meson: Serialize npm install for the agents

Parallel installs of the frida binding race on the prebuild cache, which
breaks the build on Windows.

agents/build.py

@@ -3,19 +3,22 @@ import platform
 import shutil
 import subprocess
 import sys
+import time
+from contextlib import contextmanager
 from pathlib import Path
 from typing import List
 
 
 def main(argv: List[str]):
     npm = argv[1]
-    paths = [Path(p).resolve() for p in argv[2:]]
+    npm_lock = Path(argv[2])
+    paths = [Path(p).resolve() for p in argv[3:]]
     inputs = paths[:-2]
     output_js = paths[-2]
     priv_dir = paths[-1]
 
     try:
-        build(npm, inputs, output_js, priv_dir)
+        build(npm, npm_lock, inputs, output_js, priv_dir)
     except subprocess.CalledProcessError as e:
         print(e, file=sys.stderr)
         print("Output:\n\t| " + "\n\t| ".join(e.output.strip().split("\n")), file=sys.stderr)
@@ -25,7 +28,7 @@ def main(argv: List[str]):
         sys.exit(1)
 
 
-def build(npm: Path, inputs: List[Path], output_js: Path, priv_dir: Path):
+def build(npm: Path, npm_lock: Path, inputs: List[Path], output_js: Path, priv_dir: Path):
     pkg_file = next((f for f in inputs if f.name == "package.json"))
     pkg_parent = pkg_file.parent
     entrypoint = inputs[0].relative_to(pkg_parent)
@@ -40,14 +43,39 @@ def build(npm: Path, inputs: List[Path], output_js: Path, priv_dir: Path):
 
         shutil.copy(srcfile, dstfile)
 
-    subprocess.run(
-        [npm, "install"], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, encoding="utf-8", cwd=priv_dir, check=True
-    )
+    with exclusive_lock(npm_lock):
+        subprocess.run(
+            [npm, "install"],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            encoding="utf-8",
+            cwd=priv_dir,
+            check=True,
+        )
 
     frida_compile = priv_dir / "node_modules" / ".bin" / f"frida-compile{script_suffix()}"
     subprocess.run([frida_compile, entrypoint, "-S", "-c", "-o", output_js], cwd=priv_dir, check=True)
 
 
+@contextmanager
+def exclusive_lock(path: Path):
+    with path.open("ab") as f:
+        if platform.system() == "Windows":
+            import msvcrt
+
+            while True:
+                try:
+                    msvcrt.locking(f.fileno(), msvcrt.LK_LOCK, 1)
+                    break
+                except OSError:
+                    time.sleep(0.1)
+        else:
+            import fcntl
+
+            fcntl.flock(f, fcntl.LOCK_EX)
+        yield
+
+
 def script_suffix() -> str:
     return ".cmd" if platform.system() == "Windows" else ""
 

agents/meson.build

@@ -1,4 +1,4 @@
-build_agent = [python, files('build.py'), npm]
+build_agent = [python, files('build.py'), npm, meson.current_build_dir() / 'npm-install.lock']
 
 subdir('fs')
 subdir('repl')