Ghost: Make special user directory query thread safe

Similar to e0ff7731e0.

Noticed a data race when working on blender/blender!130543, which calls
this function from a thread. Make this thread safe by avoiding returning
of static memory, instead use an optional `std::string`.

Pull Request: https://projects.blender.org/blender/blender/pulls/141083

intern/ghost/GHOST_ISystemPaths.hh

@@ -8,6 +8,9 @@
 
 #pragma once
 
+#include <optional>
+#include <string>
+
 #include "GHOST_Types.h"
 
 class GHOST_ISystemPaths {
@@ -60,9 +63,9 @@ class GHOST_ISystemPaths {
 
   /**
    * Determine a special ("well known") and easy to reach user directory.
-   * \return Unsigned char string pointing to user directory (eg `~/Documents/`).
+   * \return If successfull, a string containing the user directory path (eg `~/Documents/`).
    */
-  virtual const char *getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const = 0;
+  virtual std::optional<std::string> getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const = 0;
 
   /**
    * Determine the directory of the current binary

intern/ghost/GHOST_Path-api.hh

@@ -8,6 +8,9 @@
 
 #pragma once
 
+#include <optional>
+#include <string>
+
 #include "GHOST_Types.h"
 
 GHOST_DECLARE_HANDLE(GHOST_SystemPathsHandle);
@@ -46,9 +49,9 @@ extern const char *GHOST_getUserDir(int version, const char *versionstr);
 
 /**
  * Determine a special ("well known") and easy to reach user directory.
- * \return Unsigned char string pointing to user directory (eg `~/Documents/`).
+ * \return If successfull, a string containing the user directory path (eg `~/Documents/`).
  */
-extern const char *GHOST_getUserSpecialDir(GHOST_TUserSpecialDirTypes type);
+extern std::optional<std::string> GHOST_getUserSpecialDir(GHOST_TUserSpecialDirTypes type);
 
 /**
  * Determine the directory in which the binary file is found.

intern/ghost/intern/GHOST_Path-api.cc

@@ -34,7 +34,7 @@ const char *GHOST_getUserDir(int version, const char *versionstr)
   return systemPaths ? systemPaths->getUserDir(version, versionstr) : nullptr;
 }
 
-const char *GHOST_getUserSpecialDir(GHOST_TUserSpecialDirTypes type)
+std::optional<std::string> GHOST_getUserSpecialDir(GHOST_TUserSpecialDirTypes type)
 {
   const GHOST_ISystemPaths *systemPaths = GHOST_ISystemPaths::get();
   /* Shouldn't be `nullptr`. */

intern/ghost/intern/GHOST_SystemPathsCocoa.hh

@@ -12,6 +12,9 @@
 #  error Apple OSX only!
 #endif  // __APPLE__
 
+#include <optional>
+#include <string>
+
 #include "GHOST_SystemPaths.hh"
 
 class GHOST_SystemPathsCocoa : public GHOST_SystemPaths {
@@ -42,9 +45,9 @@ class GHOST_SystemPathsCocoa : public GHOST_SystemPaths {
 
   /**
    * Determine a special ("well known") and easy to reach user directory.
-   * \return Unsigned char string pointing to user directory (eg `~/Documents/`).
+   * \return If successfull, a string containing the user directory path (eg `~/Documents/`).
    */
-  const char *getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
+  std::optional<std::string> getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
 
   /**
    * Determine the directory of the current binary.

intern/ghost/intern/GHOST_SystemPathsCocoa.mm

@@ -5,6 +5,9 @@
 #import <AppKit/NSDocumentController.h>
 #import <Foundation/Foundation.h>
 
+#include <optional>
+#include <string>
+
 #include "GHOST_Debug.hh"
 #include "GHOST_SystemPathsCocoa.hh"
 
@@ -46,9 +49,10 @@ const char *GHOST_SystemPathsCocoa::getUserDir(int /* version */, const char *ve
   return GetApplicationSupportDir(versionstr, NSUserDomainMask, tempPath, sizeof(tempPath));
 }
 
-const char *GHOST_SystemPathsCocoa::getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const
+std::optional<std::string> GHOST_SystemPathsCocoa::getUserSpecialDir(
+    GHOST_TUserSpecialDirTypes type) const
 {
-  static char tempPath[512] = "";
+  char tempPath[512] = "";
   @autoreleasepool {
     NSSearchPathDirectory ns_directory;
 
@@ -78,12 +82,12 @@ const char *GHOST_SystemPathsCocoa::getUserSpecialDir(GHOST_TUserSpecialDirTypes
         GHOST_ASSERT(
             false,
             "GHOST_SystemPathsCocoa::getUserSpecialDir(): Invalid enum value for type parameter");
-        return nullptr;
+        return std::nullopt;
     }
 
     NSArray *paths = NSSearchPathForDirectoriesInDomains(ns_directory, NSUserDomainMask, YES);
     if (paths.count == 0) {
-      return nullptr;
+      return std::nullopt;
     }
     NSString *basePath = [paths objectAtIndex:0];
 
@@ -94,6 +98,9 @@ const char *GHOST_SystemPathsCocoa::getUserSpecialDir(GHOST_TUserSpecialDirTypes
     memcpy(tempPath, basePath_cstr, basePath_len);
     tempPath[basePath_len] = '\0';
   }
+  if (!tempPath[0]) {
+    return std::nullopt;
+  }
   return tempPath;
 }
 

intern/ghost/intern/GHOST_SystemPathsUnix.cc

@@ -6,6 +6,7 @@
  * \ingroup GHOST
  */
 
+#include <optional>
 #include <sstream>
 
 #include "GHOST_SystemPathsUnix.hh"
@@ -100,10 +101,10 @@ const char *GHOST_SystemPathsUnix::getUserDir(int version, const char *versionst
   return user_path.c_str();
 }
 
-const char *GHOST_SystemPathsUnix::getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const
+std::optional<std::string> GHOST_SystemPathsUnix::getUserSpecialDir(
+    GHOST_TUserSpecialDirTypes type) const
 {
   const char *type_str;
-  static string path;
 
   switch (type) {
     case GHOST_kUserSpecialDirDesktop:
@@ -133,16 +134,15 @@ const char *GHOST_SystemPathsUnix::getUserSpecialDir(GHOST_TUserSpecialDirTypes
       /* If `XDG_CACHE_HOME` is not set, then `$HOME/.cache is used`. */
       const char *home_dir = home_dir_get();
       if (home_dir == nullptr) {
-        return nullptr;
+        return std::nullopt;
       }
-      path = string(home_dir) + "/.cache";
-      return path.c_str();
+      return string(home_dir) + "/.cache";
     }
     default:
       GHOST_ASSERT(
           false,
           "GHOST_SystemPathsUnix::getUserSpecialDir(): Invalid enum value for type parameter");
-      return nullptr;
+      return std::nullopt;
   }
 
   /* Pipe `stderr` to `/dev/null` to avoid error prints. We will fail gracefully still. */
@@ -150,7 +150,7 @@ const char *GHOST_SystemPathsUnix::getUserSpecialDir(GHOST_TUserSpecialDirTypes
 
   FILE *fstream = popen(command.c_str(), "r");
   if (fstream == nullptr) {
-    return nullptr;
+    return std::nullopt;
   }
   std::stringstream path_stream;
   while (!feof(fstream)) {
@@ -163,11 +163,11 @@ const char *GHOST_SystemPathsUnix::getUserSpecialDir(GHOST_TUserSpecialDirTypes
   }
   if (pclose(fstream) == -1) {
     perror("GHOST_SystemPathsUnix::getUserSpecialDir failed at pclose()");
-    return nullptr;
+    return std::nullopt;
   }
 
-  path = path_stream.str();
-  return path[0] ? path.c_str() : nullptr;
+  std::string path = path_stream.str();
+  return path[0] ? std::optional(path) : std::nullopt;
 }
 
 const char *GHOST_SystemPathsUnix::getBinaryDir() const

intern/ghost/intern/GHOST_SystemPathsUnix.hh

@@ -8,6 +8,9 @@
 
 #pragma once
 
+#include <optional>
+#include <string>
+
 #include "../GHOST_Types.h"
 #include "GHOST_SystemPaths.hh"
 
@@ -40,9 +43,9 @@ class GHOST_SystemPathsUnix : public GHOST_SystemPaths {
 
   /**
    * Determine a special ("well known") and easy to reach user directory.
-   * \return Unsigned char string pointing to user directory (eg `~/Documents/`).
+   * \return If successfull, a string containing the user directory path (eg `~/Documents/`).
    */
-  const char *getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
+  std::optional<std::string> getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
 
   /**
    * Determine the directory of the current binary.

intern/ghost/intern/GHOST_SystemPathsWin32.cc

@@ -62,7 +62,8 @@ const char *GHOST_SystemPathsWin32::getUserDir(int, const char *versionstr) cons
   return user_dir;
 }
 
-const char *GHOST_SystemPathsWin32::getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const
+std::optional<std::string> GHOST_SystemPathsWin32::getUserSpecialDir(
+    GHOST_TUserSpecialDirTypes type) const
 {
   const char *special_dir = nullptr;
 
@@ -93,7 +94,7 @@ const char *GHOST_SystemPathsWin32::getUserSpecialDir(GHOST_TUserSpecialDirTypes
       GHOST_ASSERT(
           false,
           "GHOST_SystemPathsWin32::getUserSpecialDir(): Invalid enum value for type parameter");
-      return nullptr;
+      return std::nullopt;
   }
 
   static char knownpath[MAX_PATH * 3] = {0};
@@ -105,6 +106,10 @@ const char *GHOST_SystemPathsWin32::getUserSpecialDir(GHOST_TUserSpecialDirTypes
     special_dir = knownpath;
   }
 
+  if ((special_dir == nullptr) || (special_dir[0] == '\0')) {
+    return std::nullopt;
+  }
+
   CoTaskMemFree(knownpath_16);
   return special_dir;
 }

intern/ghost/intern/GHOST_SystemPathsWin32.hh

@@ -8,6 +8,9 @@
 
 #pragma once
 
+#include <optional>
+#include <string>
+
 #ifndef WIN32
 #  error WIN32 only!
 #endif  // WIN32
@@ -49,9 +52,9 @@ class GHOST_SystemPathsWin32 : public GHOST_SystemPaths {
 
   /**
    * Determine a special ("well known") and easy to reach user directory.
-   * \return Unsigned char string pointing to user directory (eg `~/Documents/`).
+   * \return If successfull, a string containing the user directory path (eg `~/Documents/`).
    */
-  const char *getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
+  std::optional<std::string> getUserSpecialDir(GHOST_TUserSpecialDirTypes type) const override;
 
   /**
    * Determine the directory of the current binary.