FileUtil: mark readFull() functions as FOLLY_NODISCARD

Summary:
Update readFull() and related functions to warn if their return value is
ignored.  Callers should pretty much always need to examine the return value,
as they need this to determine how much valid data was read.

Reviewed By: yfeldblum, meyering

Differential Revision: D9526760

fbshipit-source-id: c090044256bfe4bf9907a24ec6090eeae04296b3

folly/FileUtil.h

@@ -78,10 +78,10 @@ ssize_t writevNoInt(int fd, const iovec* iov, int count);
  * readv and preadv.  The contents of iov after these functions return
  * is unspecified.
  */
-ssize_t readFull(int fd, void* buf, size_t n);
-ssize_t preadFull(int fd, void* buf, size_t n, off_t offset);
-ssize_t readvFull(int fd, iovec* iov, int count);
-ssize_t preadvFull(int fd, iovec* iov, int count, off_t offset);
+FOLLY_NODISCARD ssize_t readFull(int fd, void* buf, size_t n);
+FOLLY_NODISCARD ssize_t preadFull(int fd, void* buf, size_t n, off_t offset);
+FOLLY_NODISCARD ssize_t readvFull(int fd, iovec* iov, int count);
+FOLLY_NODISCARD ssize_t preadvFull(int fd, iovec* iov, int count, off_t offset);
 
 /**
  * Similar to readFull and preadFull above, wrappers around write() and

folly/logging/test/AsyncFileWriterTest.cpp

@@ -253,7 +253,8 @@ TEST(AsyncFileWriter, flush) {
   // Now read from the pipe
   std::vector<char> buf;
   buf.resize(paddingSize);
-  readFull(readPipe.fd(), buf.data(), buf.size());
+  auto bytesRead = readFull(readPipe.fd(), buf.data(), buf.size());
+  EXPECT_EQ(bytesRead, paddingSize);
 
   // Make sure flush completes successfully now
   std::move(future).get(10ms);