Add basic fuzzing jig

.github/workflows/test.yml

@@ -76,13 +76,52 @@ jobs:
       - name: go test
         run: |
           go test
+  fuzz:
+    name: 10s fuzz test
+    runs-on: ubuntu-24.04
+    continue-on-error: true
+    permissions:
+      contents: read # Required to clone the repo
+    steps:
+      - name: Harden the runner
+        uses: step-security/harden-runner@8d3c67de8e2fe68ef647c8db1e6a09f647780f40 # v2.19.0
+        with:
+          egress-policy: audit
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - name: Set up Go
+        uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
+        with:
+          go-version-file: go.mod
+      - name: go fuzz
+        run: |
+          go test -fuzz=FuzzTemplatefile -fuzztime=10s
+      - name: Collect any new fuzz corpora
+        id: corpus
+        if: failure()
+        run: |
+          {
+            printf '%s\n' 'paths<<EOF'
+            git ls-files --others --exclude-standard -- testdata/fuzz/
+            printf '%s\n' 'EOF'
+          } >> "$GITHUB_OUTPUT"
+      - name: Upload fuzz failure corpora
+        if: failure() && steps.corpus.outputs.paths != ''
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: fuzz-failure-corpus
+          path: ${{ steps.corpus.outputs.paths }}
+          retention-days: 14
   no-more-jobs:
     name: No more jobs
     runs-on: ubuntu-24.04
     needs:
       - govulncheck
       - pinact-verify
       - fmt-test
+      - fuzz
     steps:
       - name: Harden the runner
         uses: step-security/harden-runner@8d3c67de8e2fe68ef647c8db1e6a09f647780f40 # v2.19.0

templatefile.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"fmt"
+
 	"github.com/hashicorp/hcl/v2/ext/tryfunc"
 	"github.com/hashicorp/terraform/lang/funcs"
 	ctyyaml "github.com/zclconf/go-cty-yaml"
@@ -138,5 +140,8 @@ func templatefile(baseDir string, path string, vars string) (string, error) {
 		return "", err
 	}
 
-	return res.AsString(), err
+	if res.Type() != cty.String {
+		return "", fmt.Errorf("template result is %s, not string", res.Type().FriendlyName())
+	}
+	return res.AsString(), nil
 }

templatefile_test.go

@@ -2,9 +2,11 @@ package main
 
 import (
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"unicode/utf8"
 
 	"github.com/stretchr/testify/require"
 )
@@ -33,3 +35,20 @@ func TestTemplatefileRecursive(t *testing.T) {
 	require.NotNil(t, err)
 	require.Contains(t, err.Error(), "Call to unknown function")
 }
+
+func FuzzTemplatefile(f *testing.F) {
+	f.Add("Hello, ${name}!", "name: world\n")
+	f.Add("${val}", "val: works\n")
+	f.Add(`The items are ${join(", ", list)}`, "list:\n  - foo\n  - bar\n")
+	f.Add("%{ for x in list ~}\n- ${x}\n%{ endfor ~}", "list:\n  - foo\n")
+
+	f.Fuzz(func(t *testing.T, tmpl, vars string) {
+		tmp := t.TempDir()
+		path := filepath.Join(tmp, "fuzz.tmpl")
+		require.NoError(t, os.WriteFile(path, []byte(tmpl), 0o600))
+
+		// Assert that the parser doesn't panic or hang, and produces valid UTF-8
+		actual, err := templatefile(tmp, path, vars)
+		require.True(t, err != nil || utf8.ValidString(actual), "output is invalid UTF-8")
+	})
+}

testdata/fuzz/FuzzTemplatefile/a3034cdd38718cd7

@@ -0,0 +1,3 @@
+go test fuzz v1
+string("${val}")
+string("val:")