libutil: convert drainFD to a Bytes generator

the `*Source` name is a slight misnomer since we do also have a
Source type, but we can probably live with this for time being.

Change-Id: I54eb2e59a4009014e324797f16b80b962759c7d3

src/libutil/file-descriptor.cc

@@ -81,12 +81,12 @@ std::string drainFD(int fd, bool block, const size_t reserveSize)
     // the parser needs two extra bytes to append terminating characters, other users will
     // not care very much about the extra memory.
     StringSink sink(reserveSize + 2);
-    drainFD(fd, sink, block);
+    sink << drainFDSource(fd, block);
     return std::move(sink.s);
 }
 
 
-void drainFD(int fd, Sink & sink, bool block)
+Generator<Bytes> drainFDSource(int fd, bool block)
 {
     // silence GCC maybe-uninitialized warning in finally
     int saved = 0;
@@ -115,7 +115,7 @@ void drainFD(int fd, Sink & sink, bool block)
                 throw SysError("reading from file");
         }
         else if (rd == 0) break;
-        else sink({(char *) buf.data(), (size_t) rd});
+        else co_yield std::span{(char *) buf.data(), (size_t) rd};
     }
 }
 

src/libutil/file-descriptor.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "error.hh"
+#include "generator.hh"
 
 namespace nix {
 
@@ -35,7 +36,7 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts = true);
  */
 std::string drainFD(int fd, bool block = true, const size_t reserveSize=0);
 
-void drainFD(int fd, Sink & sink, bool block = true);
+Generator<Bytes> drainFDSource(int fd, bool block = true);
 
 class AutoCloseFD
 {

src/libutil/processes.cc

@@ -315,7 +315,7 @@ void runProgram2(const RunOptions & options)
     out.writeSide.close();
 
     if (options.standardOut)
-        drainFD(out.readSide.get(), *options.standardOut);
+        *options.standardOut << drainFDSource(out.readSide.get());
 
     /* Wait for the child to finish. */
     int status = pid.wait();