From f0b4f9e5bb263ea63207d9355c87d02f9f04efed Mon Sep 17 00:00:00 2001
From: Rob Landley <rob@landley.net>
Date: Thu, 2 Feb 2023 21:19:48 -0600
Subject: [PATCH] Make head lseek backwards to return unused data from seekable
 inputs.

---
 tests/head.test   | 2 ++
 toys/posix/head.c | 7 +++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tests/head.test b/tests/head.test
index 729dfa7e..e23b70d7 100755
--- a/tests/head.test
+++ b/tests/head.test
@@ -32,3 +32,5 @@ testcmd "-c 3" "-c 3" "one" "" "one\ntwo"
 testcmd "-c bigger than input" "-c 3" "a" "" "a"
 testcmd "-c 3 -n 1" "-c 3 -n 1" "one\n" "" "one\ntwo"
 testcmd "-n 1 -c 3" "-n 1 -c 3" "one" "" "one\ntwo"
+testing "unget" 'while read i; do echo =$i; head -n 1; done < input' \
+  '=one\ntwo\n=three\nfour\n=five\n' 'one\ntwo\nthree\nfour\nfive\n' ''
diff --git a/toys/posix/head.c b/toys/posix/head.c
index d5c0700c..04e1658c 100644
--- a/toys/posix/head.c
+++ b/toys/posix/head.c
@@ -34,7 +34,7 @@ GLOBALS(
 
 static void do_head(int fd, char *name)
 {
-  long i, len, lines=TT.n, bytes=TT.c;
+  long i = 0, len = 0, lines = TT.n, bytes = TT.c;
 
   if ((toys.optc > 1 && !FLAG(q)) || FLAG(v)) {
     // Print an extra newline for all but the first file
@@ -50,11 +50,14 @@ static void do_head(int fd, char *name)
     if (bytes) {
       i = bytes >= len ? len : bytes;
       bytes -= i;
-    } else for(i=0; i<len;) if (toybuf[i++] == '\n' && !--lines) break;
+    } else for(i = 0; i<len;) if (toybuf[i++] == '\n' && !--lines) break;
 
     xwrite(1, toybuf, i);
   }
 
+  // attempt to unget extra data
+  if (len>i) lseek(fd, i-len, SEEK_CUR);
+
   TT.file_no++;
 }