rgw: fix interface compliance of RGWCivetWeb::write_data().

This is a potential fix for partial writes and handling of error
codes that might come from mg_write() of the CivetWeb front-end.
None of the potential issues has been observed in testing.

The commit also documents the same aspects regarding the ASIO
and FastCGI front-ends.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>

src/rgw/rgw_asio_client.cc

@@ -86,8 +86,12 @@ size_t RGWAsioClientIO::write_data(const char* const buf,
   if (ec) {
     derr << "write_data failed: " << ec.message() << dendl;
     throw rgw::io::Exception(ec.value(), std::system_category());
+  } else {
+    /* According to the documentation of boost::asio::write if there is
+     * no error (signalised by ec), then bytes == len. We don't need to
+     * take care of partial writes in such situation. */
+    return bytes;
   }
-  return bytes;
 }
 
 size_t RGWAsioClientIO::read_data(char* const buf, const size_t max)

src/rgw/rgw_civetweb.cc

@@ -14,14 +14,18 @@
 
 size_t RGWCivetWeb::write_data(const char *buf, size_t len)
 {
-  const int ret = mg_write(conn, buf, len);
-  if (ret == 0) {
-    /* didn't send anything, error out */
-    throw rgw::io::Exception(EIO, std::system_category());
-  } else if (ret < 0) {
-    throw rgw::io::Exception(-ret, std::system_category());
+  while (len) {
+    const int ret = mg_write(conn, buf, len);
+    if (ret < 0 || ! ret) {
+      /* According to the documentation of mg_write() it always returns -1 on
+       * error. The details aren't available, so we will just throw EIO. Same
+       * goes to 0 that is associated with writing to a closed connection. */
+      throw rgw::io::Exception(EIO, std::system_category());
+    } else {
+      len -= static_cast<size_t>(ret);
+    }
   }
-  return ret;
+  return len;
 }
 
 RGWCivetWeb::RGWCivetWeb(mg_connection* const conn, const int port)
@@ -36,7 +40,7 @@ size_t RGWCivetWeb::read_data(char *buf, size_t len)
 {
   const int ret = mg_read(conn, buf, len);
   if (ret < 0) {
-    throw rgw::io::Exception(-ret, std::system_category());
+    throw rgw::io::Exception(EIO, std::system_category());
   }
   return ret;
 }

src/rgw/rgw_fcgi.cc

@@ -6,7 +6,9 @@
 
 size_t RGWFCGX::write_data(const char* const buf, const size_t len)
 {
-  const auto ret = FCGX_PutStr(buf, len, fcgx->out);
+ /* According to the documentation of FCGX_PutStr if there is no error
+ * (signalised by negative return value), then always ret == len. */
+ const auto ret = FCGX_PutStr(buf, len, fcgx->out);
   if (ret < 0) {
     throw rgw::io::Exception(-ret, std::system_category());
   }