Link twice with EXPORTED_FUNCTIONS='@exported_list.txt'

[carlopi]

For the statically linked extension build path, this should be basically irrelevant, moving from inlined list of functions in lib/CMakeLists.txt to a file hosting the list, now at lib/base_exported_list.txt.

For the loadable extension path, the more interesting, this move to a two step linking process:

1. first, duckdb-wasm is built regularly (as now)
2. then, duckdb-wasm is linked, also here as currently done, passing -s MAIN_MODULE=1, this means there is limited pruning of functions, but on the other side this means the built WebAssembly module will offer the duckdb-functionality, skipping the needs to duplicate that in each extension
3. a list of exported functions is obtained. Here solution is an hack, but I will review a (duck-based?) tool to do this more properly with less bash glue.
4. duckdb-wasm is linked a second time, now with -s MAIN_MODULE=2 and -s EXPORTED_FUNCTIONS=@exported_list.txt. MAIN_MODULE=2 means more aggressive dead code elimination is now perfomed, but keeping intact the list exported at the WebAssembly level (so that say the v-table for all relevant class is not actually pruned)
5. usual cleanup of JavaScript-level export is performed

This works around some limitation in current tooling, and allows to bring in specific informations that are otherwise not propagated.

Result should be moving the on-the-wire (=compressed) size of duckdb-wasm from 7.2 MB of Wasm and 384 KB of JS to X MB of Wasm and Y of JS.

There is a follow up planned to further reduce via splitting the main duckdb-wasm module in two parts, but that's for the follow-up.

[Tishj]

warning: /opt/homebrew/Cellar/emscripten/4.0.6/libexec/src/postamble.js: invalid item in EXPORTED_RUNTIME_METHODS: createDyncallWrapper
em++: warning: warnings in JS library compilation [-Wjs-compiler]
[100%] Built target duckdb_wasm
+ '[' yes == yes ']'
+ make TARGET=eh update_exported_list
cd build/relsize/eh && wasm2wat duckdb_wasm.wasm --enable-all -o duckdb-wasm.wat
/bin/sh: wasm2wat: command not found
make[1]: *** [update_exported_list] Error 127
make: *** [build_loadable] Error 2

Fixed by:

brew install wabt

But this isn't mentioned anywhere outside of the github action files