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Fix fdRead: handle io.Reader corner cases #740

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Aug 12, 2022
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Fix fdRead: handle io.Reader corner cases #740

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Fix fdRead: handle io.Reader corner cases
ncruces Aug 8, 2022
2cb0f0e
backfills docs on rationale
Aug 11, 2022
884c394
backfills tests
Aug 12, 2022
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5 changes: 5 additions & 0 deletions wasi_snapshot_preview1/fs.go
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Original file line number Diff line number Diff line change
Expand Up @@ -411,7 +411,12 @@ var fdRead = wasm.NewGoFunc(
if errors.Is(err, io.EOF) {
break
} else if err != nil {
if n != 0 { // Let callers process the n > 0 bytes returned before considering the error err.
break
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doesn't this swallow the error? how can they consider the err if there's none returned?

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@ncruces ncruces Aug 10, 2022 •
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TLDR: they handle the error on their next call to readv (fread, etc).

If you assume the error is stable (e.g. an external disk was removed), once they readv again they'll get the same error.

OTOH, if the error is transient (which is unlikely for real filesystems), once they call readv again the error will be lost, but at least there is no data loss.

So yes, this may swallow errors in 2 situations:

  1. if n bytes are enough for the caller, and they don't readv again.
  2. if err is transient, and isn't returned on a future call to readv.

Both seem legitimate reasons to ignore the error, rather than loosing/ignoring the data.

}
return ErrnoIo
} else if n < len(b) { // Partial read, don't read into the next buffer.
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cool

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Yes, this was the one that worried me the most.

break
}
}
if !mod.Memory().WriteUint32Le(ctx, resultSize, nread) {
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