feat(pkg): compat.sycl-runtime 2026.09.07 carries the driver, so a SYCL project need not know CUDA is underneath (#355)

2026.09.06 farms the SYCL runtime and its dlopen chain, and a SYCL artifact
using the CUDA back end still needed `compat:cuda-runtime` declared beside it,
because `libur_adapter_cuda.so.0` reaches `libcuda.so.1` through the artifact's
own search path. That is a true statement about the implementation and a poor
one to put in a user's manifest: which back end the SYCL runtime dlopens is the
runtime's business, not the project's.

2026.09.07 declares `xim:libcuda-host-link` and links the same versioned soname
`compat.cuda-runtime` links -- the same sentinel, the same file, the same
soname, so a program that has both farms on its path loads it once. Only the
versioned name: mcpp puts `runtime.library_dirs` on the LINK line as well, and
an unversioned `libcuda.so` would be found by `-lcuda` and bind the build to
one machine's driver.

Nothing is required of the machine. On one with no NVIDIA driver the sentinel
installs a deliberately dangling link and warns, and the SYCL runtime reports
its CUDA back end unavailable -- which is the correct answer and the one it
would give anyway.

Measured: a consumer declaring `compat:sycl-runtime = "2026.09.07"` and NOTHING
else from compat, with `xim:dpcpp`, `xim:gcc` and `xim:cuda-nvcc` in its
workspace, answers `12 24 36 48` on an RTX 4080.

2026.09.06 is kept and frozen; a consumer already pinning it keeps resolving
and installs the farm it always did.

Co-authored-by: speak-agent <x.d2learn.org@gmail.com>

Author: Sunrisepeak

pkgs/c/compat.sycl-runtime.lua

@@ -63,7 +63,29 @@ package = {
             -- developer machine that had zlib installed for other reasons the
             -- omission was invisible; a fresh runner reported
             -- `FAIL libsycl.so.9: libz.so.1: cannot open shared object file`.
-            deps = { "xim:dpcpp@7.1.0", "xim:zlib" },
+            --
+            -- `xim:libcuda-host-link` is the third, and it is the one the
+            -- 2026.09.06 version was missing. `libur_adapter_cuda.so.0` needs
+            -- `libcuda.so.1`, and until the dpcpp payload gained a search path
+            -- of its own that adapter had none -- so it inherited the
+            -- ARTIFACT's DT_RPATH, which `compat.cuda-runtime` had already put
+            -- the driver on. The moment the payload's libraries acquired
+            -- `RUNPATH = $ORIGIN`, that inheritance switched off for them and
+            -- the adapter could no longer see a driver two farms away. The
+            -- sentinel is the one package permitted to know where the driver
+            -- is; this package declares it and links the same file
+            -- compat.cuda-runtime links, under the same soname, so a program
+            -- that has both farms on its path loads it once.
+            deps = { "xim:dpcpp@7.1.0", "xim:zlib", "xim:libcuda-host-link" },
+            -- 2026.09.06 is kept and frozen: a consumer already pinning it
+            -- keeps resolving, and it installs the farm it always did. It is
+            -- missing `libcuda.so.1`, which on a machine whose dpcpp payload
+            -- predates the search-path repair is invisible, because the
+            -- adapter then inherits the artifact's path instead.
+            ["2026.09.07"] = {
+                url    = "https://raw.githubusercontent.com/intel/llvm/v7.1.0/sycl/LICENSE.TXT",
+                sha256 = "410f3a23b4bbacbd246310d8c014a20af18cfc8c0d740ddf0f673ea20894da9c",
+            },
             ["2026.09.06"] = {
                 -- Nothing downloaded matters: the content is the set of
                 -- symlinks install() creates. A stable, tiny anchor keeps the
@@ -72,7 +94,7 @@ package = {
                 url    = "https://raw.githubusercontent.com/intel/llvm/v7.1.0/sycl/LICENSE.TXT",
                 sha256 = "410f3a23b4bbacbd246310d8c014a20af18cfc8c0d740ddf0f673ea20894da9c",
             },
-            ["latest"] = { ref = "2026.09.06" },
+            ["latest"] = { ref = "2026.09.07" },
         },
     },
 
@@ -260,6 +282,29 @@ function install()
         return true
     end
 
+    -- The driver, through the sentinel and never through a probe of our own.
+    -- Only the versioned soname: mcpp puts runtime.library_dirs on the LINK
+    -- line as well, and an unversioned `libcuda.so` here would be picked up by
+    -- `-lcuda` and bind the build to one machine's driver.
+    local drv = 0
+    for _, root in ipairs({ path.directory(path.directory(src)) }) do
+        local f = io.popen(string.format(
+            [[ls -1d "%s"/xim-x-libcuda-host-link/*/lib/libcuda.so.1 2>/dev/null | sort -V | tail -1]], root))
+        local hit = f and (f:read("l") or "") or ""
+        if f then f:close() end
+        if hit ~= "" then
+            os.exec(string.format([[ln -sfn "%s" "%s"]], hit, path.join(outdir, "libcuda.so.1")))
+            drv = 1
+        end
+    end
+    if drv == 0 then
+        -- Not fatal, and not this package's business to decide: a machine with
+        -- no NVIDIA driver is a legitimate configuration, and the SYCL runtime
+        -- reports an unavailable back end itself.
+        log.warn("compat.sycl-runtime: no libcuda-host-link found; the CUDA "
+                 .. "back end of the SYCL runtime will report itself unavailable")
+    end
+
     local stubs = farm_libc_stubs(outdir, src)
     log.info("compat.sycl-runtime: %d versioned sonames from %s, %d C-library stubs",
              n, src, stubs)

tests/examples/sycl-runtime/mcpp.toml

@@ -9,4 +9,4 @@ name    = "sycl-runtime-tests"
 version = "0.1.0"
 
 [target.'cfg(linux)'.dependencies.compat]
-sycl-runtime = "2026.09.06"
+sycl-runtime = "2026.09.07"