fix(cli): reject hard-linked aliases of program inputs as outputs

README.md

@@ -116,6 +116,9 @@ curl -fsSL https://bend-lang.com/install.sh | sh
 This `bend` ignores a project's `bunfig.toml` and `.env`; `bun bend2/main.ts`
 from a checkout reads them, so check untrusted code with `bend`.
 
+Compilation rejects `-o` paths that match a program input by path or file
+identity, including symlinks and hard links.
+
 ### 2. Tell your agent to use Bend:
 
 Add this to your `AGENTS.md`:

bend2/main.ts

@@ -311,10 +311,9 @@ async function cli_file(args: string[]): Promise<void> {
       return;
     }
     const ins = new Set([...seen.keys(), ...Object.values(book.tlds).flatMap((t) =>
-      t.$ === "Def" && t.i !== undefined ? t.i.map(path_real) : [])]);
+      t.$ === "Def" && t.i !== undefined ? t.i : [])].map(path_id)).add("");
     for (const out of outs) {
-      const at = path_real(out);
-      if (ins.has(at) || (fs.existsSync(at) && fs.statSync(at).isDirectory())) {
+      if (ins.has(path_id(out))) {
         cli_fail("-o " + out + " is a file the program reads, or a directory");
       }
       cli_emit(book, out);
@@ -356,8 +355,12 @@ async function cli_checkup(file: string): Promise<void> {
   }
 }
 
-function path_real(p: string): string {
-  return fs.existsSync(p) ? fs.realpathSync(p) : path.resolve(p);
+// A path names a file by device and inode, so links to one file agree, and
+// every directory is "".
+function path_id(p: string): string {
+  const s = fs.statSync(p, { bigint: true, throwIfNoEntry: false });
+  return s === undefined ? path.resolve(p)
+    : s.isDirectory() ? "" : s.dev + ":" + s.ino;
 }
 
 function cli_emit(book: Bend.Book, out: string): void {

gates/ping.ts

@@ -300,6 +300,12 @@ try {
     && page.err.includes("page.ts:1:") && !page.err.includes("Bun v"));
   const bad_file = path.join(TMP, "bad.bend");
   const sum_file = path.join(TMP, "sum.bend");
+  const sum_alias = path.join(TMP, "sum-alias.js");
+  fs.linkSync(sum_file, sum_alias);
+  const linked_out = await bend([sum_file, "-o", sum_alias]);
+  const linked_run = await bend([sum_file]);
+  check("an output hard link cannot destroy its input",
+    linked_out.code === 1 && linked_run.code === 0 && linked_run.out === "5n\n");
   const unsafe_file = path.join(TMP, "unsafe.bend");
   const checkup = path.join(TMP, "checkup.bend");
   const good_checkup = path.join(TMP, "good_checkup.bend");