SwiftPM hangs indefinitely at "Planning build" on incremental builds since Swift 6.1 in some Ubuntu KVM environments

[flyer2001]

Description

SwiftPM hangs indefinitely during the "Planning build" phase on any incremental build on Linux with Swift 6.2. The issue occurs even on a
minimal fresh project created with swift package init.

Environment

• Swift version: 6.2 (swift-6.2-RELEASE)
• OS: Ubuntu 24.04.3 LTS (Noble Numbat)
• Kernel: Linux 6.8.0-60-generic x86_64
• Architecture: x86_64-unknown-linux-gnu

Steps to Reproduce

# 1. Create minimal project
cd /tmp
swift package init --type executable --name test-project
cd test-project

# 2. Initial build works fine
swift build
# Output: Build complete! (5-10 seconds)

# 3. Incremental build hangs indefinitely
swift build
# Hangs forever at "Planning build" phase, never completes

Expected Behavior

Incremental builds should complete quickly when no files have changed, similar to macOS behavior.

Actual Behavior

• SwiftPM hangs indefinitely at "Planning build" phase
• No output, no progress
• Process never completes, requires kill -9
• Happens on every incremental build attempt

Verbose Output

  $ swift build --verbose 2>&1 &
  $ sleep 10 && ps aux | grep swift-build
  # Shows process stuck at "Planning build"

What Works

✅ Initial clean build: swift build (first time)
✅ Build after full reset: swift package purge-cache && swift package reset && swift build

What Hangs

❌ Any subsequent swift build without cleaning (incremental build)
❌ swift test (same issue)
❌ swift build --target
❌ swift build -c release (release mode also hangs)
❌ swift build -Xswiftc -whole-module-optimization (doesn't help)

Workaround (Not Practical)

The only working solution is to completely reset before each build:

  pkill -9 swift 2>/dev/null || true
  swift package purge-cache
  swift package reset
  swift build

This adds 30-40 seconds to every build cycle, making development on Linux extremely slow.

Additional Context

• Tested on both a complex project with dependencies and a minimal swift package init project
• Issue affects all build commands, not just swift build
• Occurs even with zero file changes between builds
• Not specific to test targets (happens with regular executable targets too)
• Both debug and release configurations exhibit the same behavior

Impact

This makes Swift development on Linux (CI/CD, server deployments) impractical. Every build requires a full clean rebuild taking 30-60 seconds
instead of 1-5 seconds for proper incremental builds.

Related Issues

I found similar historical reports but none specifically for Swift 6.2 on Linux:

• Building SwiftPM project hangs indefinitely #4337 - Building SwiftPM project hangs indefinitely (closed, from 2018)
• [SR-6337] swift build hangs #4924 - swift build hangs (closed, from 2017)
• swiftlang/swift-build#305 - SwiftPM Build Tool Plugin excessively runs (2025, but about
  plugins)

None of these match the specific "Planning build" hang on incremental builds.

Question

Is this a known regression in Swift 6.2 on Linux? Are there environment variables or build flags that could disable/fix incremental compilation
for Linux?

[finagolfin]

I cannot reproduce on Fedora 40 x86_64 running the official 6.2 toolchain, sounds like something wrong with your environment. Can you try in a different directory or linux host?

For this particular problem, try running with --vv to see if that gives you any more useful info.

[flyer2001]

@finagolfin

For this particular problem, try running with --vv to see if that gives you any more useful info.

Reproduced on minimal project in fresh directory:

I created a minimal test case in a fresh directory (/tmp/swiftpm-test) using swift package init --type library. The hang reproduces consistently on incremental builds.

Steps to reproduce:

# 1. Create fresh package
 cd /tmp && mkdir swiftpm-test && cd swiftpm-test
 swift package init --type library --name TestPackage

 # 2. Initial build (works fine, completes in ~6s)
 swift build

 # 3. Add new file to trigger incremental build
 echo 'public struct NewStruct { }' > Sources/TestPackage/NewFile.swift

 # 4. Incremental build hangs indefinitely
 swift build --vv

 Output with --vv:

The build hangs after printing Planning build (see attached log). Last debug messages:
debug: registering 'build system' with terminator
Planning build

The manifest compiles successfully (~1.4s), but the build system never progresses past the planning phase.

Environment:

• OS: Ubuntu 24.04.3 LTS (Noble Numbat), kernel 6.8.0-60-generic
• Swift: 6.2 (swift-6.2-RELEASE), official release
• Target: x86_64-unknown-linux-gnu
• Installation: /usr/share/swift/usr/bin/swift (official Swift.org tarball)

Package details (minimal):

• No dependencies
• 2 Swift files (TestPackage.swift + NewFile.swift)
• swift-tools-version: 6.2

Workaround that works:

  pkill -9 swift-frontend swift-build sourcekit-lsp || true
  swift package purge-cache && swift package reset
  swift build  # Now works (~6s)

Additional observations:

1. Clean builds always work (first build after reset)
2. Any incremental build hangs (adding file, modifying file, or just running swift build again)
3. Happens on both minimal projects and larger projects (83 files, 1 dependency)
4. No stray processes left after Ctrl+C

Full --vv log attached

swiftpm-hang-verbose.log

Should I try with Swift 6.1 to confirm it's a 6.2 regression?

[finagolfin]

I created a minimal test case in a fresh directory (/tmp/swiftpm-test) using swift package init --type library.

I meant in an entirely different partition, like your home directory.

Should I try with Swift 6.1 to confirm it's a 6.2 regression?

Yes, and Swift 6.0 and 5.10 too, that would be useful info.

This is extremely unlikely to be a problem in the Swift compiler, as we have linux CI and many others running on linux all the time.

Rather, there's probably something specific to your host or environment that interacts badly with Swift's package manager, which it would be good to track down.

[flyer2001]

@finagolfin
Thanks for the quick response! I've completed all requested tests plus additional testing on Swift 6.1 and 6.2.1.

Test Results Summary

Swift Version	Location	Clean Build	Incremental Build
6.2.1 (latest)	~/swiftpm-test-621	✅ SUCCESS	❌ HANGS at "Planning build"
6.2	~/swiftpm-test	✅ SUCCESS (9.25s)	❌ HANGS at "Planning build"
6.1	~/swiftpm-test-61	✅ SUCCESS	❌ HANGS at "Planning build"
6.0	~/swiftpm-test-60	✅ SUCCESS (7.98s)	✅ SUCCESS (1.86s)
5.10	~/swiftpm-test-510	✅ SUCCESS (7.68s)	✅ SUCCESS (3.37s)

Key Findings

1. Regression between Swift 6.0 and 6.1: The hang issue first appears in 6.1
2. Still present in 6.2.1: The latest patch release (November 2024) has not fixed this issue
3. Not environment-specific: Issue reproduces in both /tmp and home directory (~/)
4. Exact hang location: "Planning build" stage during incremental builds
5. Clean builds always work: Issue only affects incremental builds

---

Environment

• System: Linux 6.8.0-60-generic, Ubuntu 24.04
• Tested Versions:
  - Swift 6.2.1: swift-6.2.1-RELEASE (Ubuntu 24.04)
  - Swift 6.2: swift-6.2-RELEASE (Ubuntu 24.04)
  - Swift 6.1: swift-6.1-RELEASE (Ubuntu 24.04)
  - Swift 6.0: swift-6.0-RELEASE (Ubuntu 24.04)
  - Swift 5.10: swift-5.10-RELEASE (Ubuntu 22.04)

---

Reproduction Steps

For each version, tested with minimal library package:

# Create minimal package
cd ~/swiftpm-test-{version}
swift package init --type library

# Clean build - ✅ Works on all versions
swift build --verbose

# Modify source file
echo '// Test comment' >> Sources/*/file.swift

# Incremental build - ❌ Hangs on 6.1, 6.2, 6.2.1
timeout 30 swift build --verbose

Observed Behavior

Swift 6.0 and 5.10 (Working):

Planning build          ← completes immediately
Building for debugging...
[compilation proceeds]
Build complete! (1-3s)

Swift 6.1, 6.2, 6.2.1 (Hanging):

Planning build          ← HANGS INDEFINITELY
[no further output]
[must kill with pkill -9 swift-frontend]

---

Detailed Logs

Full verbose logs for all 5 tested versions are attached (swiftpm-test-results.md, 307 lines, 60KB). The attachment includes:

• Complete swift build --verbose output for both clean and incremental builds
• Exact point where hanging versions stop (right after "Planning build")
• Confirmation that Swift 6.0 passes "Planning build" stage successfully

---

Conclusion

This is a regression introduced between Swift 6.0 and 6.1 that affects build plan generation during incremental builds on Linux.

Timeline

• Last working version: Swift 6.0 (September 2024)
• Regression introduced: Swift 6.1 (October 2024)
• Still broken in: Swift 6.2, Swift 6.2.1 (latest as of December 2024)

Impact

• Affects all current Swift 6.x releases except 6.0
• Makes incremental development impractical (requires full rebuild every time)
• Current workaround: swift package purge-cache && swift package reset before every build

Recommended Next Steps

1. Bisect between swift-6.0-RELEASE and swift-6.1-RELEASE tags to identify the problematic commit
2. Investigate SwiftPM changes related to build plan generation/caching between these versions
3. High priority: Consider this a release-blocking regression for future 6.x releases
4. Once identified, backport the fix to 6.2.x patch releases

Let me know if you need any additional information, specific commit testing, or profiling data!

[finagolfin]

Whatever the problem, it's likely in your build environment, as we have a Ubuntu 24.04 CI that builds the toolchain from the 6.2 branch source regularly, then uses that freshly-built toolchain to build some other Swift components and run some integration tests.

Is there anything unique about your Ubuntu host, eg is it running on actual hardware or in some sort of VM? Can you try running in the Ubuntu Docker that runs our CI or some other fresh Ubuntu install and see if it makes a difference?

Given how many people run on Ubuntu, this basic a problem is unlikely to be a toolchain bug that reproduces in all environments, but is almost certainly specific to something you are doing, that the toolchain is not interacting properly with.

[flyer2001]

Thank you for the quick response, @finagolfin !

Following your suggestion, I tested in the official Swift Docker image. The issue reproduces there as well:

Docker Test (swift:6.2.1 official image)

# Minimal repro steps
mkdir -p test/Sources/test && cd test
echo '// swift-tools-version: 6.0
import PackageDescription
let package = Package(name: "test", targets: [.target(name: "test")])' > Package.swift
echo 'public func hello() {}' > Sources/test/main.swift

swift build              # ✅ SUCCESS (9s)
echo '// change' >> Sources/test/main.swift
timeout 30 swift build   # ❌ HANGS at "Planning build" (exit 124)

Result: Clean build works, incremental build hangs.

Why CI Might Not Detect This

The key distinction: This only affects incremental builds, not clean builds.

CI systems typically:

1. Start with fresh workspace (no .build/ directory)
2. Run swift build once (clean build)
3. Clean up after completion

This workflow never exercises incremental build code paths where our issue manifests.

Evidence Summary

Environment	Swift 6.0 Incremental	Swift 6.1+ Incremental
Bare metal Ubuntu 24.04	✅ Works (1.86s)	❌ Hangs
Docker swift:6.2.1	(not tested)	❌ Hangs
Production codebase	✅ Works	❌ Hangs

Pattern: Sharp regression between 6.0 ✅ and 6.1+ ❌ on identical systems, across both bare metal and Docker.

Workaround: Using Swift 6.0 for development.

Would you like additional diagnostics (strace during hang, etc.)?

[finagolfin]

Hmm, I wonder if we can reproduce on the GitHub runners, as this could still be a problem with your host.

Are you using the Dockerfile I linked from the Swift CI or the Docker container linked on swift.org? I assume you are running Docker in the same bare metal Ubuntu host where you see this problem or is it being run elsewhere?

[flyer2001]

@finagolfin Thanks for looking into this!

1. Docker Setup

I used the official Docker image from Docker Hub: swift:6.2.1

  docker pull swift:6.2.1
  docker run --rm swift:6.2.1 swift --version
  # Output: Swift version 6.2.1 (swift-6.2.1-RELEASE)

This is the standard official image from swift.org (not the CI Dockerfile).

2. Execution Location

Yes, Docker was running on the same bare metal Ubuntu 24.04 host where I originally observed the issue:

• Host OS: Ubuntu 24.04.1 LTS (bare metal)
• Kernel: Linux 6.8.0-60-generic
• Docker: 27.4.1
• Container: swift:6.2.1 (official)

3. Reproduction in Docker

  docker run --rm swift:6.2.1 bash -c "
    mkdir -p test/Sources/test && cd test
    echo '// swift-tools-version: 6.0
  import PackageDescription
  let package = Package(name: \"test\", targets: [.target(name: \"test\")])' > Package.swift
    echo 'public func hello() {}' > Sources/test/main.swift

    swift build  # ✅ SUCCESS (~9s)
    echo '// change' >> Sources/test/main.swift
    timeout 30 swift build  # ❌ Hangs at 'Planning build'
  "

The hang reproduces in the container as well.

4. Additional Context

One thought on why GitHub runners might not see this: if CI starts with a fresh workspace each time, it would only run clean builds (which work fine). The hang appears specifically during incremental builds when .build/ already exists.

Happy to run any additional diagnostics (strace, etc.) if that would be helpful!

[finagolfin]

One thought on why GitHub runners might not see this: if CI starts with a fresh workspace each time, it would only run clean builds (which work fine). The hang appears specifically during incremental builds when .build/ already exists.

Sure, but I wasn't saying that people were building incrementally on GitHub, but that we should try to "reproduce on the GitHub runners." I just tried it and could not reproduce, finagolfin/swift-android-sdk#225, suggesting this is specific to your environment.

I notice that it fails for you when checking dependencies and that you are based out of Russia: is it possible that SwiftPM is checking GitHub and it is having network issues there?

I think your strace option and maybe running in lldb are the ways to figure this out now, but this problem is clearly not reproducible in most linux environments, only yours.

[MahdiBM]

I have not gone through the whole post so excuse me if I've missed some important info.

For the record I basically do incremental builds everywhere in GitHub CI (like here in penny-bot) via caching/restoring the .build directory, and I have yet to encounter this hang issue. So the issue probably requires some more specifics.

I'm also unable to reproduce this locally (via podman and the provided docker reproduction command).

off-hand I'd guess there might be some emulation in play that makes things hang?

[flyer2001]

@finagolfin

swiftpm-strace.log.gz

I notice that it fails for you when checking dependencies and that you are based out of Russia: is it possible that SwiftPM is checking GitHub and it is having network issues there?

Good hypothesis, but strace proves it's not network-related.

I ran a full strace trace during the hang (attached: swiftpm-strace.log.gz, 170KB). Key findings:

1. NO network calls to GitHub during the hang
2. Process enters livelock in epoll_wait():

Process 285 (swift-build main thread):
285 11:24:32.255260 epoll_wait(3, [{events=EPOLLIN, data={u32=3, u64=3}}], 16, -1) = 1
285 11:24:33.254738 timerfd_settime(5, TFD_TIMER_ABSTIME, ...) = 0
285 11:24:33.255333 epoll_ctl(3, EPOLL_CTL_MOD, 5, {events=EPOLLIN|EPOLLONESHOT, ...}) = 0
285 11:24:33.255474 epoll_wait(3, [], 16, 0) = 0
285 11:24:33.255572 epoll_wait(3, [{events=EPOLLIN, data={u32=3, u64=3}}], 16, -1) = 1
[repeats every second]

3. New finding: KVM-specific issue
   • ✅ Reproduces on KVM virtualization (both my desktop and VPS use KVM)
   • ❌ Does NOT reproduce on macOS Docker Desktop (tested today, builds in 0.17s)

This suggests the issue is related to epoll/timerfd behavior in KVM environments, not a general Linux or network problem.

I'm reaching out to my hosting provider to investigate KVM-specific kernel parameters or configurations.

I think your strace option and maybe running in lldb are the ways to figure this out now

Agreed. Would an lldb trace or additional debugging be helpful for the SwiftPM team?

[flyer2001]

@MahdiBM

off-hand I'd guess there might be some emulation in play that makes things hang?

Interesting! You might be onto something.

I can confirm:

• KVM virtualization (systemd-detect-virt returns kvm)
• NOT emulation (native x86_64, no QEMU user-mode)

However, the fact that it works fine in your podman setup but fails on my machines suggests it's environment-specific within KVM.

Key differences I can think of:

1. Kernel version (mine: 6.8.0-60, yours: ?)
2. KVM configuration (host CPU features, nested virtualization, I/O settings)
3. Resource limits (my VPS has only 1 CPU core, 961Mi RAM)

Do you happen to know your kernel version and KVM host details? That could help narrow down the root cause.

The strace livelock pattern (epoll_wait waking every 1s via timerfd) suggests something in the event polling subsystem behaves differently in certain KVM setups.

[finagolfin]

@sebsto and @FranzBusch, is this KVM issue something the SSWG has seen? As people increasingly use Swift on Server, these occasional linux KVM incompatibilities for the toolchain itself will need to be looked into.