fix(cli): reject hard-linked input files as outputs

bend2/main.ts

@@ -316,9 +316,12 @@ async function cli_file(args: string[]): Promise<void> {
       t.$ === "Def" && t.i !== undefined ? t.i.map(path_real) : [])]);
     for (const out of outs) {
       const at = path_real(out);
-      if (ins.has(at) || (fs.existsSync(at) && fs.statSync(at).isDirectory())) {
+      if (ins.has(at) || (fs.existsSync(at) && (fs.statSync(at).isDirectory()
+        || [...ins].some((input) => same_file(at, input))))) {
         cli_fail("-o " + out + " is a file the program reads, or a directory");
       }
+    }
+    for (const out of outs) {
       cli_emit(book, out);
     }
   } catch (e) {

tests/io/output_input_links.bend

@@ -0,0 +1,12 @@
+# Output files must not overwrite inputs through a path, symlink or hard link.
+import Base
+
+def Output.input_links() -> IO(U32):
+  import "./output_input_links.js"
+
+def main() -> IO(Unit):
+  do IO<Unit>:
+    result : U32 <- Output.input_links()
+    IO.print(U32.show(result))
+
+#|0

tests/io/output_input_links.js

@@ -0,0 +1,63 @@
+function output_input_links() {
+  const fs = require("node:fs");
+  const os = require("node:os");
+  const path = require("node:path");
+  const child = require("node:child_process");
+  // Outside the repo, so a run the alarm kills leaves no files in the tree.
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "bend-output-input-"));
+  const inputs = new Map([
+    ["main.bend", 'import Base\nimport ./lib.bend as Lib\n'
+      + 'def Probe.value() -> IO(U32):\n'
+      + '  import "./effect.js"\n  import "./effect.c"\n'
+      + 'def main() -> U32:\n  Lib.value()\n'],
+    ["lib.bend", "import Base\ndef value() -> U32:\n  42\n"],
+    ["effect.js", "io_eff(CID(Probe.value), () => 42);\n"],
+    ["effect.c", "// Foreign input.\n"],
+  ].map(([name, text]) => [path.join(dir, name), text]));
+  const main = path.join(dir, "main.bend");
+  const options = { encoding: "utf8", timeout: 4000,
+    env: { ...process.env, HOME: dir, BEND_ORIGIN: "http://127.0.0.1:9",
+      BEND_NO_TELEMETRY: "1" },
+  };
+  const build = (...outs) => child.spawnSync(process.execPath,
+    [path.resolve("bend2/main.ts"), main, ...outs.flatMap((out) => ["-o", out])],
+    options);
+  const intact = () => [...inputs].every(([file, text]) =>
+    fs.readFileSync(file, "utf8") === text);
+  const rejects = (got, msg) => got.status === 1 && got.stderr.includes(msg)
+    && intact();
+  try {
+    for (const [file, text] of inputs) fs.writeFileSync(file, text);
+    const link = path.join(dir, "out.js");
+    const ok = path.join(dir, "ok.js");
+    let code = 1;
+    if (!rejects(build(main), "is a file the program reads")) return code;
+    code++;
+    fs.symlinkSync(main, link);
+    if (!rejects(build(link), "is a file the program reads")) return code;
+    fs.unlinkSync(link);
+    code++;
+    for (const file of inputs.keys()) {
+      fs.linkSync(file, link);
+      if (!rejects(build(link), "is a file the program reads")) return code;
+      fs.unlinkSync(link);
+      code++;
+    }
+    if (!rejects(build(dir), "or a directory")) return code;
+    code++;
+    // A rejected output fails the build before any output is written.
+    fs.linkSync(main, link);
+    if (!rejects(build(ok, link), "is a file the program reads")
+      || fs.existsSync(ok)) return code;
+    fs.unlinkSync(link);
+    code++;
+    if (build(ok).status !== 0 || !intact()) return code;
+    const run = child.spawnSync(process.execPath, [ok], options);
+    if (run.status !== 0 || run.stdout.trim() !== "42") return code;
+    return 0;
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+}
+
+io_eff(CID(Output.input_links), output_input_links);