Move some Compilation mutex locking into PerThread.zig

src/Compilation.zig

@@ -3601,12 +3601,6 @@ fn performAllTheWorkInner(
 
         if (comp.module) |zcu| {
             {
-                // Worker threads may append to zcu.files and zcu.import_table
-                // so we must hold the lock while spawning those tasks, since
-                // we access those tables in this loop.
-                comp.mutex.lock();
-                defer comp.mutex.unlock();
-
                 while (comp.astgen_work_queue.readItem()) |file_index| {
                     // Pre-load these things from our single-threaded context since they
                     // will be needed by the worker threads.
@@ -4344,10 +4338,10 @@ fn workerAstGenFile(
             if (mem.eql(u8, import_path, "builtin")) continue;
 
             const import_result, const imported_path_digest, const imported_root_type = blk: {
+                const res = pt.importFile(file, import_path) catch continue;
+
                 comp.mutex.lock();
                 defer comp.mutex.unlock();
-
-                const res = pt.importFile(file, import_path) catch continue;
                 if (!res.is_pkg) {
                     res.file.addReference(pt.zcu, .{ .import = .{
                         .file = file_index,

src/Zcu/PerThread.zig

@@ -327,6 +327,9 @@ pub fn updateZirRefs(pt: Zcu.PerThread) Allocator.Error!void {
     const ip = &zcu.intern_pool;
     const gpa = zcu.gpa;
 
+    zcu.comp.mutex.lock();
+    defer zcu.comp.mutex.unlock();
+
     // We need to visit every updated File for every TrackedInst in InternPool.
     var updated_files: std.ArrayListUnmanaged(UpdatedFile) = .{};
     defer cleanupUpdatedFiles(gpa, &updated_files);
@@ -1370,6 +1373,9 @@ pub fn importPkg(pt: Zcu.PerThread, mod: *Module) !Zcu.ImportFileResult {
     var keep_resolved_path = false;
     defer if (!keep_resolved_path) gpa.free(resolved_path);
 
+    zcu.comp.mutex.lock();
+    defer zcu.comp.mutex.unlock();
+
     const gop = try zcu.import_table.getOrPut(gpa, resolved_path);
     errdefer _ = zcu.import_table.pop();
     if (gop.found_existing) {
@@ -1481,6 +1487,9 @@ pub fn importFile(
     var keep_resolved_path = false;
     defer if (!keep_resolved_path) gpa.free(resolved_path);
 
+    zcu.comp.mutex.lock();
+    defer zcu.comp.mutex.unlock();
+
     const gop = try zcu.import_table.getOrPut(gpa, resolved_path);
     errdefer _ = zcu.import_table.pop();
     if (gop.found_existing) {