feat: Allow for negations in repo allowlist (runatlantis#3414)

* feat: Omit repos from allowlist

* Add quote in comment

* Better comment

* Remove test

runatlantis.io/docs/security.md

@@ -46,6 +46,7 @@ For example:
 * Specific repositories: `--repo-allowlist=github.com/runatlantis/atlantis,github.com/runatlantis/atlantis-tests`
 * Your whole organization: `--repo-allowlist=github.com/runatlantis/*`
 * Every repository in your GitHub Enterprise install: `--repo-allowlist=github.yourcompany.com/*`
+* You can also omit specific repos: `--repo-allowlist='github.com/runatlantis/*,!github.com/runatlantis/untrusted-repo'`
 * All repositories: `--repo-allowlist=*`. Useful for when you're in a protected network but dangerous without also setting a webhook secret.
 
 This flag ensures your Atlantis install isn't being used with repositories you don't control. See `atlantis server --help` for more details.

runatlantis.io/docs/server-configuration.md

@@ -810,6 +810,7 @@ This is useful when you have many projects and want to keep the pull request cle
   * Accepts a comma separated list, ex. `definition1,definition2`
   * Format is `{hostname}/{owner}/{repo}`, ex. `github.com/runatlantis/atlantis`
   * `*` matches any characters, ex. `github.com/runatlantis/*` will match all repos in the runatlantis organization
+  * An entry beginning with `!` negates it, ex. `github.com/foo/*,!github.com/foo/bar` will match all github repos in the `foo` owner *except* `bar`.
   * For Bitbucket Server: `{hostname}` is the domain without scheme and port, `{owner}` is the name of the project (not the key), and `{repo}` is the repo name
     * User (not project) repositories take on the format: `{hostname}/{full name}/{repo}` (e.g., `bitbucket.example.com/Jane Doe/myatlantis` for username `jdoe` and full name `Jane Doe`, which is not very intuitive)
   * For Azure DevOps the allowlist takes one of two forms: `{owner}.visualstudio.com/{project}/{repo}` or `dev.azure.com/{owner}/{project}/{repo}`
@@ -820,6 +821,8 @@ This is useful when you have many projects and want to keep the pull request cle
     * `--repo-allowlist=github.com/myorg/repo1,github.com/myorg/repo2`
   * Allowlist all repos under `myorg` on `github.com`
     * `--repo-allowlist='github.com/myorg/*'`
+  * Allowlist all repos under `myorg` on `github.com`, excluding `myorg/untrusted-repo`
+    * `--repo-allowlist='github.com/myorg/*,!github.com/myorg/untrusted-repo'`
   * Allowlist all repos in my GitHub Enterprise installation
     * `--repo-allowlist='github.yourcompany.com/*'`
   * Allowlist all repos under `myorg` project `myproject` on Azure DevOps

server/events/repo_allowlist_checker.go

@@ -24,28 +24,42 @@ const Wildcard = "*"
 // RepoAllowlistChecker implements checking if repos are allowlisted to be used with
 // this Atlantis.
 type RepoAllowlistChecker struct {
-	rules []string
+	includeRules []string
+	omitRules    []string
 }
 
 // NewRepoAllowlistChecker constructs a new checker and validates that the
 // allowlist isn't malformed.
 func NewRepoAllowlistChecker(allowlist string) (*RepoAllowlistChecker, error) {
-	rules := strings.Split(allowlist, ",")
-	for _, rule := range rules {
+	includeRules := make([]string, 0)
+	omitRules := make([]string, 0)
+	for _, rule := range strings.Split(allowlist, ",") {
 		if strings.Contains(rule, "://") {
 			return nil, fmt.Errorf("allowlist %q contained ://", rule)
 		}
+		if len(rule) > 1 && rule[0] == '!' {
+			omitRules = append(omitRules, rule[1:])
+		} else {
+			includeRules = append(includeRules, rule)
+		}
 	}
 	return &RepoAllowlistChecker{
-		rules: rules,
+		includeRules: includeRules,
+		omitRules:    omitRules,
 	}, nil
 }
 
 // IsAllowlisted returns true if this repo is in our allowlist and false
 // otherwise.
 func (r *RepoAllowlistChecker) IsAllowlisted(repoFullName string, vcsHostname string) bool {
 	candidate := fmt.Sprintf("%s/%s", vcsHostname, repoFullName)
-	for _, rule := range r.rules {
+	shouldInclude := r.matchesAtLeastOneRule(r.includeRules, candidate)
+	shouldOmit := r.matchesAtLeastOneRule(r.omitRules, candidate)
+	return shouldInclude && !shouldOmit
+}
+
+func (r *RepoAllowlistChecker) matchesAtLeastOneRule(rules []string, candidate string) bool {
+	for _, rule := range rules {
 		if r.matchesRule(rule, candidate) {
 			return true
 		}

server/events/repo_allowlist_checker_test.go

@@ -175,6 +175,20 @@ func TestRepoAllowlistChecker_IsAllowlisted(t *testing.T) {
 			"github.com",
 			true,
 		},
+		{
+			"should exclude with negative match",
+			"github.com/owner/*,!github.com/owner/badrepo",
+			"owner/badrepo",
+			"github.com",
+			false,
+		},
+		{
+			"should match if with negative rule doesn't match",
+			"github.com/owner/*,!github.com/owner/badrepo",
+			"owner/otherrepo",
+			"github.com",
+			true,
+		},
 	}
 
 	for _, c := range cases {