From a8864dc5900a877d621415d0601da651ee4ece63 Mon Sep 17 00:00:00 2001
From: Robin Burchell <robin+git@viroteck.net>
Date: Mon, 20 May 2019 01:18:33 +0200
Subject: [PATCH] Kernel: Report EAGAIN from read() on a non-blocking socket if
 the buffer is empty

This is not EOF, and never should have been so -- can trip up other code
when porting.

Also updates LibGUI and WindowServer which both relied on the old
behaviour (and didn't work without changes). There may be others, but I
didn't run into them with a quick inspection.
---
 Kernel/Net/LocalSocket.cpp           | 14 ++++++++++++--
 LibGUI/GEventLoop.cpp                | 15 +++++++--------
 Servers/WindowServer/WSEventLoop.cpp |  2 +-
 3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/Kernel/Net/LocalSocket.cpp b/Kernel/Net/LocalSocket.cpp
index 28ab02439fd..e0887103d65 100644
--- a/Kernel/Net/LocalSocket.cpp
+++ b/Kernel/Net/LocalSocket.cpp
@@ -153,10 +153,20 @@ bool LocalSocket::can_read(FileDescriptor& descriptor) const
 ssize_t LocalSocket::read(FileDescriptor& descriptor, byte* buffer, ssize_t size)
 {
     auto role = descriptor.socket_role();
-    if (role == SocketRole::Accepted)
+    if (role == SocketRole::Accepted) {
+        if (!descriptor.is_blocking()) {
+            if (m_for_server.is_empty())
+                return -EAGAIN;
+        }
         return m_for_server.read(buffer, size);
-    if (role == SocketRole::Connected)
+    }
+    if (role == SocketRole::Connected) {
+        if (!descriptor.is_blocking()) {
+            if (m_for_client.is_empty())
+                return -EAGAIN;
+        }
         return m_for_client.read(buffer, size);
+    }
     ASSERT_NOT_REACHED();
 }
 
diff --git a/LibGUI/GEventLoop.cpp b/LibGUI/GEventLoop.cpp
index ba29e4e916e..7999c88e49c 100644
--- a/LibGUI/GEventLoop.cpp
+++ b/LibGUI/GEventLoop.cpp
@@ -324,22 +324,22 @@ void GEventLoop::process_unprocessed_bundles()
 
 bool GEventLoop::drain_messages_from_server()
 {
-    bool is_first_pass = true;
     for (;;) {
         WSAPI_ServerMessage message;
         ssize_t nread = read(s_windowserver_fd, &message, sizeof(WSAPI_ServerMessage));
         if (nread < 0) {
+            if (errno == EAGAIN) {
+                return true;
+            }
             perror("read");
             quit(1);
             return false;
         }
         if (nread == 0) {
-            if (is_first_pass) {
-                fprintf(stderr, "EOF on WindowServer fd\n");
-                quit(1);
-                return false;
-            }
-            return true;
+            fprintf(stderr, "EOF on WindowServer fd\n");
+            quit(1);
+            exit(-1);
+            return false;
         }
         assert(nread == sizeof(message));
         ByteBuffer extra_data;
@@ -355,7 +355,6 @@ bool GEventLoop::drain_messages_from_server()
             ASSERT(extra_nread == message.extra_size);
         }
         m_unprocessed_bundles.append({ move(message), move(extra_data) });
-        is_first_pass = false;
     }
 }
 
diff --git a/Servers/WindowServer/WSEventLoop.cpp b/Servers/WindowServer/WSEventLoop.cpp
index 904dfca2fc2..c2fe108b6fe 100644
--- a/Servers/WindowServer/WSEventLoop.cpp
+++ b/Servers/WindowServer/WSEventLoop.cpp
@@ -301,7 +301,7 @@ void WSEventLoop::drain_client(WSClientConnection& client)
         WSAPI_ClientMessage message;
         // FIXME: Don't go one message at a time, that's so much context switching, oof.
         ssize_t nread = read(client.fd(), &message, sizeof(WSAPI_ClientMessage));
-        if (nread == 0) {
+        if (nread == 0 || (nread == -1 && errno == EAGAIN)) {
             if (!messages_received)
                 post_event(client, make<WSClientDisconnectedNotification>(client.client_id()));
             break;