Add Filesystem::restorecwd()

This allows restoring the working directory to its original value, which may not actually be the DF root. See #1671, dfhack/scripts#152

docs/Lua API.rst

@@ -2160,6 +2160,10 @@ unless otherwise noted.
 
   Changes the current directory to ``path``. Use with caution.
 
+* ``dfhack.filesystem.restorecwd()``
+
+  Restores the current working directory to what it was when DF started.
+
 * ``dfhack.filesystem.mkdir(path)``
 
   Creates a new directory. Returns ``false`` if unsuccessful, including if ``path`` already exists.

library/Core.cpp

@@ -1618,7 +1618,10 @@ bool Core::Init()
         freopen("stderr.log", "w", stderr);
     #endif
 
-    fprintf(stderr, "DFHack build: %s\n", Version::git_description());
+    Filesystem::init();
+
+    cerr << "DFHack build: " << Version::git_description() << "\n"
+         << "Starting with working directory: " << Filesystem::getcwd() << endl;
 
     // find out what we are...
     #ifdef LINUX_BUILD

library/LuaApi.cpp

@@ -2371,6 +2371,7 @@ static const luaL_Reg dfhack_screen_funcs[] = {
 
 static const LuaWrapper::FunctionReg dfhack_filesystem_module[] = {
     WRAPM(Filesystem, getcwd),
+    WRAPM(Filesystem, restorecwd),
     WRAPM(Filesystem, chdir),
     WRAPM(Filesystem, mkdir),
     WRAPM(Filesystem, mkdir_recursive),

library/include/modules/Filesystem.h

@@ -146,8 +146,10 @@ enum _filetype {
 
 namespace DFHack {
     namespace Filesystem {
+        DFHACK_EXPORT void init ();
         DFHACK_EXPORT bool chdir (std::string path);
         DFHACK_EXPORT std::string getcwd ();
+        DFHACK_EXPORT bool restorecwd ();
         DFHACK_EXPORT bool mkdir (std::string path);
         // returns true on success or if directory already exists
         DFHACK_EXPORT bool mkdir_recursive (std::string path);

library/modules/Filesystem.cpp

@@ -52,6 +52,18 @@ SOFTWARE.
 
 using namespace DFHack;
 
+static bool initialized = false;
+static std::string initial_cwd;
+
+void Filesystem::init ()
+{
+    if (!initialized)
+    {
+        initialized = true;
+        initial_cwd = Filesystem::getcwd();
+    }
+}
+
 bool Filesystem::chdir (std::string path)
 {
     return ::chdir(path.c_str()) == 0;
@@ -71,6 +83,11 @@ std::string Filesystem::getcwd ()
     return result;
 }
 
+bool Filesystem::restorecwd ()
+{
+    return Filesystem::chdir(initial_cwd);
+}
+
 bool Filesystem::mkdir (std::string path)
 {
     int fail;