path: introduce git_fs_path_find_executable

Provide a helper function to find an executable in the current process's
PATH.

src/fs_path.c

@@ -1851,3 +1851,59 @@ cleanup:
 	return ret;
 #endif
 }
+
+int git_fs_path_find_executable(git_str *fullpath, const char *executable)
+{
+#ifdef GIT_WIN32
+	git_win32_path fullpath_w, executable_w;
+	int error;
+
+	if (git__utf8_to_16(executable_w, GIT_WIN_PATH_MAX, executable) < 0)
+		return -1;
+
+	error = git_win32_path_find_executable(fullpath_w, executable_w);
+
+	if (error == 0)
+		error = git_str_put_w(fullpath, fullpath_w, wcslen(fullpath_w));
+
+	return error;
+#else
+	git_str path = GIT_STR_INIT;
+	const char *current_dir, *term;
+	bool found = false;
+
+	if (git__getenv(&path, "PATH") < 0)
+		return -1;
+
+	current_dir = path.ptr;
+
+	while (*current_dir) {
+		if (! (term = strchr(current_dir, GIT_PATH_LIST_SEPARATOR)))
+			term = strchr(current_dir, '\0');
+
+		git_str_clear(fullpath);
+		if (git_str_put(fullpath, current_dir, (term - current_dir)) < 0 ||
+		    git_str_putc(fullpath, '/') < 0 ||
+		    git_str_puts(fullpath, executable) < 0)
+			return -1;
+
+		if (git_fs_path_isfile(fullpath->ptr)) {
+			found = true;
+			break;
+		}
+
+		current_dir = term;
+
+		while (*current_dir == GIT_PATH_LIST_SEPARATOR)
+			current_dir++;
+	}
+
+	git_str_dispose(&path);
+
+	if (found)
+		return 0;
+
+	git_str_clear(fullpath);
+	return GIT_ENOTFOUND;
+#endif
+}

src/fs_path.h

@@ -743,4 +743,10 @@ bool git_fs_path_supports_symlinks(const char *dir);
  */
 int git_fs_path_validate_system_file_ownership(const char *path);
 
+/**
+ * Search the current PATH for the given executable, returning the full
+ * path if it is found.
+ */
+int git_fs_path_find_executable(git_str *fullpath, const char *executable);
+
 #endif

src/win32/path_w32.c

@@ -151,6 +151,137 @@ int git_win32_path_canonicalize(git_win32_path path)
 	return (int)(to - path);
 }
 
+static int git_win32_path_join(
+	git_win32_path dest,
+	const wchar_t *one,
+	size_t one_len,
+	const wchar_t *two,
+	size_t two_len)
+{
+	size_t backslash = 0;
+
+	if (one_len && two_len && one[one_len - 1] != L'\\')
+		backslash = 1;
+
+	if (one_len + two_len + backslash > MAX_PATH) {
+		git_error_set(GIT_ERROR_INVALID, "path too long");
+		return -1;
+	}
+
+	memmove(dest, one, one_len * sizeof(wchar_t));
+
+	if (backslash)
+		dest[one_len] = L'\\';
+
+	memcpy(dest + one_len + backslash, two, two_len * sizeof(wchar_t));
+	dest[one_len + backslash + two_len] = L'\0';
+
+	return 0;
+}
+
+struct win32_path_iter {
+	wchar_t *env;
+	const wchar_t *current_dir;
+};
+
+static int win32_path_iter_init(struct win32_path_iter *iter)
+{
+	DWORD len = GetEnvironmentVariableW(L"PATH", NULL, 0);
+
+	if (!len && GetLastError() == ERROR_ENVVAR_NOT_FOUND) {
+		iter->env = NULL;
+		iter->current_dir = NULL;
+		return 0;
+	} else if (!len) {
+		git_error_set(GIT_ERROR_OS, "could not load PATH");
+		return -1;
+	}
+
+	iter->env = git__malloc(len * sizeof(wchar_t));
+	GIT_ERROR_CHECK_ALLOC(iter->env);
+
+	len = GetEnvironmentVariableW(L"PATH", iter->env, len);
+
+	if (len == 0) {
+		git_error_set(GIT_ERROR_OS, "could not load PATH");
+		return -1;
+	}
+
+	iter->current_dir = iter->env;
+	return 0;
+}
+
+static int win32_path_iter_next(
+	const wchar_t **out,
+	size_t *out_len,
+	struct win32_path_iter *iter)
+{
+	const wchar_t *start;
+	wchar_t term;
+	size_t len = 0;
+
+	if (!iter->current_dir || !*iter->current_dir)
+		return GIT_ITEROVER;
+
+	term = (*iter->current_dir == L'"') ? *iter->current_dir++ : L';';
+	start = iter->current_dir;
+
+	while (*iter->current_dir && *iter->current_dir != term) {
+		iter->current_dir++;
+		len++;
+	}
+
+	*out = start;
+	*out_len = len;
+
+	if (term == L'"' && *iter->current_dir)
+		iter->current_dir++;
+
+	while (*iter->current_dir == L';')
+		iter->current_dir++;
+
+	return 0;
+}
+
+static void win32_path_iter_dispose(struct win32_path_iter *iter)
+{
+	if (!iter)
+		return;
+
+	git__free(iter->env);
+	iter->env = NULL;
+	iter->current_dir = NULL;
+}
+
+int git_win32_path_find_executable(git_win32_path fullpath, wchar_t *exe)
+{
+	struct win32_path_iter path_iter;
+	const wchar_t *dir;
+	size_t dir_len, exe_len = wcslen(exe);
+	bool found = false;
+
+	if (win32_path_iter_init(&path_iter) < 0)
+		return -1;
+
+	while (win32_path_iter_next(&dir, &dir_len, &path_iter) != GIT_ITEROVER) {
+		if (git_win32_path_join(fullpath, dir, dir_len, exe, exe_len) < 0)
+			continue;
+
+		if (_waccess(fullpath, 0) == 0) {
+			found = true;
+			break;
+		}
+	}
+
+	win32_path_iter_dispose(&path_iter);
+
+	if (found)
+		return 0;
+
+	fullpath[0] = L'\0';
+	return GIT_ENOTFOUND;
+}
+
 static int win32_path_cwd(wchar_t *out, size_t len)
 {
 	int cwd_len;

src/win32/path_w32.h

@@ -86,4 +86,6 @@ size_t git_win32_path_trim_end(wchar_t *str, size_t len);
  */
 size_t git_win32_path_remove_namespace(wchar_t *str, size_t len);
 
+int git_win32_path_find_executable(git_win32_path fullpath, wchar_t* exe);
+
 #endif

tests/core/path.c

@@ -2,6 +2,20 @@
 #include "futils.h"
 #include "fs_path.h"
 
+static char *path_save;
+
+void test_core_path__initialize(void)
+{
+	path_save = cl_getenv("PATH");
+}
+
+void test_core_path__cleanup(void)
+{
+	cl_setenv("PATH", path_save);
+	git__free(path_save);
+	path_save = NULL;
+}
+
 static void
 check_dirname(const char *A, const char *B)
 {
@@ -60,6 +74,20 @@ check_joinpath_n(
 	git_str_dispose(&joined_path);
 }
 
+static void check_setenv(const char* name, const char* value)
+{
+    char* check;
+
+    cl_git_pass(cl_setenv(name, value));
+    check = cl_getenv(name);
+
+    if (value)
+	cl_assert_equal_s(value, check);
+    else
+	cl_assert(check == NULL);
+
+    git__free(check);
+}
 
 /* get the dirname of a path */
 void test_core_path__00_dirname(void)
@@ -651,3 +679,61 @@ void test_core_path__16_resolve_relative(void)
 	assert_common_dirlen(6, "a/b/c/foo.txt", "a/b/c/d/e/bar.txt");
 	assert_common_dirlen(7, "/a/b/c/foo.txt", "/a/b/c/d/e/bar.txt");
 }
+
+static void fix_path(git_str *s)
+{
+#ifndef GIT_WIN32
+	GIT_UNUSED(s);
+#else
+	char* c;
+
+	for (c = s->ptr; *c; c++) {
+		if (*c == '/')
+			*c = '\\';
+	}
+#endif
+}
+
+void test_core_path__find_exe_in_path(void)
+{
+	char *orig_path;
+	git_str sandbox_path = GIT_STR_INIT;
+	git_str new_path = GIT_STR_INIT, full_path = GIT_STR_INIT,
+	        dummy_path = GIT_STR_INIT;
+
+#ifdef GIT_WIN32
+	static const char *bogus_path_1 = "c:\\does\\not\\exist\\";
+	static const char *bogus_path_2 = "e:\\non\\existent";
+#else
+	static const char *bogus_path_1 = "/this/path/does/not/exist/";
+	static const char *bogus_path_2 = "/non/existent";
+#endif
+
+	orig_path = cl_getenv("PATH");
+
+	git_str_puts(&sandbox_path, clar_sandbox_path());
+	git_str_joinpath(&dummy_path, sandbox_path.ptr, "dummmmmmmy_libgit2_file");
+	cl_git_rewritefile(dummy_path.ptr, "this is a dummy file");
+
+	fix_path(&sandbox_path);
+	fix_path(&dummy_path);
+
+	cl_git_pass(git_str_printf(&new_path, "%s%c%s%c%s%c%s",
+		bogus_path_1, GIT_PATH_LIST_SEPARATOR,
+		orig_path, GIT_PATH_LIST_SEPARATOR,
+		sandbox_path.ptr, GIT_PATH_LIST_SEPARATOR,
+		bogus_path_2));
+
+	check_setenv("PATH", new_path.ptr);
+
+	cl_git_fail_with(GIT_ENOTFOUND, git_fs_path_find_executable(&full_path, "this_file_does_not_exist"));
+	cl_git_pass(git_fs_path_find_executable(&full_path, "dummmmmmmy_libgit2_file"));
+
+	cl_assert_equal_s(full_path.ptr, dummy_path.ptr);
+
+	git_str_dispose(&full_path);
+	git_str_dispose(&new_path);
+	git_str_dispose(&dummy_path);
+	git_str_dispose(&sandbox_path);
+	git__free(orig_path);
+}