libtorrent: Fix libatomic detection with optimized builds

The configure check for -latomic links a program that loads from a
local std::atomic<uint64_t>, which clang folds away at -O2. On riscv32
it therefore reports that libatomic is not needed, and libtorrent.so is
left with undefined __atomic_fetch_add_8, __atomic_fetch_sub_8,
__atomic_load_8 and __atomic_store_8 references and no dependency on
libatomic.

Add a patch that makes the test atomic global and modifies it with
fetch_add() so the check cannot be optimized away.

AI-Generated: Uses Claude Code
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-connectivity/libtorrent/libtorrent/0001-checks.m4-Keep-the-atomic-test-program-from-being-opt.patch

@@ -0,0 +1,57 @@
+From: Khem Raj <khem.raj@oss.qualcomm.com>
+Date: Wed, 21 Oct 2026 13:00:00 -0700
+Subject: [PATCH] checks.m4: Keep the atomic test program from being optimized
+ away
+
+TORRENT_CHECK_ATOMIC decides whether -latomic is needed by linking a
+program that loads from a local std::atomic<uint64_t>. The atomic never
+escapes, so with -O2 in CXXFLAGS clang folds the load to a constant and
+no __atomic_* call is emitted. The check then reports that -latomic is
+not required even on targets that need it for 64-bit atomics, such as
+riscv32, and linking fails later:
+
+  ld.lld: error: undefined symbol: __atomic_load_8
+
+Make the atomic global and modify it with fetch_add() so the atomic
+operations are emitted at any optimization level.
+
+Upstream-Status: Submitted [https://github.com/rakshasa/libtorrent/pull/1020]
+Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>
+---
+ scripts/checks.m4 | 13 +++++++++----
+ 1 file changed, 9 insertions(+), 4 deletions(-)
+
+--- a/scripts/checks.m4
++++ b/scripts/checks.m4
+@@ -99,13 +99,17 @@
+ ])
+ 
+ 
++# The atomic is global and modified with a read-modify-write operation,
++# otherwise the compiler can optimize the operations away and the program
++# links without libatomic even on targets that need it.
+ AC_DEFUN([TORRENT_CHECK_ATOMIC], [
+   AC_MSG_CHECKING([whether 64-bit atomic operations require -latomic])
+   AC_LANG_PUSH(C++)
+ 
+   AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <cstdint>
+-                                    #include <atomic>]],
+-                                    [[std::atomic<uint64_t> x(0);
++                                    #include <atomic>
++                                    std::atomic<uint64_t> x(0);]],
++                                    [[x.fetch_add(1);
+                                     return x.load();]])],
+     [
+       AC_MSG_RESULT([no])
+@@ -116,8 +120,9 @@
+       LIBS="$LIBS -latomic"
+ 
+       AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <cstdint>
+-                                        #include <atomic>]],
+-                                        [[std::atomic<uint64_t> x(0);
++                                        #include <atomic>
++                                        std::atomic<uint64_t> x(0);]],
++                                        [[x.fetch_add(1);
+                                         return x.load();]])],
+         [
+           AC_MSG_RESULT([yes])

meta-oe/recipes-connectivity/libtorrent/libtorrent_0.16.25.bb

@@ -6,7 +6,9 @@ LIC_FILES_CHKSUM = "file://COPYING;md5=393a5ca445f6965873eca0259a17f833"
 
 DEPENDS = "zlib curl libsigc++-2.0 openssl cppunit"
 
-SRC_URI = "git://github.com/rakshasa/libtorrent;branch=master;protocol=https;tag=v${PV}"
+SRC_URI = "git://github.com/rakshasa/libtorrent;branch=master;protocol=https;tag=v${PV} \
+           file://0001-checks.m4-Keep-the-atomic-test-program-from-being-opt.patch \
+           "
 SRCREV = "6ad3fecf3e7e7aa86855445587b532a91256f344"
 
 UPSTREAM_CHECK_GITTAGREGEX = "v(?P<pver>\d+(\.\d+)+)"