Autodetect variables to export (#32)

Instead of using a hardcoded list of variables to export, use a more
smart approach: first inspect the original environment, then look what
variables have changed as a result of "vcvarsall.bat" invocation, and
export all those new values.

Also, log the variables we export to be more debugging-friendly.

index.js

@@ -11,21 +11,6 @@ const VERSIONS = ['2019', '2017']
 
 const VSWHERE_PATH = `${PROGRAM_FILES_X86}\\Microsoft Visual Studio\\Installer`
 
-const InterestingVariables = [
-    'INCLUDE',
-    'LIB',
-    'LIBPATH',
-    'VCINSTALLDIR',
-    'Path',
-    'Platform',
-    'VisualStudioVersion',
-    'UCRTVersion',
-    'UniversalCRTSdkDir',
-    /^VCTools/,
-    /^VSCMD_/,
-    /^WindowsSDK/i,
-]
-
 function findWithVswhere(pattern) {
     try {
         let installationPath = child_process.execSync(`vswhere -products * -latest -prerelease -property installationPath`).toString().trim()
@@ -113,15 +98,17 @@ function main() {
         args.push('-vcvars_spectre_libs=spectre')
     }
 
-    const command = `"${findVcvarsall()}" ${args.join(' ')} && set`
-    core.debug(`Running: ${command}`)
-    const environment = child_process.execSync(command, {shell: "cmd"}).toString().split('\r\n')
+    const vcvars = `"${findVcvarsall()}" ${args.join(' ')}`
+    core.debug(`vcvars command-line: ${vcvars}`)
+
+    const old_environment = child_process.execSync(`set`, {shell: "cmd"}).toString().split('\r\n')
+    const new_environment = child_process.execSync(`${vcvars} && set`, {shell: "cmd"}).toString().split('\r\n')
 
     // If vsvars.bat is given an incorrect command line, it will print out
     // an error and *still* exit successfully. Parse out errors from output
     // which don't look like environment variables, and fail if appropriate.
     var failed = false
-    for (let line of environment) {
+    for (let line of new_environment) {
         if (line.match(/^\[ERROR.*\]/)) {
             failed = true
             // Don't print this particular line which will be confusing in output.
@@ -135,15 +122,32 @@ function main() {
         throw new Error('invalid parameters')
     }
 
-    for (let string of environment) {
+    // Convert old environment lines into a dictionary for easier lookup.
+    let old_env_vars = {}
+    for (let string of old_environment) {
         const [name, value] = string.split('=')
-        for (let pattern of InterestingVariables) {
-            if (name.match(pattern)) {
-                core.exportVariable(name, value)
-                break
-            }
+        old_env_vars[name] = value
+    }
+
+    // Now look at the new environment and export everything that changed.
+    // These are the variables set by vsvars.bat. Also export everything
+    // that was not there during the first sweep: those are new variables.
+    core.startGroup('Environment variables')
+    for (let string of new_environment) {
+        // vsvars.bat likes to print some fluff at the beginning.
+        // Skip lines that don't look like environment variables.
+        if (!string.includes('=')) {
+            continue;
+        }
+        const [name, new_value] = string.split('=')
+        const old_value = old_env_vars[name]
+        // For new variables "old_value === undefined".
+        if (new_value !== old_value) {
+            core.info(`Setting ${name}`)
+            core.exportVariable(name, new_value)
         }
     }
+    core.endGroup()
 
     core.info(`Configured Developer Command Prompt`)
 }