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fix: return a safe file handle in dry-run mode - #879
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Create() and CreateTemp() returned `&os.File{}` in dry-run mode, which
panics on Name() (nil inner *file). Handing out os.NewFile(0, os.DevNull)
instead would alias stdin, so doDownload's file.Close() closed fd 0 and
`s5cmd --dry-run run < commands.txt` failed after the first download.
Open os.DevNull for writing and return that handle instead.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This was referenced Sep 17, 2026
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Context
cp --dry-runfor a download panicked on the destination file handle, and the obvious fix (a/dev/nullhandle fromos.NewFile(0, …)) introduces a worse bug: fd 0 is stdin.Root cause
Filesystem.Create()andCreateTemp()returned&os.File{}in dry-run mode. Its inner*fileis nil, soName()panics. Wrapping fd 0 instead aliases stdin:doDownloadcallsfile.Close(), which closes stdin, ands5cmd --dry-run run < commands.txtstops reading after the first download (it hangs).Fix
Return
os.OpenFile(os.DevNull, os.O_WRONLY, 0)— a real, independent handle that is safe to write, name and close. No file is created on disk.Tests
storage/fs_test.go:TestFilesystemCreateDryRun(Create and CreateTemp) — non-nil handle,Name() == os.DevNull,Fd() != 0, Write and Close succeed, stdin still usable afterwards, nothing on disk.e2e/run_test.go:TestRunDryRunDownloadFromStdin— pipes 200cp s3://… dir/lines into--dry-run run. Hangs (10 mingo testtimeout) with an fd-0 handle; passes with this fix.Linux, Go 1.24,
go test -count=1 -race ./...: all packagesok(command,e2e,orderedwriter,progressbar,storage,storage/url,strutil).gofmt,go vet,staticcheck,unparam,semgrep: clean.🤖 Generated with Claude Code