Copy lock file only when needed

[ben-z]

Description

Terragrunt copies lock files into the working directory every time Terraform is initialized. This is inconsistent with the Terraform behaviour where the lock file is only written to when changed.

This is important when the working directory is on a readonly filesystem. I work with an organization that does this to prevent unintentional code changes during provisioning.

This PR makes it so that copying the lock file is avoided when the file has no changes.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Updated the lock file copy mechanism to avoid copying when the lock file is unchanged. Useful for using a readonly working directory.

[denis256]

[INFO] Initializing environment for https://github.com/gruntwork-io/pre-commit.
Terraform fmt............................................................Passed
goimports................................................................Failed
- hook id: goimports
- files were modified by this hook

util/file.go
util/file.go

[ben-z]

@denis256 I had some trailing whitespaces. Fixed in the latest commit by running pre-commit run --all-files:

terragrunt/.circleci/config.yml

Line 105 in e5081e1

pre-commit run --all-files

[denis256]

util/file.go:613:5: stringXbytes: suggestion: bytes.Equal(sourceContents, destinationContents) (gocritic)
        if string(sourceContents) == string(destinationContents) {

[ben-z]

Resolved in b3b9287

[denis256]

Failed integration tests:

TestRenderJSONConfig
TestLocalDownload
TestLocalDownloadWithHiddenFolder
TestLocalDownloadWithRelativePath
TestLocalWithBackend
TestRemoteDownload
TestRemoteDownloadWithRelativePath
TestRemoteDownloadOverride
TestRemoteWithBackend
TestExcludeDirs
TestIncludeDirs
TestIncludeDirsDependencyConsistencyRegression
TestTerraformRegistryFetchingRootModule
TestTerraformRegistryFetchingRootShorthandModule
TestTerraformRegistryFetchingSubdirModule
TestTerraformRegistryFetchingSubdirWithReferenceModule
TestTerragruntInitHookWithSourceNoBackend
TestTerragruntInitHookWithSourceWithBackend
TestTerragruntCatchErrorsInTerraformExecution
TestTerragruntStdOut

[denis256]

Will be also helpful to add an integration test which will track that the lock copy works as expected.

[ben-z]

Will be also helpful to add an integration test which will track that the lock copy works as expected.

I added some unit tests. Integration tests seem a lot more involved and I'm not super familiar with Go. Are unit tests sufficient?