Pub is unable to solve version contraints in some cases

[DartBot]

Originally opened as dart-lang/sdk#21325

This issue was originally filed by luiz.mine...@gmail.com

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What steps will reproduce the problem?

1. clone https://github.com/luizmineo/dart-pub-errors
2. run pub get in error2/test

What is the expected output? What do you see instead?

Pub should download and install all dependencies. Instead, it hangs for about 20 minutes, and exits with the follwing message:

Resolving dependencies... (13:27.3s)
Connection closed before full header was received

What version of the product are you using?

Dart 1.7.0-dev.4.6

On what operating system?

Ubuntu 14.04 x64

Please provide any additional information below.

For more details, please see issue #1163

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Conservatively assuming that this is a release blocker.

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cc @munificent.
cc @alan-knight.
cc @ricowind.
cc @whesse.
Set owner to @nex3.
Added this to the 1.7 milestone.
Removed Priority-Unassigned label.
Added Priority-Critical, Area-Pub, Accepted labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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We should figure out if this reproduces with 1.6.x stable, 1.6.0-dev.x.x, and also the 1.7.x stable release candidates.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3476088?v=3" align="left" width="48" height="48"hspace="10"> Comment by alan-knight

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I think the only issue here is that pub could fail more gracefully in this case. I think the given constraints are unsolvable, but not in a way that's easily recognized, so it thrashes trying an exponential number of possibilities. But Nathalie is back and can speak to it much more knowledgeably.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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If this is a "known" issue and it's only about failing more gracefully in case of unsolvable constraints, we need to figure out what's causing this with 1.7. Is this a regression? A change of -dev version semantics? We need to understand the implications of rolling this out to all stable channel users. Please enlighten us :-)

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3476088?v=3" align="left" width="48" height="48"hspace="10"> Comment by alan-knight

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It does not appear to be a regression or a change in semantics. If you remove the SDK constraints on 1.7-dev and loosen the stack_trace constraint which is incompatible with pub 1.6, then this reproduces in 1.6.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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This is definitely the same issue that we've seen before with pub failing poorly on an unsolvable version graph. I'm merging this into the tracking bug for that issue, and I don't think it should block the release.

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Added Duplicate label.
Marked as being merged into #912.