Allow non-TTY stdin watch mode (postcss#448)

The presence of stdin doesn't necessarily mean there's an allocated
tty. This breaks watch mode in non-TTY stdin contexts (e.g. docker,
foreman, etc). A simple process.stdin.isTTY check would theoretically
be enough but unfortunately, subprocesses don't have the same API,
which are used extensively to test via calls to `spawn`.

A simple solution is to inject an env var dependency where we tell
the process that it's indeed a TTY-allocated process and so, watch mode
with exit handling is good to go.

Co-authored-by: Thiago Brandão <194487+thiagobrandam@users.noreply.github.com>

Author: 0xradical

index.js

@@ -61,7 +61,13 @@ let configFile
 if (argv.env) process.env.NODE_ENV = argv.env
 if (argv.config) argv.config = path.resolve(argv.config)
 
-if (argv.watch) {
+let { isTTY } = process.stdin
+
+if (process.env.FORCE_IS_TTY === 'true') {
+  isTTY = true
+}
+
+if (argv.watch && isTTY) {
   process.stdin.on('end', () => process.exit(0))
   process.stdin.resume()
 }

test/watch.js

@@ -205,13 +205,18 @@ testCb('--watch does exit on closing stdin (Ctrl-D/EOF)', (t) => {
 
   const cp = spawn(`./index.js test/fixtures/a.css -o ${tmp()} -w --no-map`, {
     shell: true,
+    env: {
+      ...process.env,
+      FORCE_IS_TTY: true,
+    },
   })
 
   cp.on('error', t.end)
   cp.on('exit', (code) => {
     t.is(code, 0)
     t.end()
   })
+
   cp.stdin.end()
 })