[APT-1640] Add style guides (#143)

Author: ebeneliason

_docs-sources/guides/index.md

@@ -116,4 +116,9 @@ The Gruntwork IaC library empowers you to construct your own bespoke architectur
 
 </Grid>
 
+## Style Guides
+
+- [Terraform Style Guide](/docs/guides/style/terraform-style-guide)
+- [Go Style Guide](/docs/guides/style/golang-style-guide)
+
 </CenterLayout>

_docs-sources/guides/style/golang-style-guide.md

@@ -0,0 +1,261 @@
+---
+category: Style Guides
+excerpt: The style guide for the Go programming language that is used by all Gruntwork-owned code repositories that contain Go code.
+tags: ["go", "golang", "code", "style"]
+---
+
+# Go Style Guide
+
+This is Gruntwork’s style guide for the Go programming language. It aims to help us ensure that the code we write is
+clear, readable, idiomatic Go code. The conventions detailed in this guide are our preferences and should be thought of
+as guidelines rather than hard rules.
+
+## Starting point
+
+We use the excellent [Effective Go](https://golang.org/doc/effective_go.html) guide as the starting point for our style
+guide. **Unless explicitly mentioned otherwise, we default to following the conventions outlined in Effective Go.**
+
+Another helpful resource is the [CodeReviewComments](https://github.com/golang/go/wiki/CodeReviewComments) section of the
+Go GitHub wiki page.
+
+Below you will find the list of conventions that differ from the above mentioned documents, either because they are
+specific to Gruntwork, or because we consciously chose to go against what Effective Go and CodeReviewComments recommend.
+
+## Additional conventions
+
+### General
+
+Before committing any Go code, run `go fmt`. We run this as part of the CI build, so this will prevent your build from failing.
+
+### Comments
+
+Every public function `Foo` should have a `//` comment of the style `Foo <explanation>`,
+where explanation says what `Foo` does, and more importantly, why it does that, what assumptions it makes, and other
+aspects not obvious from the function name.
+
+### Control structures
+
+When possible, avoid nesting `if`/`else` statements. (Of course, a single, non-nested `if`/`else` is perfectly fine.)
+Prefer multiple `return` statements over nested code. This makes the code more readable and avoids complexity.
+
+E.g:
+
+```go
+// Do this
+if something {
+        doThis()
+        return this
+}
+if orOther {
+        return that
+}
+return theOther
+```
+
+```go
+// Don't do this
+if something {
+        doThis()
+} else {
+        if orOther {
+                return that
+        }
+}
+```
+
+### Error handling
+
+Prefer using the `errors` standard library package for handling single errors. For operations that can produce multiple
+errors, leverage the [`MultiError`](https://github.com/gruntwork-io/terragrunt/blob/master/errors/multierror.go)
+package by [accumulating
+all the errors into a single `MultiError` and returning that](https://github.com/gruntwork-io/terragrunt/blob/cb369119bf5c6f3031e914e8554ffe056dcf9e22/cli/hclfmt.go#L62), rather than returning every error individually as it comes up.
+
+[Don’t panic](https://github.com/golang/go/wiki/CodeReviewComments#dont-panic) (_hat tip, Douglas Adams_). Any method that
+can have an error should return that error as its final value. This should be passed up through each layer, which can
+decide what to do with it, all the way up to the very entrypoint of the app (or `main`) if necessary.
+You should almost NEVER use `panic`.
+
+Use custom error types. Create your own types that implement the `error` interface so that error messages are clear
+and have well-defined types you can check against. For some examples of this, see e.g. the custom errors in the
+[aws](https://github.com/gruntwork-io/terratest/blob/master/modules/aws/errors.go) package of `terratest`.
+
+Include stack traces. In most of our code, we have to wrap errors with
+[`errors.WithStackTrace(e)`](https://github.com/gruntwork-io/gruntwork-cli/blob/master/errors/errors.go#L22) to add the stack trace.
+Go annoyingly doesn’t do this by default, but without it, sorting out an error can be very tricky.
+
+### Pointer usage
+
+Prefer using value type over pointers, unless necessary. Generally speaking, there are only a few cases where pointer
+usage would be justified:
+
+1.  You have very large structs containing lots of data. Instead of copying these around, passing pointers may be more
+    efficient.
+
+2.  You need to mutate a variable you passed to a function. Generally speaking, you should avoid this anyway (see the
+    section on immutability below), but sometimes it is necessary (think "rename" methods, etc).
+
+3.  You need to return `nil` as the default zero value (the default zero value for pointers is `nil`, as opposed to other data
+    types that have non-nil default zero values).
+
+### Testing
+
+In terms of testing, we don’t necessarily aim for 100% test coverage. Rather, our goal is to have enough testing to give
+us confidence that our code works and allow us to update it quickly. When adding tests, keep in mind these factors:
+
+1.  The likelihood of bugs: some code is harder to get right than others, and merits correspondingly more tests.
+
+2.  The cost of bugs: some bugs are much more costly than others — e.g., bugs in payment systems or security functionality —
+    so it makes sense to invest more in testing there.
+
+3.  The cost of tests: some types of tests are cheap and easy to write and maintain, whereas others are very costly and brittle.
+
+Unless there’s a reason not to, tests should run in parallel. This is done by including a call to `t.Parallel` in the test function.
+
+#### Setup and Teardown pattern
+
+In some cases you will want to write a group of tests that use a common resource, such as a Docker image or VPC.
+In this case, you will want to setup the common resource once, run a bunch of tests, and then teardown the resource.
+To achieve this, you can follow [the subtest pattern](https://blog.golang.org/subtests) of Go.
+
+Always use [table driven tests](https://dave.cheney.net/2019/05/07/prefer-table-driven-tests) where possible to make the
+subtest routines maintainable. Briefly, this means that you group your test cases using a test struct that reflects
+the unique parameters of the test cases. Then you can conveniently loop over the test cases in parallel, taking advantage
+of uniformity and speed.
+
+Note that the subtest pattern has gotchas when running tests in parallel:
+
+- The main test function will not wait for the subtest to run if it uses `t.Parallel`. To avoid this, you need to wrap
+  the parallel subtests in a synchronous, blocking subtest. In the example below, the `group` subtest is synchronous
+  (no call to `t.Parallel`) and thus the main function will wait until that test finishes. The `group` test does not
+  finish until all the subtests it spawns are finished, even if they are non-blocking and parallel, and thus the
+  `tearDownVPC` call does not happen until all subtests are done.
+
+- If you are using table driven tests, the range variable will be updated to the next iteration before it is used within
+  the subtest. That is, in the example below, if we did not have the `testCase := testCase` line in the range block,
+  the `testCase` reference used in the subtest after the `t.Parallel` call will correspond to the last `testCase` in the
+  `testCases` list. To avoid this, we create a new variable in the scope of the range block so that it does not get
+  updated during the loop.
+
+Example:
+
+```go
+func TestECS(t *testing.T) {
+    t.Parallel()
+
+    defer tearDownVPC()
+    deployVPC()
+
+    // Wrap the parallel tests in a synchronous test group to
+    // ensure that the main test function (the one calling
+    // `tearDownVPC` and `deployVPC`) waits until all the
+    // subtests are done before running the deferred function.
+    t.Run("group", func(t *testing.T) {
+        for _, testCase := range testCases {
+            // To avoid the range variable from getting updated in the
+                // parallel tests, we bind a new name that is within the
+                // scope of the for block.
+            testCase := testCase
+            t.Run(testCase.name, func(t *testing.T) {
+                t.Parallel()
+                testCase.testCode()
+            })
+        }
+    })
+}
+```
+
+### Naming things
+
+Prefer descriptive names over short ones. In particular, avoid one-letter variable names, other than in case of very well
+known and widely understood conventions, such as `i` for `index` (e.g. in a loop). A more descriptive name helps with
+code understanding and maintenance, at very little cost, given the auto-complete feature in most IDEs and editors.
+
+If a name contains an acronym, only capitalize the first letter of the acronym. E.g. use `someEksCluster` rather than
+`someEKSCluster`. We go against the [recommendation](https://github.com/golang/go/wiki/CodeReviewComments#initialisms)
+here in order to follow the convention already in use by some third party packages we heavily rely on (e.g. `aws-sdk-go`).
+
+#### Constants
+
+Since many languages use `ALL_CAPS` for constants, it is worth calling out explicitly that
+[Effective Go](https://golang.org/doc/effective_go.html#mixed-caps) recommends using `MixedCaps` for all names, including constants.
+Therefore, `region` or `testRegion` for private constants and `Region` or `TestRegion` for public ones is preferred over
+`REGION` or `TEST_REGION`.
+
+### Functional programming practices
+
+#### Immutability
+
+Prefer returning a new value rather than mutating an existing one.
+
+```go
+// Don't do this
+var result int = 0
+
+func main() {
+    add(1, 1, &result)
+    fmt.Println(result)
+}
+
+func add(a, b int, result *int) {
+    *result = a + b
+}
+```
+
+```go
+// Do this instead
+func main() {
+    fmt.Println(add(1, 1))
+}
+
+func add(a, b int) int {
+    return a + b
+}
+```
+
+#### Pure functions
+
+Prefer functions that take all their inputs as function parameters, instead of reading those inputs via side effects
+(e.g., reading from disk or the network or global vars), and whose entire behavior is to return values
+(note: errors are values too!), rather than performing side effects (e.g. by writing to disk or the network or global
+vars). Of course, you can’t avoid side effects forever if you want your code to do something useful, but try to do as
+much of your logic with pure functions as you can, try to pass everything around as explicit parameters and return
+everything as explicit values, and centralize the side effecting code to a few isolated places.
+
+#### Composition
+
+Build your code out of small, reusable, named functions, that you compose together.
+
+```go
+// Don't do this
+func main() {
+    fmt.Println(mulOfSums(1, 1))
+}
+
+func mulOfSums(a, b int) int {
+    return (a + b) * (a + b)
+}
+```
+
+```go
+// Do this instead
+func main() {
+    fmt.Println(mul(add(1, 1), add(1, 1)))
+}
+
+func add(a, b int) int {
+    return a + b
+}
+
+func mul(a, b int) int {
+    return a * b
+}
+```
+
+### Repo-specific conventions
+
+#### terratest
+
+Note the existence of methods in terratest which are suffixed with the letter `E`, e.g.
+[GetAccountIdE](https://github.com/gruntwork-io/terratest/blob/master/modules/aws/account.go#L23). Methods that have the
+suffix `E` return an error as the last return value; methods without `E` mark the test as failed
+(e.g., via calling `t.Fail()`) instead of returning an error.