[kernel|gen-tcp-socket|doc] Document option nodelay discrepancy

We do *not* hide that the nodelay option does not work when
used with Unix Domain Sockets when gen_tcp is used with
inet_backend = socket (which it is when used with
inet_backend = inet). This is now documented.

OTP-17156

2 files changed, 30 insertions, 10 deletions

diff --git a/lib/kernel/doc/src/gen_tcp.xml b/lib/kernel/doc/src/gen_tcp.xml
index a7deb1562f..7f23c16f67 100644
--- a/lib/kernel/doc/src/gen_tcp.xml
+++ b/lib/kernel/doc/src/gen_tcp.xml
@@ -97,14 +97,30 @@ do_recv(Sock, Bs) ->
 	message (<c>{tcp_closed, ...}</c> will be received (not an error). </p>
 	</item>
 
-        <item><p>The option
-	<seeerl marker="inet#option-show_econnreset">show_econnreset</seeerl>
-	basically do <em>not</em>
-	work as described when used with <c>inet_backend = socket</c>.
-	The "issue" is that a remote close (as described above)
-	<em>do</em> allow a reader to extract what is in the read buffers
-	before a close is "delivered".
-	</p></item>
+        <item>
+	  <p>The option
+	  <seeerl marker="inet#option-show_econnreset">show_econnreset</seeerl>
+	  basically do <em>not</em>
+	  work as described when used with <c>inet_backend = socket</c>.
+	  The "issue" is that a remote close (as described above)
+	  <em>do</em> allow a reader to extract what is in the read buffers
+	  before a close is "delivered".</p>
+	</item>
+
+        <item>
+	  <p>The option
+	  <seeerl marker="inet#option-nodelay">nodelay</seeerl>
+	  is a TCP specific option that is <em>not</em> compatible with
+	  <c>domain = local</c>. </p>
+	  <p>When using <c>inet_backend = socket</c>, trying to create a socket
+	  (via listen or connect) with <c>domain = local</c>
+	  (for example with option {ifaddr, {local,"/tmp/test"}})
+	  <em>will fail</em> with <c>{error, enotsup}</c>. </p>
+	  <p>This does not actually work for <c>inet_backend = inet</c> either,
+	  but in that case the error is simply <em>ignored</em>,
+	  which is a <em>bad</em> idea. We have choosen to <em>not</em>
+	  ignore this error for <c>inet_backend = socket</c>. </p>
+	</item>
 
 	<item><p><seeerl marker="#async_shutdown_write">Async shutdown write</seeerl></p></item>
       </list>
diff --git a/lib/kernel/doc/src/inet.xml b/lib/kernel/doc/src/inet.xml
index 903f58a1c4..2bf7c1066e 100644
--- a/lib/kernel/doc/src/inet.xml
+++ b/lib/kernel/doc/src/inet.xml
@@ -1206,12 +1206,16 @@ setcap cap_sys_admin,cap_sys_ptrace,cap_dac_read_search+epi beam.smp</code>
           <tag><c>binary</c></tag>
           <item>
             <p>Received <c>Packet</c> is delivered as a binary.</p>
+            <marker id="option-nodelay"></marker>
           </item>
           <tag><c>{nodelay, Boolean}</c>(TCP/IP sockets)</tag>
           <item>
             <p>If <c>Boolean == true</c>, option <c>TCP_NODELAY</c>
-              is turned on for the socket, which means that also small
-              amounts of data are sent immediately.</p>
+            is turned on for the socket, which means that also small
+            amounts of data are sent immediately.</p>
+	    <p>This option is <em>not</em> supported for
+	    <c>domain = local</c>, but if <c>inet_backend =/= socket</c>
+	    this error will be <em>ignored</em>. </p>
           </item>
           <tag><c>{nopush, Boolean}</c>(TCP/IP sockets)</tag>
           <item>