Update ConfigureChecks, only do the mtime hack for Linux.. on Linux

- Only check for HAVE_CLOCK_GETTIME and HAVE_FUTIMENS on Linux, since
they are only used to implement a Linux-specific workaround related
to mtime precision.
- Make sure that hack is limited to Linux builds
- HAVE_SYS_SYSCTL_H was unused, but we should have been using it
- HAVE_TERMIOS_H was unused, remove it

The only functional change is that unix machines with clock_gettime
and futimens will not bother with a Linux-specific hack, and won't
waste time checking for either during cmake configuration either.

cmake/ConfigureChecks.cmake

@@ -74,15 +74,21 @@ include(CheckCXXSourceCompiles)
 include(CheckTypeSize)
 include(CMakePushCheckState)
 check_cxx_symbol_exists(backtrace_symbols execinfo.h HAVE_BACKTRACE_SYMBOLS)
-check_cxx_symbol_exists(clock_gettime time.h HAVE_CLOCK_GETTIME)
+
+# workaround for lousy mtime precision on a Linux kernel
+if (CMAKE_SYSTEM_NAME MATCHES "Linux|Android")
+    check_cxx_symbol_exists(clock_gettime time.h HAVE_CLOCK_GETTIME)
+    check_cxx_symbol_exists(futimens sys/stat.h HAVE_FUTIMENS)
+    if ((HAVE_CLOCK_GETTIME) AND (HAVE_FUTIMENS))
+        set(UVAR_FILE_SET_MTIME_HACK 1)
+    endif()
+endif()
+
 check_cxx_symbol_exists(ctermid_r stdio.h HAVE_CTERMID_R)
 check_struct_has_member("struct dirent" d_type dirent.h HAVE_STRUCT_DIRENT_D_TYPE LANGUAGE CXX)
 check_cxx_symbol_exists(dirfd "sys/types.h;dirent.h" HAVE_DIRFD)
 check_include_file_cxx(execinfo.h HAVE_EXECINFO_H)
 check_cxx_symbol_exists(flock sys/file.h HAVE_FLOCK)
-# futimens is new in OS X 10.13 but is a weak symbol.
-# Don't assume it exists just because we can link - it may be null.
-check_cxx_symbol_exists(futimens sys/stat.h HAVE_FUTIMENS)
 check_cxx_symbol_exists(getifaddrs ifaddrs.h HAVE_GETIFADDRS)
 check_cxx_symbol_exists(getpwent pwd.h HAVE_GETPWENT)
 check_cxx_symbol_exists(getrusage sys/resource.h HAVE_GETRUSAGE)
@@ -110,7 +116,6 @@ check_struct_has_member("struct stat" st_mtim.tv_nsec "sys/stat.h" HAVE_STRUCT_S
 check_include_file_cxx(sys/ioctl.h HAVE_SYS_IOCTL_H)
 check_include_file_cxx(sys/select.h HAVE_SYS_SELECT_H)
 check_include_files("sys/types.h;sys/sysctl.h" HAVE_SYS_SYSCTL_H)
-check_include_file_cxx(termios.h HAVE_TERMIOS_H) # Needed for TIOCGWINSZ
 
 check_cxx_symbol_exists(eventfd sys/eventfd.h HAVE_EVENTFD)
 check_cxx_symbol_exists(pipe2 unistd.h HAVE_PIPE2)

config_cmake.h.in

@@ -4,9 +4,6 @@
 /* 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
-
 /* Define to 1 if you have the `ctermid_r' function. */
 #cmakedefine HAVE_CTERMID_R 1
 
@@ -22,9 +19,6 @@
 /* Define to 1 if you have the `flock' function. */
 #cmakedefine HAVE_FLOCK 1
 
-/* Define to 1 if you have the `futimens' function. */
-#cmakedefine HAVE_FUTIMENS 1
-
 /* Define to 1 if you have the `getifaddrs' function. */
 #cmakedefine HAVE_GETIFADDRS 1
 
@@ -97,9 +91,6 @@
 /* Define to 1 if you have the <sys/sysctl.h> header file. */
 #cmakedefine HAVE_SYS_SYSCTL_H 1
 
-/* Define to 1 if you have the <termios.h> header file. */
-#cmakedefine HAVE_TERMIOS_H 1
-
 /* Define to 1 if you have the <term.h> header file. */
 #cmakedefine HAVE_TERM_H 1
 
@@ -130,6 +121,9 @@
 /* Define to 1 if std::make_unique is available. */
 #cmakedefine HAVE_STD__MAKE_UNIQUE 1
 
+/* Define to use clock_gettime and futimens to hack around Linux mtime issue */
+#cmakedefine UVAR_FILE_SET_MTIME_HACK 1
+
 /* Define to 1 to disable ncurses macros that conflict with the STL */
 #define NCURSES_NOMACROS 1
 

src/common.cpp

@@ -56,9 +56,10 @@
 #include "wutil.h"  // IWYU pragma: keep
 
 // Keep after "common.h"
-#ifdef __BSD__
+#ifdef HAVE_SYS_SYSCTL_H
 #include <sys/sysctl.h>
-#elif defined(__APPLE__)
+#endif
+#if defined(__APPLE__)
 #include <mach-o/dyld.h>
 #endif
 

src/env_universal_common.cpp

@@ -709,9 +709,13 @@ bool env_universal_t::save(const wcstring &directory, const wcstring &vars_path)
         // necessary because Linux aggressively reuses inodes, causing the ABA problem; on other
         // platforms we tend to notice the file has changed due to a different inode (or file size!)
         //
+        // The current time within the Linux kernel is cached, and generally only updated on a timer
+        // interrupt. So if the timer interrupt is running at 10 milliseconds, the cached time will
+        // only be updated once every 10 milliseconds.
+        //
         // It's probably worth finding a simpler solution to this. The tests ran into this, but it's
         // unlikely to affect users.
-#if HAVE_CLOCK_GETTIME && HAVE_FUTIMENS
+#if defined(UVAR_FILE_SET_MTIME_HACK)
         struct timespec times[2] = {};
         times[0].tv_nsec = UTIME_OMIT;  // don't change ctime
         if (0 == clock_gettime(CLOCK_REALTIME, &times[1])) {