[Compiler download] Allow multiple parallel downloads of different compilers

.changeset/sweet-seas-shake.md

@@ -0,0 +1,5 @@
+---
+"hardhat": patch
+---
+
+Allow multiple parallel downloads of different compilers ([7946](https://github.com/NomicFoundation/hardhat/pull/7946)).

v-next/hardhat/src/internal/builtin-plugins/solidity/build-system/compiler/downloader.ts

@@ -153,7 +153,6 @@ export class CompilerDownloaderImplementation implements CompilerDownloader {
   readonly #compilersDir: string;
   readonly #downloadFunction: typeof download;
 
-  readonly #mutexCompiler = new MultiProcessMutex("compiler-download");
   readonly #mutexCompilerList = new MultiProcessMutex("compiler-download-list");
 
   /**
@@ -200,11 +199,14 @@ export class CompilerDownloaderImplementation implements CompilerDownloader {
   }
 
   public async downloadCompiler(version: string): Promise<boolean> {
-    // Since only one process at a time can acquire the mutex, we avoid the risk of downloading the same compiler multiple times.
-    // This is because the mutex blocks access until a compiler has been fully downloaded, preventing any new process
-    // from checking whether that version of the compiler exists. Without mutex it might incorrectly
-    // return false, indicating that the compiler isn't present, even though it is currently being downloaded.
-    return this.#mutexCompiler.use(async () => {
+    // A per-version mutex ensures that only one process at a time can download a given compiler version,
+    // while still allowing different compiler versions to be downloaded in parallel.
+    // Without the mutex, a concurrent process might check whether a version exists, incorrectly
+    // find it missing (because another process is still downloading it), and start a redundant download.
+
+    const mutex = new MultiProcessMutex(`compiler-download-${version}`);
+
+    return mutex.use(async () => {
       const isCompilerDownloaded = await this.isCompilerDownloaded(version);
 
       if (isCompilerDownloaded === true) {