Fix assert with temporary directories beginning with "//"

- Skip leading forward slashes when setting the temp directory.
- Add a utility function to set the temporary directory
  which is used for the user preferences & environment variables.

This issue was raised by #95411 where "//" resolves to "/",
then asserts when passed to Blender's file-system functions.
However the crash referenced in this report looks to be caused
by Collada failing to write to the temporary directory which
can be handled separately.

Ref !118872

source/blender/blenkernel/intern/appdir.cc

@@ -1118,16 +1118,9 @@ void BKE_appdir_app_templates(ListBase *templates)
  */
 static void where_is_temp(char *tempdir, const size_t tempdir_maxncpy, const char *userdir)
 {
-
-  tempdir[0] = '\0';
-
-  if (userdir && userdir[0] != '\0' && BLI_is_dir(userdir)) {
-    BLI_strncpy(tempdir, userdir, tempdir_maxncpy);
-    /* Add a trailing slash if needed. */
-    BLI_path_slash_ensure(tempdir, tempdir_maxncpy);
+  if (BLI_temp_directory_path_copy_if_valid(tempdir, tempdir_maxncpy, userdir)) {
     return;
   }
-
   BLI_temp_directory_path_get(tempdir, tempdir_maxncpy);
 }
 

source/blender/blenlib/BLI_tempfile.h

@@ -14,6 +14,14 @@
 extern "C" {
 #endif
 
+/**
+ * Sets `temp_directory` from `dirpath` when it's a valid directory.
+ * Simple sanitize operations are performed and a trailing slash is ensured.
+ */
+bool BLI_temp_directory_path_copy_if_valid(char *temp_directory,
+                                           const size_t buffer_size,
+                                           const char *dirpath);
+
 /* Get the path to a directory suitable for temporary files.
  *
  * The return path is guaranteed to exist and to be a directory, as well as to contain a trailing

source/blender/blenlib/intern/tempfile.c

@@ -8,6 +8,34 @@
 #include "BLI_path_util.h"
 #include "BLI_string.h"
 
+bool BLI_temp_directory_path_copy_if_valid(char *temp_directory,
+                                           const size_t buffer_size,
+                                           const char *dirpath)
+{
+  if (dirpath == NULL) {
+    return false;
+  }
+
+  /* Disallow paths starting with two forward slashes. While they are valid paths,
+   * Blender interprets them as relative in situations relative paths aren't supported,
+   * see #95411. */
+  while (UNLIKELY(dirpath[0] == '/' && dirpath[1] == '/')) {
+    dirpath++;
+  }
+  if (dirpath[0] == '\0') {
+    return false;
+  }
+  if (!BLI_is_dir(dirpath)) {
+    return false;
+  }
+
+  BLI_strncpy(temp_directory, dirpath, buffer_size);
+
+  /* Add a trailing slash if needed. */
+  BLI_path_slash_ensure(temp_directory, buffer_size);
+  return true;
+}
+
 void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
 {
   temp_directory[0] = '\0';
@@ -24,9 +52,9 @@ void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
   };
 
   for (int i = 0; i < ARRAY_SIZE(env_vars); i++) {
-    const char *tmp = BLI_getenv(env_vars[i]);
-    if (tmp && (tmp[0] != '\0') && BLI_is_dir(tmp)) {
-      BLI_strncpy(temp_directory, tmp, buffer_size);
+    if (BLI_temp_directory_path_copy_if_valid(
+            temp_directory, buffer_size, BLI_getenv(env_vars[i])))
+    {
       break;
     }
   }
@@ -34,10 +62,6 @@ void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
   if (temp_directory[0] == '\0') {
     BLI_strncpy(temp_directory, "/tmp/", buffer_size);
   }
-  else {
-    /* Add a trailing slash if needed. */
-    BLI_path_slash_ensure(temp_directory, buffer_size);
-  }
 
   BLI_dir_create_recursive(temp_directory);
 }