Replace the writer spin lock with a blocking mutex

term_write_begin spun on an atomic sequence word. A waiter burned CPU,
and with no priority inheritance a SCHED_FIFO waiter could starve a
preempted holder on the same CPU. The writer section is now a pthread
mutex with PTHREAD_PRIO_INHERIT where the platform has it, and an SRWLOCK
on Windows. Re-entry on the owning thread still only counts the depth.

The platform wrapper (init, lock, unlock, destroy) is in compat.h. term_new
initializes the mutex, and term_free destroys it. Meson links the thread
library into the core, term_dep, and term_internal_dep. The README,
term.h, and the amalgamation note say that POSIX needs -pthread.

Tests: the feed and snapshot thread test now checks the final screen and
history against a serial reference. A new test checks that term_lock
nests and that another thread waits for the last term_unlock.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

1 files changed, 4 insertions, 14 deletions

diff --git a/src/internal.h b/src/internal.h
index 8c02c91..d67e67e 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -108,7 +108,8 @@ struct resize_scratch {
 
 
 struct term {
-    _Atomic uint64_t seq;
+    /* The writer section. A waiting thread sleeps on it. */
+    term_mutex write_lock;
     _Atomic uint64_t writer_tid;
     uint32_t write_depth;
     int cols;
@@ -375,18 +376,7 @@ static inline void term_write_begin(struct term *t)
         t->write_depth++;
         return;
     }
-    uint64_t s;
-    for (;;) {
-        s = atomic_load_explicit(&t->seq, memory_order_relaxed);
-        if (s & 1) {
-            term_cpu_relax();
-            continue;
-        }
-        if (atomic_compare_exchange_weak_explicit(&t->seq, &s, s + 1,
-                                                  memory_order_acquire,
-                                                  memory_order_relaxed))
-            break;
-    }
+    term_mutex_lock(&t->write_lock);
     atomic_store_explicit(&t->writer_tid, self, memory_order_relaxed);
     t->write_depth = 1;
 }
@@ -396,7 +386,7 @@ static inline void term_write_end(struct term *t)
     if (!t) return;
     if (--t->write_depth == 0) {
         atomic_store_explicit(&t->writer_tid, 0, memory_order_relaxed);
-        atomic_fetch_add_explicit(&t->seq, 1, memory_order_release);
+        term_mutex_unlock(&t->write_lock);
     }
 }