Eliminate buffering in stream.transform()

lib/streams.js

@@ -217,12 +217,19 @@ function transformWithCancel(customCancel) {
 
 /**
  * Transform a stream using helper functions which are called on each chunk, and on stream close, respectively.
+ * Takes an optional queuing strategy for the resulting readable stream;
+ * see https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream#queuingstrategy.
+ * By default, the queueing strategy is non-buffering. When the `process`
+ * function is asynchronous, it may be useful to pass a buffering
+ * queuing strategy to enable multiple chunks to be processed in parallel;
+ * e.g. pass `{ highWaterMark: 4 }` to process up to 4 chunks in parallel.
  * @param {ReadableStream|Uint8array|String} input
  * @param {Function} process
  * @param {Function} finish
+ * @param {Object} queuingStrategy
  * @returns {ReadableStream|Uint8array|String}
  */
-function transform(input, process = () => undefined, finish = () => undefined) {
+function transform(input, process = () => undefined, finish = () => undefined, queuingStrategy = { highWaterMark: 0 }) {
   if (isArrayStream(input)) {
     const output = new ArrayStream();
     (async () => {
@@ -243,24 +250,47 @@ function transform(input, process = () => undefined, finish = () => undefined) {
     return output;
   }
   if (isStream(input)) {
-    return transformRaw(input, {
-      async transform(value, controller) {
-        try {
-          const result = await process(value);
-          if (result !== undefined) controller.enqueue(result);
-        } catch(e) {
-          controller.error(e);
-        }
+    let reader;
+    let allDone = false;
+    return new ReadableStream({
+      start() {
+        reader = input.getReader();
       },
-      async flush(controller) {
+      async pull(controller) {
+        if (allDone) {
+          controller.close();
+          input.releaseLock();
+          return;
+        }
         try {
-          const result = await finish();
-          if (result !== undefined) controller.enqueue(result);
-        } catch(e) {
+          // 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) {
+            const { value, done } = await reader.read();
+            allDone = done;
+            const result = await (done ? finish : process)(value);
+            if (result !== undefined) {
+              controller.enqueue(result);
+              return; // `pull` will get called again
+            }
+            if (done) {
+              // If `finish` didn't return a chunk to enqueue, call
+              // `close` here. Otherwise, it will get called in the
+              // next call to `pull`, above (since `allDone == true`).
+              controller.close();
+              input.releaseLock();
+              return;
+            }
+          }
+        } catch (e) {
           controller.error(e);
         }
+      },
+      async cancel(reason) {
+        await reader.cancel(reason);
       }
-    });
+    }, queuingStrategy);
   }
   const result1 = process(input);
   const result2 = finish();
@@ -438,19 +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 = [];