From c1441910e00044d85bdb7642bc6d19cd526da5e7 Mon Sep 17 00:00:00 2001
From: Michael Krayer <michael.krayer@posteo.de>
Date: Fri, 26 Mar 2021 11:06:51 +0100
Subject: [PATCH] scrollback buffer patch by Justin Bogner

---
 dtach.h  |  1 +
 master.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/dtach.h b/dtach.h
index c4d39cb..6f4236c 100644
--- a/dtach.h
+++ b/dtach.h
@@ -122,6 +122,7 @@ struct packet
 ** buffer used for the text stream.
 */
 #define BUFSIZE 4096
+#define SCROLLSIZE 8192
 
 /* This hopefully moves to the bottom of the screen */
 #define EOS "\033[999H"
diff --git a/master.c b/master.c
index 5c053e0..e94dd63 100644
--- a/master.c
+++ b/master.c
@@ -52,6 +52,9 @@ struct client
 static struct client *clients;
 /* The pseudo-terminal created for the child process. */
 static struct pty the_pty;
+/* The scrollback buffer */
+static unsigned char scrollback[SCROLLSIZE];
+static int scroll_end, scroll_full;
 
 #ifndef HAVE_FORKPTY
 pid_t forkpty(int *amaster, char *name, struct termios *termp,
@@ -238,6 +241,66 @@ update_socket_modes(int exec)
 		chmod(sockname, newmode);
 }
 
+/* Write the data in *buf into the scrollback buffer */
+static void
+write_scrollback(unsigned char *buf, int len)
+{
+	int n;
+	if (len > SCROLLSIZE)
+	{
+		buf = buf + (len - SCROLLSIZE);
+		len = SCROLLSIZE;
+	}
+
+	n = SCROLLSIZE - scroll_end;
+	if (n >= len)
+	{
+		memcpy(scrollback + scroll_end, buf,
+		       len * sizeof(unsigned char));
+		scroll_end += len * sizeof(unsigned char);
+		scroll_full = 1;
+	}
+	else
+	{
+		memcpy(scrollback + scroll_end, buf,
+		       n * sizeof(unsigned char));
+		scroll_end = len - n;
+		memcpy(scrollback, buf,
+		       scroll_end * sizeof(unsigned char));
+	}
+}
+
+/* Send the data in the scrollback buffer to client */
+static void
+send_scrollback(struct client *p)
+{
+	int written, len;
+	if (scroll_full)
+	{
+		len = SCROLLSIZE - scroll_end;
+		written = 0;
+		while (written < len)
+		{
+			int n = write(p->fd, scrollback + scroll_end + written,
+			              len - written);
+			if (n > 0)
+				written += n;
+			else if (n == 0 || errno != EINTR)
+				break;
+		}
+	}
+	len = scroll_end;
+	written = 0;
+	while (written < len)
+	{
+		int n = write(p->fd, scrollback + written, len - written);
+		if (n > 0)
+			written += n;
+		else if (n == 0 || errno != EINTR)
+			break;
+	}
+}
+
 /* Process activity on the pty - Input and terminal changes are sent out to
 ** the attached clients. If the pty goes away, we die. */
 static void
@@ -320,6 +383,8 @@ top:
 	/* Try again if nothing happened. */
 	if (!FD_ISSET(s, &readfds) && nclients == 0)
 		goto top;
+
+  write_scrollback( buf, len );
 }
 
 /* Process activity on the control socket */
@@ -348,6 +413,8 @@ control_activity(int s)
 	if (p->next)
 		p->next->pprev = &p->next;
 	*(p->pprev) = p;
+
+  send_scrollback( p );
 }
 
 /* Process activity from a client. */