Fix stdin decetion in child-processes

There’s no way to accuratly detect *if* text is going to be
piped in, especially when using child-processes (e.g., in gulp)
This fixes that by only throwing errors when the CLI is
sure nothing is going to be piped in.

See the below issue for more info.

Closes GH-58.

lib/cli/file-set-pipeline/stdin.js

@@ -13,14 +13,19 @@
  */
 
 var debug = require('debug')('mdast:cli:file-set-pipeline:stdin');
+var fs = require('fs');
 var toVFile = require('to-vfile');
 var concat = require('concat-stream');
 
 /*
  * Constants.
  */
 
-var expextPipeIn = !process.stdin.isTTY;
+var isTTY = process.stdin.isTTY;
+var isFIFO = fs.fstatSync(0).isFIFO();
+
+var definitelyTTY = isTTY === true || isFIFO === true;
+var expextPipeIn = !isTTY;
 
 /**
  * Read from standard in.
@@ -36,7 +41,7 @@ function stdin(program, callback) {
     if (program.files.length) {
         debug('Ignoring stdin');
 
-        if (expextPipeIn) {
+        if (definitelyTTY && expextPipeIn) {
             err = new Error('mdast does not accept both files and stdin');
         } else if (program.filePath) {
             err = new Error(
@@ -50,7 +55,7 @@ function stdin(program, callback) {
         return;
     }
 
-    if (!expextPipeIn) {
+    if (definitelyTTY && !expextPipeIn) {
         callback(new Error('No input'));
 
         return;

test/cli.sh

@@ -72,6 +72,10 @@ it "Should accept stdin"
     cat $markdown | $COMMAND > /dev/null 2>&1
     assert $? 0
 
+it "Should accept stdin as herestring"
+    $COMMAND <<< $(cat $markdown) > /dev/null 2>&1
+    assert $? 0
+
 it "Should fail without input"
     $COMMAND > /dev/null 2>&1
     assert $? 1
@@ -88,6 +92,10 @@ it "Should fail on stdin and files"
     cat $markdown | $COMMAND $markdown > /dev/null 2>&1
     assert $? 1
 
+it "Should ignore herestring when with files (for now)"
+    $COMMAND $markdown <<< "$(cat $markdown)" > /dev/null 2>&1
+    assert $? 0
+
 #
 # `--ast`.
 #