docs: Documenting commit ref support in CAS

Author: yhakbar

docs/src/content/docs/03-features/07-caching/04-cas.mdx

@@ -48,6 +48,20 @@ terraform {
 }
 ```
 
+<Since version="1.0.4">
+
+`ref=` also accepts commit SHAs (full or abbreviated), not only branch and tag names.
+
+```hcl
+terraform {
+  source = "git@github.com:acme/infrastructure-modules.git//vpc?ref=a1b2c3d4e5f67890abcdef1234567890deadbeef"
+}
+```
+
+The first cold clone of a repository pinned to a commit SHA fetches the full history of every branch. Shallow fetches require a ref name, and fetching a commit SHA at limited depth depends on a server option (`uploadpack.allowAnySHA1InWant`) that is not universally enabled, so CAS fetches all branches at full depth and resolves the SHA locally. Subsequent clones reuse the cached repository and never touch the network for the same commit.
+
+</Since>
+
 ### Stack Usage
 
 <Before version="1.0.3">
@@ -195,9 +209,9 @@ When Terragrunt needs to clone a repository using the CAS it does the following,
 
 For cold clones, where the content is not already in the CAS:
 
-1. Terragrunt resolves the Git reference (branch/tag) to a commit hash
+1. Terragrunt resolves the Git reference to a commit hash. Branch and tag refs resolve via `git ls-remote`. `ls-remote` lists named refs and does not resolve commit SHAs, so for SHA refs Terragrunt fetches the full history of every branch into the central Git store and resolves the SHA locally.
 2. The tree related to the commit hash is not found in the CAS
-3. Terragrunt clones the repository to a temporary directory
+3. Terragrunt fetches the repository contents (shallow for branch/tag refs, full history for commit SHAs)
 4. All blobs and trees required to reproduce the repository are extracted
 5. Content is stored in the CAS, partitioned by hash prefix
 6. The tree structure is read from the CAS and hard links are created to the target directory
@@ -206,7 +220,7 @@ For cold clones, where the content is not already in the CAS:
 
 For warm clones, where the content is already in the CAS:
 
-1. Terragrunt resolves the Git reference to a commit hash
+1. Terragrunt resolves the Git reference to a commit hash. For commit-SHA refs the local central Git store answers without contacting the remote when the SHA is already cached.
 2. CAS checks if the content exists
 3. The tree structure is read directly from the CAS
 4. Hard links are created from CAS to the target directory

docs/src/data/changelog/v1.0.4/cas-commit-refs.mdx

@@ -0,0 +1,18 @@
+---
+version: "v1.0.4"
+category: "experiments-updated"
+---
+
+#### `cas` — Commit SHAs accepted in `ref=`
+
+Source URLs of the form `git::<url>?ref=<commit-sha>` now resolve through CAS. Previously these clones failed because Terragrunt asked the remote to look up the SHA as a symbolic reference, which Git servers don't support.
+
+Both full SHAs (SHA-1 and SHA-256) and abbreviated SHAs are accepted. Abbreviated SHAs must disambiguate inside the repository, the same rule Git itself applies.
+
+```hcl
+terraform {
+  source = "git::https://github.com/acme/infrastructure-modules.git//vpc?ref=a1b2c3d4e5f67890abcdef1234567890deadbeef"
+}
+```
+
+The first cold clone of a repository pinned to a commit SHA fetches the full history of every branch. Shallow fetches require a ref name, and fetching a commit SHA at limited depth depends on a server option (`uploadpack.allowAnySHA1InWant`) that is not universally enabled, so CAS fetches all branches at full depth and resolves the SHA locally. Subsequent clones reuse the cached repository and never touch the network for the same commit. Branch and tag refs continue to use the existing shallow path.

test/fixtures/download/remote-commit-ref/terragrunt.hcl

@@ -0,0 +1,10 @@
+inputs = {
+  name = "World"
+}
+
+terraform {
+  # Pinned to the commit v0.93.2 resolves to so this fixture exercises
+  # the commit-SHA path in the CAS getter without depending on whatever
+  # the tag points to in the future.
+  source = "github.com/gruntwork-io/terragrunt.git//test/fixtures/download/hello-world-no-remote?ref=dd7913e04f0e812b51ccf2c4f35a0fda16a356a1"
+}

test/integration_download_test.go

@@ -957,6 +957,27 @@ func TestDownloadWithCASEnabled(t *testing.T) {
 	assert.Contains(t, stderr.String(), "Downloading Terraform configurations")
 }
 
+func TestDownloadWithCASCommitRef(t *testing.T) {
+	t.Parallel()
+
+	fixturePath := "fixtures/download/remote-commit-ref"
+
+	tmpEnvPath := helpers.CopyEnvironment(t, fixturePath)
+	testPath := filepath.Join(tmpEnvPath, fixturePath)
+	helpers.CleanupTerraformFolder(t, testPath)
+
+	var (
+		stdout bytes.Buffer
+		stderr bytes.Buffer
+	)
+
+	cmd := "terragrunt apply --auto-approve --non-interactive --experiment cas --log-level debug --working-dir " + testPath
+	err := helpers.RunTerragruntCommand(t, cmd, &stdout, &stderr)
+	require.NoError(t, err)
+
+	assert.Contains(t, stderr.String(), "Downloading Terraform configurations")
+}
+
 func TestCASStorageDirectory(t *testing.T) {
 	t.Parallel()