Cleanup: minor changes to temp directory API

- Pass null instead of an empty string to BKE_tempdir_init
  because the string isn't meant to be used.
- Never pass null to BLI_temp_directory_path_copy_if_valid
  (the caller must check).
- Additional comments for which checks are performed & why
  from discussion about #95411.

source/blender/asset_system/tests/asset_library_service_test.cc

@@ -62,7 +62,7 @@ class AssetLibraryServiceTest : public testing::Test {
    * The returned path ends in a slash. */
   CatalogFilePath use_temp_path()
   {
-    BKE_tempdir_init("");
+    BKE_tempdir_init(nullptr);
     const CatalogFilePath tempdir = BKE_tempdir_session();
     temp_library_path_ = tempdir + "test-temporary-path" + SEP_STR;
     return temp_library_path_;

source/blender/asset_system/tests/asset_library_test_common.hh

@@ -79,7 +79,7 @@ class AssetLibraryTestBase : public testing::Test {
    * The returned path ends in a slash. */
   std::string use_temp_path()
   {
-    BKE_tempdir_init("");
+    BKE_tempdir_init(nullptr);
     const std::string tempdir = BKE_tempdir_session();
     temp_library_path_ = tempdir + "test-temporary-path" + SEP_STR;
     return temp_library_path_;

source/blender/blenkernel/intern/appdir.cc

@@ -1118,7 +1118,7 @@ void BKE_appdir_app_templates(ListBase *templates)
  */
 static void where_is_temp(char *tempdir, const size_t tempdir_maxncpy, const char *userdir)
 {
-  if (BLI_temp_directory_path_copy_if_valid(tempdir, tempdir_maxncpy, userdir)) {
+  if (userdir && BLI_temp_directory_path_copy_if_valid(tempdir, tempdir_maxncpy, userdir)) {
     return;
   }
   BLI_temp_directory_path_get(tempdir, tempdir_maxncpy);
@@ -1166,7 +1166,7 @@ static void tempdir_session_create(char *tempdir_session,
 
 void BKE_tempdir_init(const char *userdir)
 {
-  /* Sets #g_app.temp_dirname_base to \a userdir if specified and is a valid directory,
+  /* Sets #g_app.temp_dirname_base to `userdir` if specified and is a valid directory,
    * otherwise chooses a suitable OS-specific temporary directory.
    * Sets #g_app.temp_dirname_session to a #mkdtemp
    * generated sub-dir of #g_app.temp_dirname_base. */

source/blender/blenlib/BLI_tempfile.h

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include "BLI_compiler_attrs.h"
 #include "BLI_sys_types.h"
 
 #ifdef __cplusplus
@@ -15,21 +16,23 @@ extern "C" {
 #endif
 
 /**
- * Sets `temp_directory` from `dirpath` when it's a valid directory.
+ * Sets `tempdir` 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);
+bool BLI_temp_directory_path_copy_if_valid(char *tempdir,
+                                           const size_t tempdir_maxncpy,
+                                           const char *dirpath) ATTR_NONNULL(1, 3);
 
-/* Get the path to a directory suitable for temporary files.
+/**
+ * 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);
+ * At maximum the `tempdir_maxncpy` number of characters is written to the `tempdir`.
+ * The directory path is always null-terminated.
+ */
+void BLI_temp_directory_path_get(char *tempdir, const size_t tempdir_maxncpy) ATTR_NONNULL(1);
 
 #ifdef __cplusplus
 }

source/blender/blenlib/intern/tempfile.c

@@ -8,13 +8,19 @@
 #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,
+bool BLI_temp_directory_path_copy_if_valid(char *tempdir,
+                                           const size_t tempdir_maxncpy,
                                            const char *dirpath)
 {
-  if (dirpath == NULL) {
-    return false;
-  }
+  /* NOTE(@ideasman42): it is *not* the purpose of this function to check that
+   * `dirpath` is writable under all circumstances.
+   * Only check `dirpath` doesn't resolve to an empty string & points to a directory.
+   *
+   * While other checks could be added to avoid problems writing temporary files:
+   * (read-only, permission failure, out-of-I-nodes, disk-full... etc)
+   * it's out of scope for this function as these characteristics can change at run-time.
+   * In general temporary file IO should handle failure properly with sufficient user feedback,
+   * without attempting to *solve* the problem by anticipating file-system issues ahead of time. */
 
   /* Disallow paths starting with two forward slashes. While they are valid paths,
    * Blender interprets them as relative in situations relative paths aren't supported,
@@ -29,16 +35,16 @@ bool BLI_temp_directory_path_copy_if_valid(char *temp_directory,
     return false;
   }
 
-  BLI_strncpy(temp_directory, dirpath, buffer_size);
+  BLI_strncpy(tempdir, dirpath, tempdir_maxncpy);
 
   /* Add a trailing slash if needed. */
-  BLI_path_slash_ensure(temp_directory, buffer_size);
+  BLI_path_slash_ensure(tempdir, tempdir_maxncpy);
   return true;
 }
 
-void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
+void BLI_temp_directory_path_get(char *tempdir, const size_t tempdir_maxncpy)
 {
-  temp_directory[0] = '\0';
+  tempdir[0] = '\0';
 
   const char *env_vars[] = {
 #ifdef WIN32
@@ -52,16 +58,18 @@ void BLI_temp_directory_path_get(char *temp_directory, const size_t buffer_size)
   };
 
   for (int i = 0; i < ARRAY_SIZE(env_vars); i++) {
-    if (BLI_temp_directory_path_copy_if_valid(
-            temp_directory, buffer_size, BLI_getenv(env_vars[i])))
-    {
+    const char *tempdir_test = BLI_getenv(env_vars[i]);
+    if (tempdir_test == NULL) {
+      continue;
+    }
+    if (BLI_temp_directory_path_copy_if_valid(tempdir, tempdir_maxncpy, tempdir_test)) {
       break;
     }
   }
 
-  if (temp_directory[0] == '\0') {
-    BLI_strncpy(temp_directory, "/tmp/", buffer_size);
+  if (tempdir[0] == '\0') {
+    BLI_strncpy(tempdir, "/tmp/", tempdir_maxncpy);
   }
 
-  BLI_dir_create_recursive(temp_directory);
+  BLI_dir_create_recursive(tempdir);
 }

source/blender/io/ply/tests/io_ply_exporter_test.cc

@@ -42,7 +42,7 @@ class PLYExportTest : public BlendfileLoadingBaseTest {
   {
     BlendfileLoadingBaseTest::SetUp();
 
-    BKE_tempdir_init("");
+    BKE_tempdir_init(nullptr);
   }
 
   void TearDown() override

source/blender/io/stl/tests/stl_exporter_tests.cc

@@ -60,7 +60,7 @@ class STLExportTest : public BlendfileLoadingBaseTest {
   void SetUp() override
   {
     BlendfileLoadingBaseTest::SetUp();
-    BKE_tempdir_init("");
+    BKE_tempdir_init(nullptr);
   }
 
   void TearDown() override

source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc

@@ -143,7 +143,7 @@ class ObjExporterWriterTest : public testing::Test {
  protected:
   void SetUp() override
   {
-    BKE_tempdir_init("");
+    BKE_tempdir_init(nullptr);
   }
 
   void TearDown() override