duplicates:

• SR-6638 Occasionally need to rm -rf when changing dependencies

relates to:

• SR-8380 Crash when a dependency depends on itself (cycle)

Issue Description:

I'm using Vapor for this example as that is what I was using originally.

Steps to reproduce:

1. `vapor new UpdateTest --template=api --branch=beta`

2. `vapor xcode` (not necessary I don't think but it is what I am using)

3. add `

.package(url: "https://github.com/vapor/auth.git", from: "2.0.0-rc.3.0.1"),` to your dependencies in `Package.swift` and add `Authentication` to the `App` target so it looks like the following:

```

// swift-tools-version:4.0

import PackageDescription

let package = Package(

name: "UpdateTest",

dependencies: \[

    // A server-side Swift web framework.

    .package(url: "https://github.com/vapor/vapor.git", from: "3.0.0-rc.2"),



    // Swift ORM (queries, models, relations, etc) built on SQLite 3.

    .package(url: "https://github.com/vapor/fluent-sqlite.git", from: "3.0.0-rc.2"),

    

    .package(url: "https://github.com/vapor/auth.git", from: "2.0.0-rc.3.0.1"),

\],

targets: \[

    .target(name: "App", dependencies: \["FluentSQLite", "Vapor", "Authentication"\]),

    .target(name: "Run", dependencies: \["App"\]),

    .testTarget(name: "AppTests", dependencies: \["App"\])

\]

)

```

4. Run `vapor update` (which runs swift package update), this should succeed

5. Change the auth depedency to read `

.package(url: "https://github.com/Noobish1/auth.git", .branch("token-optional-fix")),` so your `Package.swift` looks like the following:

```

// swift-tools-version:4.0

import PackageDescription

let package = Package(

name: "UpdateTest",

dependencies: \[

    // A server-side Swift web framework.

    .package(url: "https://github.com/vapor/vapor.git", from: "3.0.0-rc.2"),



    // Swift ORM (queries, models, relations, etc) built on SQLite 3.

    .package(url: "https://github.com/vapor/fluent-sqlite.git", from: "3.0.0-rc.2"),

    

    .package(url: "https://github.com/Noobish1/auth.git", .branch("token-optional-fix")),

\],

targets: \[

    .target(name: "App", dependencies: \["FluentSQLite", "Vapor", "Authentication"\]),

    .target(name: "Run", dependencies: \["App"\]),

    .testTarget(name: "AppTests", dependencies: \["App"\])

\]

)

```

6. Run `vapor update` again

Expected result: `vapor update` should succeed and our dependencies should be updated

Actual result: `vapor update` fails with the following message:

```

Error: backgroundExecute(code: 1, error: "error: terminated(128): git -C '/Users/Blair/Documents/iPhone Development/UpdateTest/.build/checkouts/auth.git-9199502942084726963' checkout -f 8bc7cc84b6fe7b6767ace857acb0592216c92c90 output:\n \n\n", output: "Fetching https://github.com/Noobish1/auth.git\nUpdating https://github.com/vapor/vapor.git\nUpdating https://github.com/vapor/fluent-sqlite.git\nUpdating https://github.com/vapor/validation.git\nUpdating https://github.com/vapor/service.git\nUpdating https://github.com/vapor/crypto.git\nUpdating https://github.com/vapor/routing.git\nUpdating https://github.com/vapor/template-kit.git\nUpdating https://github.com/vapor/console.git\nUpdating https://github.com/vapor/engine.git\nUpdating https://github.com/vapor/core.git\nUpdating https://github.com/vapor/database-kit.git\nUpdating https://github.com/apple/swift-nio.git\nUpdating https://github.com/apple/swift-nio-zlib-support.git\nUpdating https://github.com/apple/swift-nio-ssl.git\nUpdating https://github.com/apple/swift-nio-ssl-support.git\nUpdating https://github.com/vapor/fluent.git\nUpdating https://github.com/vapor/sqlite.git\nResolving https://github.com/Noobish1/auth.git at token-optional-fix\n")

```

Running `swift package update` also fails with this error:

```

Updating https://github.com/Noobish1/auth.git
Updating https://github.com/vapor/fluent-sqlite.git
Updating https://github.com/vapor/vapor.git
Updating https://github.com/vapor/routing.git
Updating https://github.com/vapor/database-kit.git
Updating https://github.com/vapor/crypto.git
Updating https://github.com/vapor/console.git
Updating https://github.com/vapor/core.git
Updating https://github.com/vapor/engine.git
Updating https://github.com/vapor/service.git
Updating https://github.com/vapor/template-kit.git
Updating https://github.com/vapor/validation.git
Updating https://github.com/apple/swift-nio.git
Updating https://github.com/apple/swift-nio-zlib-support.git
Updating https://github.com/apple/swift-nio-ssl.git
Updating https://github.com/apple/swift-nio-ssl-support.git
Updating https://github.com/vapor/sqlite.git
Updating https://github.com/vapor/fluent.git
Resolving https://github.com/Noobish1/auth.git at token-optional-fix
error: terminated(128): git -C '/Users/Blair/Documents/iPhone Development/UpdateTest/.build/checkouts/auth.git-9199502942084726963' checkout -f 8bc7cc84b6fe7b6767ace857acb0592216c92c90 output:

```