From dc517a87103f66262f6933bfb6970ec8ca0a498d Mon Sep 17 00:00:00 2001
From: Mikael Pettersson <mikpelinux@gmail.com>
Date: Wed, 6 Sep 2023 20:28:31 +0200
Subject: [PATCH] pdp10_stdio:handle_fopen/2: handling writes after seeks may
 require reads, so try to get read access even if the user did not ask for it

---
 erlang/apps/lib/src/pdp10_stdio.erl | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/erlang/apps/lib/src/pdp10_stdio.erl b/erlang/apps/lib/src/pdp10_stdio.erl
index 6c9fac4..b0fbe81 100644
--- a/erlang/apps/lib/src/pdp10_stdio.erl
+++ b/erlang/apps/lib/src/pdp10_stdio.erl
@@ -243,11 +243,21 @@ code_change(_OldVsn, State, _Extra) -> {ok, State}.
 
 handle_fopen(Path, Modes) ->
   case iodir(Modes) of
-    {ok, {Read, Write}} ->
-      case file_open(Path, fopen_modes(Modes)) of
-        {ok, IoDev} -> {ok, {IoDev, Read, Write}};
-        {error, _Reason} = Error -> Error
+    {ok, {Read = false, Write = true}} ->
+      %% A write after a seek to a non octet-aligned nonet needs to reload the
+      %% shiftreg, which requires a read. Therefore if the user did not request
+      %% read access try to get it anyway.
+      case file_open(Path, fopen_modes([read | Modes])) of
+        {ok, IoDev} -> {ok, {IoDev, _Read = true, Write}};
+        {error, _Reason} -> handle_fopen(Path, Modes, Read, Write)
       end;
+    {ok, {Read, Write}} -> handle_fopen(Path, Modes, Read, Write);
+    {error, _Reason} = Error -> Error
+  end.
+
+handle_fopen(Path, Modes, Read, Write) ->
+  case file_open(Path, fopen_modes(Modes)) of
+    {ok, IoDev} -> {ok, {IoDev, Read, Write}};
     {error, _Reason} = Error -> Error
   end.