Fetching/decompressing: use magic numbers (#31589)

Spack currently depends on parsing filenames of downloaded files to
determine what type of archive they are and how to decompress them.
This commit adds a preliminary check based on magic numbers to
determine archive type (but falls back on name parsing if the
extension type cannot be determined).

As part of this work, this commit also enables decompression of
.tar.xz-compressed archives on Windows.

lib/spack/llnl/util/filesystem.py

@@ -24,7 +24,7 @@
 from llnl.util.lang import dedupe, memoized
 from llnl.util.symlink import islink, symlink
 
-from spack.util.executable import Executable
+from spack.util.executable import CommandNotFoundError, Executable, which
 from spack.util.path import path_to_os_path, system_path_filter
 
 is_windows = _platform == "win32"
@@ -113,6 +113,69 @@ def path_contains_subdirectory(path, root):
     return norm_path.startswith(norm_root)
 
 
+@memoized
+def file_command(*args):
+    """Creates entry point to `file` system command with provided arguments"""
+    try:
+        file_cmd = which("file", required=True)
+    except CommandNotFoundError as e:
+        if is_windows:
+            raise CommandNotFoundError("`file` utility is not available on Windows")
+        else:
+            raise e
+    for arg in args:
+        file_cmd.add_default_arg(arg)
+    return file_cmd
+
+
+@memoized
+def _get_mime_type():
+    """Generate method to call `file` system command to aquire mime type
+    for a specified path
+    """
+    return file_command("-b", "-h", "--mime-type")
+
+
+@memoized
+def _get_mime_type_compressed():
+    """Same as _get_mime_type but attempts to check for
+    compression first
+    """
+    mime_uncompressed = _get_mime_type()
+    mime_uncompressed.add_default_arg("-Z")
+    return mime_uncompressed
+
+
+def mime_type(filename):
+    """Returns the mime type and subtype of a file.
+
+    Args:
+        filename: file to be analyzed
+
+    Returns:
+        Tuple containing the MIME type and subtype
+    """
+    output = _get_mime_type()(filename, output=str, error=str).strip()
+    tty.debug("==> " + output)
+    type, _, subtype = output.partition("/")
+    return type, subtype
+
+
+def compressed_mime_type(filename):
+    """Same as mime_type but checks for type that has been compressed
+
+    Args:
+        filename (str): file to be analyzed
+
+    Returns:
+        Tuple containing the MIME type and subtype
+    """
+    output = _get_mime_type_compressed()(filename, output=str, error=str).strip()
+    tty.debug("==> " + output)
+    type, _, subtype = output.partition("/")
+    return type, subtype
+
+
 #: This generates the library filenames that may appear on any OS.
 library_extensions = ["a", "la", "so", "tbd", "dylib"]