Clarify extract-resize-extract operation ordering

docs/api-resize.md

@@ -214,7 +214,7 @@ Extract/crop a region of the image.
 
 - Use `extract` before `resize` for pre-resize extraction.
 - Use `extract` after `resize` for post-resize extraction.
-- Use `extract` before and after for both.
+- Use `extract` twice and `resize` once for extract-then-resize-then-extract in a fixed operation order.
 
 
 **Throws**:

lib/resize.js

@@ -250,6 +250,9 @@ function resize (widthOrOptions, height, options) {
   if (isResizeExpected(this.options)) {
     this.options.debuglog('ignoring previous resize options');
   }
+  if (this.options.widthPost !== -1) {
+    this.options.debuglog('operation order will be: extract, resize, extract');
+  }
   if (is.defined(widthOrOptions)) {
     if (is.object(widthOrOptions) && !is.defined(options)) {
       options = widthOrOptions;
@@ -437,7 +440,7 @@ function extend (extend) {
  *
  * - Use `extract` before `resize` for pre-resize extraction.
  * - Use `extract` after `resize` for post-resize extraction.
- * - Use `extract` before and after for both.
+ * - Use `extract` twice and `resize` once for extract-then-resize-then-extract in a fixed operation order.
  *
  * @example
  * sharp(input)

test/unit/extract.js

@@ -319,5 +319,16 @@ describe('Partial image extraction', function () {
       s.extract(options);
       assert.strictEqual(warningMessage, 'ignoring previous extract options');
     });
+
+    it('Multiple extract+resize emits warning', () => {
+      let warningMessage = '';
+      const s = sharp();
+      s.on('warning', function (msg) { warningMessage = msg; });
+      const options = { top: 0, left: 0, width: 1, height: 1 };
+      s.extract(options).extract(options);
+      assert.strictEqual(warningMessage, '');
+      s.resize(1);
+      assert.strictEqual(warningMessage, 'operation order will be: extract, resize, extract');
+    });
   });
 });