picolibc: fixup various cross-compilation/broken issues

Went thru many of the pkgsCross options to see what actually can/can’t
be built. Split up hosted vs. non-hosted flags. Found broken platforms &
other flags that needed to be toggled.

Assisted-by: Big Pickle

pkgs/by-name/pi/picolibc/package.nix

@@ -15,6 +15,7 @@
 
 let
   canExecute = stdenvNoLibc.buildPlatform.canExecute stdenvNoLibc.hostPlatform;
+  isHosted = !stdenvNoLibc.hostPlatform.isNone;
 in
 stdenvNoLibc.mkDerivation (finalAttrs: {
   pname = "picolibc";
@@ -43,8 +44,14 @@ stdenvNoLibc.mkDerivation (finalAttrs: {
   ];
 
   mesonFlags = [
-    # Always non-default from upstream
-    (lib.mesonBool "use-stdlib" true)
+    # use-stdlib builds against a system C library, requiring a Linux-ABI libc
+    # whose headers we can compile against (libc/native needs statx() & 64-bit
+    # off_t/time_t)… so native glibc/musl only.
+    (lib.mesonBool "use-stdlib" (
+      isHosted
+      && stdenvNoLibc.hostPlatform == stdenvNoLibc.buildPlatform
+      && (stdenvNoLibc.hostPlatform.libc == "glibc" || stdenvNoLibc.hostPlatform.libc == "musl")
+    ))
     (lib.mesonOption "specsdir" "${placeholder "dev"}/lib")
 
     # Multi-lib (cross-compilation)
@@ -53,17 +60,23 @@ stdenvNoLibc.mkDerivation (finalAttrs: {
     (lib.mesonOption "multilib-exclude" (lib.concatStringsSep "," multilib-exclude))
 
     # Platform-dependent: picolibc defaults are for bare-metal; adjust for
-    # hosted (non-none) platforms.
-    (lib.mesonBool "picolib" stdenvNoLibc.hostPlatform.isNone)
-    (lib.mesonBool "semihost" stdenvNoLibc.hostPlatform.isNone)
-    (lib.mesonBool "picocrt" stdenvNoLibc.hostPlatform.isNone)
-    (lib.mesonBool "posix-console" (!stdenvNoLibc.hostPlatform.isNone))
-    (lib.mesonOption "tls-model" (
-      if stdenvNoLibc.hostPlatform.isNone then "local-exec" else "global-dynamic"
-    ))
-    (lib.mesonOption "errno-function" (
-      if stdenvNoLibc.hostPlatform.isNone then "false" else "auto"
-    ))
+    # hosted platforms.
+    (lib.mesonBool "semihost" (!isHosted))
+    (lib.mesonBool "picocrt" (!isHosted))
+    (lib.mesonBool "posix-console" isHosted)
+    (lib.mesonBool "initfini-array" (!isHosted))
+    (lib.mesonOption "tls-model" (if isHosted then "global-dynamic" else "local-exec"))
+    (lib.mesonOption "errno-function" (if isHosted then "auto" else "false"))
+
+    # For hosted builds, disable os-fallback since sbrk.c in the fallback
+    # library references __heap_start/__heap_end which are only defined for
+    # bare-metal targets with an internal heap.
+    (lib.mesonOption "os-fallback" (if isHosted then "false" else "auto"))
+  ]
+  ++ lib.optionals (stdenvNoLibc.hostPlatform.libc == "fblibc") [
+    # Linkless cross toolchain: meson’s TLS auto-detect enables TLS then
+    # rejects -ftls-model; fall back to non-thread-local errno.
+    (lib.mesonBool "thread-local-storage" false)
   ]
   ++ lib.optionals finalAttrs.doCheck [
     (lib.mesonBool "tests" true)
@@ -73,12 +86,17 @@ stdenvNoLibc.mkDerivation (finalAttrs: {
     (lib.mesonOption "tests-cdefs" "false")
   ];
 
-  doCheck = stdenvNoLibc.buildPlatform.canExecute stdenvNoLibc.hostPlatform;
+  # The cross no-libc toolchains cannot link (no crt files), so meson’s
+  # link-based probes (meaning the --defsym alias check that enables the printf
+  # tests) all fail. Keep tests on the native build only.
+  doCheck = canExecute && stdenvNoLibc.hostPlatform == stdenvNoLibc.buildPlatform;
 
   passthru = {
     updateScript = nix-update-script { };
     tests = {
       arm = pkgsCross.arm-embedded.picolibc;
+      musl64 = pkgsCross.musl64.picolibc;
+      riscv64 = pkgsCross.riscv64-embedded.picolibc;
     };
   };
 
@@ -97,6 +115,7 @@ stdenvNoLibc.mkDerivation (finalAttrs: {
       bsd3
     ];
     maintainers = with lib.maintainers; [ toastal ];
+    badPlatforms = [ lib.systems.inspect.patterns.isMinGW ];
     # https://github.com/picolibc/picolibc/tree/${finalAttrs.version}?tab=readme-ov-file#supported-architectures
     platforms = lib.platforms.all;
   };