settings: allow swapping out memory allocator

Tie in the newly created infrastructure for swapping out memory
allocators into our settings code. A user can now simply use the new
option "GIT_OPT_SET_ALLOCATOR" with `git_libgit2_opts`, passing in an
already initialized allocator structure as vararg.

include/git2/common.h

@@ -183,6 +183,7 @@ typedef enum {
 	GIT_OPT_GET_WINDOWS_SHAREMODE,
 	GIT_OPT_SET_WINDOWS_SHAREMODE,
 	GIT_OPT_ENABLE_STRICT_HASH_VERIFICATION,
+	GIT_OPT_SET_ALLOCATOR
 } git_libgit2_opt_t;
 
 /**
@@ -345,6 +346,12 @@ typedef enum {
  *		> additional checksum calculation on each object. This defaults
  *		> to enabled.
  *
+ *	 opts(GIT_OPT_SET_ALLOCATOR, git_allocator *allocator)
+ *
+ *		> Set the memory allocator to a different memory allocator. This
+ *		> allocator will then be used to make all memory allocations for
+ *		> libgit2 operations.
+ *
  * @param option Option key
  * @param ... value to set the option
  * @return 0 on success, <0 on failure

include/git2/sys/alloc.h

@@ -72,6 +72,30 @@ typedef struct {
 	void (*gfree)(void *ptr);
 } git_allocator;
 
+/**
+ * Initialize the allocator structure to use the `stdalloc` pointer.
+ *
+ * Set up the structure so that all of its members are using the standard
+ * "stdalloc" allocator functions. The structure can then be used with
+ * `git_allocator_setup`.
+ *
+ * @param allocator The allocator that is to be initialized.
+ * @return An error code or 0.
+ */
+int git_stdalloc_init_allocator(git_allocator *allocator);
+
+/**
+ * Initialize the allocator structure to use the `crtdbg` pointer.
+ *
+ * Set up the structure so that all of its members are using the "crtdbg"
+ * allocator functions. Note that this allocator is only available on Windows
+ * platforms and only if libgit2 is being compiled with "-DMSVC_CRTDBG".
+ *
+ * @param allocator The allocator that is to be initialized.
+ * @return An error code or 0.
+ */
+int git_win32_crtdbg_init_allocator(git_allocator *allocator);
+
 GIT_END_DECL
 
 #endif

src/alloc.c

@@ -29,3 +29,12 @@ int git_allocator_setup(git_allocator *allocator)
 	memcpy(&git__allocator, allocator, sizeof(*allocator));
 	return 0;
 }
+
+#if !defined(GIT_MSVC_CRTDBG)
+int git_win32_crtdbg_init_allocator(git_allocator *allocator)
+{
+	GIT_UNUSED(allocator);
+	giterr_set(GIT_EINVALID, "crtdbg memory allocator not available");
+	return -1;
+}
+#endif

src/settings.c

@@ -16,6 +16,7 @@
 #endif
 
 #include <git2.h>
+#include "alloc.h"
 #include "sysdir.h"
 #include "cache.h"
 #include "global.h"
@@ -260,6 +261,10 @@ int git_libgit2_opts(int key, ...)
 		git_odb__strict_hash_verification = (va_arg(ap, int) != 0);
 		break;
 
+	case GIT_OPT_SET_ALLOCATOR:
+		error = git_allocator_setup(va_arg(ap, git_allocator *));
+		break;
+
 	default:
 		giterr_set(GITERR_INVALID, "invalid option key");
 		error = -1;