chore: update server detection script for legacy server (#208000)

* chore: update server detection script for legacy server

* chore: address review feedback

build/gulpfile.reh.js

@@ -376,6 +376,7 @@ function packageTask(type, platform, arch, sourceFolderName, destinationFolderNa
 		if (platform === 'linux' || platform === 'alpine') {
 			result = es.merge(result,
 				gulp.src(`resources/server/bin/helpers/check-requirements-linux.sh`, { base: '.' })
+					.pipe(replace('@@SERVER_APPLICATION_NAME@@', product.serverApplicationName))
 					.pipe(rename(`bin/helpers/check-requirements.sh`))
 					.pipe(util.setExecutableBit())
 			);

resources/server/bin/helpers/check-requirements-linux.sh

@@ -5,6 +5,15 @@
 
 set -e
 
+# The script checks necessary server requirements for the classic server
+# scenarios. Currently, the script can exit with any of the following
+# 3 exit codes and should be handled accordingly on the extension side.
+#
+# 0: All requirements are met, use the default server.
+# 99: Unsupported OS, abort server startup with appropriate error message.
+# 100: Use legacy server.
+#
+
 # Do not remove this check.
 # Provides a way to skip the server requirements check from
 # outside the install flow. A system process can create this
@@ -19,6 +28,12 @@ if [ -f "/tmp/vscode-skip-server-requirements-check" ]; then
 	exit 0
 fi
 
+# Default to legacy server if the following file is present.
+if [ -f "$HOME/@@SERVER_APPLICATION_NAME@@-use-legacy" ]; then
+    echo "!!! WARNING: Using legacy server due to the presence of $HOME/@@SERVER_APPLICATION_NAME@@-use-legacy !!!"
+    exit 100
+fi
+
 ARCH=$(uname -m)
 found_required_glibc=0
 found_required_glibcxx=0
@@ -144,7 +159,7 @@ else
 fi
 
 if [ "$found_required_glibc" = "0" ] || [ "$found_required_glibcxx" = "0" ]; then
-	echo "Error: Missing required dependencies. Please refer to our FAQ https://aka.ms/vscode-remote/faq/old-linux for additional information."
+	echo "Warning: Missing required dependencies. Please refer to our FAQ https://aka.ms/vscode-remote/faq/old-linux for additional information."
 	# Custom exit code based on https://tldp.org/LDP/abs/html/exitcodes.html
-	#exit 99
+	exit 100
 fi