Remove unnecessary os.Stat in eval cache (#624)

* Remove unnecessary `os.Stat` in eval cache
Any time that stat returns a file stat, we're going to read the file anyways. Might as well jump straight to the read
`os.ReadFile` returns an error that matches `os.IsNotExist` just the same as `os.Stat`

* Add caching test

pkg/jsonnet/eval_test.go

@@ -1,24 +1,104 @@
 package jsonnet
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
+const importTreeResult = `[
+   {
+      "breed": "apple",
+      "color": "red",
+      "creates": "o2",
+      "eats": "co2",
+      "keeps": "the world healthy",
+      "kind": "tree",
+      "needs": "water",
+      "size": "m"
+   },
+   {
+      "breed": "cherry",
+      "color": "red",
+      "creates": "o2",
+      "eats": "co2",
+      "keeps": "the world healthy",
+      "kind": "tree",
+      "needs": "water",
+      "size": "xs"
+   },
+   {
+      "breed": "peach",
+      "color": "orange",
+      "creates": "o2",
+      "eats": "co2",
+      "keeps": "the world healthy",
+      "kind": "tree",
+      "needs": "water",
+      "size": "s"
+   }
+]
+`
+
+const thisFileResult = `{
+   "test": "testdata/thisFile/main.jsonnet"
+}
+`
+
 // To be consistent with the jsonnet executable,
 // when evaluating a file, `std.thisFile` should point to the given path
 func TestEvaluateFile(t *testing.T) {
 	result, err := EvaluateFile("testdata/thisFile/main.jsonnet", Opts{})
 	assert.NoError(t, err)
-	assert.Equal(t, `{
-   "test": "testdata/thisFile/main.jsonnet"
-}
-`, result)
+	assert.Equal(t, thisFileResult, result)
 }
 
 func TestEvaluateFileDoesntExist(t *testing.T) {
 	result, err := EvaluateFile("testdata/doesnt-exist/main.jsonnet", Opts{})
 	assert.EqualError(t, err, "open testdata/doesnt-exist/main.jsonnet: no such file or directory")
 	assert.Equal(t, "", result)
 }
+
+func TestEvaluateFileWithCaching(t *testing.T) {
+	tmp, err := os.MkdirTemp("", "test-tanka-caching")
+	require.NoError(t, err)
+	defer os.RemoveAll(tmp)
+	cachePath := filepath.Join(tmp, "cache") // Should be created during caching
+
+	// Evaluate two files
+	result, err := EvaluateFile("testdata/thisFile/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, thisFileResult, result)
+	result, err = EvaluateFile("testdata/importTree/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, importTreeResult, result)
+
+	// Check that we have two entries in the cache
+	readCache, err := os.ReadDir(cachePath)
+	require.NoError(t, err)
+	assert.Len(t, readCache, 2)
+
+	// Evaluate two files again, same result
+	result, err = EvaluateFile("testdata/thisFile/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, thisFileResult, result)
+	result, err = EvaluateFile("testdata/importTree/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, importTreeResult, result)
+
+	// Modify the cache items
+	for _, entry := range readCache {
+		require.NoError(t, os.WriteFile(filepath.Join(cachePath, entry.Name()), []byte(entry.Name()), 0666))
+	}
+
+	// Evaluate two files again, modified cache is returned instead of the actual result
+	result, err = EvaluateFile("testdata/thisFile/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, "BYfdlr1ZOVwiOfbd89JYTcK-eRQh05bi8ky3k1vVW5o=.json", result)
+	result, err = EvaluateFile("testdata/importTree/main.jsonnet", Opts{CachePath: cachePath})
+	assert.NoError(t, err)
+	assert.Equal(t, "R_3hy-dRfOwXN-fezQ50ZF4dnrFcBcbQ9LztR_XWzJA=.json", result)
+}

pkg/jsonnet/evalcache.go

@@ -26,8 +26,7 @@ func (c *FileEvalCache) Get(hash string) (string, error) {
 		return "", err
 	}
 
-	if _, err := os.Stat(cachePath); err == nil {
-		bytes, err := os.ReadFile(cachePath)
+	if bytes, err := os.ReadFile(cachePath); err == nil {
 		return string(bytes), err
 	} else if !os.IsNotExist(err) {
 		return "", err