view tests/test-bad-extension.t @ 40021:c537144fdbef

wireprotov2: support response caching

One of the things I've learned from managing VCS servers over the
years is that they are hard to scale. It is well known that some
companies have very beefy (read: very expensive) servers to power
their VCS needs. It is also known that specialized servers for
various VCS exist in order to facilitate scaling servers. (Mercurial
is in this boat.)

One of the aspects that make a VCS server hard to scale is the
high CPU load incurred by constant client clone/pull operations.

To alleviate the scaling pain associated with data retrieval
operations, I want to integrate caching into the Mercurial wire
protocol server as robustly as possible such that servers can
aggressively cache responses and defer as much server load as
possible.

This commit represents the initial implementation of a general
caching layer in wire protocol version 2.

We define a new interface and behavior for a wire protocol cacher
in repository.py. (This is probably where a reviewer should look
first to understand what is going on.)

The bulk of the added code is in wireprotov2server.py, where we
define how a command can opt in to being cached and integrate
caching into command dispatching.

From a very high-level:

* A command can declare itself as cacheable by providing a callable
  that can be used to derive a cache key.
* At dispatch time, if a command is cacheable, we attempt to
  construct a cacher and use it for serving the request and/or
  caching the request.
* The dispatch layer handles the bulk of the business logic for
  caching, making cachers mostly "dumb content stores."
* The mechanism for invalidating cached entries (one of the harder
  parts about caching in general) is by varying the cache key when
  state changes. As such, cachers don't need to be concerned with
  cache invalidation.

Initially, we've hooked up support for caching "manifestdata" and
"filedata" commands. These are the simplest to cache, as they should
be immutable over time. Caching of commands related to changeset
data is a bit harder (because cache validation is impacted by
changes to bookmarks, phases, etc). This will be implemented later.
(Strictly speaking, censoring a file should invalidate caches. I've
added an inline TODO to track this edge case.)

To prove it works, this commit implements a test-only extension
providing in-memory caching backed by an lrucachedict. A new test
showing this extension behaving properly is added. FWIW, the
cacher is ~50 lines of code, demonstrating the relative ease with
which a cache can be added to a server.

While the test cacher is not suitable for production workloads, just
for kicks I performed a clone of just the changeset and manifest data
for the mozilla-unified repository. With a fully warmed cache (of just
the manifest data since changeset data is not cached), server-side
CPU usage dropped from ~73s to ~28s. That's pretty significant and
demonstrates the potential that response caching has on server
scalability!

Differential Revision: https://phab.mercurial-scm.org/D4773

ensure that failing ui.atexit handlers report sensibly

  $ cat > $TESTTMP/bailatexit.py <<EOF
  > from mercurial import util
  > def bail():
  >     raise RuntimeError('ui.atexit handler exception')
  > 
  > def extsetup(ui):
  >     ui.atexit(bail)
  > EOF
  $ hg -q --config extensions.bailatexit=$TESTTMP/bailatexit.py \
  >  help help
  hg help [-ecks] [TOPIC]
  
  show help for a given topic or a help overview
  error in exit handlers:
  Traceback (most recent call last):
    File "*/mercurial/dispatch.py", line *, in _runexithandlers (glob)
      func(*args, **kwargs)
    File "$TESTTMP/bailatexit.py", line *, in bail (glob)
      raise RuntimeError('ui.atexit handler exception')
  RuntimeError: ui.atexit handler exception
  [255]

  $ rm $TESTTMP/bailatexit.py

another bad extension

  $ echo 'raise Exception("bit bucket overflow")' > badext.py
  $ abspathexc=`pwd`/badext.py

  $ cat >baddocext.py <<EOF
  > """
  > baddocext is bad
  > """
  > EOF
  $ abspathdoc=`pwd`/baddocext.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > gpg =
  > hgext.gpg =
  > badext = $abspathexc
  > baddocext = $abspathdoc
  > badext2 =
  > EOF

  $ hg -q help help 2>&1 |grep extension
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  *** failed to import extension badext2: No module named badext2

show traceback

  $ hg -q help help --traceback 2>&1 | egrep ' extension|^Exception|Traceback|ImportError'
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  Traceback (most recent call last):
  Exception: bit bucket overflow
  *** failed to import extension badext2: No module named badext2
  Traceback (most recent call last):
  ImportError: No module named badext2

names of extensions failed to load can be accessed via extensions.notloaded()

  $ cat <<EOF > showbadexts.py
  > from mercurial import commands, extensions, registrar
  > cmdtable = {}
  > command = registrar.command(cmdtable)
  > @command(b'showbadexts', norepo=True)
  > def showbadexts(ui, *pats, **opts):
  >     ui.write('BADEXTS: %s\n' % ' '.join(sorted(extensions.notloaded())))
  > EOF
  $ hg --config extensions.badexts=showbadexts.py showbadexts 2>&1 | grep '^BADEXTS'
  BADEXTS: badext badext2

#if no-extraextensions
show traceback for ImportError of hgext.name if devel.debug.extensions is set

  $ (hg help help --traceback --debug --config devel.debug.extensions=yes 2>&1) \
  > | grep -v '^ ' \
  > | egrep 'extension..[^p]|^Exception|Traceback|ImportError|not import'
  debug.extensions: loading extensions
  debug.extensions: - processing 5 entries
  debug.extensions:   - loading extension: 'gpg'
  debug.extensions:   > 'gpg' extension loaded in * (glob)
  debug.extensions:     - validating extension tables: 'gpg'
  debug.extensions:     - invoking registered callbacks: 'gpg'
  debug.extensions:     > callbacks completed in * (glob)
  debug.extensions:   - loading extension: 'badext'
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  Traceback (most recent call last):
  Exception: bit bucket overflow
  debug.extensions:   - loading extension: 'baddocext'
  debug.extensions:   > 'baddocext' extension loaded in * (glob)
  debug.extensions:     - validating extension tables: 'baddocext'
  debug.extensions:     - invoking registered callbacks: 'baddocext'
  debug.extensions:     > callbacks completed in * (glob)
  debug.extensions:   - loading extension: 'badext2'
  debug.extensions:     - could not import hgext.badext2 (No module named badext2): trying hgext3rd.badext2
  Traceback (most recent call last):
  ImportError: No module named *badext2 (glob)
  debug.extensions:     - could not import hgext3rd.badext2 (No module named badext2): trying badext2
  Traceback (most recent call last):
  ImportError: No module named *badext2 (glob)
  *** failed to import extension badext2: No module named badext2
  Traceback (most recent call last):
  ImportError: No module named badext2
  debug.extensions: > loaded 2 extensions, total time * (glob)
  debug.extensions: - loading configtable attributes
  debug.extensions: - executing uisetup hooks
  debug.extensions:   - running uisetup for 'gpg'
  debug.extensions:   > uisetup for 'gpg' took * (glob)
  debug.extensions:   - running uisetup for 'baddocext'
  debug.extensions:   > uisetup for 'baddocext' took * (glob)
  debug.extensions: > all uisetup took * (glob)
  debug.extensions: - executing extsetup hooks
  debug.extensions:   - running extsetup for 'gpg'
  debug.extensions:   > extsetup for 'gpg' took * (glob)
  debug.extensions:   - running extsetup for 'baddocext'
  debug.extensions:   > extsetup for 'baddocext' took * (glob)
  debug.extensions: > all extsetup took * (glob)
  debug.extensions: - executing remaining aftercallbacks
  debug.extensions: > remaining aftercallbacks completed in * (glob)
  debug.extensions: - loading extension registration objects
  debug.extensions: > extension registration object loading took * (glob)
  debug.extensions: > extension baddocext take a total of * to load (glob)
  debug.extensions: > extension gpg take a total of * to load (glob)
  debug.extensions: extension loading complete
#endif

confirm that there's no crash when an extension's documentation is bad

  $ hg help --keyword baddocext
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  *** failed to import extension badext2: No module named badext2
  Topics:
  
   extensions Using Additional Features