Don't retry invalid credentials from git credential helpers

[hmarr]

If a git credential helper returns invalid credentials, we currently get stuck in an infinite retry loop by calling the credentials callback over and over, each time returning the same invalid credentials. This change means we only invoke the credential helper once.

How to reproduce

1. Create a git credential store with some invalid credentials:

echo "https://example-user:invalid-credentials@github.com" > ~/invalid-store

2. Tell git to use that as your credential store by adding this to your ~/.gitconfig:

[credential]
	helper = store --file=/home/<user>/invalid-store

3. Add an invalid Git dependency to a Cargo.toml. For instance:

[dependencies.fake-repository]
git = "https://github.com/fake-user/fake-repository"
version = ">= 1.0.0"

4. Try to update the dependencies (e.g. with cargo update or cargo build).

Cargo hangs forever, retrying the invalid credentials.

[rust-highfive]

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[hmarr]

Looks like the test failure is unrelated - quoting the comment above the test that failed:

/// This is a form of randomized testing, if you are unlucky it can fail.
/// A failure on its own is not a big deal. If you did not change the
/// registry_strategy then feel free to retry without concern.

[alexcrichton]

Thanks for this! Could this piggy-back off cred_helper_bad perhaps? Additionally, could the comment above this function be updated to indicate why this is being checked?

[hmarr]

Thanks for your quick reply!

I've updated the comment and reused the existing variable. Let me know if there's anything else that could be improved 🙂

[alexcrichton]

@bors: r+

[bors]

📌 Commit 96ab67b has been approved by alexcrichton

[bors]

⌛ Testing commit 96ab67b with merge b33ce7f...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: alexcrichton
Pushing b33ce7f to master...