thread-utils: macros to unconditionally compile pthreads API

When built with NO_PTHREADS, the macros are used make the code build
even though pthreads header and library may be missing. The code can
still have different code paths for no threads support with
HAVE_THREADS variable.

There are of course impacts on no-pthreads builds:

- data structure may get slightly bigger because all the mutexes and
  pthread_t are present (as an int)

- code execution is not impacted much. Locking (in hot path) is
  no-op. Other wrapper function calls really should not matter much.

- the binary size grows bigger because of threaded code. But at least
  on Linux this does not matter, if some code is not executed, it's
  not mapped in memory.

This is a preparation step to remove "#ifdef NO_PTHREADS" in the code
mostly because of maintainability. As Jeff put it

> it's probably OK to stop thinking of it as "non-threaded platforms
> are the default and must pay zero cost" and more as "threaded
> platforms are the default, and non-threaded ones are OK to pay a
> small cost as long as they still work".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 45 insertions, 3 deletions

diff --git a/thread-utils.h b/thread-utils.h
index d9a769d..4961487 100644
--- a/thread-utils.h
+++ b/thread-utils.h
@@ -4,12 +4,54 @@
 #ifndef NO_PTHREADS
 #include <pthread.h>
 
-extern int online_cpus(void);
-extern int init_recursive_mutex(pthread_mutex_t*);
+#define HAVE_THREADS 1
 
 #else
 
-#define online_cpus() 1
+#define HAVE_THREADS 0
+
+/*
+ * macros instead of typedefs because pthread definitions may have
+ * been pulled in by some system dependencies even though the user
+ * wants to disable pthread.
+ */
+#define pthread_t int
+#define pthread_mutex_t int
+#define pthread_cond_t int
+#define pthread_key_t int
+
+#define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
+#define pthread_mutex_lock(mutex)
+#define pthread_mutex_unlock(mutex)
+#define pthread_mutex_destroy(mutex)
+
+#define pthread_cond_init(cond, attr) dummy_pthread_init(cond)
+#define pthread_cond_wait(cond, mutex)
+#define pthread_cond_signal(cond)
+#define pthread_cond_broadcast(cond)
+#define pthread_cond_destroy(cond)
+
+#define pthread_key_create(key, attr) dummy_pthread_init(key)
+#define pthread_key_delete(key)
+
+#define pthread_create(thread, attr, fn, data) \
+	dummy_pthread_create(thread, attr, fn, data)
+#define pthread_join(thread, retval) \
+	dummy_pthread_join(thread, retval)
+
+#define pthread_setspecific(key, data)
+#define pthread_getspecific(key) NULL
+
+int dummy_pthread_create(pthread_t *pthread, const void *attr,
+			 void *(*fn)(void *), void *data);
+int dummy_pthread_join(pthread_t pthread, void **retval);
+
+int dummy_pthread_init(void *);
 
 #endif
+
+int online_cpus(void);
+int init_recursive_mutex(pthread_mutex_t*);
+
+
 #endif /* THREAD_COMPAT_H */