Design proposal for new ring buffer API

docs/RingBuffer.md

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+# New ebpf Ring Buffer Map (proposal)
+
+## Overview
+
+The current ringbuffer uses a pure callback-based approach to reading the ringbuffer.
+Linux also supports memory-mapped polling consumers, which can't be directly supported in the current model.
+
+The new API will support 2 consumer types: callbacks and direct access to the mapped producer memory (with poll to wait for data).
+
+Callback consumer:
+
+1. Call `ring_buffer__new` to set up callback.
+2. The callback will be invoked for each record written to the ring buffer.
+
+Mapped memory consumer:
+
+1. Call `ebpf_ring_buffer_get_buffer` to get pointers to the mapped producer/consumer pages.
+2. Call `ebpf_ring_buffer_get_wait_handle` to get the wait handle.
+3. Directly read records from the producer pages (and update consumer offset as we read).
+4. Call `WaitForSingleObject`/`WaitForMultipleObject` as needed to wait for new data to be available.
+
+## API Changes
+
+### Changes to ebpf helper functions
+
+```c
+/**
+ * @brief Output record to ringbuf
+ *
+ * Note newly added flag values (to specify wakeup options).
+ *
+ * Wakeup options (flags):
+ * - 0 (auto/default): Notify if consumer has caught up.
+ * - BPF_RB_FORCE_WAKEUP - Always notify consumer.
+ * - BPF_RB_NO_WAKEUP - Never notify consumer.
+ *
+ */
+ebpf_result_t
+ebpf_ring_buffer_output(ebpf_ring_buffer_t* ring, uint8_t* data, size_t length, size_t flags)
+```
+
+### Existing libbpf functions for callback consumer
+
+The behaviour of these functions will be unchanged.
+
+Use the existing `ring_buffer__new()` to set up automatic callbacks for each record.
+Call `ebpf_ring_buffer_get_buffer()` ([New eBPF APIs](#new-ebpf-apis-for-mapped-memory-consumer))
+to get direct access to the mapped ringbuffer memory.
+
+```c
+struct ring_buffer;
+
+typedef int (*ring_buffer_sample_fn)(void *ctx, void *data, size_t size);
+
+struct ring_buffer_opts {
+	size_t sz; /* size of this struct, for forward/backward compatiblity */
+};
+
+#define ring_buffer_opts__last_field sz
+
+/**
+ * @brief Creates a new ring buffer manager.
+ *
+ * @param[in] map_fd File descriptor to ring buffer map.
+ * @param[in] sample_cb Pointer to ring buffer notification callback function.
+ * @param[in] ctx Pointer to sample_cb callback function context.
+ * @param[in] opts Ring buffer options.
+ *
+ * @returns Pointer to ring buffer manager.
+ */
+struct ring_buffer *
+ring_buffer__new(int map_fd, ring_buffer_sample_fn sample_cb, void *ctx,
+		 const struct ring_buffer_opts *opts);
+
+/**
+ * @brief Frees a ring buffer manager.
+ *
+ * @param[in] rb Pointer to ring buffer manager to be freed.
+ *
+ */
+void ring_buffer__free(struct ring_buffer *rb);
+```
+
+### New ebpf APIs for mapped memory consumer
+
+```c
+/**
+ * get pointers to mapped producer and consumer pages
+ *
+ * @param[out] producer pointer* to start of read-only mapped producer pages
+ * @param[out] consumer pointer* to start of read-write mapped consumer page
+ */
+ebpf_result_t ebpf_ring_buffer_get_buffer(fd_t map_fd, void **producer, void **consumer);
+
+/**
+ * get the wait handle to use with WaitForSingleObject/WaitForMultipleObject
+ *
+ * @returns Wait handle
+ */
+HANDLE ebpf_ring_buffer_get_wait_handle(fd_t map_fd);
+```
+
+## Ringbuffer consumer
+
+### mapped memory consumer example
+
+This consumer directly accesses the records from the producer memory and directly updates the consumer offset to show the logic.
+
+```c++
+
+//
+// == Ringbuf helpers ==
+//
+
+// Ring buffer record is 64 bit header + data.
+typedef struct _rb_header
+{
+    //NOTE: bit fields are not portable, so this is just for simpler example code -- the actual code should use bit masking to perform equivalent operations on the header bits, and ReadAcquire to read the header.
+    uint8_t locked : 1;
+    uint8_t discarded : 1;
+    uint32_t length : 30;
+    uint32_t offset; // for kernel use (offset of record in pages from start of buffer data area)
+} rb_header_t;
+
+typedef struct _rb_record
+{
+    rb_header_t header;
+    uint8_t data[];
+} rb_record_t;
+
+/**
+ * @brief clear the ringbuffer.
+ */
+void rb_flush(uint64_t *cons_offset, const uint64_t *prod_offset) {
+    WriteRelease64(cons_offset,ReadAcquire64(prod_offset));
+}
+
+
+//
+// == mmap/epoll consumer ==
+//
+
+void *rb_cons; // Pointer to read/write mapped consumer page with consumer offset.
+void *rb_prod; // Pointer to start of read-only producer pages.
+
+// Open ringbuffer.
+fd_t map_fd = bpf_obj_get(rb_map_name.c_str());
+if (map_fd == ebpf_fd_invalid) return 1;
+
+// Initialize wait handle for map.
+HANDLE wait_handle = ebpf_ring_buffer_get_wait_handle(map_fd);
+if (!wait_handle) {
+    // … log error …
+    goto Exit;
+}
+
+// get pointers to the producer/consumer pages
+int err = ebpf_ring_buffer_get_buffer(map_fd, &rb_prod, &rb_cons);
+
+if (err) {
+    goto Exit;
+}
+
+const uint64_t *prod_offset = (const uint64_t*)rb_prod; // Producer offset ptr (r only).
+uint64_t *cons_offset = (uint64_t*)rb_cons; // Consumer offset ptr (r/w mapped).
+const uint8_t *rb_data = ((const uint8_t*)rb_prod) + PAGESIZE; // Double-mapped rb data ptr (r only).
+
+uint64_t producer_offset = ReadAcquire64(prod_offset);
+uint64_t consumer_offset = ReadAcquire64(cons_offset);
+// have_data used to track whether we should wait for notification or just keep reading.
+bool have_data = producer_offset > consumer_offset;
+
+void           *lp_ctx = NULL;
+OVERLAPPED     *overlapped = NULL;
+DWORD          bytesTransferred = 0;
+
+// Now loop until error.
+For(;;) {
+  if (!have_data) { // Only wait if we have already caught up.
+    // Wait for rb to notify -- or we could spin/poll until *prod_offset > *cons_offset.
+    DWORD wait_status = WaitForSingleObject(wait_handle, INFINITE);
+
+    if (wait_status != WAIT_OBJECT_0) { // No notification
+      uint32_t wait_err = GetLastError();
+      if (wait_err == /* terminal error */) {
+        // … log error …
+        break;
+      }
+      producer_offset = ReadAcquire64(prod_offset);
+      have_data = producer_offset > consumer_offset; // It's possible we still have data.
+      if (!have_data) continue;
+    } else { // We got notified of new data.
+      have_data = true;
+    }
+  }
+  uint64_t remaining = producer_offset - consumer_offset;
+
+  if (remaining == 0) {
+    have_data = false; // Caught up to producer.
+    continue;
+  } else if (remaining < sizeof(rb_header_t)) {
+    // Bad record or consumer offset out of alignment.
+    // … log error …
+    break;
+  }
+
+  // Check header flags first, then read/skip data and update offset.
+  rb_header_t header = (rb_header_t)(&rb_data[consumer_offset % rb_size]);
+  if (header.locked) { // Next record not ready yet, wait.
+    have_data = false;
+    continue;
+    // Or we could spin/poll on ((rb_header_t*)(&rb_data[consumer_offset % rb_size]))->locked.
+  }
+  if (!header.discarded) {
+    const rb_record_t *record = *(const rb_record_t*)(&rb_data[consumer_offset % rb_size]);
+    // Read data from record->data[0 ... record->length-1].
+    // … business logic …
+  } // Else it was discarded, skip and continue.
+
+  // Update consumer offset (and pad record length to multiple of 8).
+  consumer_offset += sizeof(rb_header_t) + (record->length + 7 & ~7);
+  WriteRelease64(cons_offset, consumer_offset);
+}
+
+Exit:
+```
+
+### Simplified polling ringbuf consumer
+
+This consumer uses some possible helpers to simplify the above logic (might also want timeout).
+
+```c
+//Note: the below theoretical helpers would only need access to producers/consumer pages (offsets and data pages)
+//rb__empty(prod,cons) - check whether consumer offset == consumer offset (!empty doesn't mean data is ready)
+//rb__flush(prod,cons) - just set consumer offset = producer offset (skips all completed/in-progress records)
+//rb__next_record(prod,cons) - advance consumer offset to next record
+//rb__get_record(prod,cons,&record) - get pointer to current record (if any), skipping discarded records
+//Returns E_SUCCESS (0) if record ready, E_LOCKED if record still locked, E_EMPTY if consumer has caught up.
+
+for(;;) {
+  for(; !(err=rb__get_record(prod,cons,&record)); rb__next_record(prod,cons)) {
+    // Data is now in record->data[0 ... record->length-1].
+    // … Do record handling here …
+  }
+  // 3 cases for err:
+  // 1) Ringbuf empty - Wait on handle, or poll for !rb__empty(prod,cons).
+  // 2) Record locked - Wait on handle, or spin/poll on header lock bit.
+  // 3) Corrupt record or consumer offset - Break (could flush to continue reading from next good record).
+  if (err!=E_EMPTY && err!=E_LOCKED) {
+    // … log error …
+    break;
+  }
+  if (err == /* Handled errors, e.g. timeout */) {
+    // … log error and continue (we might still have record(s) to read) …
+  } else if (err != E_SUCCESS) {
+    // … log error …
+    break;
+  }
+}
+return err;
+```