From 55972db8668b1be2a65ef857590b5c9376961b42 Mon Sep 17 00:00:00 2001
From: kssenii <sumarokovakseniia@mail.ru>
Date: Fri, 1 Oct 2021 11:38:58 +0300
Subject: [PATCH] Small fix and more comments

---
 ...ynchronousReadIndirectBufferFromRemoteFS.h |  2 ++
 src/Disks/ReadIndirectBufferFromWebServer.cpp | 27 +++++++++++++------
 src/IO/ReadBufferFromS3.cpp                   | 13 +++++++++
 src/IO/ReadWriteBufferFromHTTP.h              | 12 +++++++++
 4 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/src/Disks/AsynchronousReadIndirectBufferFromRemoteFS.h b/src/Disks/AsynchronousReadIndirectBufferFromRemoteFS.h
index 1095a22091f..afe58ae1c6a 100644
--- a/src/Disks/AsynchronousReadIndirectBufferFromRemoteFS.h
+++ b/src/Disks/AsynchronousReadIndirectBufferFromRemoteFS.h
@@ -24,6 +24,8 @@ namespace DB
 * Buffers chain for diskWeb:
 * AsynchronousIndirectReadBufferFromRemoteFS -> ReadBufferFromRemoteFS ->
 * -> ReadIndirectBufferFromWebServer -> ReadBufferFromHttp -> ReadBufferFromIStream.
+*
+* We pass either `memory` or `prefetch_buffer` through all this chain and return it back.
 */
 class AsynchronousReadIndirectBufferFromRemoteFS : public ReadBufferFromFileBase
 {
diff --git a/src/Disks/ReadIndirectBufferFromWebServer.cpp b/src/Disks/ReadIndirectBufferFromWebServer.cpp
index f5ef1efae02..02a38c1d546 100644
--- a/src/Disks/ReadIndirectBufferFromWebServer.cpp
+++ b/src/Disks/ReadIndirectBufferFromWebServer.cpp
@@ -71,20 +71,31 @@ bool ReadIndirectBufferFromWebServer::nextImpl()
 
     if (impl)
     {
-        if (!use_external_buffer)
+        if (use_external_buffer)
         {
+            /**
+            * use_external_buffer -- means we read into the buffer which
+            * was passed to us from somewhere else. We do not check whether
+            * previously returned buffer was read or not, because this branch
+            * means we are prefetching data, each nextImpl() call we can fill
+            * a different buffer.
+            */
+            impl->set(internal_buffer.begin(), internal_buffer.size());
+            assert(working_buffer.begin() != nullptr);
+            assert(!internal_buffer.empty());
+        }
+        else
+        {
+            /**
+            * impl was initialized before, pass position() to it to make
+            * sure there is no pending data which was not read, becuase
+            * this branch means we read sequentially.
+            */
             impl->position() = position();
             assert(!impl->hasPendingData());
         }
     }
 
-    if (use_external_buffer)
-    {
-        impl->set(internal_buffer.begin(), internal_buffer.size());
-        assert(working_buffer.begin() != nullptr);
-        assert(!internal_buffer.empty());
-    }
-
     WriteBufferFromOwnString error_msg;
     for (size_t i = 0; (i < max_tries) && !successful_read && !next_result; ++i)
     {
diff --git a/src/IO/ReadBufferFromS3.cpp b/src/IO/ReadBufferFromS3.cpp
index 8b815021186..9dbe8cfd578 100644
--- a/src/IO/ReadBufferFromS3.cpp
+++ b/src/IO/ReadBufferFromS3.cpp
@@ -52,6 +52,13 @@ bool ReadBufferFromS3::nextImpl()
     {
         if (!use_external_buffer)
         {
+            /**
+            * use_external_buffer -- means we read into the buffer which
+            * was passed to us from somewhere else. We do not check whether
+            * previously returned buffer was read or not, because this branch
+            * means we are prefetching data, each nextImpl() call we can fill
+            * a different buffer.
+            */
             impl->position() = position();
             assert(!impl->hasPendingData());
         }
@@ -65,6 +72,12 @@ bool ReadBufferFromS3::nextImpl()
 
     if (use_external_buffer)
     {
+        /**
+        * use_external_buffer -- means we read into the buffer which
+        * was passed to us from somewhere else. We do not check whether
+        * previously returned buffer was read or not, because this branch
+        * means we are prefetching data.
+        */
         impl->set(internal_buffer.begin(), internal_buffer.size());
         assert(working_buffer.begin() != nullptr);
         assert(!internal_buffer.empty());
diff --git a/src/IO/ReadWriteBufferFromHTTP.h b/src/IO/ReadWriteBufferFromHTTP.h
index 2cf2c587922..33edcdb2e42 100644
--- a/src/IO/ReadWriteBufferFromHTTP.h
+++ b/src/IO/ReadWriteBufferFromHTTP.h
@@ -213,12 +213,24 @@ namespace detail
 
             if (use_external_buffer)
             {
+                /**
+                * use_external_buffer -- means we read into the buffer which
+                * was passed to us from somewhere else. We do not check whether
+                * previously returned buffer was read or not, because this branch
+                * means we are prefetching data, each nextImpl() call we can fill
+                * a different buffer.
+                */
                 impl->set(internal_buffer.begin(), internal_buffer.size());
                 assert(working_buffer.begin() != nullptr);
                 assert(!internal_buffer.empty());
             }
             else
             {
+                /**
+                * impl was initialized before, pass position() to it to make
+                * sure there is no pending data which was not read, becuase
+                * this branch means we read sequentially.
+                */
                 if (!working_buffer.empty())
                     impl->position() = position();
             }