[lucet-runtime-internals] use custom stack size on FreeBSD, use cfg-if

On FreeBSD/amd64, SIGSTKSZ == MINSIGSTKSZ(4 * 512) + 32768 == 34816.
This is not a multiple of the 4k page size, which is required, so use the custom size.

Plus, rewrite the choice using cfg_if, as manual cfgs are getting unwieldy.

Author: valpackett

lucet-runtime/lucet-runtime-internals/Cargo.toml

@@ -17,6 +17,7 @@ anyhow = "1.0"
 bitflags = "1.0"
 bincode = "1.1.4"
 byteorder = "1.3"
+cfg-if = "0.1"
 lazy_static = "1.4"
 libc = "0.2.65"
 libloading = "0.5"

lucet-runtime/lucet-runtime-internals/src/alloc/mod.rs

@@ -451,19 +451,23 @@ pub const MINSIGSTKSZ: usize = libc::MINSIGSTKSZ;
 ///
 /// [sigstksz]: https://pubs.opengroup.org/onlinepubs/009695399/basedefs/signal.h.html
 pub const DEFAULT_SIGNAL_STACK_SIZE: usize = {
-    // on Linux, `SIGSTKSZ` is too small for the signal handler when compiled in debug mode
-    #[cfg(all(debug_assertions, not(target_os = "macos")))]
-    const SIZE: usize = 12 * 1024;
-
-    // on Mac, `SIGSTKSZ` is way larger than we need; it would be nice to combine these debug cases once
-    // `std::cmp::max` is a const fn
-    #[cfg(all(debug_assertions, target_os = "macos"))]
-    const SIZE: usize = libc::SIGSTKSZ;
-
-    #[cfg(not(debug_assertions))]
-    const SIZE: usize = libc::SIGSTKSZ;
-
-    SIZE
+    cfg_if::cfg_if! {
+        if #[cfg(target_os = "freebsd")] {
+            // on FreeBSD/amd64, `SIGSTKSZ` is not a multiple of the page size
+            // (34816 == MINSIGSTKSZ(2048) + 32768)
+            12 * 1024
+        } else if #[cfg(target_os = "macos")] {
+            // on Mac, `SIGSTKSZ` is way larger than we need;
+            // it would be nice to combine these debug cases once
+            // `std::cmp::max` is a const fn
+            libc::SIGSTKSZ
+        } else if #[cfg(debug_assertions)] {
+            // on Linux, `SIGSTKSZ` is too small for the signal handler when compiled in debug mode
+            12 * 1024
+        } else {
+            libc::SIGSTKSZ
+        }
+    }
 };
 
 impl Limits {