It's Always DNS

Jun 23, 2024

The meme is real, but I think this particular case is sort of interesting, because it turned out, ultimately, to not be due to DNS configuration, but an honest-to-goodness bug in glibc.

As previously mentioned, I heavily rely on email-oauth2-proxy for my work email. Every now and then, I’d see a failure like this:

Email OAuth 2.0 Proxy: Caught network error in IMAP server at [::]:1993 (unsecured) proxying (SSL/TLS) - is there a network connection? Error type <class 'socket.gaierror'> with message: [Errno -2] Name or service not known

This always coincided with a change in my network, but - and this is the issue - the app never recovered. Even though other processes - even Python ones - could happily resolve - this long-running daemon remained stuck, until it was restarted.

A bug in the proxy?

My first suspect here was this bit of code:

1761     def create_socket(self, socket_family=socket.AF_UNSPEC, socket_type=socket.SOCK_STREAM):
1762         # connect to whichever resolved IPv4 or IPv6 address is returned first by the system
1763         for a in socket.getaddrinfo(self.server_address[0], self.server_address[1], socket_family, socket.SOCK_STREAM):
1764             super().create_socket(a[0], socket.SOCK_STREAM)
1765             return

We’re looping across the gai results, but returning after the first one, and there’s no attempt to account for the first address result being unreachable, but later ones being fine.

Makes no sense, right? My guess was that somehow getaddrinfo() was returning IPv6 results first in this list, as at the time, the IPv6 configuration on the host was a little wonky. Perhaps I needed to tweak gai.conf ?

However, while this was a proxy bug, it was not the cause of my issue.

DNS caching?

Perhaps, then, this is a local DNS cache issue? Other processes work OK, even Python test programs, so it didn’t seem likely to be the system-level resolver caching stale results. Python itself doesn’t seem to cache results.

This case triggered (sometimes) when my VPN connection died. The openconnect vpnc script had correctly updated /etc/resolv.conf back to the original configuration, and as there’s no caching in the way, then the overall system state looked correct. But somehow, this process still had wonky DNS?

A live reproduction

I was not going to get any further until I had a live reproduction and the spare time to investigate it before restarting the proxy.

The running proxy in this state could be triggered easily by waking up fetchmail, which made it much easier to investigate what was happening each time.

So what was the proxy doing on line :1763 above? Here’s an strace snippet:

[pid  1552] setsockopt(7, SOL_IP, IP_RECVERR, [1], 4) = 0
[pid  1552] connect(7, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("ELIDED")}, 16) = 0
[pid  1552] poll([{fd=7, events=POLLOUT}], 1, 0) = 1 ([{fd=7, revents=POLLOUT}])
[pid  1552] sendto(7, "\250\227\1 \0\1\0\0\0\0\0\1\7outlook\toffice365\3c"..., 50, MSG_NOSIGNAL, NULL, 0) = 50
[pid  1552] poll([{fd=7, events=POLLIN}], 1, 5000) = 1 ([{fd=7, revents=POLLERR}])
[pid  1552] close(7)                    = 0

As we might expect, we’re opening a socket, connecting over UDP to port 53, and sending out a request to the DNS server.

This indicated the proximal issue: the DNS server IP address was wrong - the DNS servers used were the ones originally set up by openconnect still. The process wasn’t incorrectly caching DNS results but the DNS servers. Forever.

Nameserver configuration itself is not something that applications typically control, so the next question was - how does this work normally? When I update /etc/resolv.conf, or the thousand other ways to configure name resolution in modern Linux systems, what makes getaddrinfo() continue to work, normally?

/etc/resolv.conf and glibc

So, how does glibc account for changes in resolver configuration?

The contents of the /etc/resolv.conf file are the canonical location for DNS server addresses for processes (like Python ones) using the standard glibc resolver. Logically then, there must be a way for updates to the file to affect running processes.

In glibc, such configuration is represented by struct resolv_context. This is lazily initialized via __resolv_context_get()->maybe_init(), which looks like this:

 68 /* Initialize *RESP if RES_INIT is not yet set in RESP->options, or if
 69    res_init in some other thread requested re-initializing.  */
 70 static __attribute__ ((warn_unused_result)) bool
 71 maybe_init (struct resolv_context *ctx, bool preinit)
 72 {
 73   struct __res_state *resp = ctx->resp;
 74   if (resp->options & RES_INIT)
 75     {
 76       if (resp->options & RES_NORELOAD)
 77         /* Configuration reloading was explicitly disabled.  */
 78         return true;
 80       /* If there is no associated resolv_conf object despite the
 81          initialization, something modified *ctx->resp.  Do not
 82          override those changes.  */
 83       if (ctx->conf != NULL && replicated_configuration_matches (ctx))
 84         {
 85           struct resolv_conf *current = __resolv_conf_get_current ();
 86           if (current == NULL)
 87             return false;
 89           /* Check if the configuration changed.  */
 90           if (current != ctx->conf)

Let’s take a look at __resolv_conf_get_current():

123 struct resolv_conf *
124 __resolv_conf_get_current (void)
125 {
126   struct file_change_detection initial;
127   if (!__file_change_detection_for_path (&initial, _PATH_RESCONF))
128     return NULL;
130   struct resolv_conf_global *global_copy = get_locked_global ();
131   if (global_copy == NULL)
132     return NULL;
133   struct resolv_conf *conf;
134   if (global_copy->conf_current != NULL
135       && __file_is_unchanged (&initial, &global_copy->file_resolve_conf))

This is the file change detection code we’re looking for: _PATH_RESCONF is /etc/resolv.conf, and __file_is_unchanged() compares the cached values of things like the file mtime and so on against the one on disk.

If it has in fact changed, then maybe_init() is supposed to go down the “reload configuration” path.

Now, in my case, this wasn’t happening. And the reason for this is line 83 above: the replicated_configuration_matches() call.

Resolution options

We already briefly mentioned gai.conf. There is also, as the resolver.3 man page says, this interface:

The resolver routines use configuration and state information
contained in a __res_state structure (either passed as the statep
argument, or in the global variable _res, in the case of the
older nonreentrant functions).  The only field of this structure
that is normally manipulated by the user is the options field.

So an application can dynamically alter options too, outside of whatever static configuration there is. And (I think) that’s why we have the replicated_configuration_matches() check:

static bool
replicated_configuration_matches (const struct resolv_context *ctx)
  return ctx->resp->options == ctx->conf->options
    && ctx->resp->retrans == ctx->conf->retrans
    && ctx->resp->retry == ctx->conf->retry
    && ctx->resp->ndots == ctx->conf->ndots;

The idea being, if the application has explicitly diverged its options, it doesn’t want them to be reverted just because the static configuration changed. Our Python application isn’t changing anything here, so this should still work as expected.

In fact, though, we find that it’s returning false: the dynamic configuration has somehow acquired the extra options RES_SNGLKUP and RES_SNGLKUPREOP. We’re now very close to the source of the problem!

A hack that bites

So what could possibly set these flags? Turns out the send_dg() function does:

 999                   {
1000                     /* There are quite a few broken name servers out
1001                        there which don't handle two outstanding
1002                        requests from the same source.  There are also
1003                        broken firewall settings.  If we time out after
1004                        having received one answer switch to the mode
1005                        where we send the second request only once we
1006                        have received the first answer.  */
1007                     if (!single_request)
1008                       {
1009                         statp->options |= RES_SNGLKUP;
1010                         single_request = true;
1011                         *gotsomewhere = save_gotsomewhere;
1012                         goto retry;
1013                       }
1014                     else if (!single_request_reopen)
1015                       {
1016                         statp->options |= RES_SNGLKUPREOP;
1017                         single_request_reopen = true;
1018                         *gotsomewhere = save_gotsomewhere;
1019                         __res_iclose (statp, false);
1020                         goto retry_reopen;
1021                       }

Now, I don’t believe the relevant nameservers have such a bug. Rather, what seems to be happening is that when the VPN connection drops, making the servers inaccessible, we hit this path. And these flags are treated by maybe_init() as if the client application set them, and has thus diverged from the static configuration. As the application itself has no control over these options being set like this, this seemd like a real glibc bug.

The fix

I originally reported this to the list back in March; I was not confident in my analysis but the maintainers confirmed the issue. More recently, they fixed it. The actual fix was pretty simple: apply the workaround flags to statp->_flags instead, so they don’t affect the logic in maybe_init(). Thanks DJ Delorie!

Scroll wheel behaviour in vim with gnome-terminal

Nov 4, 2023

I intentionally have mouse support disabled in vim, as I find not being able to select text the same way as in any other terminal screen unergonomic.

However, this has an annoying problem as a libvte / gnome-terminal user: the terminal, on switching to an “alternate screen” application like vim that has mouse support disabled, “helpfully” maps scroll wheel events to arrow up/down events.

This is possibly fine, except I use the scroll wheel click as middle-button paste, and I’m constantly accidentally pasting something in the wrong place as a result.

This is unfixable from within vim, since it only sees normal arrow key presses (not ScrollWheelUp and so on).

However, you can turn this off in libvte, by the magic escape sequence:

echo -ne '\e[?1007l'

Also known as XTERM_ALTBUF_SCROLL. This is mentioned in passing in this ticket. Documentation in general is - at best - sparse, but you can always go to the source.

A Headless Office 365 Proxy

Nov 9, 2022

As I mentioned in my last post, I’ve been experimenting with replacing davmail with Simon Robinson’s super-cool email-oauth2-proxy, and hooking fetchmail and mutt up to it. As before, here’s a specific rundown of how I configured O365 access using this.


We need some small tweaks to the shipped configuration file. It’s used for both permanent configuration and acquired tokens, but the static part looks something like this:

[[email protected]]
permission_url =
token_url =
oauth2_scope = offline_access
redirect_uri =
client_id = facd6cff-a294-4415-b59f-c5b01937d7bd
client_secret =

We’re re-using davmail’s client_id again.

Updated 2023-10-10: emailproxy now supports a proper headless mode, as discussed below.

Updated 2022-11-22: you also want to set delete_account_token_on_password_error to False: otherwise, a typo will delete the tokens, and you’ll need to re-authenticate from scratch.

We’ll configure fetchmail as follows:

poll localhost protocol IMAP port 1993
 auth password username "[email protected]"
 is localuser here
 sslmode none
 mda "/usr/bin/procmail -d %T"
 folders INBOX

and mutt like this:

set smtp_url = "smtp://[email protected]@localhost:1587/"
unset smtp_pass
set ssl_starttls=no
set ssl_force_tls=no

When you first connect, you will get a GUI pop-up and you need to interact with the tray menu to follow the authorization flow. After that, the proxy will refresh tokens as necessary.

Running in systemd

Here’s my service file I use, slightly modified from the upstream’s README:

$ cat /etc/systemd/system/emailproxy.service
Description=Email OAuth 2.0 Proxy

ExecStart=/usr/bin/python3 /home/localuser/src/email-oauth2-proxy/ --external-auth --no-gui --config-file /home/localuser/src/email-oauth2-proxy/my.config


Headless operation

Typically, only initial authorizations require the GUI, so you could easily do the initial dance then use the above systemd service.

Even better, with current versions of email-oauth2-proxy, you can operate in an entirely headless manner! With the above --external-auth and --no-gui options, the proxy will prompt on stdin with a URL you can copy into your browser; pasting the response URL back in will authorize the proxy, and store the necessary access and refresh tokens in the config file you specify.

For example:

$ sudo systemctl stop emailproxy

$ python3 ./ --external-auth --no-gui --config-file /home/localuser/src/email-oauth2-proxy/my.config

# Now connect from mutt or fetchmail.

Authorisation request received for [email protected] (external auth mode)
Email OAuth 2.0 Proxy No-GUI external auth mode: please authorise a request for account [email protected]
Please visit the following URL to authenticate account [email protected]:

Copy+paste or press [↵ Return] to visit the following URL and authenticate account [email protected]:
then paste here the full post-authentication URL from the browser's address bar (it should start with

# Paste the updated URL bar contents from your browser in response:

SMTP (localhost:1587; [email protected]) [ Successfully authenticated SMTP connection - releasing session ]
$ sudo systemctl start emailproxy

Obviously, you’ll need to do this interactively from the terminal, then restart in daemon mode.

Fetchmail and Office 365

Nov 6, 2022

I previously described accessing Office365 email (and in particular its oauth2 flow) via davmail, allowing me to continue using fetchmail, procmail and mutt. As davmail is java, it’s a pain to have around, so I thought I’d give some details on how to do this more directly in fetchmail, as all the available docs I found were a little vague, and it’s quite easy to screw up.

As it happens, I came across a generally better solution shortly after writing this post, on which more later.

Fetchmail 7

Unfortunately there is little interest in releasing a Fetchmail version with oauth2 support - the maintainer is taking a political stance on not integrating it - so you’ll need to check out the next branch from git:

cd ~/src/
git clone -b next [email protected]:fetchmail/fetchmail.git fetchmail-next
cd fetchmail-next
./ && ./configure --prefix=/opt/fetchmail7 && make && sudo make install

I used the branch as of 43c18a54 Merge branch 'legacy_6x' into next. Given that the maintainer warns us they might remove oauth2 support, you might need this exact hash…

Generate a token

We need to go through the usual flow for getting an initial token. There’s a helper script for this, but first we need a config file:

[email protected]
scope= offline_access

Replace [email protected] and localuser in the above, and put it at ~/.fetchmail.oauth2.cfg. It’s rare to find somebody mention this, but O365 does not need a client_secret, and we’re just going to borrow davmail’s client_id - it’s not a secret in any way, and trying to get your own is a royal pain. Also, if you see a reference to tenant_id anywhere, ignore it - common is what we need here.

Run the flow:

$ # This doesn't get installed...
$ chmod +x ~/src/fetchmail-next/contrib/
$ # Sigh.
$ sed -i 's+/usr/bin/python+/usr/bin/python3+' ~/src/fetchmail-next/contrib/
$ ~/src/fetchmail-next/contrib/ -c ~/.fetchmail.oauth2.cfg --obtain_refresh_token_file
To authorize token, visit this url and follow the directions:
Enter verification code:

Unlike davmail, this needs just the code, not the full returned URL, so you’ll need to be careful to dig out just the code from the response URL (watch out for any session_state parameter at the end!).

This will give you an access token that will last for around an hour.

Fetchmail configuration

Now we need an oauthbearer .fetchmailrc like this:

set daemon 60
set no bouncemail
poll protocol IMAP port 993
 auth oauthbearer username "[email protected]"
 passwordfile "/home/localuser/.fetchmail-token"
 is localuser here
 sslmode wrapped sslcertck
 folders INBOX
 mda "/usr/bin/procmail -d %T"

Replace [email protected] and localuser.

At this point, hopefully starting /opt/fetchmail7/bin/fetchmail will work!

Refresh tokens

As per the OAUTH2 README, fetchmail itself does not take care of refreshing the token, so you need something like this in your crontab:

*/2 * * * * $HOME/src/fetchmail-next/contrib/ -c $HOME/.fetchmail.oauth2.cfg --auto_refresh

Relative __FILE__ With CMake

Jun 16, 2022

I have the misfortune of maintaining some things using CMake. One major annoyance is that __FILE__ is an absolute path, and that can’t be changed in CMake itself. Like most CMake annoyances, you can find a discussion online from about 15 years ago, but no sign of an actual fix.

Instead, you need a hack: this - I think - is the simplest one.

First, in our top-level CMakeLists.txt, we’ll define this helper function:

function(add_srcs NAME)
  foreach(f IN LISTS ARGN)
    set_source_files_properties(${f} PROPERTIES COMPILE_DEFINITIONS

This will take each of the arguments, convert each file into a path relative to the top-level directory, then re-define __FILE__ on a per-source-file basis. We also set() a variable for our parent scope to use.

We’ll also need -Wno-builtin-macro-redefined.

Then, in each child CMakeLists.txt, we will do something like:

add_srcs(MYCODE_SRCS mycode.c mycode.h)
add_library(mycode ${MYCODE_SRCS})

add_srcs(CODE2_SRCS code2.c code2.h)
add_library(code2 ${CODE2_SRCS})

How I Code Review

Feb 5, 2022

I thought it might be interesting, at least to myself, to write up how I approach code reviews. My history in tech is one where close code review was emphasized and well-respected: most appreciated that a detailed review was not only worth the reviewer’s time, but mutually beneficial, and often a great learning experience for everyone. So my tiny mind can’t process ideas like post-commit reviews, that seem to be predicated on the idea that testing is some kind of replacement for code review. To me, the kind of things that tests can cover is only a very small part of what code review is useful for.

I’ve code-reviewed entire projects totalling many thousands of lines, and single-character changes, but I’m usually following at least some of the below patterns either way:

Understand the context

First thing I read is the commit message. Of course, this should be in normal form, but I’m really making sure I can understand what the change is, based solely upon the commit message. Without looking at the code, can I, as a casual observer, understand what’s changed? Is the commit title specific enough? Does the commit message’s contents describe not just what changed, but why (as usual, the GNU standard is an exemplar of what not to do here)? Is it clear? Does the message needlessly have things that belong on another tracking system (target gate)?

I will read any associated ticket for its context - especially keeping an eye out for anything that doesn’t seem to fit with the PR’s changes. This could be a missing case, or a fundamental mis-understanding of what the real underlying problem is. If there is any design doc mentioned (and they should be mentioned!) I’ll also read that and diff its contents against what actually got implemented.

I’m looking mainly for disparities between what everyone agreed we should do, and what is actually happening in the code, but especially for missing things; it’s very easy to accidentally skip somebody’s drive-by comment, but that could turn out to be crucial to the implementation.

I also consider if this change makes sense on its own, and if it could be split up: this is often a matter of appetite (and personally I find the Linux kernel approach often goes a little too far), but patch series with one logical change per commit is often much easier to review. It should hopefully go without saying that each individual commit in the series should still pass CI, but unfortunately that’s painful to do with at least github PRs.

Get an overview

Next I start looking at the actual code changes: often with one tab per file, I’m trying to understand how the changes fit together: who calls what, what new intra-code dependencies there are, what the possible impact of the changes could be.

I might well look back in git history for each of these files to understand why the old code is like it is: this is also often very useful in identifying potential issues with the new changes.

Depending on the change, I will often checkout a local copy, and use git grep, ctags, etc. to help me understand how everything fits together.

My focus at this level is often on interfaces: does a new method have a suitable name? Is it at the right level of abstraction? What is the ownership of the relevant objects? Are there any layering violations?

Are there any external dependencies we need to worry about? Equally if anyone is depending on us, are we providing well-written interfaces? Are they designed with care and attention to versioning, information hiding, and all the usual API concerns? Is this going to wear well after it’s been in production for years?

I’m also bearing in mind other ongoing work: if there’s a project underway that is directly relevant to this specific change, I might ask for some accommodation that will make the eventual merge of both easier. Equally if there’s a general desire to take a particular technical direction, I might complain if something is taking a different tack.

It’s a rare code review where I don’t have to pause to go research something: systemd service definition semantics, syscall error modes, how selinux roles work etc. As I said above, great learning experience!

Are there potential performance concerns with the change: lots of unnecessary I/O, potential for big-O issues, needless overhead etc? What are the expected limits of the objects being handled?

What if there are bugs with this change: is the error handling suitable? Is there a sufficient level of logging, exception details, etc. to identify in the field what went wrong? Is there unnecessary noise? How would a stressed-out SRE deal with this in production?

Have any necessary unit/component tests been updated or added? Do they actually test something useful?

I almost never build or test changes I’m reviewing: that’s a job for the submitter and your CI infrastructure. The only exception is if I’m struggling to understand something, and running the tests would help.

Detailed review

I’m now going to go line-by-line through the whole patch, leaving comments where necessary. Sometimes I’ll reach something I don’t understand, add leave a “FIXME” for myself: if, after reading the whole change, I still don’t understand what’s happening, this will often re-formulate itself into a question for the submitter, along with a request for expanded code comments, but usually I can just delete these later.

If I find major - architectural level - issues with what I’m looking at, that’s often a good prompt to take the discussion elsewhere, perhaps to a Zoom call or design document discussion. Doing design review inside a PR is not fun for anyone.

I’ve noticed a tendency to “review the diffs”: the idea that only the changed lines are relevant to the review - that tools expand 10 lines at a time is a symptom of this. This is very wrong-headed in my opinion, and I often find myself in the rest of the code to make sure I can properly review what has changed.

Comb for nits

Everyone has a different appetite for code review nits: generally, I will always point out actual typos (often just once, if it’s repeated, expecting the submitter to apply my comment to all instances). If I have a substantive comment, I might also suggest some style-level improvements. I never expect someone to make these sort of changes for existing code: the general idea is to leave the code in a slightly better place than it was, not re-write whole files for cosmetic nits.

Often these stylistic nits are marked “optional”: if the submitter feels like it, they could change it, but it’s no big deal if not.

I’ll very often have style comments on things like:

  • unnecessary comments that just describe the code
  • missing comments
  • variable, function naming
  • function size and decomposition
  • local customs

Many of these things can be a matter of opinion, so I try to bear in mind other ways of thinking, up to a point. I’m never going to be happy seeing a ton of CamelCase and Hungarian notation in code that doesn’t have it already.


I haven’t yet found a code review tool that’s ideal at iteration: gerrit is still pretty hopeless at tracking outstanding comment state. PRs in github are better at displaying this, but have the fatal flaw that any history rewrite means all context is lost.

Regardless, when I get a new version of the changes, I’ll often review both the incremental diff and the whole change, checking that:

  • my review comments have been acted upon and the fixes look good
  • the change as a whole still makes sense:
    • is the commit message still correct?
    • are there now unnecessary changes, due to follow-on fixes?
    • do the updates go far enough?

Long review cycles for a PR can be grueling, both for reviewers and the PR owner. But in my opinion it’s almost always worth the effort, especially for complex changes: this code is probably going to live a lot longer than you’d think, and be maintained by people other than you.

Even worse, it’s often work not given the respect it’s due: PR owners can see it as a combative process, and management can see it as overhead. I don’t really know what to do about that.

Pay it forward!

Testing a C Library With Python

Dec 22, 2021

It’s still common for a systems library to be written in the default lingua franca, C, although Rust is encroaching, for good reasons.

However, when it comes to testing, things get tedious quickly: writing unit or component tests in C is a slow, bug-prone exercise. With libvfio-user, after fixing too many bugs that were due to the test rather than the test subject, I decided it would be worth looking at alternative approaches. The aim was to reduce the time it takes to develop unit/component tests.

Up until this point, we’d been using ctest, along with cmocka when we needed to mock out certain functions (such as socket handling). Leaving aside my strong feelings on these tools, this was rather unsatisfactory: libvfio-user effectively implements a (UNIX) socket server, but we weren’t actually testing round-trip interactions for the most part. In terms of code coverage, very little useful could be done via this unit testing approach, but the “sample” client/server was tedious to work with for testing purposes.

Python-based testing

After a quick proof of concept, it became clear that using Python would be a great choice to cover most of our testing needs. libvfio-user doesn’t ship with any client bindings, and, given that the main clients are qemu, cloud-hypervisor and SPDK, Python bindings would be of dubious utility.

As a result, we decided against “proper” Python bindings, auto-generated or otherwise, in favour of a small and simple approach. In particular, by using the terrible magic of ctypes, we could easily set up both client and server test cases that fully represent how the library works in real life.

So, instead of auto-generated bindings, we write - by hand - simple, thin, layers of type wrappers:

class vfio_irq_info(Structure):
    _pack_ = 1
    _fields_ = [
        ("argsz", c.c_uint32),
        ("flags", c.c_uint32),
        ("index", c.c_uint32),
        ("count", c.c_uint32),

small harness routines for socket handling

def connect_client(ctx):
    sock = connect_sock()

    json = b'{ "capabilities": { "max_msg_fds": 8 } }'
    # struct vfio_user_version
    payload = struct.pack("HH%dsc" % len(json), LIBVFIO_USER_MAJOR,
                          LIBVFIO_USER_MINOR, json, b'\0')
    hdr = vfio_user_header(VFIO_USER_VERSION, size=len(payload))
    sock.send(hdr + payload)

… interacting with the library on the server side

def get_pci_header(ctx):
    ptr = lib.vfu_pci_get_config_space(ctx)
    return c.cast(ptr, c.POINTER(vfu_pci_hdr_t)).contents

… and so on. Writing this by hand might seem immensely tedious, but in practice, as it’s pretty much all boilerplate, it’s very quick to write and modify, and easily understandable; something that can rarely be said for any kind of auto-generated code.

Client/server interactions

Another observation was that, for the purposes of these tests, we really didn’t need a client process and a server process: in fact, we don’t even need more than one thread of execution. If we make each test round-robin between acting as the client, then acting as the server, it becomes trivial to follow the control flow, and understanding logs, debugging, etc. is much easier. This is illustrated by the msg() helper:

def msg(ctx, sock, cmd, payload=bytearray(), expect_reply_errno=0, fds=None,
        rsp=True, expect_run_ctx_errno=None):
    Round trip a request and reply to the server. vfu_run_ctx will be
    called once for the server to process the incoming message,
    @expect_run_ctx_errrno checks the return value of vfu_run_ctx. If a
    response is not expected then @rsp must be set to False, otherwise this
    function will block indefinitely.
    # FIXME if expect_run_ctx_errno == errno.EBUSY then shouldn't it implied
    # that rsp == False?
    hdr = vfio_user_header(cmd, size=len(payload))

    if fds:
        sock.sendmsg([hdr + payload], [(socket.SOL_SOCKET, socket.SCM_RIGHTS,
                                        struct.pack("I" * len(fds), *fds))])
        sock.send(hdr + payload)

    ret = vfu_run_ctx(ctx, expect_errno=expect_run_ctx_errno)
    if expect_run_ctx_errno is None:
        assert ret >= 0, os.strerror(c.get_errno())

    if not rsp:

    return get_reply(sock, expect=expect_reply_errno)

We are operating as the client when we do the sendmsg(); the server then processes that message via vfu_run_ctx(), before we “become” the client again and receive the response via get_reply().

We can then implement an individual test like this:

def test_dma_region_too_big():
    global ctx, sock

    payload = vfio_user_dma_map(argsz=len(vfio_user_dma_map()),
        flags=(VFIO_USER_F_DMA_REGION_READ |
        offset=0, addr=0x10000, size=MAX_DMA_SIZE + 4096)

    msg(ctx, sock, VFIO_USER_DMA_MAP, payload, expect_reply_errno=errno.ENOSPC)

which we can run via make pytest:

___________________________ test_dma_region_too_big ____________________________
----------------------------- Captured stdout call -----------------------------
DEBUG: quiescing device
DEBUG: device quiesced immediately
DEBUG: adding DMA region [0x10000, 0x80000011000) offset=0 flags=0x3
ERROR: DMA region size 8796093026304 > max 8796093022208
ERROR: failed to add DMA region [0x10000, 0x80000011000) offset=0 flags=0x3: No space left on device
ERROR: msg0x62: cmd 2 failed: No space left on device

This is many times easier to write and test than trying to do this in C, whether as a client/server, or attempting to use mocking. And we can be reasonably confident that the test is meaningful, as we are really executing all of the library’s message handling.

Debugging/testing tools

With a little bit of tweaking, we can also use standard C-based tools like valgrind and gcov. Code coverage is simple: after defeating the mini-boss of cmake, we can run make gcov and get code-coverage results for all C code invoked via the Python tests - it just works!

Running Python tests with valgrind was a little harder: for leak detection, we need to make sure the tests clean up after themselves explicitly. But Python itself also has a lot of valgrind noise. Eventually we found that this valgrind invocation worked well:

	valgrind \
	--suppressions=$(CURDIR)/test/py/valgrind.supp \
	--quiet \
	--track-origins=yes \
	--errors-for-leak-kinds=definite \
	--show-leak-kinds=definite \
	--leak-check=full \
	--error-exitcode=1 \

We need to force Python to use the system allocator, and add a number of suppressions for internal Python valgrind complaints - I was unable to find a working standard suppression file for Python, so had to construct this myself based upon the Python versions in our CI infrastructure.

Unfortunately, at least on our test systems, ASAN was completely incompatible, so we couldn’t directly run that for the Python tests.


The approach I’ve described here has worked really well for us: it no longer feels immensely tedious to add tests along with library changes, which can only help improve overall code quality. They are quick to run and modify, and for the most part easy to understand what the tests are actually doing.

There’s been a few occasions where ctypes has been difficult to work with - for me the documentation is particularly sparse, and callbacks from the C library into Python are distinctly non-obvious - but we’ve so far always managed to battle through, and twist it to our needs.

Doing things this way has a few other drawbacks: it’s not clear, for example, how we might test intermittent allocation failures, or other failure injection scenarios. It’s also not really suitable for any kind of performance or scalability testing.

I’m curious if others have taken a similar approach, and what their experiences might be.

Github Pull Requests

Feb 6, 2021

Stefan Hajnoczi recently posted about clean commit history.

It’s a controversial viewpoint that not everyone agrees with - there is a sizable population in favour of “never rewrite history”. For me, though, the points he makes there are totally correct: each commit should be a logical change, main (neé master) should stay green, and CI should pass at every single point in main’s history. More than just CI though: regardless of whether it passes CI, the main branch should be of good quality at all times, if you want to avoid the Quality Death Spiral.

Unfortunately, Github pull requests make this model a little difficult for a few reasons:

You can’t ever rebase a PR undergoing review

It’s important that a non-draft PR is never rebased, or re-written in any way. Why? Well, aside from making it difficult for a reviewer to see what’s changed since last looking, if you rebase, the commits previously on the PR disappear off into reflog hyperspace.

The View Changes button on review comments is attached to that particular commit hash, which is no longer in the history for that branch, and you get the dreaded:

We went looking everywhere, but couldn’t find those commits.

Note that if your PR is still a draft, you’re fine to edit the history whichever way you like: in fact, it’s often useful for review purposes to have multiple commits even at the start of a PR review before you move it from draft. Up to you.

The only other safe time to rebase is on final approach. At that point, presuming you are keeping to the “single main commit per PR” approach (see below), you’ll be wanting to squash the entire branch history into a single commit to main. For this, I usually use prr: it’s handy for picking up Reviewed-by automatically, and merging commit comments together for final editing.

Github CI only runs on branch tips

You probably don’t want to have a PR where you’re going to merge more than one commit into main. This is because CI only runs on the top-level commit: if an ancestor commit breaks the build, you’ll never know. Stefan mentions using git rebase --exec for checking commits in a stack, which indeed works great, but unless you’re running exactly the same CI that’s running under Github Actions, you can’t rely on it.

If that’s the case, what if you have one or more changes that depend on another? This is where “stacked PRs” come in, and they’re a bit of a pain…

Stacked PRs are cumbersome

Gerrit has a really useful model for reviewing stacks of changes: instead of the full history, each “patchset” corresponds to the single logical change Stefan talks about above. Every time you push to Gerrit, you’re supposed to have collapsed and rebased additional changes into single commits corresponding to each Gerrit CR. The model has some disadvantages as well (in particular, it’s a bit of a pain to keep a full history locally), but the Gerrit review UI doesn’t suffer from the rebasing issues Github does1.

Presuming - as there is no CI available - gerrithub is a non-starter, the only option available on Github is to use multiple PRs. This is better than it used to be, but is still a little painful.

Essentially, a stacked PR is one that’s opened not against the main branch, but against another PR branch. Say we have changes A and B, where B is dependent on A. You’d create a local branch with A, then push it to Github and open a PR. You’d have another local branch with A and B, then push that branch to Github and open a separate PR.

Now we need to make the B PR be based against the A PR. You can do this via the web UI by clicking Edit, though there is annoying bug here: it doesn’t reset the title and description. You can use gh pr create --base ... to avoid this problem.

Now, in the second PR, you’ll just see the commit for B. Each PR can be reviewed separately, and each PR gets its own CI run.

You also might want to merge additional changes up the stack. Let’s say that you have commit A2 on the A PR, that you want in PR B and C: the best - if rather tedious - way to do this, is to merge A into B, then B into C. That’s a lot of merge commits, but remember we’re squashing a PR every time before merging a PR to main.

You’ll see on the web recommendations to “merge downwards”: you wait for commit approval for the whole stack, then merge the top PR (B) into the PR underneath it (A), and so on, until you merge to main.

I don’t think that’s necessary these days2. Instead, when you have approval for the base PR - and logically, it will make sense that is reviewed first - you can merge it to main. Github will then offer to delete the PR branch. If you do this, the stacked PR gets automatically reset such that its merge base is now main !

There is an annoying thing here though: because of that squash during the merge to main, git, and Github, needs you to merge main back into the commit history of the PR that just changed bases. If you already merged the parent PR, you can always do git merge -Xours master to fix this, since there shouldn’t be any actual diff difference between the PR branch diffs as a whole, and what was merged to master. Or, if you didn’t merge in the parent PR, you’ll need a normal git merge master.

Another bug (as far as I’m concerned) is that if you ask for review on a stacked PR, it doesn’t get tagged with “Review required”, since, technically, you could merge the PR into its parent without approval. And there is no “Review requested” tag.

I would love all this to have some tooling: something that lets me do everything on my local stacked branches, automate merges up, keep track of dependencies, and updating the branches in Github. But I haven’t been able to find anything that can do it.

[2022-05-12 update]: I just came across spr which is so far proving excellent in solving some of these problems. I love it!

  1. Gerrit uses Change-ID embedded in the commit message to map commits onto CRs. It’s clumsy but effective. ↩︎

  2. I think it dates from before Github automatically reset a PR when its merge base was deleted ↩︎

Mutt and Office365

Nov 6, 2020

For reasons, I now need to interact with Office365 mail and calendar. It should go without saying that the Outlook webapp is almost painfully unusable (there really is no key binding for “next unread email”). Thus began the quest to get mutt interacting with the O365 server. This was a rather painful process: so much of the information on the web refers to earlier authentication schemes, Microsoft-special protocols, things that don’t support 2FA, dead Linux software, useless Linux software, etc.

After many false starts, I eventually found a working solution that allows mutt usage (and my treasured key binding for “mark current thread as read and move to the next unread email”). That solution is davmail. Yes, it’s on sourceforge, and yes, it’s Java, but it works perfectly.

It’s not very well-documented, but you can run it in headless mode and still do the interactive OAuth2 flow needed with modern O365. Your settings should include:


When davmail starts, it will ask you to visit a URL and paste the resulting URL back - this contains the necessary OAuth2 tokens it needs. No need for any GUI!

Once davmail is running, your .fetchmailrc can be:

poll localhost protocol IMAP port 1143
 auth password username "[email protected]"
 is localuser here
 sslmode none
 mda "/usr/bin/procmail -d %T"
 folders INBOX,etc,etc

Note that since davmail is running locally, there’s not really any need for SSL, though you can set that up if you like.

When you start fetchmail, enter your password, and that will initiate the auth flow against the davmail instance. Note that you’re not storing passwords anywhere, unlike the old-style app password approach you might have used previously on gmail and the like.

I don’t need to send mail often, so I have mutt set up like this:

set smtp_url= "smtp://[email protected]@localhost:1025/"
unset smtp_pass
set ssl_starttls=no
set ssl_force_tls=no

Having to enter my password each time is not a big deal for me.

Equally I have my calendar app set up to pull over caldav from davmail. Works great. I’d love to be able to pull my O365 calendar into Google Calendar, but apparently Google and Microsoft are unable - or more likely unwilling - to make this work in any meaningful way.

I’m pretty sure it’s possible to adapt Google’s OAuth2 scripts to directly use fetchmail with O365’s modern auth stuff, but I’m not sure I have the energy to figure it out - and as far as I can find, nobody else has?

procmail and threads

Sep 14, 2020

I’m apparently old-school enough to find gmail and co painfully inefficient for handling significant amounts of mail. I still find procmail+mutt hard to beat. One thing mutt can’t do, however, is filter threads automatically - there’s no “mute” facility like gmail has; threads have to processed manually.

Equally, procmail itself has no threading facilities or understanding of Message-Id or References.

Matching email threads

It can be done, though, with some cheesy awk:


# If a mail message has a References: value found in the refs file, then
# add the requested header.
# Usage:
# cat mail_msgs | ~/.mail.refs.muted "Muted: true"


cat - >$mail

newrefs="$(cat $mail | formail -x references -x message-id | tr -d '\n')"

touch $ref_file

cat $ref_file | awk -v newrefs="$newrefs" '

		found = 0;
		split(newrefs, tmp);
		for (i in tmp) {

	# Each thread will have one line in the ref file, with
	# space-separated references. So we just need to look for any
	# reference from the mail.
		for (ref in refs) {
			if (index($0, ref) != 0) {
				found = 1;

	END {
		exit(found ? 0 : 1);

if [[ $? = 0 ]]; then
	cat $mail | formail -i "$header"
	cat $mail

rm $mail

Essentially, we record all the References in the thread we’re trying to act on. Then we can trigger the above to see if the new mail is part of the thread of interest.

(This seems like the sort of thing formail could do, given its -D option has a message ID cache, but I haven’t even bothered to take a look at how hard that would be…)

procmail usage

In .procmailrc, we’d use this like so:

:0 Wfh: formail.lock
| $HOME/src/procmail-thread/ $HOME/.refs.muted "Procmail-Muted: true"

:0 Wfh: formail.lock
| $HOME/src/procmail-thread/ $HOME/.refs.watched "Procmail-Watched: true"

This will add the given header if we find any of the email’s References values in our “database”.

Then, we can do what we like with the mails, like deliver them as already-read, carbon copy them to the inbox, etc.:

* Procmail-Muted: true

* Procmail-Watched: true
        :0 c:



mutt usage

To actually watch or mute a thread, we add a couple of mutt macros:

macro index,pager "M" "|~/src/procmail-thread/ ~/.refs.muted<return>"
macro index,pager "W" "|~/src/procmail-thread/ ~/.refs.watched<return>"

The script is similar to the above, but populates the refs file with all message IDs found in the given email.

I put all this in a git repo.