Minimal backport to support libnfs 6 on v0.23

This is based on the following commits in master:
- 31e583e
- 58e3b83

also changed cpp_std=c++20 to facilitate the use of span

Author: BlackIkeEagle

meson.build

@@ -6,8 +6,8 @@ project(
   default_options: [
     'c_std=c11',
     'build.c_std=c11',
-    'cpp_std=c++17',
-    'build.cpp_std=c++17',
+    'cpp_std=c++20',
+    'build.cpp_std=c++20',
     'warning_level=3',
 
     # If we build those libraries as Meson subproject, they shall be

src/lib/nfs/Connection.cxx

@@ -103,11 +103,25 @@ NfsConnection::CancellableCallback::Stat(nfs_context *ctx,
 
 inline void
 NfsConnection::CancellableCallback::Read(nfs_context *ctx, struct nfsfh *fh,
-					 uint64_t offset, size_t size)
+					 uint64_t offset,
+#ifdef LIBNFS_API_2
+					 std::span<std::byte> dest
+#else
+					 std::size_t size
+#endif
+	)
 {
 	assert(connection.GetEventLoop().IsInside());
 
-	int result = nfs_pread_async(ctx, fh, offset, size, Callback, this);
+	int result = nfs_pread_async(ctx, fh,
+#ifdef LIBNFS_API_2
+				     dest.data(), dest.size(),
+#endif
+				     offset,
+#ifndef LIBNFS_API_2
+				     size,
+#endif
+				     Callback, this);
 	if (result < 0)
 		throw FormatRuntimeError("nfs_pread_async() failed: %s",
 					 nfs_get_error(ctx));
@@ -329,15 +343,26 @@ NfsConnection::Stat(struct nfsfh *fh, NfsCallback &callback)
 }
 
 void
-NfsConnection::Read(struct nfsfh *fh, uint64_t offset, size_t size,
+NfsConnection::Read(struct nfsfh *fh, uint64_t offset,
+#ifdef LIBNFS_API_2
+		    std::span<std::byte> dest,
+#else
+		    std::size_t size,
+#endif
 		    NfsCallback &callback)
 {
 	assert(GetEventLoop().IsInside());
 	assert(!callbacks.Contains(callback));
 
 	auto &c = callbacks.Add(callback, *this, false);
 	try {
-		c.Read(context, fh, offset, size);
+		c.Read(context, fh, offset,
+#ifdef LIBNFS_API_2
+		       dest
+#else
+		       size
+#endif
+			);
 	} catch (...) {
 		callbacks.Remove(c);
 		throw;

src/lib/nfs/Connection.hxx

@@ -71,7 +71,13 @@ class NfsConnection {
 		void Open(nfs_context *context, const char *path, int flags);
 		void Stat(nfs_context *context, struct nfsfh *fh);
 		void Read(nfs_context *context, struct nfsfh *fh,
-			  uint64_t offset, size_t size);
+			  uint64_t offset,
+#ifdef LIBNFS_API_2
+			  std::span<std::byte> dest
+#else
+			  std::size_t size
+#endif
+			);
 
 		/**
 		 * Cancel the operation and schedule a call to
@@ -193,7 +199,12 @@ public:
 	/**
 	 * Throws std::runtime_error on error.
 	 */
-	void Read(struct nfsfh *fh, uint64_t offset, size_t size,
+	void Read(struct nfsfh *fh, uint64_t offset,
+#ifdef LIBNFS_API_2
+		  std::span<std::byte> dest,
+#else
+		  std::size_t size,
+#endif
 		  NfsCallback &callback);
 
 	void Cancel(NfsCallback &callback) noexcept;

src/lib/nfs/FileReader.cxx

@@ -129,15 +129,28 @@ NfsFileReader::Read(uint64_t offset, size_t size)
 {
 	assert(state == State::IDLE);
 
+#ifdef LIBNFS_API_2
+	assert(!read_buffer);
+	// TOOD read into caller-provided buffer
+	read_buffer = std::make_unique<std::byte[]>(size);
+	connection->Read(fh, offset, {read_buffer.get(), size}, *this);
+#else
 	connection->Read(fh, offset, size, *this);
+#endif
+
 	state = State::READ;
 }
 
 void
 NfsFileReader::CancelRead() noexcept
 {
 	if (state == State::READ) {
+#ifdef LIBNFS_API_2
+		assert(read_buffer);
+		read_buffer.release();
+#endif
 		connection->Cancel(*this);
+
 		state = State::IDLE;
 	}
 }

src/lib/nfs/FileReader.hxx

@@ -32,6 +32,10 @@
 
 #include <sys/stat.h>
 
+#ifdef LIBNFS_API_2
+#include <memory>
+#endif
+
 struct nfsfh;
 class NfsConnection;
 
@@ -68,6 +72,10 @@ class NfsFileReader : NfsLease, NfsCallback {
 	 */
 	InjectEvent defer_open;
 
+#ifdef LIBNFS_API_2
+	std::unique_ptr<std::byte[]> read_buffer;
+#endif
+
 public:
 	NfsFileReader() noexcept;
 	~NfsFileReader() noexcept;

src/lib/nfs/meson.build

@@ -4,6 +4,13 @@ if not nfs_dep.found()
   subdir_done()
 endif
 
+if nfs_dep.version().version_compare('>=6')
+  # libnfs has no version macro therefore we must detect the API
+  # version 2 at configure time
+  nfs_dep = declare_dependency(compile_args: '-DLIBNFS_API_2',
+                               dependencies: nfs_dep)
+endif
+
 nfs = static_library(
   'nfs',
   'Connection.cxx',