From 12bdab620f29373c0db31a2be67be1bc16e02af0 Mon Sep 17 00:00:00 2001
From: Amaan Qureshi <git@amaanq.com>
Date: Mon, 9 Mar 2026 18:21:29 -0400
Subject: [PATCH] repl: fix history truncation of long lines

Editline's `read_history` uses `fgets(buf, 256, fp)` internally, splitting
any line longer than 255 characters into multiple history entries. This
commit bypasses it with `FdSource::readLine` and per-line `add_history`
calls, which handle arbitrary lengths. GNU readline's `read_history` has no
such limit and is left as-is, and `write_history` already writes full lines
via `fprintf` so it needs no change either.
---
 src/libcmd/repl-interacter.cc | 22 +++++++++++++++++++++-
 src/libutil/file-system.cc    |  2 +-
 tests/repl-completion.nix     | 25 ++++++++++++++++++++++++-
 3 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/src/libcmd/repl-interacter.cc b/src/libcmd/repl-interacter.cc
index 8eebeec25b5e..50c3dcea3e42 100644
--- a/src/libcmd/repl-interacter.cc
+++ b/src/libcmd/repl-interacter.cc
@@ -22,6 +22,7 @@ extern "C" {
 #include "nix/util/finally.hh"
 #include "nix/cmd/repl-interacter.hh"
 #include "nix/util/file-system.hh"
+#include "nix/util/serialise.hh"
 #include "nix/cmd/repl.hh"
 #include "nix/util/environment-variables.hh"
 
@@ -124,8 +125,27 @@ ReadlineLikeInteracter::Guard ReadlineLikeInteracter::init(detail::ReplCompleter
     }
 #if !USE_READLINE
     el_hist_size = 1000;
-#endif
+    // editline's read_history uses a fixed 256-byte buffer (SCREEN_INC),
+    // which silently splits lines longer than 255 characters into separate
+    // history entries. Read the file ourselves to avoid the length limit.
+    auto fd = openFileReadonly(historyFile);
+    if (!fd) {
+        NativeSysError err("opening file %s", PathFmt(historyFile));
+        if (!err.is(std::errc::no_such_file_or_directory) && !err.is(std::errc::not_a_directory))
+            logWarning(err.info());
+    } else {
+        try {
+            FdSource source(fd.get());
+            while (true)
+                add_history(source.readLine().c_str());
+        } catch (EndOfFile &) {
+        } catch (SystemError & e) {
+            logWarning(e.info());
+        }
+    }
+#else
     read_history(historyFile.string().c_str());
+#endif
     auto oldRepl = curRepl;
     curRepl = repl;
     Guard restoreRepl([oldRepl] { curRepl = oldRepl; });
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index d2983ec7d321..16e4e6ce8dee 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -227,7 +227,7 @@ std::string readFile(const std::filesystem::path & path)
 {
     auto fd = openFileReadonly(path);
     if (!fd)
-        throw NativeSysError("opening file %1%", PathFmt(path));
+        throw NativeSysError("opening file %s", PathFmt(path));
     return readFile(fd.get());
 }
 
diff --git a/tests/repl-completion.nix b/tests/repl-completion.nix
index 9ea45a7b1b03..853f87077df3 100644
--- a/tests/repl-completion.nix
+++ b/tests/repl-completion.nix
@@ -50,7 +50,9 @@ runCommand "repl-completion"
       }
       exit 0
     '';
-    passAsFile = [ "expectScript" ];
+    passAsFile = [
+      "expectScript"
+    ];
   }
   ''
     export NIX_STORE=$TMPDIR/store
@@ -60,5 +62,26 @@ runCommand "repl-completion"
 
     nix-store --init
     expect $expectScriptPath
+
+    # Write a 300-char line to the history file, then run a REPL session
+    # that reads it back (read_history) and writes it out (write_history).
+    histFile=$HOME/.local/share/nix/repl-history
+    mkdir -p "$(dirname "$histFile")"
+    printf '%0300d\n' 0 | tr '0' 'a' > "$histFile"
+    echo "short" >> "$histFile"
+
+    # unbuffer allocates a pty so nix repl runs the interactive
+    # ReadlineLikeInteracter path (read_history on init, write_history
+    # on exit). Plain piped input skips history entirely.
+    echo ":q" | unbuffer -p nix repl --offline --extra-experimental-features nix-command 2>/dev/null || true
+
+    # Verify the long line survived the read/write cycle.
+    maxLen=$(awk '{ print length }' "$histFile" | sort -rn | head -1)
+    if [ "$maxLen" -lt 300 ]; then
+      echo "FAIL: long history line was truncated (max length: $maxLen)"
+      exit 1
+    fi
+    echo "Long history line preserved (length: $maxLen)."
+
     touch $out
   ''