`--dir .` and `--dir /` conflict with each other

[kkebo]

Description

When I run wasmkit run --dir . --dir / <Wasm binary>, it succeeds or fails randomly. When I use --dir .build --dir / or --dir . --dir /Users instead, it always succeeds. So I think --dir . and --dir / conflict with each other.

Steps to reproduce

1. Run mkdir foo && cd foo

2. Run swift package init --type library

3. Edit Sources/foo/foo.swift

   import Foundation
   
   @main
   struct foo {
       static func main() {
           print(try! String(contentsOf: URL(filePath: #filePath), encoding: .utf8))
           print(Bundle.main)
       }
   }

4. Run swift build --swift-sdk swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a_wasm

5. Run wasmkit run --dir . --dir / .build/wasm32-unknown-wasip1/debug/foo.wasm several times

Expected behavior

It succeeds every time. Or it fails every time with the same error messages.

Actual behavior

It always fails, but its error messages randomly change each time it is executed.

Pattern 1:

import Foundation                                                                                                                                    
                                                                                                                                                     
@main                                                                                                                                                
struct foo {                                                                                                                                         
    static func main() {                                                                                                                             
        print(try! String(contentsOf: URL(filePath: #filePath), encoding: .utf8))                                                                    
        print(Bundle.main)                                                                                                                           
    }                                                                                                                                                
}                                                                                                                                                    
                                                                                                                                                     
Error: Trap: unreachable                                                                                                                             
    0: $s10Foundation6BundleC05_mainB033_6273FB07A96EB35BCC520B3A80F5C3CDLL_WZ                                                                       
    1: swift_once                                                                                                                                    
    2: $s10Foundation6BundleC4mainACvgZ
    3: $s3fooAAV4mainyyFZ            
    4: $s3fooAAV5$mainyyFZ
    5: main
    6: __main_void
    7: _start
    8: unknown

Pattern 2:

foo/foo.swift:9: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=NSCocoaErrorDomain Code=260 "The file doesn’t exist."UserI
nfo={NSFilePath=/Users/kebo/foo/Sources/foo/foo.swift, NSURL=file:///Users/kebo/foo/Sources/foo/foo.swift, NSUnderlyingError=Error Domain=NSPOSIXErro
rDomain Code=44 "No such file or directory"}
Error: Trap: unreachable
    0: $ss17_assertionFailure__4file4line5flagss5NeverOs12StaticStringV_SSAHSus6UInt32VtF
    1: swift_unexpectedError
    2: $s3fooAAV4mainyyFZ
    3: $s3fooAAV5$mainyyFZ
    4: main
    5: __main_void
    6: _start
    7: unknown

Environemnt

$ uname -a
Darwin Xylotrupes-gideon.local 25.1.0 Darwin Kernel Version 25.1.0: Sun Oct  5 21:11:28 PDT 2025; root:xnu-12377.40.120~10/RELEASE_ARM64_T8132 arm64
$ which swift
/Users/kebo/Library/Application Support/swiftly/bin/swift
$ swiftly list
Installed release toolchains
----------------------------
Swift 6.2.0 (default)

Installed snapshot toolchains
-----------------------------
main-snapshot-2025-10-02 (in use)
main-snapshot-2025-09-20
main-snapshot-2025-09-06
$ swift --version
Apple Swift version 6.3-dev (LLVM 0d0246569621d5b, Swift 199240b3fe97eda)
Target: arm64-apple-macosx26.0
Build config: +assertions
$ which wasmkit
/Users/kebo/Library/Application Support/swiftly/bin/wasmkit
$ wasmkit --version
0.1.6

[kkebo]

Since swiftlang/swift-package-manager#9114, SwiftPM has started passing --dir . to WasmKit when running swift test for Wasm targets. Because of this, I can't add --dir / to testRunner's extraCLIOptions in toolset.json.

[kkebo]

FYI, I did some print debugging.

diff

diff --git a/Sources/WASI/WASI.swift b/Sources/WASI/WASI.swift
index 5bd648c..f0e22dc 100644
--- a/Sources/WASI/WASI.swift
+++ b/Sources/WASI/WASI.swift
@@ -1410,6 +1410,7 @@ public class WASIBridgeToHost: WASI {
                 _ = try fdTable.push(.directory(DirEntry(preopenPath: guestPath, fd: fd)))
             }
         }
+        print(fdTable)
         self.fdTable = fdTable
         self.wallClock = wallClock
         self.monotonicClock = monotonicClock
@@ -1774,6 +1775,7 @@ public class WASIBridgeToHost: WASI {
         fsRightsInheriting: WASIAbi.Rights,
         fdflags: WASIAbi.Fdflags
     ) throws -> WASIAbi.Fd {
+        print("path_open", dirFd)
         #if os(Windows)
             throw WASIAbi.Errno.ENOTSUP
         #else

results:

$ ~/WasmKit/.build/release/wasmkit-cli run --dir .build --dir / .build/wasm32-unknown-wasip1/debug/foo.wasm
FdTable(map: [2: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 2), accessMode: WASI.FileAccessMode(rawValue: 2))),
 1: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 1), accessMode: WASI.FileAccessMode(rawValue: 2))), 0: WASI.FdEn
try.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 0), accessMode: WASI.FileAccessMode(rawValue: 1))), 3: WASI.FdEntry.directory
(WASI.DirEntry(preopenPath: Optional("/"), fd: SystemPackage.FileDescriptor(rawValue: 3))), 4: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Opti
onal(".build"), fd: SystemPackage.FileDescriptor(rawValue: 4)))], nextFd: 5)                                                                         
path_open 3                                                                                                                                          
import Foundation                                                                                                                                    
                                                                                                                                                     
@main                                                                                                                                                
struct foo {                                                                                                                                         
    static func main() {                                                                                                                             
        print(try! String(contentsOf: URL(filePath: #filePath), encoding: .utf8))                                                                    
        print(Bundle.main)                                                                                                                           
    }                                                                                                                                                
}                                                                                                                                                    
                                                                                                                                                     
path_open 4                                                                                                                                          
path_open 4                                                                                                                                          
Bundle </.build/wasm32-unknown-wasip1/debug> (loaded)                                                                                                
$ ~/WasmKit/.build/release/wasmkit-cli run --dir . --dir /Users .build/wasm32-unknown-wasip1/debug/foo.wasm
FdTable(map: [3: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Optional("."), fd: SystemPackage.FileDescriptor(rawValue: 3))), 2: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 2), accessMode: WASI.FileAccessMode(rawValue: 2))), 0: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 0), accessMode: WASI.FileAccessMode(rawValue: 1))), 1: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 1), accessMode: WASI.FileAccessMode(rawValue: 2))), 4: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Optional("/Users"), fd: SystemPackage.FileDescriptor(rawValue: 4)))], nextFd: 5)
path_open 4
import Foundation

@main
struct foo {
    static func main() {
        print(try! String(contentsOf: URL(filePath: #filePath), encoding: .utf8))
        print(Bundle.main)
    }
}

path_open 3
path_open 3
Bundle </.build/wasm32-unknown-wasip1/debug> (loaded)
$ ~/WasmKit/.build/release/wasmkit-cli run --dir . --dir / .build/wasm32-unknown-wasip1/debug/foo.wasm
FdTable(map: [4: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Optional("."), fd: SystemPackage.FileDescriptor(rawValue: 4))), 0: WASI.FdEntry.fi
le(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 0), accessMode: WASI.FileAccessMode(rawValue: 1))), 2: WASI.FdEntry.file(WASI.Stdio
FileEntry(fd: SystemPackage.FileDescriptor(rawValue: 2), accessMode: WASI.FileAccessMode(rawValue: 2))), 1: WASI.FdEntry.file(WASI.StdioFileEntry(fd:
 SystemPackage.FileDescriptor(rawValue: 1), accessMode: WASI.FileAccessMode(rawValue: 2))), 3: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Opti
onal("/"), fd: SystemPackage.FileDescriptor(rawValue: 3)))], nextFd: 5)
path_open 4
foo/foo.swift:6: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=NSCocoaErrorDomain Code=260 "The file doesn’t exist."UserInfo={NSUnderlyingError=Error Domain=NSPOSIXErrorDomain Code=44 "No such file or directory", NSFilePath=/Users/kebo/foo/Sources/foo/foo.swift, NSURL=file:///Users/kebo/foo/Sources/foo/foo.swift}
Error: Trap: unreachable    0: $ss17_assertionFailure__4file4line5flagss5NeverOs12StaticStringV_SSAHSus6UInt32VtF
    1: swift_unexpectedError
    2: $s3fooAAV4mainyyFZ
    3: $s3fooAAV5$mainyyFZ
    4: main
    5: __main_void
    6: _start
    7: unknown
$ ~/WasmKit/.build/release/wasmkit-cli run --dir . --dir / .build/wasm32-unknown-wasip1/debug/foo.wasm
FdTable(map: [0: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 0), accessMode: WASI.FileAccessMode(rawValue: 1))), 3: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Optional("."), fd: SystemPackage.FileDescriptor(rawValue: 3))), 1: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 1), accessMode: WASI.FileAccessMode(rawValue: 2))), 4: WASI.FdEntry.directory(WASI.DirEntry(preopenPath: Optional("/"), fd: SystemPackage.FileDescriptor(rawValue: 4))), 2: WASI.FdEntry.file(WASI.StdioFileEntry(fd: SystemPackage.FileDescriptor(rawValue: 2), accessMode: WASI.FileAccessMode(rawValue: 2)))], nextFd: 5)
path_open 4
import Foundation

@main
struct foo {
    static func main() {
        print(try! String(contentsOf: URL(filePath: #filePath), encoding: .utf8))
        print(Bundle.main)
    }
}

path_open 4
Error: Trap: unreachable
    0: $s10Foundation6BundleC05_mainB033_6273FB07A96EB35BCC520B3A80F5C3CDLL_WZ
    1: swift_once
    2: $s10Foundation6BundleC4mainACvgZ
    3: $s3fooAAV4mainyyFZ
    4: $s3fooAAV5$mainyyFZ
    5: main
    6: __main_void
    7: _start
    8: unknown

When using --dir . --dir /, a value passed to dirFd seems wrong.

[MaxDesiatov]

Would you be able to clarify what your use case is for exposing / in addition to .? How would you expect it to work? No matter what CWD is, / will overlap with all other directories, I don't think we have any defined strategy for merging overlapping source trees. Given that, I'm inclined to say that supporting multiple --dir options should not have been supported in the first place, and maybe even should be disabled until we come up with some clearly defined and desirable behavior for multiple options.

[kkebo]

@MaxDesiatov

Would you be able to clarify what your use case is for exposing / in addition to .? How would you expect it to work? No matter what CWD is, / will overlap with all other directories, I don't think we have any defined strategy for merging overlapping source trees. Given that, I'm inclined to say that supporting multiple --dir options should not have been supported in the first place, and maybe even should be disabled until we come up with some clearly defined and desirable behavior for multiple options.

My use case is to run swift-syntax's tests to ensure that swift-syntax is not broken when building for wasm32-unknown-wasi. This is for maintaining kkebo/swift-format (a Wasm port of swift-format). I have been running swift-syntax's tests on the wasm32-unknown-wasi target in CI since 2022.

kkebo/swift-syntax#414

Since the test runner depends on Bundle.main, Bundle.main requires --dir .build or --dir . as you described in swiftlang/swift-package-manager#9114. On the other hand, swift-syntax's tests require to access other directories, such as #filePath. So --dir .build or --dir . is insufficient. However, --dir / doesn't cover them, so both are needed.

--dir .build --dir / always succeeds. However, --dir . --dir .build --dir / succeeds or fails randomly, and --dir . --dir / always fails.

[kkebo]

If you run Bundle.main with wasmkit run --dir / <binary>, you will encounter the following error:

Error: Trap: unreachable
    0: $s10Foundation6BundleC05_mainB033_6273FB07A96EB35BCC520B3A80F5C3CDLL_WZ
    1: swift_once
    2: $s10Foundation6BundleC4mainACvgZ
    3: $s3fooAAV4mainyyFZ
    4: $s3fooAAV5$mainyyFZ
    5: main
    6: __main_void
    7: _start
    8: unknown

And if you run print(Bundle.main.bundlePath) with wasmkit run --dir . <binary>, you can see something like /.build/wasm32-unknown-wasip1/debug. This is why Bundle.main requires --dir .build or --dir ., not --dir /.