Support building on Mac OS X by using pthread_getspecific for TLS

The Mach-O format does not permit gcc to implement the __thread
TLS specification, so we must instead emulate it using a single
int cell allocated from memory and stored inside of the thread
specific data associated with the current pthread.

What makes this tricky is git_errno must be a valid lvalue, so
we really need to return a pointer to the caller and deference it
as part of the git_errno macro.

The GCC-specific __attribute__((constructor)) extension is used
to ensure the pthread_key_t is allocated before any Git functions
are executed in the library, as this is necessary to access our
thread specific storage.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/errors.c b/src/errors.c
index b3e014dd4..75636f477 100644
--- a/src/errors.c
+++ b/src/errors.c
@@ -1,9 +1,32 @@
 #include "common.h"
 #include "thread-utils.h" /* for GIT_TLS */
 
+#if defined(GIT_TLS)
 /* compile-time constant initialization required */
 GIT_TLS int git_errno = 0;
 
+#elif defined(GIT_HAS_PTHREAD)
+
+static pthread_key_t errno_key;
+
+static void init_errno(void) __attribute__((constructor));
+static void init_errno(void)
+{
+	pthread_key_create(&errno_key, free);
+}
+
+int *git__errno_storage(void)
+{
+	int *e = pthread_getspecific(errno_key);
+	if (!e) {
+		e = calloc(1, sizeof(*e));
+		pthread_setspecific(errno_key, e);
+	}
+	return e;
+}
+
+#endif
+
 static struct {
 	int num;
 	const char *str;