alloc: add GIT_DEBUG_STRICT_ALLOC

Add `GIT_DEBUG_STRICT_ALLOC` to help identify problematic callers of
allocation code that pass a `0` size to the allocators and then expect a
non-`NULL` return.

When given a 0-size allocation, `malloc` _may_ return either a `NULL`
_or_ a pointer that is not writeable.  Most systems return a non-`NULL`
pointer; AIX is an outlier.  We should be able to cope with this AIXy
behavior, so this adds an option to emulate it.

CMakeLists.txt

@@ -49,6 +49,7 @@ OPTION(USE_GSSAPI			"Link with libgssapi for SPNEGO auth"			OFF)
 OPTION(USE_STANDALONE_FUZZERS		"Enable standalone fuzzers (compatible with gcc)"	OFF)
 OPTION(USE_LEAK_CHECKER			"Run tests with leak checker"				OFF)
 OPTION(DEBUG_POOL			"Enable debug pool allocator"				OFF)
+OPTION(DEBUG_STRICT_ALLOC		"Enable strict allocator behavior"			OFF)
 OPTION(ENABLE_WERROR			"Enable compilation with -Werror"			OFF)
 OPTION(USE_BUNDLED_ZLIB    		"Use the bundled version of zlib. Can be set to one of Bundled(ON)/Chromium. The Chromium option requires a x86_64 processor with SSE4.2 and CLMUL"			OFF)
    SET(USE_HTTP_PARSER			"" CACHE STRING "Specifies the HTTP Parser implementation; either system or builtin.")

src/CMakeLists.txt

@@ -6,6 +6,11 @@ IF(DEBUG_POOL)
 ENDIF()
 ADD_FEATURE_INFO(debugpool GIT_DEBUG_POOL "debug pool allocator")
 
+IF(DEBUG_STRICT_ALLOC)
+	SET(GIT_DEBUG_STRICT_ALLOC 1)
+ENDIF()
+ADD_FEATURE_INFO(debugalloc GIT_DEBUG_STRICT_ALLOC "debug strict allocators")
+
 INCLUDE(PkgBuildConfig)
 INCLUDE(SanitizeBool)
 

src/allocators/stdalloc.c

@@ -9,34 +9,56 @@
 
 static void *stdalloc__malloc(size_t len, const char *file, int line)
 {
-	void *ptr = malloc(len);
+	void *ptr;
 
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-	if (!ptr) git_error_set_oom();
+#ifdef GIT_DEBUG_STRICT_ALLOC
+	if (!len)
+		return NULL;
+#endif
+
+	ptr = malloc(len);
+
+	if (!ptr)
+		git_error_set_oom();
+
 	return ptr;
 }
 
 static void *stdalloc__calloc(size_t nelem, size_t elsize, const char *file, int line)
 {
-	void *ptr = calloc(nelem, elsize);
+	void *ptr;
 
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-	if (!ptr) git_error_set_oom();
+#ifdef GIT_DEBUG_STRICT_ALLOC
+	if (!elsize)
+		return NULL;
+#endif
+
+	ptr = calloc(nelem, elsize);
+
+	if (!ptr)
+		git_error_set_oom();
+
 	return ptr;
 }
 
 static char *stdalloc__strdup(const char *str, const char *file, int line)
 {
-	char *ptr = strdup(str);
+	char *ptr;
 
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-	if (!ptr) git_error_set_oom();
+	ptr = strdup(str);
+
+	if (!ptr)
+		git_error_set_oom();
+
 	return ptr;
 }
 
@@ -48,7 +70,7 @@ static char *stdalloc__strndup(const char *str, size_t n, const char *file, int
 	length = p_strnlen(str, n);
 
 	if (GIT_ADD_SIZET_OVERFLOW(&alloclength, length, 1) ||
-		!(ptr = stdalloc__malloc(alloclength, file, line)))
+	    !(ptr = stdalloc__malloc(alloclength, file, line)))
 		return NULL;
 
 	if (length)
@@ -65,7 +87,7 @@ static char *stdalloc__substrdup(const char *start, size_t n, const char *file,
 	size_t alloclen;
 
 	if (GIT_ADD_SIZET_OVERFLOW(&alloclen, n, 1) ||
-		!(ptr = stdalloc__malloc(alloclen, file, line)))
+	    !(ptr = stdalloc__malloc(alloclen, file, line)))
 		return NULL;
 
 	memcpy(ptr, start, n);
@@ -75,12 +97,21 @@ static char *stdalloc__substrdup(const char *start, size_t n, const char *file,
 
 static void *stdalloc__realloc(void *ptr, size_t size, const char *file, int line)
 {
-	void *new_ptr = realloc(ptr, size);
+	void *new_ptr;
 
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-	if (!new_ptr) git_error_set_oom();
+#ifdef GIT_DEBUG_STRICT_ALLOC
+	if (!size)
+		return NULL;
+#endif
+
+	new_ptr = realloc(ptr, size);
+
+	if (!new_ptr)
+		git_error_set_oom();
+
 	return new_ptr;
 }
 

src/features.h.in

@@ -2,6 +2,8 @@
 #define INCLUDE_features_h__
 
 #cmakedefine GIT_DEBUG_POOL 1
+#cmakedefine GIT_DEBUG_STRICT_ALLOC 1
+
 #cmakedefine GIT_TRACE 1
 #cmakedefine GIT_THREADS 1
 #cmakedefine GIT_WIN32_LEAKCHECK 1