App crashing on boot

[philipengberg]

Out of the blue I get this error when my server instance boots on Heroku:

Run: error while loading shared libraries: libswiftCore.so: cannot open shared object file: No such file or directory

This is the buildpack I'm using:

https://buildpack-registry.s3.amazonaws.com/buildpacks/vapor/vapor.tgz

I'm using Heroku-20 stack.

[philipengberg]

This is the start of the build log:

-----> Building on the Heroku-20 stack
-----> Using buildpack: vapor/vapor
-----> Swift app detected
-----> Using Swift 6.0 (default)
-----> Installing swiftenv
-----> Installing Swift 6.0
Downloading https://swift.org/builds/swift-6.0-release/ubuntu2004/swift-6.0-RELEASE/swift-6.0-RELEASE-ubuntu20.04.tar.gz
/tmp/swiftenv-6.0- /tmp/codon/tmp/buildpacks/2c03c92bc6b8e2a37603d34bfa3d352f037ddcc4

Bare in mind, I don't know if this is the right place to report this.

[vzsg]

You're at the right place.

---

And the issue is perplexing. That file should not be necessary since I included the --static-swift-stdlib argument for swift build... But apparently there is a constellation in which this is not enough. Does your project have any exotic dependencies? Macros, swift-syntax, anything else?

---

If Swift 6.0 is not a hard requirement, you could fall back to the old version of the buildpack that kept the Swift dynamic libraries in the image for now:

heroku buildpacks:set https://github.com/vapor-community/heroku-buildpack.git#5101-release

In the mean time, I'll probably add a flag that reverts to the old behavior.

[vzsg]

Alternatively, could you try bumping the stack to heroku-22 or heroku-24? Both would be interesting data points.

[philipengberg]

I'll try heroku-22 first ☁️ (I tried it earlier, where it never finished building)

[philipengberg]

Yes, heroku-22 just stalls building with this as the last thing in the log:

[1809/1811] Compiling Run main.swift
[1809/1811] Write Objects.LinkFileList

[philipengberg]

I'll try the old version of the buildpack now

[philipengberg]

The downgrade on the buildpack seems to work 👀 Why though? 😅

[MahdiBM]

@philipengberg looks like the Swift 6 build regression I've reported here: swiftlang/swift#76555

So downgrading the Swift compiler version enabled the builder to not run out of memory.

[philipengberg]

Interesting! Makes sense.

[vzsg]

I've pushed two relevant releases to the buildpack.

If you switch back to vapor/vapor now, you'll get:

• the option to try Swift 6.0.1 (the new default);
• and the old dynamic linking behavior if you enable it with heroku config:set SWIFT_DYNAMIC_STDLIB=true.

Anecdotally, dynamic linking has required less memory than static linking, so it might be worth a try.
It would be a very interesting data point too, if you could test this new option with 5.10.1 and 6.0.1.

---

tl;dr:

heroku buildpacks:set vapor/vapor
heroku config:set SWIFT_DYNAMIC_STDLIB=true
git push heroku main

☝🏻 should result in a successful build even with Swift 6.