Auto merge of #110427 - Nilstrieb:parallel-bootstrap-startup, r=albertlarsan68

Parallelize initial Rust download in bootstrap

Parallelize the initial download of Rust in `bootstrap.py`

`time ./x.py --help` after `rm -r build`
Before: 33s
After: 27s

src/bootstrap/bootstrap.py

@@ -13,6 +13,7 @@ import tarfile
 import tempfile
 
 from time import time
+from multiprocessing import Pool, cpu_count
 
 try:
     import lzma
@@ -406,6 +407,48 @@ class Stage0Toolchain:
         return self.version + "-" + self.date
 
 
+class DownloadInfo:
+    """A helper class that can be pickled into a parallel subprocess"""
+
+    def __init__(
+        self,
+        base_download_url,
+        download_path,
+        bin_root,
+        tarball_path,
+        tarball_suffix,
+        checksums_sha256,
+        pattern,
+        verbose,
+    ):
+        self.base_download_url = base_download_url
+        self.download_path = download_path
+        self.bin_root = bin_root
+        self.tarball_path = tarball_path
+        self.tarball_suffix = tarball_suffix
+        self.checksums_sha256 = checksums_sha256
+        self.pattern = pattern
+        self.verbose = verbose
+
+def download_component(download_info):
+    if not os.path.exists(download_info.tarball_path):
+        get(
+            download_info.base_download_url,
+            download_info.download_path,
+            download_info.tarball_path,
+            download_info.checksums_sha256,
+            verbose=download_info.verbose,
+        )
+
+def unpack_component(download_info):
+    unpack(
+        download_info.tarball_path,
+        download_info.tarball_suffix,
+        download_info.bin_root,
+        match=download_info.pattern,
+        verbose=download_info.verbose,
+    )
+
 class RustBuild(object):
     """Provide all the methods required to build Rust"""
     def __init__(self):
@@ -460,17 +503,49 @@ class RustBuild(object):
                     )
                     run_powershell([script])
                 shutil.rmtree(bin_root)
+
+            key = self.stage0_compiler.date
+            cache_dst = os.path.join(self.build_dir, "cache")
+            rustc_cache = os.path.join(cache_dst, key)
+            if not os.path.exists(rustc_cache):
+                os.makedirs(rustc_cache)
+
             tarball_suffix = '.tar.gz' if lzma is None else '.tar.xz'
-            filename = "rust-std-{}-{}{}".format(
-                rustc_channel, self.build, tarball_suffix)
-            pattern = "rust-std-{}".format(self.build)
-            self._download_component_helper(filename, pattern, tarball_suffix)
-            filename = "rustc-{}-{}{}".format(rustc_channel, self.build,
-                                              tarball_suffix)
-            self._download_component_helper(filename, "rustc", tarball_suffix)
-            filename = "cargo-{}-{}{}".format(rustc_channel, self.build,
-                                            tarball_suffix)
-            self._download_component_helper(filename, "cargo", tarball_suffix)
+
+            toolchain_suffix = "{}-{}{}".format(rustc_channel, self.build, tarball_suffix)
+
+            tarballs_to_download = [
+                ("rust-std-{}".format(toolchain_suffix), "rust-std-{}".format(self.build)),
+                ("rustc-{}".format(toolchain_suffix), "rustc"),
+                ("cargo-{}".format(toolchain_suffix), "cargo"),
+            ]
+
+            tarballs_download_info = [
+                DownloadInfo(
+                    base_download_url=self.download_url,
+                    download_path="dist/{}/{}".format(self.stage0_compiler.date, filename),
+                    bin_root=self.bin_root(),
+                    tarball_path=os.path.join(rustc_cache, filename),
+                    tarball_suffix=tarball_suffix,
+                    checksums_sha256=self.checksums_sha256,
+                    pattern=pattern,
+                    verbose=self.verbose,
+                )
+                for filename, pattern in tarballs_to_download
+            ]
+
+            # Download the components serially to show the progress bars properly.
+            for download_info in tarballs_download_info:
+                download_component(download_info)
+
+            # Unpack the tarballs in parallle.
+            # In Python 2.7, Pool cannot be used as a context manager.
+            p = Pool(min(len(tarballs_download_info), cpu_count()))
+            try:
+                p.map(unpack_component, tarballs_download_info)
+            finally:
+                p.close()
+
             if self.should_fix_bins_and_dylibs():
                 self.fix_bin_or_dylib("{}/bin/cargo".format(bin_root))
 
@@ -486,13 +561,9 @@ class RustBuild(object):
                 rust_stamp.write(key)
 
     def _download_component_helper(
-        self, filename, pattern, tarball_suffix,
+        self, filename, pattern, tarball_suffix, rustc_cache,
     ):
         key = self.stage0_compiler.date
-        cache_dst = os.path.join(self.build_dir, "cache")
-        rustc_cache = os.path.join(cache_dst, key)
-        if not os.path.exists(rustc_cache):
-            os.makedirs(rustc_cache)
 
         tarball = os.path.join(rustc_cache, filename)
         if not os.path.exists(tarball):