Use close_range() to close large numbers of files when available.

close_range() is a newish, Linux and FreeBSD system call to close a
large range of file descriptors as is commonly done by the child
process after a fork.  Linux now supports extremely large limits and
the traditional, brute force way of closing file descriptors can incur
a several minute delay in those situations.  This change checks for
and uses close_range() when it is available.

mythtv/configure

@@ -1934,6 +1934,7 @@ MYTHTV_HAVE_LIST='
     libudev
     stdint_h
     sync_file_range
+    close_range
     sys_endian_h
 '
 
@@ -5644,6 +5645,17 @@ int main(int argc, char **argv){
 }
 EOF
 
+# test for close_range (linux system call since 5.9 and also FreeBSD)
+check_ld "cc" <<EOF && enable close_range
+#define _GNU_SOURCE
+#include <unistd.h>
+
+int main(int argc, char **argv){
+    close_range(0,0,0);
+    return 0;
+}
+EOF
+
 # test for sizeof(int)
 for sizeof in 1 2 4 8 16; do
     check_cc <<EOF && _sizeof_int=$sizeof && break

mythtv/libs/libmythbase/mythsystemunix.cpp

@@ -1110,8 +1110,12 @@ void MythSystemLegacyUnix::Fork(std::chrono::seconds timeout)
         }
 
         /* Close all open file descriptors except stdin/stdout/stderr */
+#if HAVE_CLOSE_RANGE
+        close_range(3, sysconf(_SC_OPEN_MAX) - 1, 0);
+#else
         for( int fd = sysconf(_SC_OPEN_MAX) - 1; fd > 2; fd-- )
             close(fd);
+#endif
 
         /* set directory */
         if( directory && chdir(directory) < 0 )

mythtv/libs/libmythtv/recorders/ExternalStreamHandler.cpp

@@ -21,6 +21,7 @@
 #include <QFile>
 
 // MythTV headers
+#include "config.h"
 #include "libmythbase/exitcodes.h"
 
 #include "ExternalChannel.h"
@@ -401,8 +402,12 @@ void ExternIO::Fork(void)
     }
 
     /* Close all open file descriptors except stdin/stdout/stderr */
+#if HAVE_CLOSE_RANGE
+    close_range(3, sysconf(_SC_OPEN_MAX) - 1, 0);
+#else
     for (int i = sysconf(_SC_OPEN_MAX) - 1; i > 2; --i)
         close(i);
+#endif
 
     /* Set the process group id to be the same as the pid of this
      * child process.  This ensures that any subprocesses launched by this

mythtv/programs/mythbackend/mythbackend.cpp

@@ -21,6 +21,8 @@
 #include <QProcessEnvironment>
 #endif
 
+#include <unistd.h>
+
 // MythTV
 #include "libmythbase/cleanupguard.h"
 #include "libmythbase/compat.h"
@@ -86,8 +88,12 @@ int main(int argc, char **argv)
     }
 
 #ifndef _WIN32
+#if HAVE_CLOSE_RANGE
+    close_range(UNUSED_FILENO, sysconf(_SC_OPEN_MAX) - 1, 0);
+#else
     for (long i = UNUSED_FILENO; i < sysconf(_SC_OPEN_MAX) - 1; ++i)
         close(i);
+#endif
     QCoreApplication a(argc, argv);
 #else
     // MINGW application needs a window to receive messages

mythtv/programs/mythpreviewgen/mythpreviewgen.cpp

@@ -149,8 +149,12 @@ int main(int argc, char **argv)
     }
 
 #ifndef _WIN32
+#if HAVE_CLOSE_RANGE
+    close_range(UNUSED_FILENO, sysconf(_SC_OPEN_MAX) - 1, 0);
+#else
     for (long i = UNUSED_FILENO; i < sysconf(_SC_OPEN_MAX) - 1; ++i)
         close(i);
+#endif
     QCoreApplication a(argc, argv);
 #else
     // MINGW application needs a window to receive messages