internal/declaration/declaration.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"reflect"
 	"sort"
 	"strings"
@@ -569,7 +570,24 @@ func strictDecode(raw []byte, into any) error {
 	// field the host does not know is a thing the control plane believes it asked for.
 	dec := json.NewDecoder(bytes.NewReader(raw))
 	dec.DisallowUnknownFields()
-	return dec.Decode(into)
+	if err := dec.Decode(into); err != nil {
+		return err
+	}
+	// And **nothing after it**. A decoder reads one value and stops, so a file holding a
+	// declaration followed by anything at all — a truncated rewrite, two declarations
+	// concatenated, a stray line from whatever wrote the file — parses as the first value and the
+	// rest is never looked at.
+	//
+	// That is the same fault this host refuses everywhere else, in its quietest form: the machine
+	// applies something, reports success, and what it applied is not what the file says. Found
+	// when a test harness appended a line to a bundle by accident and every apply kept working.
+	if _, err := dec.Token(); err != io.EOF {
+		return fmt.Errorf(
+			"there is more in this file after the declaration ends. Refused whole: a file with " +
+				"something after it may be a truncated rewrite or two declarations run together, " +
+				"and applying the first would be applying something nobody wrote")
+	}
+	return nil
 }
 
 // unknownFields names every JSON key the kind's struct has no field for.

internal/declaration/declaration_test.go

@@ -313,3 +313,34 @@ func TestSomethingInsideAValueIsNotAComment(t *testing.T) {
 		t.Fatalf("a value was mangled: %q", file.Content)
 	}
 }
+
+// A declaration followed by anything at all is refused whole.
+//
+// A JSON decoder reads one value and stops, so a file holding a declaration and then a stray line
+// parses as the declaration and the rest is never looked at. The machine applies something,
+// reports success, and what it applied is not what the file says.
+//
+// Not hypothetical: a test harness appended a line to the substrate bundle by accident, every
+// apply kept working, and nothing said so for the entire time it was wrong.
+func TestSomethingAfterTheDeclarationIsRefused(t *testing.T) {
+	good := `{"declaration":1,"resources":[{"id":"a","type":"file","path":"/tmp/a",` +
+		`"content":"x","mode":"0644"}]}`
+	if _, err := ParseFileTrusted([]byte(good)); err != nil {
+		t.Fatalf("an ordinary declaration was refused: %v", err)
+	}
+	for _, after := range []string{
+		"MESHBUNDLE 2>&1; echo \"__exit=$?\"",
+		good,
+		"garbage",
+	} {
+		_, err := ParseFileTrusted([]byte(good + "\n" + after))
+		if err == nil {
+			t.Fatalf("a file with %q after the declaration was accepted", after)
+		}
+	}
+	// Trailing whitespace is not "something after it", and refusing it would make every file
+	// written by an editor unusable.
+	if _, err := ParseFileTrusted([]byte(good + "\n\n  \n")); err != nil {
+		t.Fatalf("a declaration with a trailing newline was refused: %v", err)
+	}
+}