working on todo.md features

src/capture.cpp

@@ -0,0 +1,238 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "capture.hpp"
+
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include <cstdio>
+#include <cstring>
+#include <ctime>
+
+#include <chrono>
+#include <vector>
+
+namespace udpscope {
+namespace {
+
+/// The file header is one 64-byte block: fixed fields first, zeroed padding after.
+constexpr std::uint64_t kHeaderBytes = 64;
+/// `seq` lives at the *end* of a slot, at a fixed offset, because the reader must
+/// find it without trusting any other field.
+constexpr std::uint64_t kSeqBytes = 8;
+
+void put_u32(std::uint8_t* p, std::uint32_t v) { std::memcpy(p, &v, 4); }
+void put_u16(std::uint8_t* p, std::uint16_t v) { std::memcpy(p, &v, 2); }
+void put_f64(std::uint8_t* p, double v) { std::memcpy(p, &v, 8); }
+std::uint32_t get_u32(const std::uint8_t* p) {
+  std::uint32_t v;
+  std::memcpy(&v, p, 4);
+  return v;
+}
+double get_f64(const std::uint8_t* p) {
+  double v;
+  std::memcpy(&v, p, 8);
+  return v;
+}
+
+double wall_seconds() {
+  struct timespec ts;
+  clock_gettime(CLOCK_REALTIME, &ts);
+  return double(ts.tv_sec) + double(ts.tv_nsec) * 1e-9;
+}
+
+/// `seq` is published with a release store *after* the bytes are down, so a reader
+/// that sees a non-zero `seq` sees the whole slot -- the reason no `fsync` is needed
+/// for the file to be readable after a crash. Plain __atomic builtins, not
+/// `std::atomic_ref`: the target is mmap'd memory the class does not own as an object.
+void publish_seq(std::uint8_t* slot, std::uint64_t seq) {
+  __atomic_store_n(reinterpret_cast<std::uint64_t*>(slot + kSlotBytes - kSeqBytes), seq,
+                   __ATOMIC_RELEASE);
+}
+
+std::uint64_t read_seq(const std::uint8_t* slot) {
+  return __atomic_load_n(reinterpret_cast<const std::uint64_t*>(slot + kSlotBytes - kSeqBytes),
+                         __ATOMIC_ACQUIRE);
+}
+
+}  // namespace
+
+void PacketCapture::set_error(const char* what) noexcept {
+  std::snprintf(error_, sizeof(error_), "%s", what);
+  error_set_.store(true, std::memory_order_release);
+}
+
+void PacketCapture::error(char* out, std::size_t cap) const noexcept {
+  if (cap == 0) return;
+  out[0] = '\0';
+  if (!error_set_.load(std::memory_order_acquire)) return;
+  std::snprintf(out, cap, "%s", error_);
+}
+
+bool PacketCapture::open(const std::string& path, std::uint32_t slot_count) {
+  close();
+  path_ = path;
+  slot_count_ = slot_count;
+  seq_ = 0;
+  cursor_ = 0;
+  stored_.store(0, std::memory_order_relaxed);
+  truncated_.store(0, std::memory_order_relaxed);
+  error_set_.store(false, std::memory_order_relaxed);
+
+  const std::chrono::steady_clock::time_point t0 = std::chrono::steady_clock::now();
+
+  // Keep the previous capture one deep: re-enabling must not destroy the capture the
+  // user enabled *for*. A failure here is reported, and the old previous kept.
+  std::string prev = path;
+  const std::size_t slash = prev.rfind('/');
+  const std::size_t dot = prev.rfind('.');
+  if (dot != std::string::npos && (slash == std::string::npos || dot > slash)) {
+    prev.insert(dot, "-prev");
+  } else {
+    prev += "-prev";
+  }
+  ::rename(path.c_str(), prev.c_str());
+
+  fd_ = ::open(path.c_str(), O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0600);
+  if (fd_ < 0) {
+    set_error(strerror(errno));
+    return false;
+  }
+
+  // Reserved, not sparse: a store into a page the filesystem cannot back raises
+  // SIGBUS and kills udpscope outright, and an out-of-space condition must be an
+  // error at the moment the user clicked, not a crash ten minutes later.
+  const std::uint64_t total = kHeaderBytes + std::uint64_t(slot_count) * kSlotBytes;
+  if (::posix_fallocate(fd_, 0, off_t(total)) != 0) {
+    set_error(strerror(errno));
+    ::close(fd_);
+    fd_ = -1;
+    ::unlink(path.c_str());
+    return false;
+  }
+
+  map_ = static_cast<std::uint8_t*>(
+      ::mmap(nullptr, total, PROT_READ | PROT_WRITE, MAP_SHARED, fd_, 0));
+  if (map_ == MAP_FAILED) {
+    map_ = nullptr;
+    set_error(strerror(errno));
+    ::close(fd_);
+    fd_ = -1;
+    ::unlink(path.c_str());
+    return false;
+  }
+  map_bytes_ = total;
+
+  std::uint8_t* h = map_;
+  std::memset(h, 0, kHeaderBytes);
+  std::memcpy(h, "UDPSCAP\0", 8);
+  put_u32(h + 8, 1);                      // version
+  put_u32(h + 12, slot_count);
+  put_u32(h + 16, kSlotBytes);
+  put_u32(h + 20, 0);                     // cursor: a hint, `seq` is the order
+  put_f64(h + 24, wall_seconds());        // the epoch recv_time is relative to
+  put_u32(h + 32, peer_addr_);
+  put_u16(h + 36, peer_port_);
+  put_u16(h + 38, 0);  // local_port: the client does not expose its own socket
+  const double ms =
+      std::chrono::duration<double, std::milli>(std::chrono::steady_clock::now() - t0).count();
+  open_ms_.store(ms, std::memory_order_relaxed);
+
+  enabled_.store(true, std::memory_order_release);
+  return true;
+}
+
+void PacketCapture::close() noexcept {
+  if (map_ != nullptr) {
+    ::munmap(map_, map_bytes_);
+    map_ = nullptr;
+  }
+  if (fd_ >= 0) {
+    ::close(fd_);
+    fd_ = -1;
+  }
+  map_bytes_ = 0;
+  enabled_.store(false, std::memory_order_relaxed);
+}
+
+void PacketCapture::store(const std::uint8_t* bytes, std::size_t len, double recv_time) noexcept {
+  if (!enabled_.load(std::memory_order_relaxed) || map_ == nullptr) return;
+
+  std::uint8_t* slot = map_ + kHeaderBytes + std::uint64_t(cursor_) * kSlotBytes;
+  std::uint32_t orig = std::uint32_t(len);
+  std::size_t fit = len;
+  if (fit > kMaxDatagramBytes) {
+    fit = kMaxDatagramBytes;
+    truncated_.fetch_add(1, std::memory_order_relaxed);
+  }
+
+  put_f64(slot, recv_time);
+  put_u32(slot + 8, std::uint32_t(fit));
+  put_u32(slot + 12, orig);
+  std::memcpy(slot + kRecordHeaderBytes, bytes, fit);
+  ++seq_;
+  publish_seq(slot, seq_);  // last: a non-zero `seq` is what makes a slot complete
+
+  cursor_ = (cursor_ + 1) % slot_count_;
+  put_u32(map_ + 20, cursor_);  // a hint for a human in xxd; `seq` is the order
+  stored_.fetch_add(1, std::memory_order_relaxed);
+}
+
+bool read_capture(const std::string& path, CaptureReadStats* out) {
+  if (out == nullptr) return false;
+  *out = CaptureReadStats{};
+
+  FILE* f = std::fopen(path.c_str(), "rb");
+  if (f == nullptr) return false;
+  std::fseek(f, 0, SEEK_END);
+  const long size = std::ftell(f);
+  std::fseek(f, 0, SEEK_SET);
+  out->file_bytes = std::uint64_t(size < 0 ? 0 : size);
+
+  std::uint8_t h[kHeaderBytes];
+  if (std::fread(h, 1, sizeof(h), f) != sizeof(h)) {
+    std::fclose(f);
+    return false;
+  }
+  out->header_ok = std::memcmp(h, "UDPSCAP\0", 8) == 0 && get_u32(h + 8) == 1;
+  out->slot_count = get_u32(h + 12);
+  out->slot_bytes = get_u32(h + 16);
+  out->cursor = get_u32(h + 20);
+  out->t0_realtime = get_f64(h + 24);
+  if (!out->header_ok || out->slot_count == 0 || out->slot_bytes < kRecordHeaderBytes + 8) {
+    std::fclose(f);
+    return false;
+  }
+
+  const std::uint32_t payload = out->slot_bytes - kRecordHeaderBytes - kSeqBytes;
+  std::vector<std::uint8_t> slot(out->slot_bytes);
+  for (std::uint32_t i = 0; i < out->slot_count; ++i) {
+    if (std::fseek(f, long(kHeaderBytes + std::uint64_t(i) * out->slot_bytes), SEEK_SET) != 0) {
+      break;
+    }
+    if (std::fread(slot.data(), 1, slot.size(), f) != slot.size()) break;
+    const std::uint64_t seq = read_seq(slot.data());
+    const std::uint32_t len = get_u32(slot.data() + 8);
+    const std::uint32_t orig = get_u32(slot.data() + 12);
+    if (seq == 0 && len == 0 && orig == 0) {
+      ++out->empty;
+      continue;
+    }
+    if (seq == 0) {
+      ++out->torn;  // bytes down, ordinal never published: a capture that died mid-copy
+      continue;
+    }
+    ++out->stored;
+    out->payload += len;
+    if (orig > payload) ++out->truncated;
+    if (out->first_seq == 0 || seq < out->first_seq) out->first_seq = seq;
+    if (seq > out->last_seq) out->last_seq = seq;
+  }
+  std::fclose(f);
+  return true;
+}
+
+}  // namespace udpscope