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

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

@@ -21,10 +21,76 @@
 #include <string.h>
 #include <libgen.h>
 #include <stdlib.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 exec0(const char *path, char *const argv[], char *const envp[], const char *dirpath, int channels[3]) {
     int pipe0[2], pipe1[2], pipe2[2];
     pid_t childpid;
@@ -83,14 +149,7 @@ pid_t exec0(const char *path, char *const argv[], char *const envp[], const char
         }
 
         /* 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();
 
         setpgid(getpid(), getpid());