fixes #21273; fixes an io.readLine off by one bug [backport 1.0] (#21276)

fixes #21273; io.readLine off by one

1 files changed, 4 insertions, 3 deletions

diff --git a/lib/std/syncio.nim b/lib/std/syncio.nim
index 46f113450..83b2d6d13 100644
--- a/lib/std/syncio.nim
+++ b/lib/std/syncio.nim
@@ -484,11 +484,12 @@ 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