vala: Compile inside a temporary directory

This allows to compile projects without granting access to the filesystem.
Additionally, all sessions opened that compile Vala code will use
the same build directory, allowing us to reuse the same meson
configuration.

Author: Diego-Ivan

src/langs/vala/Compiler.js

@@ -1,19 +1,41 @@
 import Gio from "gi://Gio";
 import dbus_previewer from "../../Previewer/DBusPreviewer.js";
+import { createTmpDir, copy } from "../../util.js";
+import { setupValaProject } from "./vala.js";
+
+let ready_to_build = false;
+const session_build_dir = await createTmpDir("vala");
 
 export default function ValaCompiler({ session }) {
   const { file } = session;
 
   const meson_builddir = "builddir";
-  const module_file = file
+  const module_file = session_build_dir
     .get_child(meson_builddir)
     .get_child("libworkbenchcode.so");
 
   async function compile() {
+    if (!ready_to_build) {
+      try {
+        await setupValaProject(session_build_dir);
+        ready_to_build = true;
+      } catch (err) {
+        console.error(err);
+        return false;
+      }
+    }
+
+    await copy(
+      "main.vala",
+      file,
+      session_build_dir,
+      Gio.FileCopyFlags.OVERWRITE | Gio.FileCopyFlags.ALL_METADATA,
+    );
+
     // TODO: Do not run setup if the build directory is already
     // configured
     const meson_launcher = new Gio.SubprocessLauncher();
-    meson_launcher.set_cwd(file.get_path());
+    meson_launcher.set_cwd(session_build_dir.get_path());
     const meson_setup = meson_launcher.spawnv([
       "meson",
       "setup",
@@ -26,6 +48,19 @@ export default function ValaCompiler({ session }) {
       return false;
     }
 
+    const meson_clean = meson_launcher.spawnv([
+      "meson",
+      "compile",
+      "--clean",
+      "-C",
+      meson_builddir,
+    ]);
+
+    await meson_clean.wait_async(null);
+    if (!meson_clean.get_successful()) {
+      return false;
+    }
+
     const meson_compile = meson_launcher.spawnv([
       "meson",
       "compile",