Speed up closing all file descriptors in the child process (Fixes #835)

When _SC_OPEN_MAX (max nr of open files limit per process) is a very
big number, then closing all possible file handles can take a while.
This change first tries to use the syscall close_range() if available,
falling back to use /proc/self/fd to close only open file handles,
and lastly falling back to the old way of closing all possible handles
one after another.
In general, the first or second approach should be available
which speeds up the pty spawning.

Refs JetBrains/pty4j#147
Copied from 04685d870f
(which is EPL 1.0)

Co-authored-by: Sergey Simonchik <sergey.simonchik@jetbrains.com>

core/org.eclipse.cdt.core.native/native_src/unix/exec_pty.c

@@ -24,10 +24,72 @@
 #include <libgen.h>
 #include <stdlib.h>
 #include <termios.h>
+#include <sys/syscall.h>
+#include <dirent.h>
+#include <ctype.h>
 
 /* from pfind.c */
 extern char *pfind(const char *name, char *const envp[]);
 
+static int sys_close_range_wrapper(unsigned int from_fd_inclusive) {
+    // Use fast `close_range` (https://man7.org/linux/man-pages/man2/close_range.2.html) if available.
+    // Cannot call `close_range` from libc, as it may be unavailable in older libc.
+# if defined(__linux__) && defined(SYS_close_range) && defined(CLOSE_RANGE_UNSHARE)
+    return syscall(SYS_close_range, from_fd_inclusive, ~0U, CLOSE_RANGE_UNSHARE);
+# else
+    errno = ENOSYS;
+    return -1;
+# endif
+}
+
+static int close_all_fds_using_parsing(unsigned int from_fd_inclusive) {
+    // If `opendir` is implemented using a file descriptor, we may close it accidentally.
+    // Let's close a few lowest file descriptors, in hope that `opendir` will use it.
+    int lowest_fds_to_close = 2;
+    for (int i = 0; i < lowest_fds_to_close; i++) {
+        close(from_fd_inclusive + i);
+    }
+
+#if defined(__APPLE__)
+#define FD_DIR "/dev/fd"
+#else
+#define FD_DIR "/proc/self/fd"
+#endif
+
+    DIR *dirp = opendir(FD_DIR);
+    if (dirp == NULL) return -1;
+
+    struct dirent *direntp;
+
+    while ((direntp = readdir(dirp)) != NULL) {
+        if (isdigit(direntp->d_name[0])) {
+            int fd = strtol(direntp->d_name, NULL, 10);
+            if (fd >= from_fd_inclusive + lowest_fds_to_close && fd != dirfd(dirp)) {
+                close(fd);
+            }
+        }
+    }
+
+    closedir(dirp);
+
+    return 0;
+}
+
+static void close_all_fds_fallback(unsigned int from_fd_inclusive) {
+    int fdlimit = sysconf(_SC_OPEN_MAX);
+    if (fdlimit == -1) fdlimit = 65535; // arbitrary default, just in case
+    for (int fd = from_fd_inclusive; fd < fdlimit; fd++) {
+        close(fd);
+    }
+}
+
+static void close_all_fds() {
+    unsigned int from_fd = STDERR_FILENO + 1;
+    if (sys_close_range_wrapper(from_fd) == 0) return;
+    if (close_all_fds_using_parsing(from_fd) == 0) return;
+    close_all_fds_fallback(from_fd);
+}
+
 pid_t exec_pty(const char *path, char *const argv[], char *const envp[], const char *dirpath, int channels[3],
                const char *pts_name, int fdm, int console) {
     int pipe2[2];
@@ -107,14 +169,7 @@ pid_t exec_pty(const char *path, char *const argv[], char *const envp[], const c
         }
 
         /* Close all the fd's in the child */
-        {
-            int fdlimit = sysconf(_SC_OPEN_MAX);
-            int fd = 3;
-
-            while (fd < fdlimit) {
-                close(fd++);
-            }
-        }
+        close_all_fds();
 
         if (envp && envp[0]) {
             execve(full_path, argv, envp);