video: Fix race condition if multiple video backends are downloading OpenH264 at the same time

Co-authored-by: TÖRÖK Attila <torokati44@gmail.com>

Cargo.lock

@@ -4565,6 +4565,7 @@ dependencies = [
  "ruffle_video_software",
  "slotmap",
  "swf",
+ "tempfile",
  "tracing",
 ]
 

video/external/Cargo.toml

@@ -21,6 +21,7 @@ md-5 = "0.10.6"
 reqwest = { version = "0.12.5", default-features = false, features = ["blocking"] }
 hex = "0.4.3"
 bzip2 = { version = "0.4.4", features = ["static"] }
+tempfile = "3.11.0"
 
 [package.metadata.cargo-machete]
 ignored = [

video/external/src/backend.rs

@@ -85,20 +85,24 @@ impl ExternalVideoBackend {
 
         let (filename, md5sum) = Self::get_openh264_data()?;
 
-        let filepath = std::env::current_exe()?
+        let current_exe = std::env::current_exe()?;
+        let directory = current_exe
             .parent()
-            .ok_or("Could not determine Ruffle location.")?
-            .join(filename);
+            .ok_or("Could not determine Ruffle location.")?;
+        let filepath = directory.join(filename);
 
         // If the binary doesn't exist in the expected location, download it.
         if !filepath.is_file() {
             let url = format!("{}{}{}", URL_BASE, filename, URL_SUFFIX);
             let response = reqwest::blocking::get(url)?;
-            let bytes = response.bytes()?;
-            let mut bzip2_reader = BzDecoder::new(bytes.as_ref());
+            let mut bzip2_reader = BzDecoder::new(response);
 
-            let mut file = File::create(filepath.clone())?;
-            copy(&mut bzip2_reader, &mut file)?;
+            let mut tempfile = tempfile::NamedTempFile::with_prefix_in(filename, directory)?;
+            copy(&mut bzip2_reader, &mut tempfile)?;
+            // Let's assume that if this fails, it's because another process has already put it there
+            // and loaded it, therefore it can't be overwritten (on Windows at least), but in the end,
+            // all's fine - the MD5 hash will still be checked before attempting to load the library.
+            let _ = tempfile.persist(&filepath);
         }
 
         // Regardless of whether the library was already there, or we just downloaded it, let's check the MD5 hash.