From 6c6da57ae2aa962aabde6892442227063d87e88c Mon Sep 17 00:00:00 2001
From: Lasse Collin <lasse.collin@tukaani.org>
Date: Mon, 7 Mar 2022 00:36:16 +0200
Subject: [PATCH] xz: Add initial support for threaded decompression.

If threading support is enabled at build time, this will
use lzma_stream_decoder_mt() even for single-threaded mode.
With memlimit_threading=0 the behavior should be identical.

This needs some work like adding --memlimit-threading=LIMIT.

The original patch from Sebastian Andrzej Siewior included
a method to get currently available RAM on Linux. It might
be one way to go but as it is Linux-only, the available-RAM
approach needs work for portability or using a fallback method
on other OSes.

The man page wasn't updated yet.
---
 src/xz/coder.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/xz/coder.c b/src/xz/coder.c
index 85f95439..dc70f1cc 100644
--- a/src/xz/coder.c
+++ b/src/xz/coder.c
@@ -51,7 +51,7 @@ static lzma_check check;
 /// This becomes false if the --check=CHECK option is used.
 static bool check_default = true;
 
-#if defined(HAVE_ENCODERS) && defined(MYTHREAD_ENABLED)
+#ifdef MYTHREAD_ENABLED
 static lzma_mt mt_options = {
 	.flags = 0,
 	.timeout = 300,
@@ -520,9 +520,43 @@ coder_init(file_pair *pair)
 			break;
 
 		case FORMAT_XZ:
+#	ifdef MYTHREAD_ENABLED
+			mt_options.flags = flags;
+
+			mt_options.threads = hardware_threads_get();
+
+			// TODO: Support --memlimit-threading=LIMIT.
+			mt_options.memlimit_stop
+				= hardware_memlimit_get(MODE_DECOMPRESS);
+			mt_options.memlimit_threading
+					= mt_options.memlimit_stop;
+
+			if (mt_options.threads == 1) {
+				// Single-threaded mode was requested. Force
+				// the decoder to use minimal memory, matching
+				// the behavior of lzma_stream_decoder().
+				mt_options.memlimit_threading = 0;
+
+			} else if (mt_options.memlimit_threading
+					== UINT64_MAX) {
+				// TODO: Support --memlimit-threading=LIMIT.
+				//
+				// If lzma_physmem() fails, it returns 0 and
+				// we end up with a single thread.
+				//
+				// NOTE: It is assential that we never end up
+				// with an effectively infinite value in
+				// memlimit_threading!
+				mt_options.memlimit_threading
+						= lzma_physmem() / 4;
+			}
+
+			ret = lzma_stream_decoder_mt(&strm, &mt_options);
+#	else
 			ret = lzma_stream_decoder(&strm,
 					hardware_memlimit_get(
 						MODE_DECOMPRESS), flags);
+#	endif
 			break;
 
 		case FORMAT_LZMA: