From 000892e315abacfec7a8588b4ecdfa36e923a749 Mon Sep 17 00:00:00 2001
From: Mahmoud Al-Qudsi <mqudsi@neosmart.net>
Date: Sun, 4 Mar 2018 21:13:31 -0600
Subject: [PATCH] Use constexpr for is_windows_subsystem_for_linux()

To guarantee that at runtime there will be no branching, using a CMAKE
test/define combined with a constexpr wrapper for code-friendliness.
---
 cmake/ConfigureChecks.cmake |  5 +++++
 config_cmake.h.in           |  3 +++
 src/common.cpp              | 19 -------------------
 src/common.h                | 12 ++++++++++--
 4 files changed, 18 insertions(+), 21 deletions(-)

diff --git a/cmake/ConfigureChecks.cmake b/cmake/ConfigureChecks.cmake
index 8919749fd..052a39172 100644
--- a/cmake/ConfigureChecks.cmake
+++ b/cmake/ConfigureChecks.cmake
@@ -14,6 +14,11 @@ IF(APPLE)
   SET(CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS} "-Werror=unguarded-availability")
 ENDIF()
 
+# Detect WSL. Does not match against native Windows/WIN32.
+if (CMAKE_HOST_SYSTEM_VERSION MATCHES ".*-Microsoft")
+  SET(WSL 1)
+endif()
+
 # Set up the config.h file.
 SET(PACKAGE_NAME "fish")
 SET(PACKAGE_TARNAME "fish")
diff --git a/config_cmake.h.in b/config_cmake.h.in
index f5e186c43..bd4fe4d28 100644
--- a/config_cmake.h.in
+++ b/config_cmake.h.in
@@ -1,6 +1,9 @@
 /* Define to 1 if you have the `backtrace_symbols' function. */
 #cmakedefine HAVE_BACKTRACE_SYMBOLS 1
 
+/* Define to 1 if compiled on WSL */
+#cmakedefine WSL 1
+
 /* Define to 1 if you have the `clock_gettime' function. */
 #cmakedefine HAVE_CLOCK_GETTIME 1
 
diff --git a/src/common.cpp b/src/common.cpp
index 5de98289e..0c12b99ac 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -1991,25 +1991,6 @@ void fish_mutex_t::assert_is_locked(const char *who, const char *caller) const {
     }
 }
 
-/// Detect if we are Windows Subsystem for Linux by inspecting /proc/sys/kernel/osrelease
-/// and checking if "Microsoft" is in the first line.
-/// See https://github.com/Microsoft/WSL/issues/423
-bool is_windows_subsystem_for_linux() {
-    ASSERT_IS_NOT_FORKED_CHILD();
-    static bool s_is_wsl = false;
-    static std::once_flag oflag;
-    std::call_once(oflag, []() {
-        // 'e' sets CLOEXEC if possible.
-        FILE *fp = fopen("/proc/sys/kernel/osrelease", "re");
-        if (fp) {
-            char buff[256];
-            if (fgets(buff, sizeof buff, fp)) s_is_wsl = (strstr(buff, "Microsoft") != NULL);
-            fclose(fp);
-        }
-    });
-    return s_is_wsl;
-}
-
 template <typename CharType_t>
 static CharType_t **make_null_terminated_array_helper(
     const std::vector<std::basic_string<CharType_t> > &argv) {
diff --git a/src/common.h b/src/common.h
index d3116995d..eec785c8d 100644
--- a/src/common.h
+++ b/src/common.h
@@ -757,8 +757,16 @@ 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__)
 
-/// Return whether we are running in Windows Subsystem for Linux.
-bool is_windows_subsystem_for_linux();
+/// Detect if we are Windows Subsystem for Linux by inspecting /proc/sys/kernel/osrelease
+/// and checking if "Microsoft" is in the first line.
+/// See https://github.com/Microsoft/WSL/issues/423 and Microsoft/WSL#2997 
+constexpr bool is_windows_subsystem_for_linux() {
+#ifdef WSL
+    return true;
+#else
+    return false;
+#endif
+}
 
 extern "C" {
 __attribute__((noinline)) void debug_thread_error(void);