Consider unexpected undefined symbols to be errors (#86)

## Motivation for the change, related issues

We currently live with the following undefined symbol warnings when
linking with Emscripten:
```
#84 1.878 warning: undefined symbol: SharpYuvConvert (referenced by root reference (e.g. compiled C/C++ code))                                                                                              
#84 1.878 warning: undefined symbol: SharpYuvGetConversionMatrix (referenced by root reference (e.g. compiled C/C++ code))
#84 1.878 warning: undefined symbol: SharpYuvInit (referenced by root reference (e.g. compiled C/C++ code))                 
#84 1.880 warning: undefined symbol: getcontext (referenced by root reference (e.g. compiled C/C++ code))                       
#84 1.880 warning: undefined symbol: getdtablesize (referenced by root reference (e.g. compiled C/C++ code))                                                                                                
#84 1.880 warning: undefined symbol: makecontext (referenced by root reference (e.g. compiled C/C++ code))                                                                                                  
#84 1.880 warning: undefined symbol: swapcontext (referenced by root reference (e.g. compiled C/C++ code)) 
```

If any new undefined symbols come up, we would not notice them because
we set `ERROR_ON_UNDEFINED_SYMBOLS=0`. Let's change this to error so we
do not quietly miss new undefined symbols.

## Implementation details

Set ERROR_ON_UNDEFINED_SYMBOLS=1 so the Emscripten linker fails if there
are any undefined symbols.

We have been living with a number of undefined symbols. Some are
undefined because Emscripten does not provide them, and others are
undefined because we needed to link to an additional library. This PR
addresses both cases.

In order to be explicit and intentional about undefined symbols we know
and accept, this PR adds a JS library for Emscripten that explicitly
lists known undefined symbols and declares stubs for them that fail if
called. This way, we document the undefined symbols we know about and
avoid complaints from Emscripten.

## Testing Instructions (or ideally a Blueprint)

- CI

Author: brandonpayton

packages/php-wasm/compile/php/Dockerfile

@@ -196,7 +196,7 @@ RUN if [ "$WITH_GD" = "yes" ]; \
 		else \
 			echo -n ' --with-png-dir=/root/lib --with-jpeg --with-webp --with-gd --enable-gd ' >> /root/.php-configure-flags; \
 		fi; \
-		echo -n ' -I /root/lib -I /root/install -lz -lpng16 -lwebp -ljpeg ' >> /root/.emcc-php-wasm-flags; \
+		echo -n ' -I /root/lib -I /root/install -lz -lpng16 -lwebp -lsharpyuv -ljpeg ' >> /root/.emcc-php-wasm-flags; \
 	fi;
 
 # Add openssl if needed
@@ -423,7 +423,7 @@ RUN source /root/emsdk/emsdk_env.sh && \
 	export JSPI_FLAGS=$(if [ "$WITH_JSPI" = "yes" ]; then echo "-sSUPPORT_LONGJMP=wasm -fwasm-exceptions"; else echo ""; fi) && \
 	EMCC_FLAGS=" -D__x86_64__ -sSIDE_MODULE -Dsetsockopt=wasm_setsockopt -Dphp_exec=wasm_php_exec $JSPI_FLAGS " \
 	# ...which means we must skip all the libraries - they will be provided in the final linking step.
-	EMCC_SKIP="-lz -ledit -ldl -lncurses -lzip -lpng16 -lssl -lcrypto -lxml2 -lc -lm -lsqlite3 /root/lib/lib/libxml2.a /root/lib/lib/libsqlite3.so /root/lib/lib/libsqlite3.a /root/lib/lib/libsqlite3.a /root/lib/lib/libpng16.so /root/lib/lib/libwebp.a /root/lib/lib/libjpeg.a" \
+	EMCC_SKIP="-lz -ledit -ldl -lncurses -lzip -lpng16 -lssl -lcrypto -lxml2 -lc -lm -lsqlite3 /root/lib/lib/libxml2.a /root/lib/lib/libsqlite3.so /root/lib/lib/libsqlite3.a /root/lib/lib/libsqlite3.a /root/lib/lib/libpng16.so /root/lib/lib/libwebp.a /root/lib/lib/libsharpyuv.a /root/lib/lib/libjpeg.a" \
 	emmake make -j1
 
 RUN cp -v /root/php-src/.libs/libphp*.la /root/lib/libphp.la
@@ -1000,6 +1000,7 @@ export ASYNCIFY_ONLY=$'\
 # Build the final .wasm file
 RUN mkdir /root/output
 COPY ./php/phpwasm-emscripten-library.js /root/phpwasm-emscripten-library.js
+COPY ./php/phpwasm-emscripten-library-known-undefined-functions.js /root/phpwasm-emscripten-library-known-undefined-functions.js
 ARG WITH_SOURCEMAPS
 ARG WITH_DEBUG
 RUN set -euxo pipefail; \
@@ -1046,6 +1047,7 @@ RUN set -euxo pipefail; \
 	fi; \
 	emcc $OPTIMIZATION_FLAGS \
 	--js-library /root/phpwasm-emscripten-library.js \
+	--js-library /root/phpwasm-emscripten-library-known-undefined-functions.js \
 	-I .  \
 	-I ext   \
 	-I ext/json   \
@@ -1063,7 +1065,7 @@ RUN set -euxo pipefail; \
 	-s INITIAL_MEMORY=1024MB \
 	-s ALLOW_MEMORY_GROWTH=1         \
 	-s ASSERTIONS=0                  \
-	-s ERROR_ON_UNDEFINED_SYMBOLS=0  \
+	-s ERROR_ON_UNDEFINED_SYMBOLS=1  \
 	-s NODEJS_CATCH_EXIT=0           \
 	-s NODEJS_CATCH_REJECTION=0      \
 	-s INVOKE_RUN=0                  \

packages/php-wasm/compile/php/phpwasm-emscripten-library-known-undefined-functions.js

@@ -0,0 +1,30 @@
+/**
+ * This file is an Emscripten "library" file. It is included in the
+ * build "php_<major>_<minor>.js" files and implements JavaScript functions
+ * that can be called from C code.
+ *
+ * @see https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#implement-a-c-api-in-javascript
+ */
+'use strict';
+
+// Explicitly declare known, accepted undefined functions. By doing this,
+// we can enable compiler warnings for unexpected undefined symbols.
+var knownUndefinedFunctions = [
+	// Functions used by PHP for fibers but not supported by Emscripten by default.
+	// https://github.com/php/php-src/blob/747851724efcdae1502040eec4787fa7b911b6bf/Zend/zend_fibers.c#L517
+	'getcontext',
+	'makecontext',
+	'swapcontext',
+
+	// Used by PHP when attempting direct access to file descriptors.
+	// Not currently included in our build by Emscripten.
+	// If we encounter a need for it, let's investigate and fix this.
+	// https://github.com/php/php-src/blob/747851724efcdae1502040eec4787fa7b911b6bf/ext/standard/php_fopen_wrapper.c#L321
+	'getdtablesize',
+];
+
+var LibraryKnownUndefinedFunctions = {};
+for (const name of knownUndefinedFunctions) {
+	LibraryKnownUndefinedFunctions[name] = () => abort('missing function: ${name}');
+}
+mergeInto(LibraryManager.library, LibraryKnownUndefinedFunctions);

packages/php-wasm/compile/php/phpwasm-emscripten-library.js

@@ -1,7 +1,7 @@
 /**
  * This file is an Emscripten "library" file. It is included in the
- * build "php-8.0.js" file and implements JavaScript functions that
- * called from C code.
+ * build "php_<major>_<minor>.js" files and implements JavaScript functions
+ * that can be called from C code.
  *
  * @see https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#implement-a-c-api-in-javascript
  */