Eliminate buffering in slice

Author: twiss

lib/streams.js

@@ -468,19 +468,48 @@ function slice(input, begin=0, end=Infinity) {
   }
   if (isStream(input)) {
     if (begin >= 0 && end >= 0) {
+      let reader;
       let bytesRead = 0;
-      return transformRaw(input, {
-        transform(value, controller) {
-          if (bytesRead < end) {
-            if (bytesRead + value.length >= begin) {
-              controller.enqueue(slice(value, Math.max(begin - bytesRead, 0), end - bytesRead));
+      return new ReadableStream({
+        start() {
+          reader = input.getReader();
+        },
+        async pull(controller) {
+          try {
+            // Read repeatedly until we have a chunk to enqueue or until
+            // we can close the stream, as `pull` won't get called again
+            // until we call `enqueue` or `close`.
+            while (true) {
+              if (bytesRead < end) {
+                const { value, done } = await reader.read();
+                if (done) {
+                  controller.close();
+                  input.releaseLock();
+                  return;
+                }
+                let valueToEnqueue;
+                if (bytesRead + value.length >= begin) {
+                  valueToEnqueue = slice(value, Math.max(begin - bytesRead, 0), end - bytesRead);
+                }
+                bytesRead += value.length;
+                if (valueToEnqueue) {
+                  controller.enqueue(valueToEnqueue);
+                  return; // `pull` will get called again
+                }
+              } else {
+                controller.close();
+                input.releaseLock();
+                return;
+              }
             }
-            bytesRead += value.length;
-          } else {
-            controller.terminate();
+          } catch (e) {
+            controller.error(e);
           }
+        },
+        async cancel(reason) {
+          await reader.cancel(reason);
         }
-      });
+      }, { highWaterMark: 0 });
     }
     if (begin < 0 && (end < 0 || end === Infinity)) {
       let lastBytes = [];