From cdaa179eeb2a66e685c2f28016816c9b966cf615 Mon Sep 17 00:00:00 2001
From: Arne Elster <arne@elster.li>
Date: Mon, 29 Nov 2021 19:56:29 +0100
Subject: [PATCH] LibCore: Fix relative seeking in IODevice

The recently introduced read buffer in IODevice broke relative seeking.
The amount of data in the buffer wouldn't get taken into account.
---
 Tests/LibCore/10kb.txt                  |  1 +
 Tests/LibCore/CMakeLists.txt            |  2 +-
 Tests/LibCore/TestLibCoreIODevice.cpp   | 32 +++++++++++++++++++++++++
 Userland/Libraries/LibCore/IODevice.cpp |  1 +
 4 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 Tests/LibCore/10kb.txt

diff --git a/Tests/LibCore/10kb.txt b/Tests/LibCore/10kb.txt
new file mode 100644
index 00000000000..d412b9c2985
--- /dev/null
+++ b/Tests/LibCore/10kb.txt
@@ -0,0 +1 @@
+0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678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9012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
\ No newline at end of file
diff --git a/Tests/LibCore/CMakeLists.txt b/Tests/LibCore/CMakeLists.txt
index b77b556bfb9..9372c37ed51 100644
--- a/Tests/LibCore/CMakeLists.txt
+++ b/Tests/LibCore/CMakeLists.txt
@@ -9,4 +9,4 @@ foreach(source IN LISTS TEST_SOURCES)
     serenity_test("${source}" LibCore)
 endforeach()
 
-install(FILES long_lines.txt DESTINATION usr/Tests/LibCore)
+install(FILES long_lines.txt 10kb.txt DESTINATION usr/Tests/LibCore)
diff --git a/Tests/LibCore/TestLibCoreIODevice.cpp b/Tests/LibCore/TestLibCoreIODevice.cpp
index 1148170f3f0..f8e4ba8080b 100644
--- a/Tests/LibCore/TestLibCoreIODevice.cpp
+++ b/Tests/LibCore/TestLibCoreIODevice.cpp
@@ -44,3 +44,35 @@ TEST_CASE(file_readline)
         EXPECT_EQ(inputData[i], outputData[i]);
     }
 }
+
+TEST_CASE(file_get_read_position)
+{
+    const String path = "10kb.txt";
+    auto file = Core::File::open(path, Core::OpenMode::ReadOnly).release_value();
+
+    const size_t step_size = 98;
+    for (size_t i = 0; i < 10240 - step_size; i += step_size) {
+        auto read_buffer = file->read(step_size);
+        EXPECT_EQ(read_buffer.size(), step_size);
+
+        for (size_t j = 0; j < read_buffer.size(); j++) {
+            EXPECT_EQ(static_cast<u32>(read_buffer[j] - '0'), (i + j) % 10);
+        }
+
+        off_t offset = 0;
+        VERIFY(file->seek(0, Core::SeekMode::FromCurrentPosition, &offset));
+        EXPECT_EQ(offset, static_cast<off_t>(i + step_size));
+    }
+
+    {
+        off_t offset = 0;
+        VERIFY(file->seek(0, Core::SeekMode::FromEndPosition, &offset));
+        EXPECT_EQ(offset, 10240);
+    }
+
+    {
+        off_t offset = 0;
+        VERIFY(file->seek(0, Core::SeekMode::SetPosition, &offset));
+        EXPECT_EQ(offset, 0);
+    }
+}
diff --git a/Userland/Libraries/LibCore/IODevice.cpp b/Userland/Libraries/LibCore/IODevice.cpp
index 1dff9e941cf..8ece8753547 100644
--- a/Userland/Libraries/LibCore/IODevice.cpp
+++ b/Userland/Libraries/LibCore/IODevice.cpp
@@ -245,6 +245,7 @@ bool IODevice::seek(i64 offset, SeekMode mode, off_t* pos)
         break;
     case SeekMode::FromCurrentPosition:
         m = SEEK_CUR;
+        offset -= m_buffered_data.size();
         break;
     case SeekMode::FromEndPosition:
         m = SEEK_END;