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