Make WSL detection dynamic rather than statically compiled

This resolves the issue where running pre-compiled Linux packages from
binary package manager repositories lead fish to think that we are not
running under WSL.

- Closes #5619.
- Ping neovim/neovim#7330

src/common.cpp

@@ -31,6 +31,12 @@
 #include <sys/ioctl.h>
 #endif
 
+#ifdef __linux__
+// Includes for WSL detection
+#include <cstring>
+#include <sys/utsname.h>
+#endif
+
 #ifdef __FreeBSD__
 #include <sys/sysctl.h>
 #elif __APPLE__
@@ -148,6 +154,31 @@ long convert_hex_digit(wchar_t d) {
     return -1;
 }
 
+bool is_windows_subsystem_for_linux() {
+#if defined(WSL)
+    return true;
+#elif not defined(__linux__)
+    return false;
+#endif
+
+    // We are purposely not using std::call_once as it may invoke locking, which is an unnecessary
+    // overhead since there's no actual race condition here - even if multiple threads call this
+    // routine simultaneously the first time around, we just end up needlessly querying uname(2) one
+    // more time.
+
+    static bool wsl_state = []() {
+        utsname info;
+        uname(&info);
+
+        // Sample utsname.release under WSL: 4.4.0-17763-Microsoft
+        return strstr(info.release, "Microsoft") != nullptr;
+    }();
+
+    // Subsequent calls to this function may take place after fork() and before exec() in
+    // postfork.cpp. Make sure we never dynamically allocate any memory in the fast path!
+    return wsl_state;
+}
+
 #ifdef HAVE_BACKTRACE_SYMBOLS
 // This function produces a stack backtrace with demangled function & method names. It is based on
 // https://gist.github.com/fmela/591333 but adapted to the style of the fish project.

src/common.h

@@ -866,18 +866,10 @@ void assert_is_not_forked_child(const char *who);
 #define ASSERT_IS_NOT_FORKED_CHILD_TRAMPOLINE(x) assert_is_not_forked_child(x)
 #define ASSERT_IS_NOT_FORKED_CHILD() ASSERT_IS_NOT_FORKED_CHILD_TRAMPOLINE(__FUNCTION__)
 
-/// Detect if we are Windows Subsystem for Linux by inspecting /proc/sys/kernel/osrelease
+/// Determines if we are running under Microsoft's Windows Subsystem for Linux to work around
-/// and checking if "Microsoft" is in the first line.
+/// some known limitations and/or bugs.
 /// See https://github.com/Microsoft/WSL/issues/423 and Microsoft/WSL#2997
-constexpr bool is_windows_subsystem_for_linux() {
+bool is_windows_subsystem_for_linux();
-    // This function is called after fork() and before exec() in postfork.cpp. Make sure we
-    // don't allocate any memory here!
-#ifdef WSL
-    return true;
-#else
-    return false;
-#endif
-}
 
 /// Detect if we are running under Cygwin or Cgywin64
 constexpr bool is_cygwin() {