make load_library() panic on some errors when err=0 (#47343)

load_library with `err=0` now panics on errors, provided that the
file exists. 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 it.

Fixes #47027

Author: apaz-cli

cli/loader_lib.c

@@ -13,6 +13,11 @@ extern "C" {
 /* Bring in helper functions for windows without libgcc. */
 #ifdef _OS_WINDOWS_
 #include "loader_win_utils.c"
+
+#include <fileapi.h>
+static int win_file_exists(wchar_t* wpath) {
+  return GetFileAttributesW(wpath) == INVALID_FILE_ATTRIBUTES ? 0 : 1;
+}
 #endif
 
 // Save DEP_LIBS to a variable that is explicitly sized for expansion
@@ -31,6 +36,13 @@ 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 the file does not exist,
+ * or panics if opening failed for any other reason. */
+/* Currently the only use of this function with !err is in opening libjulia-codegen,
+ * which the user can delete to save space if generating new code is not necessary.
+ * However, if it exists and cannot be loaded, that's a problem. So, we alert the user
+ * and abort the process. */
 static void * load_library(const char * rel_path, const char * src_dir, int err) {
     void * handle = NULL;
 
@@ -55,19 +67,21 @@ 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() win_file_exists(wpath)
     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, F_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("");

test/compiler/codegen.jl

@@ -689,12 +689,25 @@ mktempdir() do pfx
     cp(dirname(Sys.BINDIR), pfx; force=true)
     libpath = relpath(dirname(dlpath(libjulia_codegen_name())), dirname(Sys.BINDIR))
     libs_deleted = 0
-    for f in filter(f -> startswith(f, "libjulia-codegen"), readdir(joinpath(pfx, libpath)))
+    libfiles = filter(f -> startswith(f, "libjulia-codegen"), readdir(joinpath(pfx, libpath)))
+    for f in libfiles
         rm(joinpath(pfx, libpath, f); force=true, recursive=true)
         libs_deleted += 1
     end
     @test libs_deleted > 0
     @test readchomp(`$pfx/bin/$(Base.julia_exename()) -e 'print("no codegen!\n")'`) == "no codegen!"
+
+    # PR #47343
+    libs_emptied = 0
+    for f in libfiles
+        touch(joinpath(pfx, libpath, f))
+        libs_emptied += 1
+    end
+
+    errfile = joinpath(pfx, "stderr.txt")
+    @test libs_emptied > 0
+    @test_throws ProcessFailedException run(pipeline(`$pfx/bin/$(Base.julia_exename()) -e 'print("This should fail!\n")'`; stderr=errfile))
+    @test contains(readline(errfile), "ERROR: Unable to load dependent library")
 end
 
 # issue #42645