`FileManager.default.createFile()` doesn't work

[kkebo]

Description

FileManager.default.createFile() doesn't work.

Steps to reproduce

1. mkdir foo
2. cd foo
3. swift package init --type executable
4. Edit Sources/main.swift

   import Foundation
   
   print(FileManager.default.createFile(atPath: "/tmp/foo", contents: Data()))

5. swift build --swift-sdk <Swift SDK for Wasm>
6. wasmtime run --dir /tmp .build/debug/foo.wasm

Expected behavior

$ wasmtime run --dir /tmp .build/debug/foo.wasm
true

Actual behavior

$ wasmtime run --dir /tmp .build/debug/foo.wasm
false

Environment

$ uname -a
Linux Raspberry-beetle 6.6.51+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.6.51-1+rpt2 (2024-10-01) aarch64 GNU/Linux
$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
$ swift --version
Swift version 6.0.2-dev (LLVM 43d73cb0cc589f7, Swift aaa632cea622394)
Target: aarch64-unknown-linux-gnu
$ swift sdk list
6.0-SNAPSHOT-2024-09-18-a-wasm32-unknown-wasi
$ wasmtime -V
wasmtime 25.0.1 (b4cb894c9 2024-09-24)

Supplement

swift-foundation's createFile() implementation forces to use the option .atomic.

So we can see the detailed error by running try Data().write(to: URL(filePath: "/tmp/foo"), options: .atomic).

Swift/ErrorType.swift:253: Fatal error: Error raised at top level: FoundationEssentials.CocoaError(code: FoundationEssentials.CocoaError.Code(rawValue: 3328), userInfo: ["NSUserStringVariant": "Folder"])
Error: failed to run main module `.build/debug/hoge.wasm`

Caused by:
    0: failed to invoke command default
    1: error while executing at wasm backtrace:
           0: 0x6c1b71 - <unknown>!$ss17_assertionFailure__4file4line5flagss5NeverOs12StaticStringV_SSAHSus6UInt32VtF
           1: 0x76a8da - <unknown>!swift_errorInMain
           2: 0x2c755 - <unknown>!main
           3: 0xc2f036 - <unknown>!__main_void
           4: 0x2c2b9 - <unknown>!_start
       note: using the `WASMTIME_BACKTRACE_DETAILS=1` environment variable may show more debugging information
    2: wasm trap: wasm `unreachable` instruction executed

If options contain .atomic, createFile() will call writeToFileAux(), and writeToFileAux() will call createProtectedTemporaryFile() and createTemporaryFile(). However, SwiftWasm doesn't support createTemporaryFile().

How about stopping adding .atomic in createFile() if os(WASI)?

[kkebo]

Another solution is not to call createTemporaryFile() from createProtectedTemporaryFile() if os(WASI). createTemporaryFile() is only used in createProtectedTemporaryFile(), so I think we don't necessary have to fix createTemporaryFile() if we fix on the createProtectedTemporaryFile() side.

diff --git a/Sources/FoundationEssentials/Data/Data+Writing.swift b/Sources/FoundationEssentials/Data/Data+Writing.swift
index 7c23e1e..29815b0 100644
--- a/Sources/FoundationEssentials/Data/Data+Writing.swift
+++ b/Sources/FoundationEssentials/Data/Data+Writing.swift
@@ -237,9 +237,28 @@ private func createProtectedTemporaryFile(at destinationPath: String, inPath: Pa
     }
 #endif
     
+#if os(WASI)
+    let url = URL(fileURLWithPath: destinationPath, isDirectory: false)
+    let temporaryDirectoryPath = try FileManager.default.url(for: .itemReplacementDirectory, in: .userDomainMask, appropriateFor: url, create: true).path(percentEncoded: false)
+    let auxFile = temporaryDirectoryPath.appendingPathComponent(destinationPath.lastPathcomponent)
+    return try auxFile.withFileSystemRepresentation { auxFileFileSystemRep in
+        guard let auxFileFileSystemRep else {
+            throw CocoaError(.fileWriteInvalidFileName)
+        }
+        let fd = openFileDescriptorProtected(path: auxFileFileSystemRep, flags: O_CREAT | O_EXCL | O_RDWR, options: options)
+        if fd >= 0 {
+            return (fd, auxFile, temporaryDirectoryPath)
+        } else {
+            let savedErrno = errno
+            cleanupTemporaryDirectory(at: temporaryDirectoryPath)
+            throw CocoaError.errorWithFilePath(inPath, errno: savedErrno, reading: false, variant: variant)
+        }
+    }
+#else
     let temporaryDirectoryPath = destinationPath.deletingLastPathComponent()
     let (fd, auxFile) = try createTemporaryFile(at: temporaryDirectoryPath, inPath: inPath, prefix: ".dat.nosync", options: options, variant: variant)
     return (fd, auxFile, nil)
+#endif
 }
 
 private func write(buffer: UnsafeRawBufferPointer, toFileDescriptor fd: Int32, path: PathOrURL, parentProgress: Progress?) throws {

https://github.com/swiftlang/swift-foundation/blob/3f9171072dc842c04c77ed2fa578d1b0cd123a04/Sources/FoundationEssentials/Data/Data%2BWriting.swift#L240-L242

[kateinoigakukun]

I think .itemReplacementDirectory is not always writable on WASI, so I prefer the way of stopping adding .atomic.

[kkebo]

That makes sense, and I agree with the use of non-atomic writing for createFile().

On the other hand, though this is off the main topic of the issue, I think it would be worthwhile to support the creation of temporary files using .itemReplacementDirectory for those who want to use atomic writing (e.g. write(to: ..., options: .atomic)).

[kateinoigakukun]

How about allowing atomic file writing when TMPDIR environment variable is explicitly defined?

[kkebo]

Thank you for your consideration. However, this comment has changed my mind, and I'm beginning to think that relying on $TMPDIR is not a good idea.

[kkebo]

I created another issue #5594 for discussing atomic file writing.

[kkebo]

Resolved by swiftlang/swift-foundation#992