net/textproto: dont't ignore error in skipSpace function

[cz2h]

skipSpace ignores the error it encounters while using a reader
which seems like a bug. Refactor the function and have it
returns the errors.

Modified readContinuedLineSlice and have it ignore io.EOF
while something is ever read in the function to keep its
behaviour unchanged.

Update #53858

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Message from Ian Lance Taylor:

Patch Set 1:

(3 comments)

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Message from chuhao zeng:

Patch Set 2:

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Patch Set 2:

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Patch Set 3:

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Patch Set 3:

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[kayrus]

@zengchu2 why this PR was closed?

[cz2h]

@zengchu2 why this PR was closed?

It seems that changes to skipSpace will affect the behaviour of readContinuedLine, making it return a EOF after reading the last line of text. On the other hand, if we somehow ignore this EOF if we are returning something, then this change seems not that necessary cause it cannot fix corner case mentioned in the issue.

Probably there is another way of fixing that issue?

[kayrus]

Probably there is another way of fixing that issue?

not sure, but silently swallowing the err is also not good :\

[cz2h]

Agree with that. But I also found it little bit hard to provide a proper example test case for this refactoring.

For regular io readers, the error can be detected during the next time when readContinuedLine is called. I guess this is why the error is ignored in skipSpace