[libc][File] Fix a bug under fseek(..., SEEK_CUR).

libc/src/__support/File/CMakeLists.txt

if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS}_file.cpp)
   add_object_library(
     platform_file
     SRCS
       ${LIBC_TARGET_OS}_file.cpp
     DEPENDS
       .file
       libc.include.errno
       libc.include.fcntl
+      libc.include.stdio
       libc.include.sys_syscall
       libc.src.__support.OSUtil.osutil
       libc.src.errno.errno
   )
 endif()
 

libc/src/__support/File/file.cpp

 //===--- Implementation of a platform independent file data structure -----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "file.h"
 
 #include "src/__support/CPP/ArrayRef.h"
 
 #include <errno.h>
+#include <stdio.h>
 #include <stdlib.h>
 
 namespace __llvm_libc {
 
 size_t File::write(const void *data, size_t len) {
   FileLock lock(this);
 
   if (!write_allowed()) {

 int File::seek(long offset, int whence) {
   FileLock lock(this);
   if (prev_op == FileOp::WRITE && pos > 0) {
     size_t transferred_size = platform_write(this, buf, pos);
     if (transferred_size < pos) {
       err = true;
       return -1;
     }
+  } else if (prev_op == FileOp::READ && whence == SEEK_CUR) {
+    // More data could have been read out from the platform file than was
+    // required. So, we have to adjust the offset we pass to platform seek
+    // function. Note that read_limit >= pos is always true.
+    offset -= (read_limit - pos);
   }
   pos = read_limit = 0;
   prev_op = FileOp::SEEK;
   // Reset the eof flag as a seek might move the file positon to some place
   // readable.
   eof = false;
   return platform_seek(this, offset, whence);
 }

libc/test/src/__support/File/file_test.cpp

TEST(LlvmLibcFileTest, ReadOnly) {
   EXPECT_EQ(f->write(initial_content, sizeof(initial_content)), size_t(0));
   EXPECT_TRUE(f->error());
   EXPECT_NE(errno, 0);
   errno = 0;
 
   ASSERT_EQ(f->close(), 0);
 }
 
+TEST(LlvmLibcFileTest, ReadSeekCurAndRead) {
+  const char initial_content[] = "1234567890987654321";
+  constexpr size_t FILE_BUFFER_SIZE = sizeof(initial_content);
+  char file_buffer[FILE_BUFFER_SIZE];
+  StringFile *f = new_string_file(file_buffer, FILE_BUFFER_SIZE, 0, false, "r");
+  f->reset_and_fill(initial_content, sizeof(initial_content));
+
+  constexpr size_t READ_SIZE = 5;
+  char data[READ_SIZE];
+  data[READ_SIZE - 1] = '\0';
+  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "1234");
+  ASSERT_EQ(f->seek(5, SEEK_CUR), 0);
+  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "0987");
+  ASSERT_EQ(f->seek(-5, SEEK_CUR), 0);
+  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "9098");
+  ASSERT_EQ(f->close(), 0);
+}
+
 TEST(LlvmLibcFileTest, AppendOnly) {
   const char initial_content[] = "1234567890987654321";
   const char write_data[] = "append";
   constexpr size_t FILE_BUFFER_SIZE = sizeof(write_data) * 3 / 2;
   char file_buffer[FILE_BUFFER_SIZE];
   StringFile *f = new_string_file(file_buffer, FILE_BUFFER_SIZE, 0, false, "a");
   f->reset_and_fill(initial_content, sizeof(initial_content));
 

libc/test/src/__support/File/platform_file_test.cpp

TEST(LlvmLibcPlatformFileTest, LargeFile) {
 
   // Reading more data should trigger EOF.
   ASSERT_EQ(file->read(data, 1), size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
 }
 
+TEST(LlvmLibcPlatformFileTest, ReadSeekCurAndRead) {
+  constexpr char FILENAME[] = "testdata/read_seek_cur_and_read.test";
+  File *file = __llvm_libc::openfile(FILENAME, "w");
+  ASSERT_FALSE(file == nullptr);
+  constexpr char CONTENT[] = "1234567890987654321";
+  ASSERT_EQ(sizeof(CONTENT) - 1, file->write(CONTENT, sizeof(CONTENT) - 1));
+  ASSERT_EQ(0, file->close());
+
+  file = __llvm_libc::openfile(FILENAME, "r");
+  ASSERT_FALSE(file == nullptr);
+
+  constexpr size_t READ_SIZE = 5;
+  char data[READ_SIZE];
+  data[READ_SIZE - 1] = '\0';
+  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "1234");
+  ASSERT_EQ(file->seek(5, SEEK_CUR), 0);
+  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "0987");
+  ASSERT_EQ(file->seek(-5, SEEK_CUR), 0);
+  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_STREQ(data, "9098");
+
+  ASSERT_EQ(file->close(), 0);
+}
+
 TEST(LlvmLibcPlatformFileTest, IncorrectOperation) {
   constexpr char FILENAME[] = "testdata/incorrect_operation.test";
   char data[1] = {123};
 
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
   ASSERT_EQ(file->read(data, 1), size_t(0)); // Cannot read
   ASSERT_FALSE(file->iseof());