From ece6c19fa6e6b03db9b0274f038d8c0e295d0608 Mon Sep 17 00:00:00 2001
From: Levi Neely <141506390+lneely@users.noreply.github.com>
Date: Sat, 18 Jul 2026 17:13:01 +0200
Subject: [PATCH] =?UTF-8?q?9p:=20fix=20/net/dns=20read=20pattern=20?=
 =?UTF-8?q?=E2=80=94=20ignore=20offset=20for=20write-then-read?=
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Shell fd position advances after write, so the subsequent read
arrives with offset=N (write length). readDNS always serves from
offset 0, fixing empty reads from exec 3<>file; echo >&3; cat <&3.
---
 server/server.go | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/server/server.go b/server/server.go
index e9f7521..f99258c 100644
--- a/server/server.go
+++ b/server/server.go
@@ -757,22 +757,22 @@ func (s *Server) read(cs *connState, fc *plan9.Fcall, ctx context.Context) *plan
 		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
 	}
 
-	// /complete: return per-fid result from previous write
+	// /complete: return per-fid result from previous write (offset 0-based)
 	if path == "/complete" {
 		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
-		return s.readSlice(fc, f.completeResult)
+		return s.readDNS(fc, f.completeResult)
 	}
 
-	// /generate: return per-fid result from previous write
+	// /generate: return per-fid result from previous write (offset 0-based)
 	if path == "/generate" {
 		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
-		return s.readSlice(fc, f.generateResult)
+		return s.readDNS(fc, f.generateResult)
 	}
 
-	// /route: return per-fid result from previous write
+	// /route: return per-fid result from previous write (offset 0-based)
 	if path == "/route" {
 		s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
-		return s.readSlice(fc, f.routeResult)
+		return s.readDNS(fc, f.routeResult)
 	}
 
 	s.log.Debug("Tread path=%q offset=%d count=%d", path, fc.Offset, fc.Count)
@@ -850,6 +850,21 @@ func (s *Server) readSlice(fc *plan9.Fcall, content []byte) *plan9.Fcall {
 	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
 }
 
+// readDNS implements the Plan 9 /net/dns read pattern: the result is always
+// read from offset 0 regardless of the client's file position (which advances
+// after a write). This allows write-then-read on the same fd.
+func (s *Server) readDNS(fc *plan9.Fcall, content []byte) *plan9.Fcall {
+	var data []byte
+	if len(content) > 0 {
+		end := int(fc.Count)
+		if end > len(content) {
+			end = len(content)
+		}
+		data = content[:end]
+	}
+	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
+}
+
 func (s *Server) helpPath() string {
 	return paths.CfgDir() + "/help.md"
 }