Make storage upload atomic - #325
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Upload opened the destination directly with O_TRUNC and streamed the request body straight into it. The source is a live sandbox HTTP stream that can break mid-transfer, which is a normal occurrence, not an edge case, but a mid-copy failure had already truncated whatever was at that key. A re-upload to the same filename, which happens any time a script or session step reruns and writes the same output name again, could destroy a prior good file and replace it with a partial one while the caller was told the upload failed and nothing happened. Write to a temp file in the same directory instead and rename it over the destination only once the copy and close both succeed. Any failure along the way removes the temp file and leaves the existing key untouched.
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Summary
Upload opened the destination directly with O_TRUNC and streamed the
request body straight into it. The source is a live sandbox HTTP stream
that can break mid-transfer, which is a normal occurrence, not an edge
case, but a mid-copy failure had already truncated whatever was at that
key. A re-upload to the same filename, which happens any time a script or
session step reruns and writes the same output name again, could destroy a
prior good file and replace it with a partial one while the caller was
told the upload failed and nothing happened.
Writes to a temp file in the same directory instead and renames it over
the destination only once the copy and close both succeed. Any failure
along the way removes the temp file and leaves the existing key untouched.
Test plan
go test -race ./pkg/storage/...must survive) and a failed upload to a brand-new key (nothing should
be left behind); confirmed both fail on the old code
go build ./...,go vet ./...