoding, even happening upon a nice set of templates for LDAP so I didn't have to try to build them with the lack of documentation, but it still ended up not working well, especially given the nice model of genericity I was looking for. I played around with a node-based LDAP server (especially annoying given the current name feud in Debian that prevents the nodejs package from migrating to testing). It worked well enough for an initial test, but the problem of either driving the server from xpcshell or writing node shims combined with the fact that it only processes the protocol and has no usable backend caused me to give up that path. Desperate, I even tried to find just general BER-parsing libraries in JS on the general web and discovered that the ones that were there couldn't quite cope with the format as we use it.

Conclusions: it's possible. The only real hard part is writing the BER parsing library myself. If anyone decides they want to work on this, I can send them the partial pieces of the puzzle to finish. If not, I'll probably nibble on this here and there over the next year or two.

MIME

MIME—that's well-tested per our testsuite, right? Well, not really. A lot of the testing is just pure incidental: hooking the MIME library up to the IMAP fakeserver did a good job of fleshing out a lot of issues, but you can also find lots of small details that no one's going to notice (charsets come to mind). It turns out that MIME is one of those protocols where everybody does the same thing slightly differently, and you end up accumulating a lot of random fixes to MIME. If you want to replace the module from scratch, you become terrified of finding random regressions in real-world mail.

Perhaps unsurprisingly, there are no test suites for proper MIME parsing on the web. There is one for RFC 2231 decoding (kind of). But there's nothing that tries to determine any of the following:

• Charset detection, especially who gets priority when everyone conficts
• Whether a part is inline, attached, or not shown at all
• How attachments get detected and handled
• Test suites for the various crap that crops up when people fail at i18n
• Text-to-html or HTML sanitization issues
• Identifying headers properly (malformed References headers, etc.)
• Pseudo-MIME constructs, like TNEF, uuencode, BinHex, or yEnc
• S/MIME or PGP

Issues relating to message display could be handled with a suite of reftests. A brief test confirms that reftest specifications accept absolute URLs, including the URLs that are used to drive the message UI (this can even test it from loading the offline protocol). Reftests even allow you to set prefs before specific tests; with a bit of sugaring around the reftest list, a MIME reftest is easily doable. Attachment and header handling could also follow a MIME reftest design, but I'm not sure that is the best design. I'd also like it to be the kind of test that other people who write MIME libraries could use.

The main issue here is seeding the repository with something useful. Sampling a variety of Usenet newsgroups (especially foreign-language hierarchies) should pick up something useful for basic charset, and I can get uuencode and yEnc by trawling through some binary newsgroups. For a focus on gmail, I could probably pick up some Google Groups things (especially if I recall the magic incantations that let me at actual RFC-822 objects). Random public mailing lists might find something useful. My own private email is unlikely to provide any useful test cases, since I tend to communicate with too homogeneous an environment (i.e., I don't get enough people using Outlook). Sanitizing all of this public stuff is also going to be a pain, especially with the emails that have DKIM.

OS integration

OS integration is a nice header for everything that involves the actual OS: MAPI, import from standard system mail clients, integration with system address books. Unfortunately, my main development environment is Linux, where we have none of this stuff, so I can't really claim that I have a plan for testing here. Thanks to bug 731877, at least testing Outlook Express importing is a possibility, but true tests would probably require dumping some .psts into our tree, but we have no similar story for Mail.app. MAPI could be done with a mock app that exercises the MAPI interfaces; what it really comes down to is that we need to implement these APIs in a way that we can test them by executing in various mock environments during tests.

Performance tests

The other major hole we have is performance. Firefox measures its performance with things like Talos; Thunderbird ought to have a similar testsuite of performance benchmarks. What kind of benchmarks are useful for Thunderbird testing though? Modulo debates over where exactly to place the endpoints on the following tests, I think the following is a good list:

• Startup and shutdown time
• Time to open a "large" folder (maybe requiring rebuild?) and mem usage in doing so
• Doing message operations (mark as read, delete, move, copy, etc.) on several messages in a "large" folder. Possibly memory too
• Time to select and display a "large" message (inline parts), as well as detach/delete attachments on said message
• Cross-folder message search (with/without gloda?)
• Some sort of database resync operation
• Address book queries

For the large folders, I think having a good distribution of the size of threads (so some messages not in threads, others collected in a 50+ message thread) is necessary. Slow performance in extra-large folders is something we routinely get criticized on, so being able to track regressions is something that I think is useful. Tests that can also adequately catch some stupid things like "download a message fifteen times to display it" are extremely useful in my opinion, and I feel like there needs to be some sort of performance tests that highlight problems in IMAP code would be useful.

Mozilla and Thunderbird

Let me start by saying that I have been contributing to Thunderbird for nearly 5 years. I don't have any secret knowledge; what I know that isn't public is generally had just by talking to people in private messages on IRC. All points I make in here are my own thoughts and beliefs, and do not necessarily reflect the rest of Mozilla.

To say that the recent announcement on Thunderbird's future threw people in a tizzy would be an understatement. After all, we have nothing less than apocalyptic proclamations of the death of Thunderbird. I believe that such proclamations are as exaggerated as Samuel Clemens's death notices (apologies for making a joke that is probably inscrutable to non-en-US people).

The truth is, Thunderbird has not been a priority for Mozilla since before I started working on it. There really isn't any coordination in mozilla-central to make sure that any planned "featurectomies" don't impact Thunderbird—we typically get the same notice that add-on authors get, despite being arguably the largest binary user of the codebase outside of mozilla-central. Given also that the Fennec and B2G codebases were subsequently merged into mozilla-central (one of the arguments I heard about the Fennec merge was that "it's too difficult to maintain the project outside of mozilla-central") and that comm-central remains separate, it should be quickly clear how much apathy for Thunderbird existed prior to this announcement.

As a consequence, the community has historically played a major role in the upkeep of Thunderbird. The massive de-RDF project was driven by a lawyer-in-training. I myself have made significant changes to the address book, NNTP, testing, and MIME codes. Our QA efforts are driven in large part by a non-paid contributor. More than half of the top-ten contributors are non-employees, according to hg churn. So the end of purely-Thunderbird-focused paid developers is by no means the end of the project.

There's a lot of invective about the decision, so let me attempt to rationalize why it was made. Mozilla's primary goal is to promote the Open Web, which means in large part, that Mozilla needs to ensure that it remains relevant in markets to prevent the creation of walled gardens. I believe that Mozilla has judged that it needs to focus on the mobile market, which is where the walled gardens are starting to crop up again. In the desktop world, Mozilla has a strong browser and a strong email client, and maintaining that position is good enough. In the mobile world, Mozilla has virtually no presence right now. Hence all of the effort being put into Firefox Mobile and B2G right now.

Now, many of the decisions as to the future of the project are uncertain; unfortunately, the email laying all of this out was prematurely leaked. But it is clear that Thunderbird suffers from massive technical debt: when I was pondering parts that the Gaia email app might be able to leverage, I first considered the IMAP protocol implementation and then ran out of things to suggest. Well, maybe lightning or the chat backends (for calendaring and IM, respectively), but it's clear that most of the mammoth codebase is completely unsuitable for reincorporation into another project. To this end, I think the most useful thing that could happen in Thunderbird falls under the "maintenance" banner anyways: a replacement of these crappy components with more solid implementations that are less reliant on maybe-obsolete Gecko features and that could be shared with the Gaia email app. As a bit of a shameless plug, I have been working on a replacement MIME parser with an explicit eye towards letting Gaia's app use it. Such work would be more useful than whining about the decision, in any case.

Building mozilla with clang LTO

Ingredients:

• 1 (one) copy of clang, preferably on the new side
• 1 (one) source tree containing mozilla-central
• 4 (four) GiB (that's 2³⁰, not 10⁹) of memory
• Linux (these instructions not tested on OS X, so it may or may not work for you)

Instructions:

For the sake of example, I have clang installed to /usr/local; substitute as appropriate for your environment. Configure mozilla-central with the following mozconfig:

# -O4 tells clang to do LTO
ac_add_options --disable-debug --enable-optimize='-O4'
# Exclude this line, and your will use over 10x the amount of memory.
ac_add_options --disable-debug-symbols
# Build with clang
export CC=/usr/local/bin/clang
export CXX=/usr/local/bin/clang++
# These flags are necessary to get elfhack to compile
export LDFLAGS='-Wl,-z,norelro -flto'
# This flag is necessary to link properly
export RANLIB='ar -s --plugin /usr/local/lib/LLVMgold.so'

You may season with other configure options as desired. Configure and let build for a few hours. Admire the build process as most individual files get compiled much quicker, but be displeased when link times explode. Enjoy!

JS code coverage

My most recent adventure has been an extended foray into the depths of getting good JavaScript code coverage. The tool that I had been pointed to a few years ago was JSCoverage, which took the approach of "reserialize all JS code with implementation details," which doesn't work quite so well with the ever-shifting landscapes of Mozilla JS implementation. It also used SpiderMonkey's internal-only parser API, which, from experience, is a bloody edge to ride on. I've dithered on and off about using the reflection API to use this, but making sure that data can get extracted out again is a bit of a pain.

Now it's 2012 and the JS engine has better hooks that allow you to use debugging APIs without causing the code to come to a screeching halt in terms of performance (with the proliferation of setTimeout-based tests, drastically slowing down JS execution causes a surprising amount of tests to fail). So I started by using a modified version of Gregory Szorc's patch for development, but that immediately ran into several bugs with the JS debugging API. It also requires major contortions due to current limitations of the API. After complaining enough in #jsapi, I was pointed to some bytecode counter hooks enabled in the JS API. After finding and temporarily working my way around more bugs (I've been told that most of this stuff isn't necessary with compartment-per-global, but no one has been ripping this stuff out yet…), I produced a patch (see the previous linked bug) which hooks into xpconnect and automatically does code coverage for all JS run on the xpconnect runtime.

Since the pc counts just dumps out raw counts of hits, I need further processing to make it work; this comes in the form of post-processing script. There is also related issues that counts can't be generated for scripts it never sees, so there's also logic in my ever-growing code-coverage building script to compute all functions that get run. This is the most fragile part of the process right now, as many things can cause it to break. In addition, I ended up writing a partial replacement of lcov in python since I was getting extremely tired of its poor performance. This goes so far as actually reading the gcno and gcda files output by gcc itself, since gcov has problems reading one of them.

So the final result is posted now, and, like usual, I have a treemapified version as well. In that, I went ahead and implemented a feature which lets the URI specify the directory to start in, so you don't have to bear with several long script dialog warnings to view into a specific directory.

More code coverage treemap fun

Quite some time ago, I built a treemap visualization of code coverage. This had been inspired by the information visualization course I was taking then, and I have since found a less static version more useful for getting a feeling of code coverage than lcov's index pages. Back then, two years ago, the only decent toolkit I found for building treemaps was in Java, which would necessitate the use of applets if I tried to put it on the web. Since most of what I produced was just static images, using Java locally wasn't issue. Then again, I'm also a contributor to an organization that is pushing hard about being able to do things in pure JS. So I thought I might as well try to replicate it using a JS toolkit.

Two years has made quite a difference in terms of JS toolkits. Back in 2010, the only JS toolkit I found that had treemaps was protovis, which I found fairly unusable. Now, there are a few more higher-quality toolkits, but they've all had a fairly major problem of forcing you to shoehorn your data into a specific format instead of letting you customize how data is formatted. The JavaScript Infovis Toolkit is the one that both makes me happy in the results and annoyed to get it to work. Finally, I settled on D3, which is still too low-level to make me happy (don't ask me how painful getting animations working was).

The end result is here (the equivalent lcov result is also available). Every file is a rectangle in that represents a single file of source code. Hover over it, and you get details of coverage in that file. Click on it, and you can zoom in one level down in the source code hierarchy. If you ctrl-click, you zoom one level back. The size of the rectangles is indicative of how large the file is (you can configure it to be by number of lines or number of functions). Their color is indicative of what percentage of lines (or functions) is covered: a red color means very few are executed in tests, while a green color means most are. There is a scale that indicates what a given percentage of code looks like, and there is even internal code to affect the scale (so as to make the midpoint muddy red be not 50% but more like 70%), but this is disabled since the scale is more difficult to change. Changing parameters causes the treemap to animate to its new position, but the sheer number of elements here appears to give Gecko heartburn—it's much smoother if you zoom in to smaller directories! I do realize that this also results in a lot of boxes flying around each other, but a squarified treemap is unfortunately a very unstable algorithm.

The technology that underlies the visualization isn't complicated. I used lcov to produce a summary file, and then used a python script to convert the file into a summarized JSON version. Tooltips are provided using tipsy, which I don't particularly like all that much, but I haven't found anything that is as easy to use while also servicing my needs (most importantly, placing tooltips so they don't fall off the edge of the screen). As an aside, a visualization toolkit that doesn't have any tooltip support is extremely bad—details-on-demand is a critical component of information visualization.

There's some things I don't have: I don't scrape branch coverage yet, for example. And having a query parameter that lets you zoom into a specific directory immediately would be helpful, as would being able to cut off the tree before the leaves. I would also like to see a nice view of a single file, so I can completely kick the lcov output. Being able to select individual files would also be a boon. Another neat feature would be to select from multiple sources (e.g., different test suite coverages or even monitoring as code coverage changes through the years!).

As a postscript, let me give you a tour of top-level directories in the view. The tiny sliver in the uppper-left is, from top-to-bottom, the C++ source code of Thunderbird-specific code, lightning, and LDAP. Then there is a sizable block for mork, and the rest of the left column is mailnews code. Then there is a barely-noticeable column and another small but noticeable column which contain between them a lot of small top-level directories in mozilla. The next column (similar in width to the comm-central column) contains, from top-to-bottom, IPC, spellcheck, widget, security, accessibility, parser, and xpcom. The next column contains editor, toolkit, network, and DOM code. The right 40% of the diagram or so lacks definable columns. The upper-left of this portion (you'll notice it by the large red block with a lot of "S" letters peeking out) is the graphics code. To its right lies JS. Beneath both of these lies the code for layout, and beneath that and at the bottom is content.

And, no, my code is not on github yet because most of everything that matters is in that one HTML file. I also plan to integrate this view into DXR at some point in time.

A tour of libmime

Libmime (the C++ version, that is) exposes three services, not including the tight coupling it shares with compose code. These three services are a service that extracts headers (which many people probably didn't knew existed), a service that parses structured headers (in two place), and everything else. The first two are fairly straightforward to understand, so it's the complexity of the last one which is worth talking about here.

The first thing the astute programmer will notice is the apparent lack of a way to drive the MIME converter. This is because the converter also implements nsIStreamConverter, kind of. The MIME converter uses the fact that it derives from nsIStreamListener to receive data, and it uses the Init method to set itself up (while effectively ignoring half its parameters). There is logic to extract the URL to load from the context parameter, and this is where the fun and games begin. If no output type is manually selected, it is sniffed from the URL as follows:

1. No URL results in quoting
2. Finding part= (unless the desired type is application/vnd.mozilla.xul+xml) causes the output type to be raw, unless a type= parameter is present, which induces body display and uses that type for the stream's content-type.
3. Finding emitter=js causes a special case that is used to select gloda's JS MIME emitter
4. Finding header= does a lookup for the special mime output type; the values possible are filter, quotebody, print, only (uses header display), none (uses body display), quote, saveas, src, and raw, most of whose mappings to the appropriate value in nsMimeOutput are fairly obvious.
5. The default, if none of the above match, is to display the body.

With the URL and output format settled, the next step is choosing the emitter. The gloda hack obviously results in the gloda JS MIME emitter, and editor templates and drafts don't use an emitter, but, otherwise, either the raw emitter (attach, decrypt, raw, and source), XML emitter (header display), orHTML emitter (everybody else) is used.

After the emitter is initialized and constructed, a bridge stream is set up to actually pump the data into libmime. The output is handled differently in the case of things that get sent to compose and things that get driven by display. The former merely passes through some more options and then decomposes attachments into temporary files, while the latter is more interesting and complicated. The URL is parsed—again—and the following parts are noticed. If headers= is present and results in one of only, none, all, some, micro, cite, or citation, then opts->headers is set to signify the choice. The presence of part= sets opts->part_to_load, while rot13 enables rot13_p (did you know TB had that feature?). Some more hacks are enabled if emitter=js is present, and then the part number is checked to see if it might be an attempt to use the old (Netscape 2) MIME part numbering scheme. After all of this done, we set up charset overrides for the message if they need to be set up and then we can actually start pumping data.

The guts of libmime that actually do the parsing is in the polymorphic object system within libmime. The text initially gets fed to an instance of MimeMessage, but once the headers of one these containers gets parsed, the body is fed through another object. Going from a content-type to an object takes several lines of code, but a brief summary of the decisions should suffice. If no Content-Type header is present, the code chooses a default (ultimately, for children of MimeMessage, the default is untyped text, which sniffs for binhex, yEnc, and uuencode). Some parts can have their content-type sniffed by a found filename instead. The extension point that allows extensions to add their own content-type handlers gets run before anyone else, but, as far as I can tell, only enigmail leverages this. There is another subtle extension hook which allows people to convert any type to HTML (Lightning uses this). Some types are simple and always go to the same class: message/rfc822 and message/news both result in a MimeMessage, while message/external-body creates a MimeExternalBody. Other types get dispatched based mostly on preferences: text/html and multipart/* are the biggest cases here. Everything else (note that text/* defaults to text/plain and multipart/* to multipart/mixed) becomes a MimeExternalObject, and a paranoia preference can also turn half of the classes into a MimeExternalObject.

After creating classes, the next phase is the actual parsing logic. And this is where I'm stopping the tour for now. I've got more precise information laid out on virtual sticky notes on my laptop, but I don't think it's worth posting what amounts to several large switch statements in a brief tour.

A new JS MIME parser

Here is one of those guessing games whose answers will depress you: how many (partial) MIME parsers do we have in Thunderbird? If you guessed zero, ha-ha, very funny; now go sit in the corner. If you guessed one, you've obviously never worked with libmime before. The actual answer depends on how liberal you want to be. On the conservative end, there are no fewer than 5 parsers which go at least as far as parsing multipart messages (including two in a single file). If you choose to go liberally, counting everybody who attempts to split header values to look for a specific value, you gain an additional six that I am aware of… and I have no doubt that more are lurking behind them. I suppose this means that we now have more implementations of message header parsing than we do base64-decoding (although nowadays, it seems like most base64-decoding got stuffed into one method with several wrappers). The complete list as I know it:

• nsMsgBodyHandler
• nsMsgDBFolder::GetMsgTextFromStream
• libmime
• IMAP fakeserver (one for body parts, one for body structure, and another spot happily hunts down headers)
• NNTP fakeserver (hunts down headers)
• nsParseMailbox
• nsNNTPProtocol (hunts down headers)
• nsMsgLocalStoreUtils (hunts down headers)
• Somewhere in compose code. Probably, although it's not clear how much is being funneled back to libmime and how much is not.
• A class in necko implements in the RFC 2231 and RFC 2047 decoding.

Well, it's time for to increment that count by one. And then decrement it by at least three (two of the IMAP and the NNTP fakeserver decoders are switched over in a queued patch, and I hope to get the third IMAP fakeserver switched over). I've been working on a new JS MIME parser to Thunderbird for a bit at a time over the past several months, and I have local patches that start using it in tests (and as a replacement for nsIMimeHeaders).

So, why replace libmime instead of consolidating everyone onto it? The short answer is because libmime is a festering pile of crap. It uses an internal object system that can be summarized as "reimplementing C++ in C" and has existed largely in the same form since at least Netscape 5 (the Mozilla classic version gives a date of May 15, 1996). All of that would be manageable were it not for the fact that the architecture is clearly broken. Reliably parsing multipart MIME messages is not a trivial task (indeed, not only does one of the above do it incorrectly, but there is actually a test which may rely on it being wrong. Guess which one it is.), and the fact that so many people have variants on it is a clear indication that the API fails to expose what it ought to expose. This means that the code is in need of change, and the implementation is a form which makes changing things extremely difficult.

The choice to do it in JS was motivated mostly by Andrew Sutherland. There has been a long-term ideal in Thunderbird dating back to at least around 2008 to move more implementation of core code into JS, which would help avoid massive churn spurred on by mozilla-central; nowadays, there is the added benefit that it would aid in efforts like B2G or porting to platforms where native code is frowned upon. MIME code, being extremely self-contained (charset conversion and S/MIME encryption make up the biggest dependencies in the core parser). As of my current implementation, the only external dependency that the MIME parser has is atob, although charset conversion (via the proposed string encoding API) will be added when I get there. In other words, this code is usable by anyone who wants to write a mail client in JS, not just the internal mailnews code.

Another advantage to writing my own library is that it gives me a lot of time to lay out specifications in clearer terms. One called out in the spec are on how to handle seeing boundaries for the non-leaf-most part. My own brain went further and started musing on non-leaf parts getting QP or base64 content-transfer-encodings (which RFC 6532 went ahead and allowed anyways), or multiple nested parts having the same boundary (which the specification, in my view, hints at resolving in a particular fashion). Other developments include the fact that most MIME parsers do not permit as much CFWS as the specification indicates could be present ("Content-Type: multipart / mixed" would be unusable in every MIME parser source I read)I also produced a list of all the specifications that the parser will need to refer to that I have found so far (13 RFCs and a handful of non-RFCs, plus 9 obsoleted RFCs that may still warrant viewing). As for size? The JS implementation is about 600-700 lines right now (including over 300 lines of comments), while the equivalent C++ portions take over a thousand lines to do more or less the same thing.

As I said earlier, one of the problems with libmime is its oppressive architecture. It is primarily designed to drive the message pane while living as a streaming converter. However, it encompasses roughly three steps: the production of the raw MIME tree (or a tree that combine the raw MIME data with pseudo-MIME alternatives), conversion of that tree into a more traditional body-and-attachments view, and then the final driving of the UI. Getting it to stop any earlier is between difficult and impossible; what I have now, once it gets some more testing, can satisfy people who just need the first step. Doing the second step would require me to sit down and document how libmime makes its choices, which is not a task I look forward to doing.