load_library() panics on errors when err=0

load_library with `err=0` now panics on errors, provided that the
file is openable. It used to never panic on errors, leading to
confusion between when cases the libjuliacodegen library had been
intentionally removed and when it tried but failed to load.

Fixes JuliaLang#47027

cli/loader_lib.c

@@ -31,6 +31,9 @@ void jl_loader_print_stderr3(const char * msg1, const char * msg2, const char *
 }
 
 /* Wrapper around dlopen(), with extra relative pathing thrown in*/
+/* If err, then loads the library successfully or panics.
+ * If !err, then loads the library or returns null if it could not be opened,
+ * or panics if opening failed for any other reason. */
 static void * load_library(const char * rel_path, const char * src_dir, int err) {
     void * handle = NULL;
 
@@ -55,19 +58,21 @@ static void * load_library(const char * rel_path, const char * src_dir, int err)
     strncat(path, rel_path, sizeof(path) - 1);
 
 #if defined(_OS_WINDOWS_)
+#define PATH_EXISTS() !_waccess(wpath, EACCES)
     wchar_t wpath[2*JL_PATH_MAX + 1] = {0};
     if (!utf8_to_wchar(path, wpath, 2*JL_PATH_MAX)) {
         jl_loader_print_stderr3("ERROR: Unable to convert path ", path, " to wide string!\n");
         exit(1);
     }
     handle = (void *)LoadLibraryExW(wpath, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
 #else
+#define PATH_EXISTS() !access(path, R_OK)
     handle = dlopen(path, RTLD_NOW | (err ? RTLD_GLOBAL : RTLD_LOCAL));
 #endif
-
     if (handle == NULL) {
-        if (!err)
+        if (!err && !PATH_EXISTS())
             return NULL;
+#undef PATH_EXISTS
         jl_loader_print_stderr3("ERROR: Unable to load dependent library ", path, "\n");
 #if defined(_OS_WINDOWS_)
         LPWSTR wmsg = TEXT("");