M10: test the print entry points against a Windows without the DLLs (#2630)

Review follow-up on #2701: the test no longer requires the DLLs in the
host's System32 (the trimmed installations this supports lack them).
Instead a test-only override points the lookup at an empty folder and
submit, submit_with_progress and printers are called through: printing
is refused naming the DLL and no printers are listed, before any call
into the delay-loaded DLLs. The job names a printer that doesn't exist
and the bytes aren't a PDF, so a missing guard can't reach a real one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/print/src/spool.rs

@@ -19,11 +19,22 @@ pub(crate) fn missing_print_dll_in(system32: &std::path::Path) -> Option<&'stati
     PRINT_DLLS.into_iter().find(|dll| !system32.join(dll).is_file())
 }
 
+#[cfg(all(windows, test))]
+thread_local! {
+    /// Tests: the folder the print DLLs are looked for in, instead of the system's, so the
+    /// public entry points can be checked against a Windows that lacks them.
+    pub(crate) static SYSTEM32_FOR_TEST: std::cell::RefCell<Option<std::path::PathBuf>> = const { std::cell::RefCell::new(None) };
+}
+
 /// The first print DLL this Windows installation lacks. `None` when all are there, and when the
 /// system folder is unknown (the loader then decides, as before).
 fn missing_print_dll() -> Option<&'static str> {
     #[cfg(windows)]
     {
+        #[cfg(test)]
+        if let Some(dir) = SYSTEM32_FOR_TEST.with(|d| d.borrow().clone()) {
+            return missing_print_dll_in(&dir);
+        }
         let root = std::env::var_os("SystemRoot").or_else(|| std::env::var_os("windir"))?;
         missing_print_dll_in(&std::path::Path::new(&root).join("System32"))
     }

crates/print/src/tests.rs

@@ -1160,7 +1160,30 @@ fn missing_print_dlls_are_named() {
     std::fs::write(dir.join(PRINT_DLLS[1]), b"").unwrap();
     assert_eq!(missing_print_dll_in(&dir), None);
     let _ = std::fs::remove_dir_all(&dir);
-    // This machine's own Windows has them.
-    let system32 = std::path::Path::new(&std::env::var_os("SystemRoot").unwrap()).join("System32");
-    assert_eq!(missing_print_dll_in(&system32), None);
+}
+
+/// #2630: on a Windows without the print DLLs, the public entry points answer before any call
+/// into the delay-loaded DLLs: no printers, and printing refused naming the DLL. The job names a
+/// printer that doesn't exist and the bytes aren't a PDF, so a missing guard fails here with
+/// another error rather than reaching a real printer.
+#[cfg(windows)]
+#[test]
+fn printing_without_the_print_dlls_is_refused_before_the_spooler() {
+    use crate::spool::{PRINT_DLLS, SYSTEM32_FOR_TEST, printers, submit, submit_with_progress};
+    let dir = std::env::temp_dir().join(format!("pdfcraft-print-no-dlls-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(&dir).unwrap();
+    SYSTEM32_FOR_TEST.with(|d| *d.borrow_mut() = Some(dir.clone()));
+    let job = crate::spool::Job { printer: Some("PdfCraft test: no such printer".into()), ..Default::default() };
+    let named = |r: Result<String, crate::PrintError>| match r {
+        Err(crate::PrintError::Spool(m)) => m.contains(PRINT_DLLS[0]),
+        _ => false,
+    };
+    assert!(named(submit(b"not a PDF", &job)), "submit names the missing DLL");
+    let mut called = false;
+    assert!(named(submit_with_progress(b"not a PDF", &job, &mut |_, _| called = true)), "submit_with_progress names it");
+    assert!(!called, "no sheet reached a spooler");
+    assert!(printers().is_empty(), "no printers are listed");
+    SYSTEM32_FOR_TEST.with(|d| *d.borrow_mut() = None);
+    let _ = std::fs::remove_dir_all(&dir);
 }