From ce24a6b9614c433e2b32cffc8544250e2173b193 Mon Sep 17 00:00:00 2001
From: Christian Mesh <christianmesh1@gmail.com>
Date: Fri, 2 Aug 2024 07:29:40 -0400
Subject: [PATCH] Better var file type detection (#1881)

Signed-off-by: Christian Mesh <christianmesh1@gmail.com>
---
 internal/command/command.go        |  5 +-
 internal/command/meta_vars.go      | 10 +++-
 internal/command/meta_vars_test.go | 78 ++++++++++++++++++++++++++++++
 3 files changed, 91 insertions(+), 2 deletions(-)
 create mode 100644 internal/command/meta_vars_test.go

diff --git a/internal/command/command.go b/internal/command/command.go
index 927fd42a1d..42dda99e39 100644
--- a/internal/command/command.go
+++ b/internal/command/command.go
@@ -32,8 +32,11 @@ const DefaultPluginVendorDir = "terraform.d/plugins/" + pluginMachineName
 // DefaultStateFilename is the default filename used for the state file.
 const DefaultStateFilename = "terraform.tfstate"
 
+// DefaultVarsExtension is the default file extension used for vars
+const DefaultVarsExtension = ".tfvars"
+
 // DefaultVarsFilename is the default filename used for vars
-const DefaultVarsFilename = "terraform.tfvars"
+const DefaultVarsFilename = "terraform" + DefaultVarsExtension
 
 // DefaultBackupExtension is added to the state file to form the path
 const DefaultBackupExtension = ".backup"
diff --git a/internal/command/meta_vars.go b/internal/command/meta_vars.go
index 9811855419..0b2a98a475 100644
--- a/internal/command/meta_vars.go
+++ b/internal/command/meta_vars.go
@@ -182,7 +182,15 @@ func (m *Meta) addVarsFromFile(filename string, sourceType tofu.ValueSourceType,
 	loader.Parser().ForceFileSource(filename, src)
 
 	var f *hcl.File
-	if strings.HasSuffix(filename, ".json") {
+
+	extJSON := strings.HasSuffix(filename, ".json")
+	extTfvars := strings.HasSuffix(filename, DefaultVarsExtension)
+
+	// Only try json detection if ambiguous
+	// Ex: -var-file=<(./scripts/vars.sh)
+	detectJSON := !extJSON && !extTfvars && strings.HasPrefix(strings.TrimSpace(string(src)), "{")
+
+	if extJSON || detectJSON {
 		var hclDiags hcl.Diagnostics
 		f, hclDiags = hcljson.Parse(src, filename)
 		diags = diags.Append(hclDiags)
diff --git a/internal/command/meta_vars_test.go b/internal/command/meta_vars_test.go
new file mode 100644
index 0000000000..2e5b9656cb
--- /dev/null
+++ b/internal/command/meta_vars_test.go
@@ -0,0 +1,78 @@
+// Copyright (c) The OpenTofu Authors
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2023 HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package command
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/opentofu/opentofu/internal/backend"
+	"github.com/opentofu/opentofu/internal/tofu"
+)
+
+func TestMeta_addVarsFromFile(t *testing.T) {
+	d := t.TempDir()
+	defer testChdir(t, d)()
+
+	hclData := `foo = "bar"`
+	jsonData := `{"foo": "bar"}`
+
+	cases := []struct {
+		filename string
+		contents string
+		errors   bool
+	}{
+		{
+			filename: "input.tfvars",
+			contents: hclData,
+			errors:   false,
+		},
+		{
+			filename: "input.json",
+			contents: jsonData,
+			errors:   false,
+		},
+		{
+			filename: "input_a.unknown",
+			contents: hclData,
+			errors:   false,
+		},
+		{
+			filename: "input_b.unknown",
+			contents: jsonData,
+			errors:   false,
+		},
+		{
+			filename: "mismatch.tfvars",
+			contents: jsonData,
+			errors:   true,
+		},
+		{
+			filename: "mismatch.json",
+			contents: hclData,
+			errors:   true,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.filename, func(t *testing.T) {
+			target := filepath.Join(d, tc.filename)
+			err := os.WriteFile(target, []byte(tc.contents), 0600)
+			if err != nil {
+				t.Fatalf("err: %s", err)
+			}
+
+			m := new(Meta)
+			to := make(map[string]backend.UnparsedVariableValue)
+			diags := m.addVarsFromFile(target, tofu.ValueFromAutoFile, to)
+			if tc.errors != diags.HasErrors() {
+				t.Log(diags.Err())
+				t.Errorf("Expected: %v, got %v", tc.errors, diags.HasErrors())
+			}
+		})
+	}
+}