nix::readLine: Add eofOk parameter

src/libutil/file-descriptor.hh

@@ -77,8 +77,13 @@ void writeFull(Descriptor fd, std::string_view s, bool allowInterrupts = true);
 
 /**
  * Read a line from a file descriptor.
+ *
+ * @param fd The file descriptor to read from
+ * @param eofOk If true, return an unterminated line if EOF is reached. (e.g. the empty string)
+ *
+ * @return A line of text ending in `\n`, or a string without `\n` if `eofOk` is true and EOF is reached.
  */
-std::string readLine(Descriptor fd);
+std::string readLine(Descriptor fd, bool eofOk = false);
 
 /**
  * Write a line to a file descriptor.

src/libutil/unix/file-descriptor.cc

@@ -47,7 +47,7 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts)
 }
 
 
-std::string readLine(int fd)
+std::string readLine(int fd, bool eofOk)
 {
     std::string s;
     while (1) {
@@ -58,8 +58,12 @@ std::string readLine(int fd)
         if (rd == -1) {
             if (errno != EINTR)
                 throw SysError("reading a line");
-        } else if (rd == 0)
-            throw EndOfFile("unexpected EOF reading a line");
+        } else if (rd == 0) {
+            if (eofOk)
+                return s;
+            else
+                throw EndOfFile("unexpected EOF reading a line");
+        }
         else {
             if (ch == '\n') return s;
             s += ch;