feat: add touch-file tool and E2E test for single O_RDWR open detection

Add a `touch-file` tool that opens a file with O_RDWR in a single
syscall (unlike replace-file-content which uses separate read/write
opens). This tests that fspy correctly detects a single O_RDWR open
as both READ and WRITE for the input-modified cache skip.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

crates/vite_task_bin/tests/e2e_snapshots/fixtures/input-read-write-not-cached/packages/touch-pkg/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@test/touch-pkg",
+  "scripts": {
+    "task": "touch-file src/data.txt"
+  }
+}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/input-read-write-not-cached/packages/touch-pkg/src/data.txt

@@ -0,0 +1 @@
+original

crates/vite_task_bin/tests/e2e_snapshots/fixtures/input-read-write-not-cached/snapshots.toml

@@ -17,3 +17,9 @@ steps = ["vt run -r task", "vt run -r task"]
 name = "verbose read-write task shows path in full summary"
 cwd = "packages/rw-pkg"
 steps = ["vt run -v task"]
+
+# Single O_RDWR open (touch-file) is also detected as read-write overlap
+[[e2e]]
+name = "single O_RDWR open is not cached"
+cwd = "packages/touch-pkg"
+steps = ["vt run task", "vt run task"]

crates/vite_task_bin/tests/e2e_snapshots/fixtures/input-read-write-not-cached/snapshots/multi task with read-write shows not cached in summary.snap

@@ -6,15 +6,19 @@ expression: e2e_outputs
 ~/packages/normal-pkg$ print hello
 hello
 
+~/packages/touch-pkg$ touch-file src/data.txt
+
 ~/packages/rw-pkg$ replace-file-content src/data.txt original modified
 
 ---
-vt run: 0/2 cache hit (0%). @test/rw-pkg#task not cached because it modified its input. (Run `vt run --last-details` for full details)
+vt run: 0/3 cache hit (0%). @test/touch-pkg#task (and 1 more) not cached because they modified their inputs. (Run `vt run --last-details` for full details)
 > vt run -r task
 ~/packages/normal-pkg$ print hello ✓ cache hit, replaying
 hello
 
+~/packages/touch-pkg$ touch-file src/data.txt
+
 ~/packages/rw-pkg$ replace-file-content src/data.txt original modified
 
 ---
-vt run: 1/2 cache hit (50%), <duration> saved. @test/rw-pkg#task not cached because it modified its input. (Run `vt run --last-details` for full details)
+vt run: 1/3 cache hit (33%), <duration> saved. @test/touch-pkg#task (and 1 more) not cached because they modified their inputs. (Run `vt run --last-details` for full details)

crates/vite_task_bin/tests/e2e_snapshots/fixtures/input-read-write-not-cached/snapshots/single O_RDWR open is not cached.snap

@@ -0,0 +1,16 @@
+---
+source: crates/vite_task_bin/tests/e2e_snapshots/main.rs
+expression: e2e_outputs
+info:
+  cwd: packages/touch-pkg
+---
+> vt run task
+~/packages/touch-pkg$ touch-file src/data.txt
+
+---
+vt run: @test/touch-pkg#task not cached because it modified its input. (Run `vt run --last-details` for full details)
+> vt run task
+~/packages/touch-pkg$ touch-file src/data.txt
+
+---
+vt run: @test/touch-pkg#task not cached because it modified its input. (Run `vt run --last-details` for full details)

packages/tools/package.json

@@ -8,7 +8,8 @@
     "print-env": "./src/print-env.js",
     "print-file": "./src/print-file.ts",
     "read-stdin": "./src/read-stdin.js",
-    "replace-file-content": "./src/replace-file-content.ts"
+    "replace-file-content": "./src/replace-file-content.ts",
+    "touch-file": "./src/touch-file.ts"
   },
   "type": "module",
   "dependencies": {

packages/tools/src/touch-file.ts

@@ -0,0 +1,15 @@
+#!/usr/bin/env node
+
+// Opens a file once in read-write mode (O_RDWR) without modifying content.
+// Used to test that fspy detects a single O_RDWR open as both READ and WRITE.
+
+import { openSync, closeSync, constants } from 'node:fs';
+
+const filename = process.argv[2];
+if (!filename) {
+  console.error('Usage: touch-file <filename>');
+  process.exit(1);
+}
+
+const fd = openSync(filename, constants.O_RDWR);
+closeSync(fd);