This is part 2 of my journey adding read-only mode to tantivy. It started, when a user reported a bug of an application failing to open an index on a read-only file system.
Before implementing the read-only feature, I would like to a) reproduce the bug b) step through it with a debugger to see what causes the error.
In the previous post, we found that removing write and execute permissions on files and directory of the tantivy index doesn’t simulate a read-only file system.
So we are back to the drawing board for ideas on how to reproduce the bug.
One way of simulating a different OS configuration locally is to use Docker.
Docker is a fad system that allows you to specify, build and run snapshoted applications.
How do you build a container?
First let’s define a docker image in a Dockerfile.
FROM ubuntu:18.04
COPY open_in_dir /repro/open_in_dir
# copies file_on_host path_on_container/file_on_host
This builds on top of the ubuntu 18.04 image, which provides compatibility with my host.
The COPY command copies the open_in_dir executable to a repro directory of the container.
Make sure the Dockerfile and the open_in_dir executable are in the small_index directory locally run the following command.
$$$$ sudo docker build -t tantivy_cont .
Sending build context to Docker daemon 63.84MB
Step 1/2 : FROM ubuntu:18.04
---> 7698f282e524
Step 2/2 : COPY open_in_dir /repro/open_in_dir
---> Using cache
---> 442ddff8340e
Successfully built 442ddff8340e
Successfully tagged tantivy_cont:latest
We have built a container image and tagged it with tantivy_cont.
So you built a container, now what?
We have successfully specified a container image that includes our open_in_dir executable in a repro directory.
On Linux, Docker relies on 2 OS primitives: control groups and namespaces. One of the namespaces is the mount namespace, which allows you to mount a specific fs point into your process. To the container it looks like the subdir in a subdir on the host is one of the top directories in its file system.
When running the container, we can mount pwd, which is the small_index/ directory on the host, as a read-only small_index/ directory in the repro dir on the container.
For more information run man 7 namespaces in your terminal.
$$$$:/tantivy/small_index sudo docker run -v `pwd`:/repro/small_index:ro -it tantivy_cont
root@21541302399e:/# cd repro
root@21541302399e:/repro# ls -al
total 62328
drwxr-xr-x 1 root root 4096 Jun 18 21:09 .
drwxr-xr-x 1 root root 4096 Jun 18 21:09 ..
-rwxrwxr-x 1 root root 63810624 Jun 18 18:24 open_in_dir
drwxr-xr-x 2 1000 1000 4096 Jun 18 21:04 small_index
root@21541302399e:/repro# touch small_index/meta.json
touch: cannot touch 'small_index/meta.json': Read-only file system
root@21541302399e:/repro# cat /proc/mounts | grep "small"
/dev/sda7 /repro/small_index ext4 ro,relatime,data=ordered 0 0
We are now the root user in a running container. We can check that our executable and the small_index dir are here as expected.
If we try to touch one of the files in the small_index/ directory, we will get rejected, because it’s a read-only file system and touch wanted to modify the file.
Looking inside the container-wide /proc/mounts we find that small_index/ is indeed read-only - ro.
The stage is set. We have a read-only file system running inside the container with our executable and the index directory.
We only need to run the executable…
…and we should see the “Read-only file system” error message thrown by Index::open_in_dir as reported in the ticket.
So we run it.
root@21541302399e:/repro# ./open_in_dir
Successfully opened the index
What have I learnt?
As it stands, I have found a configuration in which tantivy fails to open an index inside a given directory. I tried to simulate a read-only file system by removing write and execute permissions from file and directory of the index. That proved useless (it threw an error, but not what was expected), so I decided to simulate the whole fs to be read-only.
Using Docker I mounted the index directory of the host into a running container, where the application successfully opened the index.
I have exhausted my creativity to come up with a repro, so I will have to implement read-only mode relying on my understanding of tantivy internals rather than fs configuration.