Limited sharp concurrency to reduce memory fragmentation

refs lovell/sharp#955 (comment)

- we've seen Ghost hogging memory whenever images are uploaded
- it seems to be due to an issue with memory fragmentation, because a
  heap snapshot of a container after the memory grows shows nothing in
  JS code
- we've been using jemalloc in production but it still seems to occur
- this change has been suggested on the referenced thread in order to
  improve fragmentation on top of using jemalloc
- can easily revert if it causes issues

Author: daniellockyer

packages/image-transform/lib/transform.js

@@ -86,6 +86,9 @@ const unsafeResizeFromBuffer = async (originalBuffer, options = {}) => {
     // Disable the internal libvips cache - https://sharp.pixelplumbing.com/api-utility#cache
     sharp.cache(false);
 
+    // Limit the concurrency of sharp tasks - https://sharp.pixelplumbing.com/api-utility#concurrency
+    sharp.concurrency(1);
+
     // It is safe to set animated to true for all formats, because if the input image doesn't contain animation
     // nothing will change.
     let animated = options.animated ?? true;

packages/image-transform/test/transform.test.js

@@ -94,6 +94,7 @@ describe('Transform', function () {
             });
 
             sharp.cache = sinon.stub().returns({});
+            sharp.concurrency = sinon.stub().returns({});
 
             testUtils.modules.mockNonExistentModule('sharp', sharp);
         });