thodg/libgit2

diff --git a/include/git2/common.h b/include/git2/common.h
index a14e096..152e23a 100644
--- a/include/git2/common.h
+++ b/include/git2/common.h
@@ -364,7 +364,8 @@ typedef enum {
  *
  *		> Set the memory allocator to a different memory allocator. This
  *		> allocator will then be used to make all memory allocations for
- *		> libgit2 operations.
+ *		> libgit2 operations.  If the given `allocator` is NULL, then the
+ *		> system default will be restored.
  *
  *	 opts(GIT_OPT_ENABLE_UNSAVED_INDEX_SAFETY, int enabled)
  *
diff --git a/src/alloc.c b/src/alloc.c
index d4e6f1e..0cac457 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -15,6 +15,15 @@
 
 git_allocator git__allocator;
 
+static int setup_default_allocator(void)
+{
+#if defined(GIT_MSVC_CRTDBG)
+	return git_win32_crtdbg_init_allocator(&git__allocator);
+#else
+	return git_stdalloc_init_allocator(&git__allocator);
+#endif
+}
+
 int git_allocator_global_init(void)
 {
 	/*
@@ -24,15 +33,14 @@ int git_allocator_global_init(void)
 	if (git__allocator.gmalloc != NULL)
 		return 0;
 
-#if defined(GIT_MSVC_CRTDBG)
-	return git_win32_crtdbg_init_allocator(&git__allocator);
-#else
-	return git_stdalloc_init_allocator(&git__allocator);
-#endif
+	return setup_default_allocator();
 }
 
 int git_allocator_setup(git_allocator *allocator)
 {
+	if (!allocator)
+		return setup_default_allocator();
+
 	memcpy(&git__allocator, allocator, sizeof(*allocator));
 	return 0;
 }
diff --git a/tests/buf/oom.c b/tests/buf/oom.c
index ec3bad9..726234e 100644
--- a/tests/buf/oom.c
+++ b/tests/buf/oom.c
@@ -1,57 +1,59 @@
 #include "clar_libgit2.h"
 #include "buffer.h"
 
-/*
- * We want to use some ridiculous size that `malloc` will fail with
- * but that does not otherwise interfere with testing.  On Linux, choose
- * a number that is large enough to fail immediately but small enough
- * that valgrind doesn't believe it to erroneously be a negative number.
- * On macOS, choose a number that is large enough to fail immediately
- * without having libc print warnings to stderr.
- */
-#if defined(GIT_ARCH_64) && defined(__linux__)
-# define TOOBIG 0x0fffffffffffffff
-#elif defined(GIT_ARCH_64)
-# define TOOBIG 0xffffffffffffff00
-#endif
-
-/**
- * If we make a ridiculously large request the first time we
- * actually allocate some space in the git_buf, the realloc()
- * will fail.  And because the git_buf_grow() wrapper always
- * sets mark_oom, the code in git_buf_try_grow() will free
- * the internal buffer and set it to git_buf__oom.
- *
- * We initialized the internal buffer to (the static variable)
- * git_buf__initbuf.  The purpose of this test is to make sure
- * that we don't try to free the static buffer.
- *
- * Skip this test entirely on 32-bit platforms; a buffer large enough
- * to guarantee malloc failures is so large that valgrind considers
- * it likely to be an error.
- */
+/* Override default allocators with ones that will fail predictably. */
+
+static git_allocator std_alloc;
+static git_allocator oom_alloc;
+
+static void *oom_malloc(size_t n, const char *file, int line)
+{
+	/* Reject any allocation of more than 100 bytes */
+	return (n > 100) ? NULL : std_alloc.gmalloc(n, file, line);
+}
+
+static void *oom_realloc(void *p, size_t n, const char *file, int line)
+{
+	/* Reject any allocation of more than 100 bytes */
+	return (n > 100) ? NULL : std_alloc.grealloc(p, n, file, line);
+}
+
+void test_buf_oom__initialize(void)
+{
+	git_stdalloc_init_allocator(&std_alloc);
+	git_stdalloc_init_allocator(&oom_alloc);
+
+	oom_alloc.gmalloc = oom_malloc;
+	oom_alloc.grealloc = oom_realloc;
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_SET_ALLOCATOR, &oom_alloc));
+}
+
+void test_buf_oom__cleanup(void)
+{
+	cl_git_pass(git_libgit2_opts(GIT_OPT_SET_ALLOCATOR, NULL));
+}
+
 void test_buf_oom__grow(void)
 {
-#ifdef GIT_ARCH_64
 	git_buf buf = GIT_BUF_INIT;
 
-	git_buf_clear(&buf);
+	cl_git_pass(git_buf_grow(&buf, 42));
+	cl_assert(!git_buf_oom(&buf));
 
-	cl_assert(git_buf_grow(&buf, TOOBIG) == -1);
+	cl_assert(git_buf_grow(&buf, 101) == -1);
 	cl_assert(git_buf_oom(&buf));
 
 	git_buf_dispose(&buf);
-#else
-    cl_skip();
-#endif
 }
 
 void test_buf_oom__grow_by(void)
 {
 	git_buf buf = GIT_BUF_INIT;
 
-	buf.size = SIZE_MAX-10;
+	cl_git_pass(git_buf_grow_by(&buf, 42));
+	cl_assert(!git_buf_oom(&buf));
 
-	cl_assert(git_buf_grow_by(&buf, 50) == -1);
+	cl_assert(git_buf_grow_by(&buf, 101) == -1);
 	cl_assert(git_buf_oom(&buf));
 }