preview: Use dumb-init as PID 1 (#2982)

build_docs.pl runs a number of git commands through its normal course of
operation. After a certain point, these git commands will implicitly run
git gc in the background. By design, the original git process forks off
the gc child process and forgets about it. The child process will get
reparented to PID 1, which is either build_docs.pl or a shell. Neither
of those will handle SIGCHLD for processes they're not aware of, so the
number of zombie processes grows indefinitely.

With a large number of zombie processes (80k processes observed in
elastic-apps-web), metricbeat starts consuming all available CPU
resources, severely degradating the performance of the cluster. This has
a side effect of reducing the network performance to around ~50Mbps and
increasing the I/O wait to ~30%.

dumb-init was picked because it's dumb and doesn't try to do anything
fancy. There should be nothing that interferes with build_docs.pl's
operation. Signals are forwarded to dumb-init's child. dumb-init will
also detach itself from the controlling TTY and attach its child process
instead.

Ticket: https://elasticco.atlassian.net/browse/SRVCS-1367

Signed-off-by: Andrew Gunnerson <[email protected]>

Dockerfile

@@ -24,8 +24,10 @@ RUN install_packages \
     # Used both to install dependencies and at run time
   bash less \
     # Just in case you have to shell into the image
-  locales
+  locales \
     # Deal with utf-8 characters properly
+  dumb-init
+    # Reap zombie git gc processes
 RUN echo 'en_US.UTF-8 UTF-8' > /etc/locale.gen && locale-gen
 ENV LANG en_US.UTF-8
 ENV LANGUAGE en_US:en

preview/Dockerfile

@@ -25,4 +25,5 @@ COPY preview /docs_build/preview
 COPY template /docs_build/template
 COPY resources /docs_build/resources
 
+ENTRYPOINT ["dumb-init", "--"]
 CMD ["/docs_build/build_docs.pl", "--in_standard_docker", "--preview"]

preview/build.sh

@@ -5,7 +5,7 @@
 set -eo pipefail
 
 export IMAGE=docker.elastic.co/docs/preview
-export VERSION=17
+export VERSION=18
 
 cd $(git rev-parse --show-toplevel)
 ./build_docs --docker-build build