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· One min read
Benjamin Ingberg

Before heads out to summer adventures I'd like to invite everyone to a cool summer Buildbar. At the Buildbar we'll eat good food, talk about interesting technical problems, new developments with Bazel and Buildbarn.

And also have a few beers.

Feel welcome to come over on Wednesday 19 June 2024, from 16 to 20.


You'll find us at our Linköping offices at Fridtunagatan 33. Currently there is some ongoing construction but follow the red lines and you'll be fine.

Fridtunagatan 33 Linköping

· 4 min read
Nils Wireklint

If you have a formatter tool that can rewrite your code you can run it automatically on all unmerged commits. This will show you how to script git-rebase to do so without any conflicts.

There are two ways to do it manually, forward or backward. The forward pass amends each commit and deals with the conflicts when stepping to the next commit. In contrast the backwards pass, formats each commit from the end, which will avoid conflicts but for long commit chains it can be almost as boring.

This pattern comes up when working with long-lived feature branches, or tasks that were almost done, and then pre-empted by other prioritized work. Here are a few oneliners you can run to tidy up your commits.

See also the full technical guide for developing this git-rebase workflow in our documentation. Which contains more details on rebasing with git, using a scriptable editor to automate the git-rebase todo-list, as well as the squashed commit messages.

Example commits

Say you have three unmerged commits:

21cc7b5 My amazing feature e05fd9f Other complimentary work acb9fae Fix annoying bug

They contain important work, but you forgot to run some linters, or the main branch added more lint requirements after the feature work was started. This will run linters that can automatically fix issues on each commit through a scripted git-rebase.

Rebase algorithm

We have a three-step process to update each commit.

  • 1: Create a fixup commit with the applied lint suggestions, which we immediately revert so the next commit still applies


    # Formatters and fixers go here.
    # Replace with your tools of choice! rustfmt, gofmt, black, ...

    # Add a new commit with the changes and revert it again.
    git add -u
    git commit --allow-empty --fixup HEAD
    # 'git-revert' does not support '--allow-empty'.
    git revert --no-commit HEAD
    git commit --allow-empty --no-edit
  • 2: Squash the fixup commit into the original feature commit

  • 3: Squash the revert down into the next feature commit

These tabs show how the commits evolve and are squashed, the extra commits are grouped to indicate the target commit. The revert of the first commit is grouped with the second feature commit, and so on. We discard the final revert.

21cc7b5 My amazing feature
01900c5 fixup! My amazing feature

55feaba Revert "fixup! My amazing feature"
e05fd9f Other complimentary work
d122da7 fixup! Other complimentary work

249b0d3 Revert "fixup! Other complimentary work"
acb9fae Fix annoying bug
50e426a fixup! Fix annoying bug

7e84259 Revert "fixup! Fix annoying bug"


git allows us to set any editor to edit the todo-list, $GIT_SEQUENCE_EDITOR, and the commit message, $EDITOR. We choose vim as it is often available, and easier to use than sed and awk. It is nice to have a scriptable interactive editor to make changes to the workflow and try out the commands.

See the full technical guide for details and more tips on git-rebase and vim.


$ env                          \
git rebase -i --exec ./ origin/main

Fixup (autosquash):

# More robust autosquash, that handles duplicated commit messages.
# If your commit messages are all unique you can use '--autosquash' instead.
# See the technical guide for more details.
$ env \
GIT_SEQUENCE_EDITOR="vim +'g/^\w* \w* fixup!/s/^pick/fixup/'" \
git rebase -i origin/main


$ env                                                                                               \
EDITOR="sed -i '1,9d'" \
GIT_SEQUENCE_EDITOR="vim +'g/^#/d' +'normal! Gdk' +'g/^pick \w* Revert \"fixup!/normal! j0ces'" \
git rebase -i origin/main

We have not developed the incantation, git-rebase command, to preserve the author date from the original commits. We will address that next!

· One min read
Nils Wireklint

We have just started a documentation series describing the Buildbarn chroot runners, and how they can be used for hermetic input roots that contain all the required tools. This includes implementation notes for a "mountat" functionality created through the new Linux mount API, how you can use this under-documented API and its shortcomings. And how this can/will be integrated into Buildbarn, with technical descriptions of the workers and runners.

The first sections are already available, with more to come!


Reference code repository:

· 2 min read
Benjamin Ingberg

This is a continuation of the previous update article and is a high level summary of what has happened in Buildbarn from 2023-02-16 to 2023-11-14.

Added support for JWTs signed with RSA

Support for JWTs signed with RSA has been added. The following JWT signing algorithms are now supported:

  • HS256
  • HS384
  • HS512
  • RS256
  • RS384
  • RS512
  • EdDSA
  • ES256
  • ES384
  • ES512

Generalized tuneables for Linux BDI options

Linux 6.2 added a sysfs attribute for toggling BDI_CAP_STRICTLIMIT on FUSE mounts. If using the FUSE backed virtual file system on Linux 6.2 adding { "strict_limit": "0" } to linux_backing_dev_info_tunables will remove the BDI_CAP_STRICTLIMIT flag from the FUSE mount.

This may improve fileystem performance especially when running build actions which uses mmap'ed files extensively.

Add support for injecting Xcode environment variables

Remote build with macOS may call into locally installed copies of Xcode. The path to the local copy of Xcode may vary and Bazel assumes that the remote execution service is capable of processing Xcode specific environment variables.

See the proto files for details.

Add a minimum timestamp to ActionResultExpiringBlobAccess

A misbehaving worker may polluted the action cache, after fixing the misbehaving worker we would rather not throw away the entire action cache.

A minimum timestamp in ActionResultExpiringBlobAccess allows us to mark a timestamp in the past before which the action should be considered invalid.

Add authentication to HTTP servers

Much like the gRPC servers are capable of authenticated configuration the http servers can now also require authentication.

This allows the bb_browser and bb_scheduler UI to authenticate access using OAuth2 without involving any other middleware.

This also allows us to add authorization configuration for administrative tasks such as draining workers or killing of jobs.

Authentication using a JSON Web Key Set

JSON Web Key Sets (JWKS) is a standard format which allows us to specify multiple different encryption keys that may have been used to sign our JWT authentication.

Buildbarn can load the JWKS specification, either inline or as a file, when specifying trusted encryption keys.

This allows us to have rotation with overlap of encryption keys.

· 8 min read
Nils Wireklint

An adventure in finding a memory thief in Starlark-land

This is a summary and follow-up to my talk at BazelCon-2023. With abridged code examples, the full instructions are available together with the code.

Problem Statement

First, we lament Bazel's out-of-memory errors, and point out that the often useful Starlark stacktrace does not always show up. Some allocation errors just crash Bazel without giving and indication of which allocation failed.


This diagram illustrates a common problem for memory errors, the allocation that fails may not be the problem, it is just the straw that breaks the camel's back. And the real thief may already have allocated its memory.

We have seen many errors when working with clients, and they typically hide in big corporate code bases. Which complicates troubleshooting, discussion and error reporting. So we create a synthetic repository to try to illustrate the problem, and have something to discuss. The code and instructions are available here.

Errors and poor performance in the analysis phase are not good at all. This is because the analysis must always be done before starting to build all actions. With big projects the number of configuration to build for can be very large, so one cannot rely on CI runners to build the same configuration over and over, to retain the analysis cache. Instead it is on the critical-path for all builds, especially if the actions themselves are cached remotely.

To illustrate (some of the problem) we have a reproduction repository with example code base with some Python and C programs. To introduce memory problems, and make it a little more complex we add two rules: one CPU intensive rule ("spinlock") and one memory intensive aspect ("traverse"). The "traverse" aspect encodes the full dependency tree of all targets and writes that to a file with ctx.actions.write. So the allocations are tied to the Action object.


We have a couple of tools available, many are discussed in the memory optimization guide, but we find that some problems can slip through the cracks.

First off, there are the post-build analysis tools in bazel:

  • bazel info
  • bazel dump --rules
  • bazel aquery --skyframe_state

These are a good starting point and have served us well on many occasions. But with this project they seem to miss some allocations We will return to that later. Additionally, these tool will not give any information if the Bazel server crashes. You will need to increase the memory and run the same build again.

Then one can use Java tools to inspect what the JVM is doing:

The best approach here is to ask Bazel to save the heap if it crashes, so it can be analyzed post-mortem: bazel --heap_dump_on_oom

And lastly, use Bazel's profiling information:

  • bazel --profile=profile.gz --generate_json_trace_profile --noslim_profile

This contains structured information and is written continuously to disk, so if Bazel crashes we can still parse it, we just need to discard partially truncated events.

Expected Memory consumption

As the two rules write their string allocations to output files we get a clear picture of the expected RAM usage (or at least a lower bound).

$ bazel clean
$ bazel build \
--aspects @example//memory:eat.bzl%traverse \
--output_groups=default,eat_memory \
# Memory intensive tree traversal (in KB)
$ find bazel-out/ -name '*.tree' | xargs du | cut -f1 | paste -sd '+' | bc
# CPU intensive spinlocks (in KB)
$ find bazel-out/ -name '*.spinlock' | xargs du | cut -f1 | paste -sd '+' | bc

Here is a table with the data:

Memory for each targetTotal
Memory intensive0-17 MB79 MB
CPU intensive136 KB3.4 MB

Reported Memory Consumption

Next, we check with the diagnostic tools.

$ bazel version
Bazelisk version: development
Build label: 6.4.0

Bazel dump --rules

$ bazel $STARTUP_FLAGS --host_jvm_args=-Xmx"10g" dump --rules
Warning: this information is intended for consumption by developers
only, and may change at any time. Script against it at your own risk!

cc_library 4 17 524,320 131,080
native_binary 1 4 524,288 524,288
cc_binary 6 54 262,176 43,696
toolchain_type 14 0 0 0
toolchain 74 0 0 0

traverse 85 81 262,432 3,087
spinlock14 35 66 524,112 14,974
spinlock15 35 66 0 0

First, there are some common rules that we do not care about here, then we have the Aspects. traverse is the memory intensive aspect, which is applied on the command line and spinlock<N> are the CPU intensive rules, with identical implementations just numbered (there are 25 of them).

It is a little surprising that only one have allocations. And the action count for each aspect does not make sense either, as this is not a transitive aspect. It just runs a single action each time the rule is instantiated. The hypothesis is that this is a display problem, with code shared between rules. There are 25 rules, with 25 distinct implementation functions, but they in turn call the same function with the action. So the "count" and "actions" columns are glued together, but the "bytes" is reported for just one of the rules (it would be bad if this was double-counted).

Either way, the total number of bytes does not add up to what we expect. Compare the output to the lower-bound determined before:

| | Memory for each target | Total | Reported Total | | ---- | ---- | ----------- | | Memory intensive | 0-17 MB | 79 MB | 262 kB | CPU intensive | 136 KB | 3.4 MB | 524 kB

Skylark Memory Profile


This is not part of the video.

The skylark memory profiler is much more advanced, and can be dumped after a successful build.

$ bazel $STARTUP_FLAGS --host_jvm_args=-Xmx"$mem" dump \
$ pprof manual/2023-10-30/10g-2/memory.pprof
Main binary filename not available.
Type: memory
Time: Oct 30, 2023 at 12:16pm (CET)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 2816.70kB, 73.34% of 3840.68kB total
Showing top 10 nodes out of 19
flat flat% sum% cum cum%
512kB 13.33% 13.33% 512kB 13.33% impl2
256.16kB 6.67% 20.00% 256.16kB 6.67% traverse_impl
256.11kB 6.67% 26.67% 256.11kB 6.67% _add_linker_artifacts_output_groups
256.09kB 6.67% 33.34% 256.09kB 6.67% alias
256.09kB 6.67% 40.00% 256.09kB 6.67% rule
256.08kB 6.67% 46.67% 256.08kB 6.67% to_list
256.06kB 6.67% 53.34% 256.06kB 6.67% impl7
256.04kB 6.67% 60.01% 256.04kB 6.67% _is_stamping_enabled
256.04kB 6.67% 66.67% 256.04kB 6.67% impl18
256.03kB 6.67% 73.34% 768.15kB 20.00% cc_binary_impl

Here the Memory intensive aspect shows up with 256kB, which is inline with the output from bazel dump --rules, but not reflecting the big allocations we know it makes.

Eclipse Memory Analyzer

The final tool we have investigated is the Java heap analysis tool Eclipse Memory Analyzer, which can easily be used with Bazel's --heap_dump_on_oom flag. On the other hand it is a little tricker to find a heap dump from a successful build.


Here we see the (very) big allocation clear as day, but have no information of its provenance.

We have not found how to track this back to a Skylark function, Skyframe evaluator or anything that could be cross-referenced with the profiling information.

Build Time

The next section of the talk shows the execution time of the build with varying memory limits.


This is benchmarked with 5 data points for each memory limit, and the plot shows failure if there was at least one crash among the data points. There is a region where the build starts to succeed more and more often, but sometimes crashes. So the Crash and not-crash graphs overlap a little, you want to have some leeway to avoid flaky builds from occasional out-of-memory crashes.

We see that the Skymeld graph requires a lot less memory than a regular build, that is because our big allocations are all tied to Action objects. Enabling Skymeld lets Bazel start executing Actions as soon as they are ready, so the resident set of Action objects does not grow so large, and the allocations can be freed much sooner.

Pessimization with limited memory


We saw a hump in the build time for the Skymeld graph, where the builds did succeed in the 300 - 400 MB range, but the build speed gradually increased, reaching a plateau at around 500 MB. This is a pattern we have seen before, where more RAM, or more efficient rules can improve build performance.

This is probably because the memory pressure and the Java Garbage Collector interferes with the Skyframe work. See Benjamin Peterson's great talk about the Skyframe for more information.

Future work

example profile

This section details future work for more tools and signals that we can find from Bazel's profile information --profile=profile.gz --generate_json_trace_profile --noslim_profile. Written in the standard chrome://tracing format it is easy to parse for both successful and failed builds.

This contains events for the garbage collector, and all executed Starlark functions.

These can be correlated to find which functions are active during, or before, garbage collection events. Additionally, one could collect this information for all failed builds, and see if some functions are overrepresented among the last active functions for each evaluator in the build.

· One min read
Fredrik Medley

Meroton visited BazelCon 2023 in Munich October 24-25, 2023. During the conference, we held three talks:

Other talks that mentioned Buildbarn were:

We are thankful for all amazing chats with the community and are looking forward to BazelCon 2024.

· 3 min read
Benjamin Ingberg

When starting out with remote caching, an error you are likely to run into is:
INVALID_ARGUMENT: Failed to store previous blob 1-<HASH>-<LARGE_NUM>:
Shard 1: Blob is <LARGE_NUM> bytes in size,
while this backend is only capable of storing blobs of up to 238608384 bytes in size

This is because your storage backend is too small. You are attempting to upload a blob larger than the largest blob accepted by your storage backend.

How do I fix it?

The largest blob you can store is the size of your your storage device divided by the number of blocks in your device.

To store larger blobs, either increase the size of your storage device or decrease the number of blocks it is split into. Larger storage devices will take more disk, while fewer blocks will decrease the granularity which your cache works with.

In bb-deployments this setting is found in storage.jsonnet.

// ...
contentAddressableStorage: {
backend: {
'local': {
// ...
oldBlocks: 8,
currentBlocks: 24,
newBlocks: 1,
blocksOnBlockDevice: {
source: {
file: {
path: '/storage-cas/blocks',
sizeBytes: 8 * 1024 * 1024 * 1024, // 8GiB
spareBlocks: 3,
// ...
// ...

To facilitate getting started bb-deployments emulates a block device by using an 8GiB large file. This file is small enough to fit most builds while not taking over the disk completely from a developers machine.

The device is then split into 36 blocks (8+24+1+3), where each block can then store a maximum of 238608384 bytes (8GiB / 36 - some alignment).

In production it is preferable to use a large raw block device for this purpose.

What does new/old/current/spare mean?

In depth documentation about all the settings are available in the configuration proto files.

In essence the storage works as a ringbuffer where the assignment of each block is rotated. Consider a 5 block configuration with 1 old, 2 current, 1 new and 1 spare block.


As data is referenced from an old block it gets written into a new block. When the new block is full the role rotates.


There are some tradeoffs in behaviour to consider when choosing your block layout. Fewer blocks will allow larger individual blobs at the cost of granularity. Here is a quick summary of the meaning of the different fields.

  • Old - Region where reads are actively copied over to new, too small value and your device behaves more like a FIFO than a LRU cache, too large and your device does a lot of uneccesary copying.
  • Current - Stable region, should be the majority of your device.
  • New - Region for writing new data to, must be 1 for AC and should be 1-4 for CAS. Having a couple of new blocks allows data to be better spread out over the device so as to not expire at the same time.
  • Spare - Region for giving ongoing reads some time to finish before data starts getting overwritten.

· 4 min read
Benjamin Ingberg

The example configuration project for buildbarn bb-deployments has gotten updates.

This is a continuation of the updates from last year article and is a high level summary of what has happened since April 2022 up to 2023-02-16.

Let ReferenceExpandingBlobAccess support GCS

ReferenceExpandingBlobAccess already supports S3 so support was extended to Google Cloud Storage buckets.

Support for prefetching Virtual Filesystems

Running workers with Fuse allows inputs for an action to be downloaded on demand. This significantly reduces the amount of data that gets sent in order to run overspecified actions. This however leads to poor performance for actions which reads a lot of their inputs synchronously.

With the prefetcher most of these actions can be recognized and data which is likely to be needed can be downloaded ahead of time.

Support for sha256tree

Buildbarn has added support for sha256tree which uses sha256 hashing over a tree structure similar to blake3.

This algorithm will allow large CAS objects to be chunked and decompositioned with guaranteed data integrity while still using sha256 hardware instructions.

Completeness checking now streams REv2 Tree objects

This change introduces a small change to the configuration schema. If you previous had this:

backend: { completenessChecking: ... },

You will now need to write something along these lines:

backend: {
completenessChecking: {
backend: ...,
maximumTotalTreeSizeBytes: 64 * 1024 * 1024,

See also the bb-storage commit 1b84fa8.

Postponed healthy service status

The healthy and serving status, i.e. HTTP /-/healthy and grpc_health_v1.HealthCheckResponse_SERVING, are now postponed until the whole service is up and running. Before, the healthy status was potentially reported before starting to listen to the gRPC ports. Kubernetes will now wait until the service is up before forwarding connections to it.

Server keepalive parameter options

The option buildbarn.configuration.grpc.ServerConfiguration.keepalive_parameters can be used for L4 load balancing, to control when to ask clients to reconnect. For default values, see keepalive.ServerParameters.

Graceful termination of LocalBlobAccess

When SIGTERM or SIGINT is received, the LocalBlobAccess now synchronize data to disk before shutting down. Deployments using persistent storage will no longer observe loss of data when restarting the bb_storage services.

Non-sector Aligned Writes to Block Device

Using sector aligned storage is wasteful for the action cache where the messages are typically very small. Buildbarn can now fill all the gaps when writing, making storage more efficient.

DAG Shaped BlobAccess Configuration

Instead of a tree shaped BlobAccess configuration, the with_labels notation allows a directed acyclic graph. See also the bb-storage commit cc295ad.

NFSv4 as worker filesystem

The bb_worker can now supply the working directory for bb_runner using NFSv4. Previously, FUSE and hard linking files from the worker cache were the only two options. This addition was mainly done to overcome the poor FUSE support on macOS.

The NFSv4 server in bb_worker only supports macOS at the moment. No effort has been spent to write custom mount logic for other systems yet.

Specify forwardMetadata with a JMESPath

Metadata forwarding is now more flexible, the JMESPath expressions can for example add authorization result data. The format is described in grpc.proto.

A common use case is to replace

forwardMetadata: ["build.bazel.remote.execution.v2.requestmetadata-bin"],


addMetadataJmespathExpression: '{

Tracing: Deprecate the Jaeger collector span exporter

This option is deprecated, as Jaeger 1.35 and later provide native support for the OpenTelemetry protocol.

bb-deployments Ubuntu 22.04 Example Runner Image

The rbe_autoconfig in bazel-toolchains has been deprecated. In bb-deployments it has been replaced by the Act image, distributed by catthehacker, used for running GitHub Actions locally under Ubuntu 22.04.

bb-deployments Integration Tests

The bare deployment and Docker Compose deployment have now got tests scripts that builds and tests @abseil-hello//:hello_test remotely, shuts down and then checks for 100% cache hit after restart. Another CI test is checking for minimal differences between the Docker Compose deployment and the Kubernetes deployment.

If there are any other changes you feel deserve a mention feel free to submit a pull request at github using the link below.

· 4 min read
Benjamin Ingberg

** UPDATE: ** Bazel has a workaround for this issue preventing the permanent build failure loop from 6.1.0 and a proper fix with the introduction of --experimental_remote_cache_ttl in Bazel 7

Starting from v6.0.0, Bazel crashes when building without the bytes. Because it sets --experimental_action_cache_store_output_metadata when using --remote_download_minimal or --remote_download_toplevel.

Effectively this leads to Bazel getting stuck in a build failure loop when your remote cache evicts an item you need from the cache.

developer@machine:~$ bazel test @abseil-hello//:hello_test --remote_download_

[0 / 6] [Prepa] BazelWorkspaceStatusAction stable-status.txt
ERROR: /home/developer/.cache/bazel/_bazel_developer/139b99b96c4ab6cba5122193
1a36e346/external/abseil-hello/BUILD.bazel:26:8: Linking external/abseil-hell
o/hello_test failed: (Exit 34): 42 errors during bulk transfer: /home/developer/.cache/bazel/_bazel_developer/
in/external/com_google_absl/absl/base/_objs/base/spinlock.pic.o (No such file
or directory)
Target @abseil-hello//:hello_test failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 3.820s, Critical Path: 0.88s
INFO: 5 processes: 4 internal, 1 remote.
FAILED: Build did NOT complete successfully
@abseil-hello//:hello_test FAILED TO BUILD

The key here is (Exit 34): xx errors during bulk transfer. 34 is Bazel's error code for Remote Error.

The recommended solution is to set the flag explicitly to false, with --experimental_action_cache_store_output_metadata=false. To quickly solve the issue on your local machine you can run bazel clean. However, this will just push the error into the future.

The bug is independent of which remote cache system you use and is tracked at GitHub.


When performing an analysis of what to build Bazel will ask the remote cache which items have already been built. Bazel will only schedule build actions for items that do not already exist in the cache. If running a build without the bytes1 the intermediary results will not be downloaded to the client.

Should the cached items be evicted then Bazel will run into an unrecoverable error. It wants the remote system to perform an action using inputs from the cache, but they have disappeared. And Bazel can not upload them, as they were never downloaded to the client. The build would then dutifully crash (some work has been put into trying to resolve this on the bazel side but it has not been considered a priority).

This puts an implicit requirement on the remote cache implementation. Artifacts need to be saved for as long as Bazel needs them. The problem here is that this is an undefined period of time. Bazel will not proactively check if the item still exists, nor in any other manner inform the cache that it will need the item in the future.

Before v6.0.0

Bazel tied the lifetime of which items already exists in the cache (the existence cache) to the analysis cache. Whenever the analysis cache was purged it would also drop the existence cache.

The analysis cache is purged quite frequently. It would therefore be rare in practice, that the existence cache would be out of date. Furthermore, since the existence cache was an in-memory cache, Bazel crashing would forcefully evict the existence cache. Thereby fixing the issue.

After v6.0.0

With the --experimental_action_cache_store_output_metadata flag enabled by default the existence cache is instead committed to disk and never dropped during normal operation.

This means two things:

  1. The implied requirement on the remote cache is effectively infinite.
  2. Should this requirement not be met the build will fail. And since the existence cache is committed to disk Bazel will just fail again the next time you run it.

Currently the only user-facing way of purging the existence cache is to run bazel clean. Which is generally considered an anti-pattern.

If you are using the bb-clientd --remote_output_service to run builds without the bytes (an alternative strategy to --remote_download_minimal) this will not affect you.


  1. When using Bazel with remote execution remote builds are run in a remote server cluster. There is therefore no need for each developer to download the partial results of build. Bazel calls this feature Remote Builds Without the Bytes. The progress of the feature can be tracked at GitHub.

· One min read
Benjamin Ingberg

After BazelCon we've all gotten a bit giddy and you might feel excited how the information presented at BazelCon might impact your development workflow. For that purpose we're inviting everyone in the wider bazel community to an open BuildBar at the Meroton offices.

Come over and digest BazelCon with high level technical discussions of the talks, great company, a pleasant atmosphere and also beer.

Feel welcome to come over this Thursday the first of December from 16 to 20.


You'll find us at our Linköping offices at Fridtunagatan 33. Currently there is some construction but follow the red lines and you'll be fine.

Fridtunagatan 33 Linköping