def text_fd_to_metric_families(fd):
    """Parse Prometheus text format from a file descriptor.

    This is a laxer parser than the main Go parser,
    so successful parsing does not imply that the parsed
    text meets the specification.

    Yields Metric's.
    """
    name = None
    allowed_names = []
    eof = False

    seen_metrics = set()

    def build_metric(name, documentation, typ, unit, samples):
        if name in seen_metrics:
            raise ValueError("Duplicate metric: " + name)
        seen_metrics.add(name)
        if typ is None:
            typ = 'unknown'
        if documentation is None:
            documentation = ''
        if unit is None:
            unit = ''
        if unit and not name.endswith("_" + unit):
            raise ValueError("Unit does not match metric name: " + name)
        if unit and typ in ['info', 'stateset']:
            raise ValueError("Units not allowed for this metric type: " + name)
        if typ in ['histogram', 'gaugehistogram']:
            _check_histogram(samples, name)
        metric = Metric(name, documentation, typ, unit)
        # TODO: check labelvalues are valid utf8
        metric.samples = samples
        return metric

    for line in fd:
        if line[-1] == '\n':
            line = line[:-1]

        if eof:
            raise ValueError("Received line after # EOF: " + line)

        if line == '# EOF':
            eof = True
        elif line.startswith('#'):
            parts = line.split(' ', 3)
            if len(parts) < 4:
                raise ValueError("Invalid line: " + line)
            if parts[2] == name and samples:
                raise ValueError("Received metadata after samples: " + line)
            if parts[2] != name:
                if name is not None:
                    yield build_metric(name, documentation, typ, unit, samples)
                # New metric
                name = parts[2]
                unit = None
                typ = None
                documentation = None
                group = None
                seen_groups = set()
                group_timestamp = None
                group_timestamp_samples = set()
                samples = []
                allowed_names = [parts[2]]

            if parts[1] == 'HELP':
                if documentation is not None:
                    raise ValueError("More than one HELP for metric: " + line)
                if len(parts) == 4:
                    documentation = _unescape_help(parts[3])
                elif len(parts) == 3:
                    raise ValueError("Invalid line: " + line)
            elif parts[1] == 'TYPE':
                if typ is not None:
                    raise ValueError("More than one TYPE for metric: " + line)
                typ = parts[3]
                if typ == 'untyped':
                    raise ValueError("Invalid TYPE for metric: " + line)
                allowed_names = {
                    'counter': ['_total', '_created'],
                    'summary': ['_count', '_sum', '', '_created'],
                    'histogram': ['_count', '_sum', '_bucket', '_created'],
                    'gaugehistogram': ['_gcount', '_gsum', '_bucket'],
                    'info': ['_info'],
                }.get(typ, [''])
                allowed_names = [name + n for n in allowed_names]
            elif parts[1] == 'UNIT':
                if unit is not None:
                    raise ValueError("More than one UNIT for metric: " + line)
                unit = parts[3]
            else:
                raise ValueError("Invalid line: " + line)
        else:
            sample = _parse_sample(line)
            if sample.name not in allowed_names:
                if name is not None:
                    yield build_metric(name, documentation, typ, unit, samples)
                # Start an unknown metric.
                name = sample.name
                documentation = None
                unit = None
                typ = 'unknown'
                samples = []
                group = None
                group_timestamp = None
                group_timestamp_samples = set()
                seen_groups = set()
                allowed_names = [sample.name]

            if typ == 'stateset' and name not in sample.labels:
                raise ValueError("Stateset missing label: " + line)
            if (typ in ['histogram', 'gaugehistogram'] and name + '_bucket' == sample.name
                    and (float(sample.labels.get('le', -1)) < 0
                         or sample.labels['le'] != floatToGoString(sample.labels['le']))):
                raise ValueError("Invalid le label: " + line)
            if (typ == 'summary' and name == sample.name
                    and (not (0 <= float(sample.labels.get('quantile', -1)) <= 1)
                         or sample.labels['quantile'] != floatToGoString(sample.labels['quantile']))):
                raise ValueError("Invalid quantile label: " + line)

            g = tuple(sorted(_group_for_sample(sample, name, typ).items()))
            if group is not None and g != group and g in seen_groups:
                raise ValueError("Invalid metric grouping: " + line)
            if group is not None and g == group:
                if (sample.timestamp is None) != (group_timestamp is None):
                    raise ValueError("Mix of timestamp presence within a group: " + line)
                if group_timestamp is not None and group_timestamp > sample.timestamp and typ != 'info':
                    raise ValueError("Timestamps went backwards within a group: " + line)
            else:
                group_timestamp_samples = set()

            series_id = (sample.name, tuple(sorted(sample.labels.items())))
            if sample.timestamp != group_timestamp or series_id not in group_timestamp_samples:
                # Not a duplicate due to timestamp truncation.
                samples.append(sample)
            group_timestamp_samples.add(series_id)

            group = g
            group_timestamp = sample.timestamp
            seen_groups.add(g)

            if typ == 'stateset' and sample.value not in [0, 1]:
                raise ValueError("Stateset samples can only have values zero and one: " + line)
            if typ == 'info' and sample.value != 1:
                raise ValueError("Info samples can only have value one: " + line)
            if typ == 'summary' and name == sample.name and sample.value < 0:
                raise ValueError("Quantile values cannot be negative: " + line)
            if sample.name[len(name):] in ['_total', '_sum', '_count', '_bucket', '_gcount', '_gsum'] and math.isnan(
                    sample.value):
                raise ValueError("Counter-like samples cannot be NaN: " + line)
            if sample.name[len(name):] in ['_total', '_sum', '_count', '_bucket', '_gcount',
                                           '_gsum'] and sample.value < 0:
                raise ValueError("Counter-like samples cannot be negative: " + line)
            if sample.exemplar and not (
                    typ in ['histogram', 'gaugehistogram']
                    and sample.name.endswith('_bucket')):
                raise ValueError("Invalid line only histogram/gaugehistogram buckets can have exemplars: " + line)

    if name is not None:
        yield build_metric(name, documentation, typ, unit, samples)

    if not eof:
        raise ValueError("Missing # EOF at end")