Amend File_system session with SEEK_TAIL support

Used to read or write from the end of a file when multiple packets may
be in transit.

Supported by ram_fs, rump_fs, and vfs servers.

Fixes #1775

repos/dde_rump/src/server/rump_fs/file.h

@@ -116,22 +116,27 @@ class File_system::File : public Node
 
 		size_t read(char *dst, size_t len, seek_off_t seek_offset)
 		{
-			ssize_t ret = rump_sys_pread(_fd, dst, len, seek_offset);
+			ssize_t ret;
+
+			if (seek_offset == SEEK_TAIL)
+				ret = rump_sys_lseek(_fd, -len, SEEK_END) != -1 ?
+					rump_sys_read(_fd, dst, len) : 0;
+			else
+				ret = rump_sys_pread(_fd, dst, len, seek_offset);
 
 			return ret == -1 ? 0 : ret;
 		}
 
 		size_t write(char const *src, size_t len, seek_off_t seek_offset)
 		{
-			/* should we append? */
-			if (seek_offset == ~0ULL) {
-				off_t off = rump_sys_lseek(_fd, 0, SEEK_END);
-				if (off == -1)
-					return 0;
-				seek_offset = off;
-			}
+			ssize_t ret;
+
+			if (seek_offset == SEEK_TAIL)
+				ret = rump_sys_lseek(_fd, 0, SEEK_END) != -1 ?
+					rump_sys_write(_fd, src, len) : 0;
+			else
+				ret = rump_sys_pwrite(_fd, src, len, seek_offset);
 
-			ssize_t ret = rump_sys_pwrite(_fd, src, len, seek_offset);
 			return ret == -1 ? 0 : ret;
 		}
 

repos/os/include/file_system_session/file_system_session.h

@@ -40,6 +40,18 @@ namespace File_system {
 
 	enum { MAX_NAME_LEN = 256, MAX_PATH_LEN = 1024 };
 
+	/**
+	 * File offset constant for reading or writing to the end of a file
+	 *
+	 * Clients are unable to reliably append to the end of a file where there
+	 * may be other writes to the same offset in the queues of other clients.
+	 * The SEEK_TAIL constant resolves this contention by aligning packet
+	 * operations with the end of the file at the time the packet is dequeued.
+	 *
+	 * SEEK_TAIL behavior with directory and symlink nodes is undefined.
+	 */
+	enum { SEEK_TAIL = ~0ULL };
+
 	typedef Genode::Rpc_in_buffer<MAX_NAME_LEN> Name;
 	typedef Genode::Rpc_in_buffer<MAX_PATH_LEN> Path;
 
@@ -129,11 +141,15 @@ class File_system::Packet_descriptor : public Genode::Packet_descriptor
 		/**
 		 * Constructor
 		 *
-		 * \param position  seek offset in bytes (by default, append)
+		 * \param position  seek offset in bytes
+		 *
+		 * Note, if 'position' is set to 'SEEK_TAIL' read operations will read
+		 * 'length' bytes from the end of the file while write operations will
+		 * append length bytes at the end of the file.
 		 */
 		Packet_descriptor(Packet_descriptor p,
 		                  Node_handle handle, Opcode op, size_t length,
-		                  seek_off_t position = ~0)
+		                  seek_off_t position = SEEK_TAIL)
 		:
 			Genode::Packet_descriptor(p.offset(), p.size()),
 			_handle(handle), _op(op),

repos/os/include/ram_fs/file.h

@@ -15,6 +15,7 @@
 #define _INCLUDE__RAM_FS__FILE_H_
 
 /* Genode includes */
+#include <file_system_session/file_system_session.h>
 #include <base/allocator.h>
 
 /* local includes */
@@ -46,7 +47,9 @@ class File_system::File : public Node
 		{
 			file_size_t const chunk_used_size = _chunk.used_size();
 
-			if (seek_offset >= _length)
+			if (seek_offset == SEEK_TAIL)
+				seek_offset = (len < _length) ? (_length - len) : 0;
+			else if (seek_offset >= _length)
 				return 0;
 
 			/*
@@ -78,8 +81,8 @@ class File_system::File : public Node
 
 		size_t write(char const *src, size_t len, seek_off_t seek_offset)
 		{
-			if (seek_offset == (seek_off_t)(~0))
-				seek_offset = _chunk.used_size();
+			if (seek_offset == SEEK_TAIL)
+				seek_offset = _length;
 
 			if (seek_offset + len >= Chunk_level_0::SIZE) {
 				len = (Chunk_level_0::SIZE-1) - seek_offset;

repos/os/src/server/vfs/node.h

@@ -128,6 +128,7 @@ class Vfs_server::File : public Node
 	private:
 
 		Vfs::Vfs_handle *_handle;
+		char const      *_leaf_path; /* offset pointer to Node::_path */
 
 	public:
 		File(Vfs::File_system  &vfs,
@@ -140,7 +141,8 @@ class Vfs_server::File : public Node
 			unsigned vfs_mode =
 				(fs_mode-1) | (create ? Vfs::Directory_service::OPEN_MODE_CREATE : 0);
 
-			assert_open(vfs.open(path(), vfs_mode, &_handle, alloc));
+			assert_open(vfs.open(file_path, vfs_mode, &_handle, alloc));
+			_leaf_path = vfs.leaf_path(file_path);
 		}
 
 		~File() { _handle->ds().close(_handle); }
@@ -160,6 +162,15 @@ class Vfs_server::File : public Node
 		{
 			Vfs::file_size res = 0;
 
+			if (seek_offset == SEEK_TAIL) {
+				typedef Directory_service::Stat_result Result;
+				Vfs::Directory_service::Stat st;
+
+				/* if stat fails, try and see if the VFS will seek to the end */
+				seek_offset = (_handle->ds().stat(_leaf_path, st) == Result::STAT_OK) ?
+					((len < st.size) ? (st.size - len) : 0) : SEEK_TAIL;
+			}
+
 			_handle->seek(seek_offset);
 			_handle->fs().read(_handle, dst, len, res);
 			return res;
@@ -169,6 +180,15 @@ class Vfs_server::File : public Node
 		{
 			Vfs::file_size res = 0;
 
+			if (seek_offset == SEEK_TAIL) {
+				typedef Directory_service::Stat_result Result;
+				Vfs::Directory_service::Stat st;
+
+				/* if stat fails, try and see if the VFS will seek to the end */
+				seek_offset = (_handle->ds().stat(_leaf_path, st) == Result::STAT_OK) ?
+					st.size : SEEK_TAIL;
+			}
+
 			_handle->seek(seek_offset);
 			_handle->fs().write(_handle, src, len, res);
 			if (res)