add custom_extensions function in a new module

for custom (non-standard) SQLite3 extension functions that may be
added in the future

with some additional explanatory comments added to the CoffeeScript

as proposed in: https://github.com/kripken/sql.js/pull/320

Makefile

@@ -65,7 +65,7 @@ EMFLAGS_DEBUG = \
 	-s ASSERTIONS=1 \
 	-O1
 
-BITCODE_FILES = out/sqlite3.bc out/extension-functions.bc
+BITCODE_FILES = out/sqlite3.bc out/extension-functions.bc out/custom_extensions.bc
 
 OUTPUT_WRAPPER_FILES = src/shell-pre.js src/shell-post.js
 
@@ -166,6 +166,10 @@ out/extension-functions.bc: sqlite-src/$(SQLITE_AMALGAMATION)/$(EXTENSION_FUNCTI
 	mkdir -p out
 	$(EMCC) $(CFLAGS) -s LINKABLE=1 sqlite-src/$(SQLITE_AMALGAMATION)/extension-functions.c -o $@
 
+out/custom_extensions.bc: src/custom_extensions/custom_extensions.c
+	mkdir -p out
+	$(EMCC) $(CFLAGS) -Isqlite-src/$(SQLITE_AMALGAMATION) -s LINKABLE=1 $^ -o $@
+
 # TODO: This target appears to be unused. If we re-instatate it, we'll need to add more files inside of the JS folder
 # module.tar.gz: test package.json AUTHORS README.md dist/sql-asm.js
 # 	tar --create --gzip $^ > $@

README.md

@@ -2,6 +2,7 @@
 
 custom build by [@brodybits (Christopher J. Brody)](https://github.com/brodybits) with some updates including:
 
+- custom functions: TBD
 - support FTS4, FTS5, R-Tree, and JSON1
 - some more OMIT build flags to omit some obsolete SQLite features
 - `dist/sql-asm-debug.js` now built with `-s ALLOW_MEMORY_GROWTH=1` to allow the allocated memory buffer to grow as needed

src/api.coffee

@@ -240,9 +240,13 @@ class Database
     constructor: (data) ->
         @filename = 'dbfile_' + (0xffffffff*Math.random()>>>0)
         if data? then FS.createDataFile '/', @filename, data, true, true
+        # open the database and register extension functions
         @handleError sqlite3_open @filename, apiTemp
         @db = getValue(apiTemp, 'i32')
+        # register built-in extension functions:
         RegisterExtensionFunctions(@db)
+        # register any custom (non-standard) extension functions:
+        custom_extensions(@db)
         @statements = {} # A list of all prepared statements of the database
         @functions = {} # A list of all user function of the database (created by create_function call)
 
@@ -404,6 +408,8 @@ class Database
         @functions={}
         @handleError sqlite3_close_v2 @db
         binaryDb = FS.readFile @filename, encoding:'binary'
+        # just open the database
+        # (no need to register extension functions)
         @handleError sqlite3_open @filename, apiTemp
         @db = getValue apiTemp, 'i32'
         binaryDb

src/custom_extensions/custom_extensions.c

@@ -0,0 +1,8 @@
+#include "sqlite3.h"
+
+/** FUTURE TBD Build this module with any custom extension module(s) included */
+
+int custom_extensions(sqlite3 * db)
+{
+  return SQLITE_OK;
+}

src/exported_functions.json

@@ -38,5 +38,6 @@
 "_sqlite3_result_int",
 "_sqlite3_result_int64",
 "_sqlite3_result_error",
+"_custom_extensions",
 "_RegisterExtensionFunctions"
 ]

src/exports.coffee

@@ -59,6 +59,9 @@ sqlite3_result_int64 = Module['cwrap'] 'sqlite3_result_int64', '', ['number', 'n
 sqlite3_result_error = Module['cwrap'] 'sqlite3_result_error', '', ['number', 'string', 'number']
 RegisterExtensionFunctions = Module['cwrap'] 'RegisterExtensionFunctions', 'number', ['number']
 
+## Support custom db extensions
+custom_extensions = Module['cwrap'] 'custom_extensions', 'number', ['number']
+
 # Export the API
 this['SQL'] = {'Database':Database}
 Module[i] = this['SQL'][i] for i of this['SQL']