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>

src/common.h

@@ -5,6 +5,9 @@
 #include "util.h"
 #include "errors.h"
 
+#ifdef GIT_HAS_PTHREAD
+# include <pthread.h>
+#endif
 #include <assert.h>
 #include <errno.h>
 #include <stdlib.h>

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;

src/git/errors.h

@@ -13,8 +13,15 @@
 GIT_BEGIN_DECL
 
 /** The git errno. */
+#if defined(GIT_TLS)
 GIT_EXTERN(int) GIT_TLS git_errno;
 
+#elif defined(GIT_HAS_PTHREAD)
+# define git_errno (*git__errno_storage())
+GIT_EXTERN(int *) git__errno_storage(void);
+
+#endif
+
 /**
  * strerror() for the Git library
  * @param num The error code to explain

src/git/thread-utils.h

@@ -9,8 +9,15 @@
 
 #define GIT_HAS_TLS 1
 
-#if defined(__GNUC__) || defined(__SUNPRO_C) || defined(__SUNPRO_CC) || \
-	defined(__xlc__) || defined(__xlC__)
+#if defined(__APPLE__) && defined(__MACH__)
+# undef GIT_TLS
+# define GIT_HAS_PTHREAD
+
+#elif defined(__GNUC__) || \
+      defined(__SUNPRO_C) || \
+      defined(__SUNPRO_CC) || \
+      defined(__xlc__) || \
+      defined(__xlC__)
 # define GIT_TLS __thread
 
 #elif defined(__INTEL_COMPILER)
@@ -20,7 +27,9 @@
 #  define GIT_TLS __thread
 # endif
 
-#elif defined(_WIN32) || defined(_WIN32_CE) || defined(__BORLANDC__)
+#elif defined(_WIN32) || \
+      defined(_WIN32_CE) || \
+      defined(__BORLANDC__)
 # define GIT_TLS __declspec(thread)
 
 #else

tests/t0000-errno.c

@@ -0,0 +1,14 @@
+#include "test_lib.h"
+#include "errors.h"
+#include <string.h>
+
+BEGIN_TEST(errno_zero_on_init)
+	must_be_true(git_errno == 0);
+END_TEST
+
+BEGIN_TEST(set_ENOTOID)
+	must_be_true(GIT_ENOTOID != 0);
+	git_errno = GIT_ENOTOID;
+	must_be_true(git_errno == GIT_ENOTOID);
+	must_pass(strcmp(git_strerror(git_errno), "Not a git oid"));
+END_TEST