git.sesse.net Git - plocate/blob - access_rx_cache.cpp

   1 #include "access_rx_cache.h"
   2
   3 #include "io_uring_engine.h"
   4
   5 #include <stdlib.h>
   6 #include <string.h>
   7 #include <unistd.h>
   8 #include <utility>
   9
  10 using namespace std;
  11
  12 void AccessRXCache::check_access(const char *filename, bool allow_async, function<void(bool)> cb)
  13 {
  14         if (!check_visibility) {
  15                 cb(true);
  16                 return;
  17         }
  18
  19         lock_guard<mutex> lock(mu);
  20         if (engine == nullptr || !engine->get_supports_stat()) {
  21                 allow_async = false;
  22         }
  23
  24         for (const char *end = strchr(filename + 1, '/'); end != nullptr; end = strchr(end + 1, '/')) {
  25                 string parent_path(filename, end - filename);  // string_view from C++20.
  26                 auto cache_it = cache.find(parent_path);
  27                 if (cache_it != cache.end()) {
  28                         // Found in the cache.
  29                         if (!cache_it->second) {
  30                                 cb(false);
  31                                 return;
  32                         }
  33                         continue;
  34                 }
  35
  36                 if (!allow_async) {
  37                         bool ok = access(parent_path.c_str(), R_OK | X_OK) == 0;
  38                         cache.emplace(parent_path, ok);
  39                         if (!ok) {
  40                                 cb(false);
  41                                 return;
  42                         }
  43                         continue;
  44                 }
  45
  46                 // We want to call access(), but it could block on I/O. io_uring doesn't support
  47                 // access(), but we can do a dummy asynchonous statx() to populate the kernel's cache,
  48                 // which nearly always makes the next access() instantaneous.
  49
  50                 // See if there's already a pending stat that matches this,
  51                 // or is a subdirectory.
  52                 auto it = pending_stats.lower_bound(parent_path);
  53                 if (it != pending_stats.end() && it->first.size() >= parent_path.size() &&
  54                     it->first.compare(0, parent_path.size(), parent_path) == 0) {
  55                         it->second.emplace_back(PendingStat{ filename, move(cb) });
  56                 } else {
  57                         it = pending_stats.emplace(filename, vector<PendingStat>{}).first;
  58                         engine->submit_stat(filename, [this, it, filename{ strdup(filename) }, cb{ move(cb) }] {
  59                                 // The stat returned, so now do the actual access() calls.
  60                                 // All of them should be in cache, so don't fire off new statx()
  61                                 // calls during that check.
  62                                 check_access(filename, /*allow_async=*/false, move(cb));
  63                                 free(filename);
  64
  65                                 // Call all others that waited for the same stat() to finish.
  66                                 // They may fire off new stat() calls if needed.
  67                                 vector<PendingStat> pending = move(it->second);
  68                                 pending_stats.erase(it);
  69                                 for (PendingStat &ps : pending) {
  70                                         check_access(ps.filename.c_str(), /*allow_async=*/true, move(ps.cb));
  71                                 }
  72                         });
  73                 }
  74                 return;  // The rest will happen in async context.
  75         }
  76
  77         // Passed all checks.
  78         cb(true);
  79 }