Re-read from start if a read from proc doesn't fill the buffer (#4716)

Followup to #4609, fixing a bug introduced in that patch.

Stats/src/main/java/io/deephaven/stats/StatsCPUCollector.java

@@ -3,6 +3,7 @@
  */
 package io.deephaven.stats;
 
+import io.deephaven.base.verify.Assert;
 import io.deephaven.configuration.Configuration;
 import io.deephaven.internal.log.LoggerFactory;
 import io.deephaven.io.logger.Logger;
@@ -223,29 +224,27 @@ private boolean peekNextLong() {
      */
     private void readToBuffer(FileChannel fileChannel, String fileName) throws IOException {
         statBuffer.clear();
-        int nb = 0;
         fileChannel.position(0);
 
+        // Filesystem entries in /proc can only be read all at once to avoid races, so using too big of a buffer isn't a
+        // problem, but too small is. Attempt to read with the current buffer. If we filled the buffer we resize it and
+        // read again from start.
         while (true) {
-            final int thisNb = fileChannel.read(statBuffer);
+            final int nb = fileChannel.read(statBuffer);
 
-            if (thisNb == -1) {
-                break;
+            if (nb <= 0) {
+                // zero bytes read is an error, -1 is EOF
+                throw new IOException(fileName + " could not be read, or was empty");
             }
-            nb += thisNb;
-            if (!statBuffer.hasRemaining()) {
-                // allocate larger read-buffer, and continue reading
-                ByteBuffer resized = ByteBuffer.allocate(statBuffer.capacity() * 2);
-                resized.put(statBuffer.flip());
-                statBuffer = resized;
+            if (statBuffer.hasRemaining()) {
+                Assert.eq(statBuffer.position(), "statBuffer.position()", nb, "nb");
+                statBuffer.flip();
+                return;
             }
-        }
 
-        if (nb == 0) {
-            throw new IOException(fileName + " zero read");
-        } else {
-            // Success, set position and limit to the data read
-            statBuffer.flip();
+            // allocate larger read-buffer, and read again from start
+            statBuffer = ByteBuffer.allocate(statBuffer.capacity() * 2);
+            fileChannel.position(0);
         }
     }