Account for already used file descriptors when computing max open files

The process already has some existing file descriptors open, e.g. stdin,
stdout and stderr, so we can't assume we can open as many files as the
RLIMIT_NOFILE tells us.

A different approach to this is to not use a static limit of maximum
open files, but instead react to EMFILE when trying to open new files,
and then close existing files, but as a stop gap we compute a more
accurate static value for the maximum open files.

Fixes #35

Author: torarnv

sparsebundlefs.cpp

@@ -26,6 +26,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <poll.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -389,15 +390,39 @@ static int sparsebundle_read_buf(const char *path, struct fuse_bufvec **bufp,
     syslog(LOG_DEBUG, "asked to read %zu bytes at offset %ju using zero-copy read",
         length, uintmax_t(offset));
 
-    static struct rlimit fd_limit;
-    static bool fd_limit_computed = false;
-    if (!fd_limit_computed) {
+    static rlim_t max_open_files = []() {
+        struct rlimit fd_limit;
         getrlimit(RLIMIT_NOFILE, &fd_limit);
-        fd_limit_computed = true;
-    }
+        auto fd_max = fd_limit.rlim_cur - 1;
+        syslog(LOG_DEBUG, "maximum file descriptor number %ju", uintmax_t(fd_max));
+
+        // Check how many of the file descriptors are available
+        std::vector<pollfd> file_descriptors(fd_max);
+        for (unsigned i = 0; i < file_descriptors.size(); ++i) {
+            file_descriptors[i].fd = i;
+            // We don't really care about these states, but we have
+            // to specify something for poll to give us the POLLNVAL
+            // we do care about.
+            file_descriptors[i].events = POLLRDNORM | POLLWRNORM;
+        }
+        if (poll(file_descriptors.data(), file_descriptors.size(), 0) == -1) {
+            syslog(LOG_ERR, "failed to resolve available file descriptors: %s", strerror(errno));
+            return fd_max;
+        }
+
+        rlim_t available_fds = 0;
+        for (auto file_descriptor : file_descriptors) {
+            if (file_descriptor.revents & POLLNVAL)
+                ++available_fds;
+        }
+        syslog(LOG_DEBUG, "%ju available file descriptors (%ju in use)",
+            uintmax_t(available_fds), uintmax_t(fd_max - available_fds));
+
+        return available_fds;
+    }();
 
     sparsebundle_t *sparsebundle = sparsebundle_current();
-    if (sparsebundle->open_files.size() + 1 >= fd_limit.rlim_cur) {
+    if (sparsebundle->open_files.size() + 1 >= max_open_files) {
         syslog(LOG_DEBUG, "hit max number of file descriptors");
         sparsebundle_read_buf_close_files();
     }