beef up write syscall tests

Added a few tests for write(2) and pwrite(2)

1. Regular Files

For write(2)

- write zero bytes should not move the offset
- write non-zero bytes should increment the offset the exact amount
- write non-zero bytes after a lseek() should move the offset the exact amount after the seek
- write non-zero bytes with O_APPEND should move the offset the exact amount after original EOF

For pwrite(2), offset is not affected when

- pwrite zero bytes
- pwrite non-zero bytes

For EOF, added a test asserting the EOF (indicated by lseek(SEEK_END)) is updated properly after writing non-zero bytes

2. Symlink

Added one pwite64() call for symlink that is written as a counterpart of the existing test using pread64()

1 files changed, 29 insertions, 0 deletions

diff --git a/test/syscalls/linux/symlink.cc b/test/syscalls/linux/symlink.cc
index a17ff62e9..aa1f32c85 100644
--- a/test/syscalls/linux/symlink.cc
+++ b/test/syscalls/linux/symlink.cc
@@ -218,6 +218,35 @@ TEST(SymlinkTest, PreadFromSymlink) {
   EXPECT_THAT(unlink(linkname.c_str()), SyscallSucceeds());
 }
 
+TEST(SymlinkTest, PwriteToSymlink) {
+  std::string name = NewTempAbsPath();
+  int fd;
+  ASSERT_THAT(fd = open(name.c_str(), O_CREAT, 0644), SyscallSucceeds());
+  ASSERT_THAT(close(fd), SyscallSucceeds());
+
+  std::string linkname = NewTempAbsPath();
+  ASSERT_THAT(symlink(name.c_str(), linkname.c_str()), SyscallSucceeds());
+
+  ASSERT_THAT(fd = open(linkname.c_str(), O_WRONLY), SyscallSucceeds());
+
+  const int data_size = 10;
+  const std::string data = std::string(data_size, 'a');
+  EXPECT_THAT(pwrite64(fd, data.c_str(), data.size(), 0), SyscallSucceedsWithValue(data.size()));
+
+  ASSERT_THAT(close(fd), SyscallSucceeds());
+  ASSERT_THAT(fd = open(name.c_str(), O_RDONLY), SyscallSucceeds());
+
+  char buf[data_size+1];
+  EXPECT_THAT(pread64(fd, buf, data.size(), 0), SyscallSucceeds());
+  buf[data.size()] = '\0';
+  EXPECT_STREQ(buf, data.c_str());
+
+  ASSERT_THAT(close(fd), SyscallSucceeds());
+
+  EXPECT_THAT(unlink(name.c_str()), SyscallSucceeds());
+  EXPECT_THAT(unlink(linkname.c_str()), SyscallSucceeds());
+}
+
 TEST(SymlinkTest, SymlinkAtDegradedPermissions_NoRandomSave) {
   // Drop capabilities that allow us to override file and directory permissions.
   ASSERT_NO_ERRNO(SetCapability(CAP_DAC_OVERRIDE, false));