fix: prevent deadlock on single-core systems and with empty arrays in utils/parallel

lib/node_modules/@stdlib/utils/parallel/lib/defaults.js

@@ -21,6 +21,7 @@
 // MODULES //
 
 var numCPUs = require( '@stdlib/os/num-cpus' );
+var max = require( '@stdlib/math/base/special/fast/max' );
 
 
 // MAIN //
@@ -36,12 +37,17 @@ var numCPUs = require( '@stdlib/os/num-cpus' );
 * // returns {...}
 */
 function defaults() {
+	var n;
+
+	// Accommodate single-core systems by ensuring at least 1 worker and 1 concurrent script:
+	n = max( 1, numCPUs - 1 );
+
 	return {
 		// Number of workers:
-		'workers': numCPUs - 1,
+		'workers': n,
 
 		// Number of scripts to execute concurrently:
-		'concurrency': numCPUs - 1,
+		'concurrency': n,
 
 		// Executable file/command:
 		'cmd': 'node',

lib/node_modules/@stdlib/utils/parallel/lib/main.js

@@ -94,11 +94,11 @@ function parallel() {
 	if ( !isFunction( clbk ) ) {
 		throw new TypeError( format( 'invalid argument. Callback argument must be a function. Value: `%s`.', clbk ) );
 	}
-	// Prevent the number of concurrent scripts exceeding the number of actual scripts to run.
+	// Prevent the number of concurrent scripts exceeding the number of actual scripts to run:
 	if ( opts.concurrency > files.length ) {
-		opts.concurrency = files.length;
+		opts.concurrency = files.length || 1;
 	}
-	// Prevent the number of workers exceeding the number of concurrent scripts (excess capacity), as some workers would never be allocated scripts to run and always be idle.
+	// Prevent the number of workers exceeding the number of concurrent scripts (excess capacity), as some workers would never be allocated scripts to run and always be idle:
 	if ( opts.workers > opts.concurrency ) {
 		opts.workers = opts.concurrency;
 	}

lib/node_modules/@stdlib/utils/parallel/test/test.main.js

@@ -305,6 +305,45 @@ tape( 'if the number of workers is greater than the concurrency, the function se
 	}
 });
 
+tape( 'the function ensures at least one worker and a concurrency greater than or equal to unity', function test( t ) {
+	var parallel;
+
+	parallel = proxyquire( './../lib/main.js', {
+		'./node': exec,
+		'./defaults.js': mockDefaults
+	});
+
+	parallel( files(), done );
+
+	function mockDefaults() {
+		return {
+			'workers': 1,
+			'concurrency': 1,
+			'cmd': 'node',
+			'ordered': false,
+			'uid': null,
+			'gid': null,
+			'encoding': 'buffer',
+			'maxBuffer': 200 * 1024 * 1024
+		};
+	}
+
+	function done( error ) {
+		if ( error ) {
+			t.ok( false, error.message );
+		} else {
+			t.ok( true, 'runs scripts successfully' );
+		}
+		t.end();
+	}
+
+	function exec( files, opts, clbk ) {
+		t.strictEqual( opts.workers, 1, 'has at least 1 worker' );
+		t.strictEqual( opts.concurrency, 1, 'has at least 1 concurrency' );
+		clbk();
+	}
+});
+
 tape( 'the function runs scripts in parallel', function test( t ) {
 	parallel( files(), done );
 	function done( error ) {