Resolving Missing AMALGAMATION_LINE_MACROS in Regenerated SQLite Configure Scripts

Understanding the AMALGAMATION_LINE_MACROS Configuration Gap

Core Problem: AMALGAMATION_LINE_MACROS Disappears After configure Regeneration

The central issue arises when rebuilding SQLite (version 3.37.0+) from source using autotools. Developers modifying the build pipeline or redistributing SQLite may attempt to regenerate the configure script using autoreconf. However, after regeneration, the AMALGAMATION_LINE_MACROS environment variable handling vanishes from the configure script. This leads to two observable failures:

1. Missing Substitution in Makefile.in: The Makefile.in template contains references to @AMALGAMATION_LINE_MACROS@, which are not replaced during configuration if the configure script lacks logic to handle this macro.
2. Build-Time Errors: Compilation fails with errors indicating unresolved variables in the Makefile, particularly when generating the amalgamation (single-file SQLite build).

The AMALGAMATION_LINE_MACROS macro controls whether #line directives are embedded in the amalgamation code. These directives aid debugging by preserving original source file/line number information. When the macro is absent from configure, developers lose the ability to enable/disable these directives via standard build configuration methods.

Autotools Chain Breakdown: configure.ac to configure

SQLite’s build system relies on autotools (autoconf, automake) to generate platform-specific configuration scripts. The configure.ac file defines macros and logic that autoconf processes into the configure script. If a configuration parameter (like AMALGAMATION_LINE_MACROS) is not explicitly declared in configure.ac, it will not propagate to the generated configure script.

In SQLite 3.37.0+, the AMALGAMATION_LINE_MACROS logic was initially added directly to the configure script without corresponding updates to configure.ac. This creates a divergence: the configure script contains custom logic for the macro, but configure.ac does not. Consequently, regenerating configure with autotools (via autoreconf) produces a script lacking the macro’s handling code. This omission breaks builds that depend on regenerated configure scripts, such as Linux distribution packaging systems or custom CI/CD pipelines.

Root Causes of AMALGAMATION_LINE_MACROS Handling Loss

Cause 1: Missing AC_SUBST Declaration in configure.ac

The AC_SUBST macro in autoconf scripts declares variables that should be substituted in template files (e.g., Makefile.in). If AMALGAMATION_LINE_MACROS is not declared with AC_SUBST, autoconf will not:

• Include the variable in the configure script’s substitution list.
• Export the variable to config.status, which performs substitutions during make.

The original SQLite implementation bypassed this by directly modifying the configure script, which is not sustainable. Autotools-generated configure scripts are meant to be derived from configure.ac, so manual edits are overwritten during regeneration.

Cause 2: Environment Variable Handling Without Autoconf Integration

The AMALGAMATION_LINE_MACROS was designed to be set via the environment (e.g., env amalgamation_line_macros=yes ./configure). However, environment variables not declared in configure.ac are not automatically recognized by the configure script. Autoconf requires explicit declarations using AC_ARG_VAR or AC_SUBST to persist such variables. Without these declarations, the regenerated configure script has no knowledge of AMALGAMATION_LINE_MACROS, leading to silent drops of the variable.

Cause 3: Incomplete Autotools Artifact Synchronization

SQLite’s build system does not fully synchronize autotools artifacts (e.g., aclocal.m4, configure, config.h.in). When developers modify build logic (e.g., adding new macros), they must update configure.ac and rerun the full autotools chain (autoreconf -fiv). Failure to do so leaves generated files inconsistent. The initial implementation of AMALGAMATION_LINE_MACROS skipped this step, relying on manual configure edits instead of proper autoconf integration.

Repairing and Stabilizing AMALGAMATION_LINE_MACROS Handling

Step 1: Update configure.ac with AC_SUBST Declarations

To make AMALGAMATION_LINE_MACROS survive autoreconf, add the following to configure.ac:

AC_ARG_VAR([AMALGAMATION_LINE_MACROS], [Enable line macros in amalgamation])  
AC_SUBST([AMALGAMATION_LINE_MACROS])  

This instructs autoconf to:

• Recognize AMALGAMATION_LINE_MACROS as a significant variable.
• Include it in the substitution list for template files.

Step 2: Apply the Upstream Patch or Manual configure.ac Edits

The SQLite team addressed this in commit 8ad1fcaa1b734e32, which updates configure.ac to properly declare AMALGAMATION_LINE_MACROS. Developers should:

1. Check if their SQLite version includes this commit.
2. If not, manually apply the equivalent changes to configure.ac:

--- configure.ac  
+++ configure.ac  
@@ -XXX,XX +XXX,XX @@  
 AC_SUBST(TARGET_EXEEXT)  
+AC_SUBST(AMALGAMATION_LINE_MACROS)  
+AMALGAMATION_LINE_MACROS=--linemacros=0  
+AC_ARG_VAR(AMALGAMATION_LINE_MACROS, [Enable line macros in amalgamation])  

Step 3: Regenerate configure with Correct Autotools Invocation

After modifying configure.ac, regenerate the configure script using:

autoreconf -fiv  

The flags ensure:

• -f: Force regeneration, ignoring timestamps.
• -i: Install missing auxiliary files (e.g., config.sub, install-sh).
• -v: Verbose output for debugging.

Verify that AMALGAMATION_LINE_MACROS appears in the generated configure script:

grep AMALGAMATION_LINE_MACROS configure  

The output should show variable assignments and substitutions.

Step 4: Set AMALGAMATION_LINE_MACROS During Configuration

When running configure, explicitly set the variable via the environment or command line:

env AMALGAMATION_LINE_MACROS=--linemacros=1 ./configure --enable-all  

Or pre-export it:

export AMALGAMATION_LINE_MACROS=--linemacros=1  
./configure --enable-all  

To disable line macros, set --linemacros=0 or omit the variable.

Step 5: Validate Makefile Substitutions

After configuration, inspect the generated Makefile for correct substitution:

grep LINEMACROS Makefile  

The output should include a line like:

AMALGAMATION_LINE_MACROS = --linemacros=1  

If absent, verify that configure.ac was updated and autoreconf was run correctly.

Step 6: Handle Legacy Build Systems Without configure.ac Modifications

For environments where modifying configure.ac is impractical (e.g., downstream packaging), apply a patch to the generated configure script. The patch should inject the missing AMALGAMATION_LINE_MACROS handling:

--- configure  
+++ configure  
@@ -XXX,XX +XXX,XX @@  
 # Initializations.  
+AMALGAMATION_LINE_MACROS=--linemacros=0  
+  
 ac_initial=:  
 ac_site=  

This hardcodes the default value but allows override via environment variables.

Step 7: Integrate with Build Scripts and CI/CD Pipelines

Ensure build scripts explicitly handle AMALGAMATION_LINE_MACROS:

# Example shell script snippet  
AMALG_FLAG="--linemacros=1"  
if [ "$ENABLE_LINE_MACROS" = "0" ]; then  
  AMALG_FLAG="--linemacros=0"  
fi  

export AMALGAMATION_LINE_MACROS="$AMALG_FLAG"  
./configure --enable-all  
make  

Step 8: Monitor Upstream SQLite Releases

SQLite’s build system evolves, and future releases may alter autotools integration. Track the SQLite changelog and configure.ac for updates. Rebase local patches when upgrading to newer versions to avoid conflicts.

Step 9: Debugging Autotools Regeneration Issues

If AMALGAMATION_LINE_MACROS still disappears after regeneration:

1. Check autoconf version: Ensure autoconf >= 2.69 is used. Older versions may mishandle macros.
2. Inspect config.log: Look for errors during autoreconf or configure execution.
3. Clean build artifacts: Run make distclean or delete autom4te.cache before regenerating.

Step 10: Alternatives to Autotools Regeneration

For projects requiring frequent configure regeneration, consider:

• Maintaining a Fork with Fixed configure.ac: Apply the AC_SUBST patch once and merge upstream changes periodically.
• Pre-Building configure Scripts: Distribute a pre-generated configure script with the macro handling intact, bypassing autotools.

By addressing the root cause (missing AC_SUBST in configure.ac) and following the steps above, developers can stabilize AMALGAMATION_LINE_MACROS handling across autotools regenerations, ensuring consistent builds with or without line directives in the SQLite amalgamation.