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, 2 deletions

diff --git a/meson.build b/meson.build
index d23c0b4..fa450f9 100644
--- a/meson.build
+++ b/meson.build
@@ -15,6 +15,8 @@ project('term', 'c',
 
 cc = meson.get_compiler('c')
 m_dep = cc.find_library('m', required: false)
+# The writer lock is a pthread mutex on POSIX and an SRWLOCK on Windows.
+thread_dep = dependency('threads')
 if get_option('arch_flag') != ''
   add_project_arguments(get_option('arch_flag'), language: 'c')
 endif
@@ -29,7 +31,7 @@ subdir('src')
 term_dep = declare_dependency(
   include_directories: term_inc,
   link_with: term_lib.get_static_lib(),
-  dependencies: m_dep,
+  dependencies: [m_dep, thread_dep],
 )
 meson.override_dependency('term', term_dep)
 
@@ -37,7 +39,7 @@ meson.override_dependency('term', term_dep)
 term_internal_dep = declare_dependency(
   include_directories: [term_inc, term_private_inc],
   link_with: term_lib.get_static_lib(),
-  dependencies: m_dep,
+  dependencies: [m_dep, thread_dep],
 )
 
 # The short forms of term.h that the term tests use. A host project can use