exec: include stream name and WARN: in outputstream log messages

If an error happens when streaming the output, always include the stream name
and WARN in the related log message.

Some internal variable names are also shortened.

internal/exec/command.go

@@ -124,11 +124,11 @@ func newMultiWriter(w ...io.Writer) io.Writer {
 }
 
 func (c *Cmd) startOutputStreamLogging(name string, useColorStderrColorfn bool) (io.Writer, func() error) {
-	logReader, logWriter := io.Pipe()
-	scannerTerminated := make(chan struct{})
+	r, w := io.Pipe()
+	done := make(chan struct{})
 
 	go func() {
-		sc := bufio.NewScanner(logReader)
+		sc := bufio.NewScanner(r)
 		// use a bigger buf to make it more unlikely that it will fail because of very long lines
 		sc.Buffer([]byte{}, outputStreamLineReaderBufSiz)
 
@@ -142,32 +142,32 @@ func (c *Cmd) startOutputStreamLogging(name string, useColorStderrColorfn bool)
 
 		if err := sc.Err(); err != nil {
 			if errors.Is(err, bufio.ErrTooLong) {
-				c.logf("streaming output failed, shown output might be complete, lines are too long, requiring newlines after latest %dBytes in output\n",
-					outputStreamLineReaderBufSiz)
+				c.logf("WARN: streaming %s output failed, shown output might be complete, lines are too long, requiring newlines after latest %dBytes in output\n",
+					name, outputStreamLineReaderBufSiz)
 			} else {
-				c.logf("streaming output failed, shown output might be complete: %s\n", err)
+				c.logf("WARN: streaming %s output failed, shown output might be complete: %s\n", name, err)
 			}
 			// We do not Close the logReader with an error because
 			// it would cause the MultiWriter to fail on all subsequent Write() operations for all streams,
 			// the command could fail because it could not write to
 			// stderr/sdout anymore.
 
 			// drain the logReader until the end, to prevent blocks of the MultiWriter
-			_, cpErr := io.Copy(io.Discard, logReader)
+			_, cpErr := io.Copy(io.Discard, r)
 			if cpErr != nil {
-				logReader.CloseWithError(fmt.Errorf("draining stream reader after line scanning failed, also failed: %w", errors.Join(err, cpErr)))
+				r.CloseWithError(fmt.Errorf("draining %s stream reader after line scanning failed, also failed: %w", name, errors.Join(err, cpErr)))
 			}
 
 		}
-		close(scannerTerminated)
+		close(done)
 	}()
 
-	return logWriter, func() error {
-		err := logWriter.Close()
-		<-scannerTerminated
+	return w, func() error {
+		err := w.Close()
+		<-done
 
 		if err != nil {
-			return fmt.Errorf("%s: closing output stream failed: %w", name, err)
+			return fmt.Errorf("closing %s output stream failed: %w", name, err)
 		}
 
 		return nil