link: fix obvious race condition

Did you know that allocators reuse addresses? If not, then don't feel
bad, because apparently I don't either! This dumb mistake was probably
responsible for the CI failures on `master` yesterday.

Author: mlugg

src/Zcu/PerThread.zig

@@ -4395,7 +4395,7 @@ pub fn runCodegen(pt: Zcu.PerThread, func_index: InternPool.Index, air: *Air, ou
     };
     // release `out.value` with this store; synchronizes with acquire loads in `link`
     out.status.store(if (success) .ready else .failed, .release);
-    zcu.comp.link_task_queue.mirReady(zcu.comp, out);
+    zcu.comp.link_task_queue.mirReady(zcu.comp, func_index, out);
     if (zcu.pending_codegen_jobs.rmw(.Sub, 1, .monotonic) == 1) {
         // Decremented to 0, so all done.
         zcu.codegen_prog_node.end();

src/link/Queue.zig

@@ -64,7 +64,7 @@ state: union(enum) {
     finished,
     /// The link thread is not running or queued, because it is waiting for this MIR to be populated.
     /// Once codegen completes, it must call `mirReady` which will restart the link thread.
-    wait_for_mir: *ZcuTask.LinkFunc.SharedMir,
+    wait_for_mir: InternPool.Index,
 },
 
 /// In the worst observed case, MIR is around 50 times as large as AIR. More typically, the ratio is
@@ -113,15 +113,15 @@ pub fn start(q: *Queue, comp: *Compilation) void {
 
 /// Called by codegen workers after they have populated a `ZcuTask.LinkFunc.SharedMir`. If the link
 /// thread was waiting for this MIR, it can resume.
-pub fn mirReady(q: *Queue, comp: *Compilation, mir: *ZcuTask.LinkFunc.SharedMir) void {
+pub fn mirReady(q: *Queue, comp: *Compilation, func_index: InternPool.Index, mir: *ZcuTask.LinkFunc.SharedMir) void {
     // We would like to assert that `mir` is not pending, but that would race with a worker thread
     // potentially freeing it.
     {
         q.mutex.lock();
         defer q.mutex.unlock();
         switch (q.state) {
             .finished, .running => return,
-            .wait_for_mir => |wait_for| if (wait_for != mir) return,
+            .wait_for_mir => |wait_for| if (wait_for != func_index) return,
         }
         // We were waiting for `mir`, so we will restart the linker thread.
         q.state = .running;
@@ -171,7 +171,7 @@ pub fn enqueueZcu(q: *Queue, comp: *Compilation, task: ZcuTask) Allocator.Error!
         }
         // Restart the linker thread, unless it would immediately be blocked
         if (task == .link_func and task.link_func.mir.status.load(.acquire) == .pending) {
-            q.state = .{ .wait_for_mir = task.link_func.mir };
+            q.state = .{ .wait_for_mir = task.link_func.func };
             return;
         }
         q.state = .running;
@@ -248,7 +248,7 @@ fn flushTaskQueue(tid: usize, q: *Queue, comp: *Compilation) void {
             defer q.mutex.unlock();
             if (status_ptr.load(.acquire) != .pending) break :pending;
             // We will stop for now, and get restarted once this MIR is ready.
-            q.state = .{ .wait_for_mir = task.link_func.mir };
+            q.state = .{ .wait_for_mir = task.link_func.func };
             q.flush_safety.unlock();
             return;
         }
@@ -273,6 +273,7 @@ const std = @import("std");
 const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const Compilation = @import("../Compilation.zig");
+const InternPool = @import("../InternPool.zig");
 const link = @import("../link.zig");
 const PrelinkTask = link.PrelinkTask;
 const ZcuTask = link.ZcuTask;