BLI: Add utility to access system-wide temporary directory

It provides path to a directory suitable for storing temporary files
which satisfies the following conditions:

- The directory exists.
- The return path has trailing directory separator.

It is based on the code used in the appdir.c to get the temporary
directory.

For the C++ people: this is similar to the temp_directory_path()
from the std::filesystem.

No functional changes expected as it is a new code which is planned
to be used on other places as a followup development.

source/blender/blenlib/BLI_tempfile.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright 2023 Blender Foundation. */
+
+/** \file
+ * \ingroup bli
+ */
+
+#pragma once
+
+#include "BLI_sys_types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* 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
+ * directory separator.
+ *
+ * At maximum the buffer_size number of characters is written to the temp_directory. The directory
+ * path is always null-terminated. */
+void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size);
+
+#ifdef __cplusplus
+}
+#endif

source/blender/blenlib/CMakeLists.txt

@@ -147,6 +147,7 @@ set(SRC
   intern/task_pool.cc
   intern/task_range.cc
   intern/task_scheduler.cc
+  intern/tempfile.c
   intern/threads.cc
   intern/time.c
   intern/timecode.c
@@ -347,6 +348,7 @@ set(SRC
   BLI_system.h
   BLI_task.h
   BLI_task.hh
+  BLI_tempfile.h
   BLI_threads.h
   BLI_timecode.h
   BLI_timeit.hh
@@ -533,6 +535,7 @@ if(WITH_GTESTS)
     tests/BLI_string_utf8_test.cc
     tests/BLI_task_graph_test.cc
     tests/BLI_task_test.cc
+    tests/BLI_tempfile_test.cc
     tests/BLI_uuid_test.cc
     tests/BLI_vector_set_test.cc
     tests/BLI_vector_test.cc

source/blender/blenlib/intern/tempfile.c

@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright 2023 Blender Foundation. */
+
+#include "BLI_tempfile.h"
+
+#include "BLI_fileops.h"
+#include "BLI_path_util.h"
+#include "BLI_string.h"
+
+void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
+{
+  temp_directory[0] = '\0';
+
+  const char *env_vars[] = {
+#ifdef WIN32
+      "TEMP",
+#else
+      /* Non standard (could be removed). */
+      "TMP",
+      /* Posix standard. */
+      "TMPDIR",
+#endif
+  };
+
+  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);
+      break;
+    }
+  }
+
+  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);
+}

source/blender/blenlib/tests/BLI_tempfile_test.cc

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+#include "BLI_tempfile.h"
+
+#include "BLI_fileops.h"
+#include "BLI_path_util.h"
+
+#include "testing/testing.h"
+
+namespace blender::tests {
+
+TEST(BLI_tempfile, BLI_temp_directory_path_get)
+{
+  char temp_dir[FILE_MAX];
+  BLI_temp_directory_path_get(temp_dir, sizeof(temp_dir));
+
+  ASSERT_STRNE(temp_dir, "");
+
+  EXPECT_EQ(temp_dir[strlen(temp_dir) - 1], SEP);
+
+  EXPECT_TRUE(BLI_exists(temp_dir));
+  EXPECT_TRUE(BLI_is_dir(temp_dir));
+
+  EXPECT_TRUE(BLI_path_is_abs_from_cwd(temp_dir));
+}
+
+}  // namespace blender::tests