fix: an empty delivered file is a broken delivery, refused by ReadFileCarrier

docs/runnable-binding-delivery.md

@@ -206,8 +206,10 @@ Everything fails closed, and as early as the information exists:
   ConfigMap and Secret it loads as the container's environment, and refuses a
   file ConfigMap or Secret above 1 MiB, the API server's object limit.
 - **Read time.** A file carrier whose path is relative, is not a regular file,
-  exceeds `resources.MaxFileCarrierBytes`, or cannot be read is an error, not
-  an absent value: an unreadable credential never reads as an unset one. A key
+  is empty, exceeds `resources.MaxFileCarrierBytes`, or cannot be read is an
+  error, not an absent value: an unreadable credential never reads as an unset
+  one. An empty file is a truncated write or a mount race, since only a value
+  above `resources.FileCarrierThreshold` is delivered by file. A key
   delivered both inline and by file is refused as two sources for one fact.
 - **Install time.** A contract digest that does not cover the contract delivered
   with it, a route that is not the operation the value names, a policy outside

resources/file_carrier.go

@@ -225,9 +225,14 @@ func CheckProcessEnvironment(envs []*EnvironmentVariable) error {
 // ResolveFileCarriers reads every file carrier of environ ("KEY=VALUE"
 // strings, as os.Environ returns them) and returns the value key of each with
 // the content of its file. A carrier whose value key is also set inline, a
-// relative path, a file that is not a regular file, is larger than
+// relative path, a file that is not a regular file, is empty, is larger than
 // MaxFileCarrierBytes or cannot be read, and a secret file accessible by
 // others, are errors: an unreadable value never reads as an unset one.
+//
+// An empty file is refused because a value is delivered by file only when it
+// is larger than FileCarrierThreshold (deliverByFile): an empty one is a
+// truncated write or a mount race, never a value the host left unset, and
+// reading it as absent would let a lookup step past a broken delivery.
 func ResolveFileCarriers(environ []string) (map[string]string, error) {
 	inline := map[string]bool{}
 	carriers := map[string]string{}
@@ -292,5 +297,8 @@ func ReadFileCarrier(key, path string) (string, error) {
 	if len(content) > MaxFileCarrierBytes {
 		return "", fmt.Errorf("%w: %s: exceeds %d bytes", ErrFileCarrier, key, MaxFileCarrierBytes)
 	}
+	if len(content) == 0 {
+		return "", fmt.Errorf("%w: %s: delivered file is empty", ErrFileCarrier, key)
+	}
 	return string(content), nil
 }

resources/file_carrier_test.go

@@ -87,6 +87,8 @@ func TestResolveFileCarriersFailsClosed(t *testing.T) {
 	dir := t.TempDir()
 	value := filepath.Join(dir, "value")
 	require.NoError(t, os.WriteFile(value, []byte("content"), 0o600))
+	empty := filepath.Join(dir, "empty")
+	require.NoError(t, os.WriteFile(empty, nil, 0o600))
 	public := "CODEFLY__WORKSPACE_CONFIGURATION__GROUP__VALUE"
 	secret := "CODEFLY__WORKSPACE_SECRET_CONFIGURATION__GROUP__VALUE"
 
@@ -99,6 +101,9 @@ func TestResolveFileCarriersFailsClosed(t *testing.T) {
 		"relative path":      {resources.FileCarrierKey(public) + "=value"},
 		"missing file":       {resources.FileCarrierKey(public) + "=" + filepath.Join(dir, "absent")},
 		"a directory":        {resources.FileCarrierKey(public) + "=" + dir},
+		// A value goes by file only above FileCarrierThreshold, so an empty
+		// file is a broken delivery, not an unset value.
+		"an empty file": {resources.FileCarrierKey(public) + "=" + empty},
 	}
 	for name, environ := range cases {
 		_, err := resources.ResolveFileCarriers(environ)