net/url: improve url parsing error messages by quoting

[stefanb]

Current implementation doesn't always make it obvious what the exact
problem with the URL is, so this makes it clearer by consistently quoting
the invalid URL, as is the norm in other parsing implementations, eg.:
strconv.Atoi(" 123") returns an error: parsing " 123": invalid syntax

Updates #29261

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[stefanb]

CLA process initiated with my employer.
#29261 is scheduled for 1.13 release in August, but this simpler, non-breaking change could be released earlier.
Example of non-obvious error message can be seen in https://play.golang.org/p/jkcYSD6ZRcO
Mentioned strconv.Atoi example can be tried in https://play.golang.org/p/7nBfovFSPdu,

[stefanb]

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Message from Stefan Baebler:

Patch Set 1:

Hey!

This is the simplest fix for #29261

It does not attempt expand the parsing algorithm to give more granular error messages, but it improves the error message by enclosing the problematic URL in quotes, consistently with other parsing functions. If the URL is empty or it starts or ends with a whitespace the improved error message should give ample hints about the root problem.

Since you are familiar with both:

• the original problem (net/url: misleading error message when url has a leading space #29261 ) and
• the complex fix (https://go-review.googlesource.com/c/go/+/155922/ ) that unfortunately got stuck

I am asking you to review this patch.

I do agree that the URL parsing should eventually be improved, but it does not seem this will happen for Go 1.13 release as planned. This simpler fix should be safer to merge without introducing bugs or breaking changes and should stay in the code even after more thorough URL parsing is implemented.

Thanks,
Štefan

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Patch Set 4:

(1 comment)

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Patch Set 5:

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Message from Daniel Martí:

Patch Set 11:

(5 comments)

Some nits; we can test once those are fixed.

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Patch Set 12:

(5 comments)

Thanks for the suggestions, all fixed!

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Patch Set 12: Run-TryBot+1 Code-Review+2

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[vagababov]

This actually caused quite a few errors in our unit tests due to quoting...

[ianlancetaylor]

Us too. See https://golang.org/cl/195978. We decided to bite the bullet and fix the tests.

[vagababov]

Yeah, in the end it wasn't that hard to patch the tests, but it was surprising :)