SQLite CLI Control-Z Suspension Failure in Precompiled Linux Binaries

Line Editing Library Configuration and Terminal Signal Handling Mismatch

Issue Overview

The core problem involves the SQLite command-line interface (CLI) failing to suspend execution when the user sends a Control-Z signal on Linux systems. This issue is observed exclusively in precompiled SQLite CLI binaries (32-bit builds) distributed via the SQLite download page. Local builds of the same SQLite source code (both 32-bit and 64-bit) handle Control-Z correctly, suspending the process as expected. The anomalous behavior manifests as:

• The CLI ignores Control-Z as a suspension signal.
• The terminal displays a Unicode replacement character (�) instead of the standard ^Z echo.
• Input containing � triggers a syntax error if not manually erased.

Key observations from debugging:

1. Line editing libraries (e.g., readline, editline, or linenoise) are central to the issue.
2. The precompiled binary uses linenoise (a minimal line-editing library) instead of readline.
3. Terminal settings (specifically ISIG flag handling) differ between precompiled and locally built CLIs.
4. Compilation flags such as THREADSAFE=0, MUTEX_OMIT, and -DHAVE_READLINE=0 correlate with the faulty behavior.

The root cause is traced to linenoise modifying terminal settings to disable signal handling (via stty -isig), which prevents the kernel from intercepting Control-Z. This configuration is absent in builds using readline or other line-editing libraries.

Critical Differences in Precompiled vs. Local Builds

The discrepancy arises from compile-time configuration choices and library dependencies in the precompiled SQLite CLI:

1. Line Editing Library Selection

   • The precompiled binary uses linenoise, which disables terminal signal handling to simplify line-editing logic. This library explicitly sets:

     rawmode.c_lflag &= ~(ISIG | ICANON | ECHO | IEXTEN);
     

     The ISIG bitmask controls whether the terminal processes signals like Control-Z (SIGTSTP) or Control-C (SIGINT). Disabling ISIG forces the CLI to handle these signals internally, which linenoise does not implement.

   • Local builds typically link against readline or libedit, which preserve ISIG and allow the kernel to handle suspension signals.

2. Thread Safety and Mutex Configuration
   The precompiled binary includes:

   +THREADSAFE=0  
   +MUTEX_OMIT  
   -MUTEX_PTHREADS  
   

   While these flags disable threading support, they indirectly affect signal handling. Single-threaded builds may bypass certain signal-safe I/O routines, exacerbating conflicts with linenoise’s terminal settings.

3. Compiler and Library Version Mismatches
   The precompiled binary uses an older compiler (gcc-5.2.0 vs. gcc-11.3.0), which might link against outdated system libraries or header definitions. For example, older versions of linenoise lack fixes for terminal signal handling (e.g., antirez/linenoise#141).

Resolving Terminal Conflicts and Reclaiming Control-Z Handling

Step 1: Diagnose Line Editing Library Usage
Determine which line-editing library the SQLite CLI is using:

ldd sqlite3 | grep -E 'readline|editline|linenoise'

• No output indicates the CLI uses linenoise (statically linked).
• Presence of libreadline.so or libedit.so confirms usage of those libraries.

Step 2: Temporarily Restore ISIG Handling
Force-enable terminal signal processing during an active SQLite session:

1. Identify the terminal device:

   tty
   # Output: /dev/pts/0
   

2. Enable ISIG without exiting SQLite:

   stty -F /dev/pts/0 isig
   

   Test Control-Z: The CLI should now suspend.

Step 3: Rebuild SQLite with Correct Line-Editing Library
To permanently resolve the issue, compile SQLite with readline or editline support:

1. Install dependencies:

   sudo apt-get install libreadline-dev libncurses-dev
   

2. Configure and build:

   ./configure --enable-readline --enable-threadsafe
   make
   

   Verify configuration:

   grep 'HAVE_READLINE' config.h
   # Should output: #define HAVE_READLINE 1
   

Step 4: Patch or Replace Linenoise
If using linenoise is mandatory:

1. Apply the ISIG preservation patch from empirical-lang#84:

   // In linenoise.c, replace:
   raw.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
   // With:
   raw.c_lflag &= ~(ECHO | ICANON | IEXTEN);
   

2. Rebuild SQLite with the patched linenoise.

Step 5: Signal Handling Workarounds
For unsanitized precompiled binaries, use wrapper scripts to enforce ISIG:

#!/bin/bash
stty isig
/path/to/sqlite3 "$@"

Final Validation
Confirm terminal settings before and after launching SQLite:

stty -a | grep isig
# Correct output: isig

If isig is absent, the CLI or its line-editing library has altered terminal flags.

This guide systematically addresses the interplay between SQLite’s line-editing dependencies, terminal configuration, and thread safety flags. By aligning compilation settings with the host environment’s signal-handling expectations, users can restore standard job control behavior.