Blog posts for tags/python

1. Removing Process.memory_maps() on macOS

   This is part of the psutil 5.6.0 release (see the full release notes).

   As of 5.6.0, Process.memory_maps() is no longer defined on macOS.

   The bug¶

   #1291: on macOS, Process.memory_maps() would either raise OSError: [Errno 22] Invalid argument or segfault the whole Python process! Both triggered from code as simple as psutil.Process().as_dict(), since Process.as_dict() iterates every attribute, and Process.memory_maps() is one of them.

   The root cause was inside Apple's undocumented proc_regionfilename() syscall. On some memory regions it returns EINVAL. On others it takes the process down. Which regions? Nobody figured out. Arnon Yaari (@wiggin15) did most of the investigation: he wrote a standalone C reproducer and walked me through what he'd tried.

   In PR-1436 I attempted a fix by reverse-engineering vmmap(1) but it didn't work. The fundamental problem is that vmmap is closed source and proc_regionfilename is undocumented. Neither my virtualized macOS (10.11.6) nor Travis CI (10.12.1) could reproduce the bug, which reproduced reliably only on 10.14.3.

   Why remove outright¶

   While removing the C code I noticed that the macOS unit test had been disabled long ago, presumably by me after recurring flaky Travis runs. Meaning that the method had been broken on some macOS versions far longer than the 2018 bug report suggested.

   Deprecating for a cycle didn't help either: raising AccessDenied breaks code that relied on a successful return, returning an empty list does the same silently, and leaving the method in place doesn't stop the segfault. Basically there was no sane solution, so since 5.6 is a major version I decided to just remove Process.memory_maps() for good.

   On macOS it never supported other processes anyway. Calling it on any PID other than the current one (or its children) raised AccessDenied, even as root.

   If someone finds a Mach API path that works, the method can return. Nobody has found one so far.

2. AIX support

   After a long wait psutil finally supports a new exotic platform: AIX!

   Honestly I'm not sure how many AIX Python users are out there (probably not many), but here it is.

   For this we have to thank Arnon Yaari, who started working on the port a couple of years ago (#605). I was skeptical at first, because AIX is the only platform I can't virtualize and test on my laptop, so that made me a bit nervous. Arnon did a great job. The final PR-1123 is huge: it required a considerable amount of work on his part, and a review of more than 140 messages exchanged between us over about a month, during which I was travelling through China.

   The end result is very good: almost all original unit tests pass, and code quality is awesome, which (I must say) is fairly unusual for an external contribution like this. Kudos to you, Arnon! ;-)

   Other changes¶

   Besides AIX support, release 5.4.0 also includes a couple of important bug fixes for psutil.sensors_temperatures() and psutil.sensors_fans() on Linux, and a fix for a bug on macOS that could cause a segmentation fault when using Process.open_files(). The complete list of bug fixes is in the changelog.

   The future¶

   Looking ahead at other exotic, still-unsupported platforms, two contributions are worth mentioning: a (still incomplete) PR for Cygwin which looks promising (PR-998), and Mingw32 compiler support on Windows (PR-845).

   psutil is gradually reaching a point where adding new features is becoming rarer, so it's a good moment to welcome new platforms while the API is mature and stable.

   Future work along these lines could also include Android and (hopefully) iOS support. Now that would be really awesome to have.

   Stay tuned.

   Discussion¶

   • Reddit
   • Blogspot

3. Announcing psutil 5.3.0

   psutil 5.3.0 is finally out. This release is a major one, bigger than any release before it in terms of improvements and bugfixes. It is interesting to notice how huge the diff between 5.2.2 and 5.3.0 is. This is because I've been travelling quite a lot this year, so I kept postponing it. It may sound weird but I consider publishing a new release and writing a blog post about it more stressful than working on the release itself. =). Anyway, here goes.

   Full Unicode support¶

   String-returning APIs (Process.exe(), Process.cwd(), Process.username(), etc.) are now Unicode-correct on both Python 2 and 3 (#1040), see detailed separate blog post.

   Improved process_iter()¶

   process_iter() now accepts attrs and ad_value parameters, letting you pre-fetch process attributes in one shot and skip the try/except NoSuchProcess boilerplate, see detailed separate blog post.

   Automatic overflow handling of numbers¶

   On very busy or long-lived systems, numbers returned by disk_io_counters() and net_io_counters() functions may wrap (restart from zero). Up to version 5.2.x you had to take this into account, while now this is automatically handled by psutil (see: #802). If a "counter" restarts from 0 psutil will add the value from the previous call for you so that numbers will never decrease. This is crucial for applications monitoring disk or network I/O in real time. Old behavior can be resumed by passing the nowrap=True argument.

   SunOS Process environ()¶

   Process.environ() is now available also on SunOS (see #1091).

   Other improvements and bug fixes¶

   Amongst others, here are a couple of important bug fixes I'd like to mention:

   • #1044: on OSX different Process methods could incorrectly raise AccessDenied for zombie processes. This was due to the poor proc_pidpath OSX API.
   • #1094: on Windows, pid_exists() may lie due to the poor OpenProcess Windows API which can return a handle even when a process PID no longer exists. This had repercussions for many Process methods such as cmdline(), environ(), cwd(), connections() and others which could have unpredictable behaviors such as returning empty data or erroneously raising NoSuchProcess exceptions. For the same reason (broken OpenProcess API), processes could unexpectedly stick around after using terminate() and wait().

   BSD systems also received some love (NetBSD and OpenBSD in particular). Different memory leaks were fixed and functions returning connected sockets were partially rewritten. The full list of enhancements and bug fixes can be seen here.

   About me¶

   I would like to spend a couple more words about my current situation. Last year (2016) I relocated to Prague and remote worked from there the whole year (it's been cool - great city!). This year I have mainly been resting in Turin (Italy) due to some health issues and travelling across Asia once I started to recover. I am currently in Shenzhen, China, and unless the current situation with North Korea gets worse I'm planning to continue my trip until November and visit Taiwan, South Korea and Japan. Once I'm finished, the plan is to briefly return to Turin (Italy) and finally return to Prague. By then I will probably be looking for a new (remote) gig again, so if you have anything for me by November feel free to send me a message. ;-)

4. Fixing Unicode across Python 2 and 3

   This one took a while. Adding proper Unicode support to psutil took four months of auditing, design decisions, and rewriting nearly every API that returned a string. The full journey is documented in #1040, and what follows is a summary.

   This can serve as a case study for any Python library with a C extension that needs to support both Python 2 and Python 3, as it will encounter the exact same set of problems.

   What was broken¶

   psutil has different APIs returning a string, many of which misbehaved when it came to unicode. There were three distinctive problems (#1040). Each API could:

   • A: raise a decoding error for non-ASCII strings (Python 3).
   • B: return unicode instead of str (Python 2).
   • C: return incorrect / invalid encoded data for non-ASCII strings (both).

   Process.memory_maps() hit all three on various OSes. disk_partitions() raised decoding errors on every UNIX except Linux. Windows service methods leaked unicode into Python 2 return values. The C extension had accumulated years of ad-hoc encode/decode decisions, with no single rule covering all of them.

   It was a mess.

   Filesystem or locale encoding?¶

   First problem was that the C extension was using 2 approaches when it came to decoding and returning a string: PyUnicode_DecodeFSDefault (filesystem encoding) for path-like APIs, and PyUnicode_DecodeLocale (user locale) for non-path strings like Process.username().

   It appeared clear that I had to use PyUnicode_DecodeFSDefault for all filesystem-related APIs like Process.exe() and Process.open_files().

   It was less clear, though, when to use PyUnicode_DecodeLocale.

   After some back and forth, I decided to use a single encoding for all APIs: the filesystem encoding (PyUnicode_DecodeFSDefault). This makes the encoding choice an implementation detail of psutil, not something the user has to care about.

   Error handling¶

   Second question was what to do in case the string cannot be correctly decoded (because invalid, corrupted or whatever). On Python 3 + UNIX the natural choice was 'surrogateescape', which is also the default for PyUnicode_DecodeFSDefault. On Windows the default is 'surrogatepass' (Python 3.6) or 'replace' as per PEP 529.

   And here come the troubles: Python 2 is different. To correctly handle all kinds of strings on Python 2 we should return unicode instead of str, but I didn't want to do that, nor have APIs which return two different types depending on the circumstance.

   Since unicode support is already broken in Python 2 and its stdlib (see bpo-18695), I was happy to always return str, use 'replace' as the error handler, and simply consider unicode support in psutil + Python 2 broken.

   Final behavior¶

   Starting from 5.3.0, psutil behaves consistently across all APIs that return a string. The rules are intentionally simple, even if the underlying implementation is not.

   The notes below apply to any method returning a string such as Process.exe() or Process.cwd(), including non-filesystem-related methods such as Process.username():

   • all strings are encoded using the OS filesystem encoding (PyUnicode_DecodeFSDefault), which varies depending on the platform you're on (e.g. 'UTF-8' on Linux, 'mbcs' on Windows).

   • no API call is supposed to crash with UnicodeDecodeError.

   • in case of badly encoded data returned by the OS, the following error handlers are used to replace the bad characters in the string:

     • Python 2: 'replace'.
     • Python 3: 'surrogateescape' on POSIX, 'replace' on Windows.

   • on Python 2 all APIs return bytes (str type), never unicode.

   • on Python 2 you can go back to unicode by doing:

     >>> unicode(proc.exe(), sys.getdefaultencoding(), errors="replace")
     

   The full journey was implemented in PR-1052, and shipped in 5.3.0 (see the changelog).

5. Improved process_iter()

   This is part of the psutil 5.3.0 release (see the changelog for the full list of changes).

   The old pattern¶

   Iterating over processes and collecting attributes requires more boilerplate than it should. A process returned by psutil.process_iter() may disappear before you access it, or require elevated privileges, so every lookup has to be guarded with a try / except:

   >>> import psutil
   >>> for proc in psutil.process_iter():
   ...     try:
   ...         pinfo = proc.as_dict(attrs=['pid', 'name'])
   ...     except (psutil.NoSuchProcess, psutil.AccessDenied):
   ...         pass
   ...     else:
   ...         print(pinfo)
   ...
   {'pid': 1, 'name': 'systemd'}
   {'pid': 2, 'name': 'kthreadd'}
   {'pid': 3, 'name': 'ksoftirqd/0'}
   

   This is not decorative. It's necessary to avoid the race condition.

   The new pattern¶

   5.3.0 adds attrs and ad_value parameters to psutil.process_iter(). With these, the loop body becomes:

   >>> import psutil
   >>> for proc in psutil.process_iter(attrs=['pid', 'name']):
   ...     print(proc.info)
   ...
   {'pid': 1, 'name': 'systemd'}
   {'pid': 2, 'name': 'kthreadd'}
   {'pid': 3, 'name': 'ksoftirqd/0'}
   

   Internally, process_iter() attaches an info dict to the Process instance. The attributes are pre-fetched in one shot. Processes that disappear during iteration are silently skipped, and attributes that would raise AccessDenied get assigned ad_value, which defaults to None:

   for p in psutil.process_iter(['name', 'username'], ad_value="N/A"):
       print(p.name(), p.username())
   

   Performance¶

   Beyond the syntactic win, the new syntax is also faster than calling individual methods in a loop. process_iter(attrs=[...]) is equivalent to using Process.oneshot() on each process (see Making psutil twice as fast for how that works): attributes that share a syscall or a /proc file are fetched together instead of re-read on every method call, which is a lot faster.

   Comprehensions¶

   With the exception boilerplate out of the way, comprehensions finally work cleanly. E.g. getting processes owned by the current user can be written as:

   >>> import getpass
   >>> from pprint import pprint as pp
   >>> pp([(p.pid, p.info['name']) for p in psutil.process_iter(attrs=['name', 'username']) if p.info['username'] == getpass.getuser()])
   [(16832, 'bash'),
    (19772, 'ssh'),
    (20492, 'python')]