From b1a0467ffd656f7ec5c2c18677a7efcb989d4600 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Thu, 26 Jan 2023 03:56:19 +0800
Subject: [PATCH] fixes #21273; fixes an io.readLine off by one bug [backport
 1.0] (#21276)

fixes #21273; io.readLine off by one

(cherry picked from commit c4d3d650baa88820dadeafa81ca75b6e495084f2)
---
 lib/system/io.nim    |  7 ++++---
 tests/stdlib/tio.nim | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/lib/system/io.nim b/lib/system/io.nim
index 2df5107ba4..f88ea80127 100644
--- a/lib/system/io.nim
+++ b/lib/system/io.nim
@@ -478,11 +478,12 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
       if last > 0 and line[last-1] == '\c':
         line.setLen(last-1)
         return last > 1 or fgetsSuccess
-        # We have to distinguish between two possible cases:
+      elif last > 0 and line[last-1] == '\0':
+        # We have to distinguish among three possible cases:
         # \0\l\0 => line ending in a null character.
         # \0\l\l => last line without newline, null was put there by fgets.
-      elif last > 0 and line[last-1] == '\0':
-        if last < pos + sp - 1 and line[last+1] != '\0':
+        #   \0\l => last line without newline, null was put there by fgets.
+        if last >= pos + sp - 1 or line[last+1] != '\0': # bug #21273
           dec last
       line.setLen(last)
       return last > 0 or fgetsSuccess
diff --git a/tests/stdlib/tio.nim b/tests/stdlib/tio.nim
index 0da64f9c26..fc1329c469 100644
--- a/tests/stdlib/tio.nim
+++ b/tests/stdlib/tio.nim
@@ -35,3 +35,21 @@ block: # readChars
         break
     doAssert n2s == @[2,2,2,1,0]
     doAssert s2 == s
+
+
+import std/strutils
+
+block: # bug #21273
+  let FILE = buildDir / "D20220119T134305.txt"
+
+  let hex = "313632313920313632343720313632353920313632363020313632393020323035363520323037323120323131353020323239393820323331303520323332313020323332343820323332363820"
+
+
+  writeFile FILE, parseHexStr(hex)
+
+  doAssert readFile(FILE).toHex == hex
+
+  let f = open(FILE)
+  var s = newString(80)
+  while f.readLine(s):
+    doAssert s.toHex == hex