Read served files with pread

Over TLS a file is copied out of its mapping in this process, so one
truncated while it was being served raised SIGBUS and took the whole
server down. Read it with pread, which comes up short instead.

httplib.h

@@ -3988,6 +3988,10 @@ public:
   bool is_open() const;
   size_t size() const;
   const char *data() const;
+  // Copies up to len bytes from offset into buf and returns how many. Unlike
+  // reading data(), it gives fewer, down to 0, for a file that has shrunk
+  // since it was opened, rather than a SIGBUS.
+  size_t read(size_t offset, char *buf, size_t len) const;
 
 private:
 #if defined(_WIN32)
@@ -6520,6 +6524,22 @@ inline const char *mmap::data() const {
   return is_open_empty_file ? "" : static_cast<const char *>(addr_);
 }
 
+inline size_t mmap::read(size_t offset, char *buf, size_t len) const {
+  if (offset >= size_) { return 0; }
+  len = (std::min)(len, size_ - offset);
+#if defined(_WIN32)
+  // Windows does not let a mapped file be truncated.
+  std::memcpy(buf, static_cast<const char *>(addr_) + offset, len);
+  return len;
+#else
+  ssize_t n;
+  do {
+    n = ::pread(fd_, buf, len, static_cast<off_t>(offset));
+  } while (n < 0 && errno == EINTR);
+  return n > 0 ? static_cast<size_t>(n) : 0;
+#endif
+}
+
 inline void mmap::close() {
 #if defined(_WIN32)
   if (addr_) {
@@ -8991,9 +9011,12 @@ inline void set_file_content_provider(Response &res,
   res.set_content_provider(
       m->size(), content_type,
       [m](size_t offset, size_t length, DataSink &sink) -> bool {
-        // Never read past the mapping.
-        if (offset >= m->size()) { return false; }
-        sink.write(m->data() + offset, (std::min)(length, m->size() - offset));
+        // Never read past the file, which ends the response early if it has
+        // shrunk since it was opened.
+        char buf[CPPHTTPLIB_SEND_BUFSIZ];
+        auto n = m->read(offset, buf, (std::min)(length, sizeof(buf)));
+        if (n == 0) { return false; }
+        sink.write(buf, n);
         return true;
       });