// Copyright 2021 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package exporter

import (
	"crypto/sha256"
	"database/sql"
	"errors"
	"fmt"
	"log/slog"
	"math"
	"os"
	"regexp"
	"strings"
	"time"

	"github.com/blang/semver/v4"
	"github.com/prometheus/client_golang/prometheus"
)

// Metric name parts.
const (
	// Namespace for all metrics.
	namespace = "pg"
	// Subsystems.
	exporter = "exporter"
	// Metric label used for static string data thats handy to send to Prometheus
	// e.g. version
	staticLabelName = "static"
	// Metric label used for server identification.
	serverLabelName = "server"
)

// ColumnUsage should be one of several enum values which describe how a
// queried row is to be converted to a Prometheus metric.
type ColumnUsage int

const (
	// DISCARD ignores a column
	DISCARD ColumnUsage = iota
	// LABEL identifies a column as a label
	LABEL ColumnUsage = iota
	// COUNTER identifies a column as a counter
	COUNTER ColumnUsage = iota
	// GAUGE identifies a column as a gauge
	GAUGE ColumnUsage = iota
	// MAPPEDMETRIC identifies a column as a mapping of text values
	MAPPEDMETRIC ColumnUsage = iota
	// DURATION identifies a column as a text duration (and converted to milliseconds)
	DURATION ColumnUsage = iota
	// HISTOGRAM identifies a column as a histogram
	HISTOGRAM ColumnUsage = iota
)

// UnmarshalYAML implements the yaml.Unmarshaller interface.
func (cu *ColumnUsage) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var value string
	if err := unmarshal(&value); err != nil {
		return err
	}

	columnUsage, err := stringToColumnUsage(value)
	if err != nil {
		return err
	}

	*cu = columnUsage
	return nil
}

// MappingOptions is a copy of ColumnMapping used only for parsing
type MappingOptions struct {
	Usage             string             `yaml:"usage"`
	Description       string             `yaml:"description"`
	Mapping           map[string]float64 `yaml:"metric_mapping"` // Optional column mapping for MAPPEDMETRIC
	SupportedVersions semver.Range       `yaml:"pg_version"`     // Semantic version ranges which are supported. Unsupported columns are not queried (internally converted to DISCARD).
}

// Mapping represents a set of MappingOptions
type Mapping map[string]MappingOptions

// Regex used to get the "short-version" from the postgres version field.
var versionRegex = regexp.MustCompile(`^\w+ ((\d+)(\.\d+)?(\.\d+)?)`)
var lowestSupportedVersion = semver.MustParse("9.1.0")

// Parses the version of postgres into the short version string we can use to
// match behaviors.
func parseVersion(versionString string) (semver.Version, error) {
	submatches := versionRegex.FindStringSubmatch(versionString)
	if len(submatches) > 1 {
		return semver.ParseTolerant(submatches[1])
	}
	return semver.Version{},
		errors.New(fmt.Sprintln("Could not find a postgres version in string:", versionString))
}

// ColumnMapping is the user-friendly representation of a prometheus descriptor map
type ColumnMapping struct {
	usage             ColumnUsage        `yaml:"usage"`
	description       string             `yaml:"description"`
	mapping           map[string]float64 `yaml:"metric_mapping"` // Optional column mapping for MAPPEDMETRIC
	supportedVersions semver.Range       `yaml:"pg_version"`     // Semantic version ranges which are supported. Unsupported columns are not queried (internally converted to DISCARD).
}

// UnmarshalYAML implements yaml.Unmarshaller
func (cm *ColumnMapping) UnmarshalYAML(unmarshal func(interface{}) error) error {
	type plain ColumnMapping
	return unmarshal((*plain)(cm))
}

// intermediateMetricMap holds the partially loaded metric map parsing.
// This is mainly so we can parse cacheSeconds around.
type intermediateMetricMap struct {
	columnMappings map[string]ColumnMapping
	master         bool
	cacheSeconds   uint64
}

// MetricMapNamespace groups metric maps under a shared set of labels.
type MetricMapNamespace struct {
	labels         []string             // Label names for this namespace
	columnMappings map[string]MetricMap // Column mappings in this namespace
	master         bool                 // Call query only for master database
	cacheSeconds   uint64               // Number of seconds this metric namespace can be cached. 0 disables.
}

// MetricMap stores the prometheus metric description which a given column will
// be mapped to by the collector
type MetricMap struct {
	discard    bool                              // Should metric be discarded during mapping?
	histogram  bool                              // Should metric be treated as a histogram?
	vtype      prometheus.ValueType              // Prometheus valuetype
	desc       *prometheus.Desc                  // Prometheus descriptor
	conversion func(interface{}) (float64, bool) // Conversion function to turn PG result into float64
}

// ErrorConnectToServer is a connection to PgSQL server error
type ErrorConnectToServer struct {
	Msg string
}

// Error returns error
func (e *ErrorConnectToServer) Error() string {
	return e.Msg
}

// TODO: revisit this with the semver system
func DumpMaps() {
	// TODO: make this function part of the exporter
	for name, cmap := range builtinMetricMaps {
		query, ok := queryOverrides[name]
		if !ok {
			fmt.Println(name)
		} else {
			for _, queryOverride := range query {
				fmt.Println(name, queryOverride.versionRange, queryOverride.query)
			}
		}

		for column, details := range cmap.columnMappings {
			fmt.Printf("  %-40s %v\n", column, details)
		}
		fmt.Println()
	}
}

var builtinMetricMaps = map[string]intermediateMetricMap{
	"pg_stat_database_conflicts": {
		map[string]ColumnMapping{
			"datid":            {LABEL, "OID of a database", nil, nil},
			"datname":          {LABEL, "Name of this database", nil, nil},
			"confl_tablespace": {COUNTER, "Number of queries in this database that have been canceled due to dropped tablespaces", nil, nil},
			"confl_lock":       {COUNTER, "Number of queries in this database that have been canceled due to lock timeouts", nil, nil},
			"confl_snapshot":   {COUNTER, "Number of queries in this database that have been canceled due to old snapshots", nil, nil},
			"confl_bufferpin":  {COUNTER, "Number of queries in this database that have been canceled due to pinned buffers", nil, nil},
			"confl_deadlock":   {COUNTER, "Number of queries in this database that have been canceled due to deadlocks", nil, nil},
		},
		true,
		0,
	},
	"pg_stat_replication": {
		map[string]ColumnMapping{
			"procpid":                  {DISCARD, "Process ID of a WAL sender process", nil, semver.MustParseRange("<9.2.0")},
			"pid":                      {DISCARD, "Process ID of a WAL sender process", nil, semver.MustParseRange(">=9.2.0")},
			"usesysid":                 {DISCARD, "OID of the user logged into this WAL sender process", nil, nil},
			"usename":                  {DISCARD, "Name of the user logged into this WAL sender process", nil, nil},
			"application_name":         {LABEL, "Name of the application that is connected to this WAL sender", nil, nil},
			"client_addr":              {LABEL, "IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.", nil, nil},
			"client_hostname":          {DISCARD, "Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.", nil, nil},
			"client_port":              {DISCARD, "TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used", nil, nil},
			"backend_start":            {DISCARD, "with time zone	Time when this process was started, i.e., when the client connected to this WAL sender", nil, nil},
			"backend_xmin":             {DISCARD, "The current backend's xmin horizon.", nil, nil},
			"state":                    {LABEL, "Current WAL sender state", nil, nil},
			"sent_location":            {DISCARD, "Last transaction log position sent on this connection", nil, semver.MustParseRange("<10.0.0")},
			"write_location":           {DISCARD, "Last transaction log position written to disk by this standby server", nil, semver.MustParseRange("<10.0.0")},
			"flush_location":           {DISCARD, "Last transaction log position flushed to disk by this standby server", nil, semver.MustParseRange("<10.0.0")},
			"replay_location":          {DISCARD, "Last transaction log position replayed into the database on this standby server", nil, semver.MustParseRange("<10.0.0")},
			"sent_lsn":                 {DISCARD, "Last transaction log position sent on this connection", nil, semver.MustParseRange(">=10.0.0")},
			"write_lsn":                {DISCARD, "Last transaction log position written to disk by this standby server", nil, semver.MustParseRange(">=10.0.0")},
			"flush_lsn":                {DISCARD, "Last transaction log position flushed to disk by this standby server", nil, semver.MustParseRange(">=10.0.0")},
			"replay_lsn":               {DISCARD, "Last transaction log position replayed into the database on this standby server", nil, semver.MustParseRange(">=10.0.0")},
			"sync_priority":            {DISCARD, "Priority of this standby server for being chosen as the synchronous standby", nil, nil},
			"sync_state":               {DISCARD, "Synchronous state of this standby server", nil, nil},
			"slot_name":                {LABEL, "A unique, cluster-wide identifier for the replication slot", nil, semver.MustParseRange(">=9.2.0")},
			"plugin":                   {DISCARD, "The base name of the shared object containing the output plugin this logical slot is using, or null for physical slots", nil, nil},
			"slot_type":                {DISCARD, "The slot type - physical or logical", nil, nil},
			"datoid":                   {DISCARD, "The OID of the database this slot is associated with, or null. Only logical slots have an associated database", nil, nil},
			"database":                 {DISCARD, "The name of the database this slot is associated with, or null. Only logical slots have an associated database", nil, nil},
			"active":                   {DISCARD, "True if this slot is currently actively being used", nil, nil},
			"active_pid":               {DISCARD, "Process ID of a WAL sender process", nil, nil},
			"xmin":                     {DISCARD, "The oldest transaction that this slot needs the database to retain. VACUUM cannot remove tuples deleted by any later transaction", nil, nil},
			"catalog_xmin":             {DISCARD, "The oldest transaction affecting the system catalogs that this slot needs the database to retain. VACUUM cannot remove catalog tuples deleted by any later transaction", nil, nil},
			"restart_lsn":              {DISCARD, "The address (LSN) of oldest WAL which still might be required by the consumer of this slot and thus won't be automatically removed during checkpoints", nil, nil},
			"pg_current_xlog_location": {DISCARD, "pg_current_xlog_location", nil, nil},
			"pg_current_wal_lsn":       {DISCARD, "pg_current_xlog_location", nil, semver.MustParseRange(">=10.0.0")},
			"pg_current_wal_lsn_bytes": {GAUGE, "WAL position in bytes", nil, semver.MustParseRange(">=10.0.0")},
			"pg_xlog_location_diff":    {GAUGE, "Lag in bytes between master and slave", nil, semver.MustParseRange(">=9.2.0 <10.0.0")},
			"pg_wal_lsn_diff":          {GAUGE, "Lag in bytes between master and slave", nil, semver.MustParseRange(">=10.0.0")},
			"confirmed_flush_lsn":      {DISCARD, "LSN position a consumer of a slot has confirmed flushing the data received", nil, nil},
			"write_lag":                {DISCARD, "Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby.", nil, semver.MustParseRange(">=10.0.0")},
			"flush_lag":                {DISCARD, "Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level remote_flush incurred while committing if this server was configured as a synchronous standby.", nil, semver.MustParseRange(">=10.0.0")},
			"replay_lag":               {DISCARD, "Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby.", nil, semver.MustParseRange(">=10.0.0")},
		},
		true,
		0,
	},
	"pg_replication_slots": {
		map[string]ColumnMapping{
			"slot_name":       {LABEL, "Name of the replication slot", nil, nil},
			"database":        {LABEL, "Name of the database", nil, nil},
			"active":          {GAUGE, "Flag indicating if the slot is active", nil, nil},
			"pg_wal_lsn_diff": {GAUGE, "Replication lag in bytes", nil, nil},
		},
		true,
		0,
	},
	"pg_stat_archiver": {
		map[string]ColumnMapping{
			"archived_count":     {COUNTER, "Number of WAL files that have been successfully archived", nil, nil},
			"last_archived_wal":  {DISCARD, "Name of the last WAL file successfully archived", nil, nil},
			"last_archived_time": {DISCARD, "Time of the last successful archive operation", nil, nil},
			"failed_count":       {COUNTER, "Number of failed attempts for archiving WAL files", nil, nil},
			"last_failed_wal":    {DISCARD, "Name of the WAL file of the last failed archival operation", nil, nil},
			"last_failed_time":   {DISCARD, "Time of the last failed archival operation", nil, nil},
			"stats_reset":        {DISCARD, "Time at which these statistics were last reset", nil, nil},
			"last_archive_age":   {GAUGE, "Time in seconds since last WAL segment was successfully archived", nil, nil},
		},
		true,
		0,
	},
	"pg_stat_activity": {
		map[string]ColumnMapping{
			"datname":          {LABEL, "Name of this database", nil, nil},
			"state":            {LABEL, "connection state", nil, semver.MustParseRange(">=9.2.0")},
			"usename":          {LABEL, "connection usename", nil, nil},
			"application_name": {LABEL, "connection application_name", nil, nil},
			"backend_type":     {LABEL, "connection backend_type", nil, nil},
			"wait_event_type":  {LABEL, "connection wait_event_type", nil, nil},
			"wait_event":       {LABEL, "connection wait_event", nil, nil},
			"count":            {GAUGE, "number of connections in this state", nil, nil},
			"max_tx_duration":  {GAUGE, "max duration in seconds any active transaction has been running", nil, nil},
		},
		true,
		0,
	},
}

// Turn the MetricMap column mapping into a prometheus descriptor mapping.
func makeDescMap(pgVersion semver.Version, serverLabels prometheus.Labels, metricMaps map[string]intermediateMetricMap, logger *slog.Logger, metricPrefix string) map[string]MetricMapNamespace {
	var metricMap = make(map[string]MetricMapNamespace)

	for namespace, intermediateMappings := range metricMaps {
		thisMap := make(map[string]MetricMap)

		namespace = strings.Replace(namespace, "pg", metricPrefix, 1)

		// Get the constant labels
		var variableLabels []string
		for columnName, columnMapping := range intermediateMappings.columnMappings {
			if columnMapping.usage == LABEL {
				variableLabels = append(variableLabels, columnName)
			}
		}

		for columnName, columnMapping := range intermediateMappings.columnMappings {
			// Check column version compatibility for the current map
			// Force to discard if not compatible.
			if columnMapping.supportedVersions != nil {
				if !columnMapping.supportedVersions(pgVersion) {
					// It's very useful to be able to see what columns are being
					// rejected.
					logger.Debug("Column is being forced to discard due to version incompatibility", "column", columnName)
					thisMap[columnName] = MetricMap{
						discard: true,
						conversion: func(_ interface{}) (float64, bool) {
							return math.NaN(), true
						},
					}
					continue
				}
			}

			// Determine how to convert the column based on its usage.
			// nolint: dupl
			switch columnMapping.usage {
			case DISCARD, LABEL:
				thisMap[columnName] = MetricMap{
					discard: true,
					conversion: func(_ interface{}) (float64, bool) {
						return math.NaN(), true
					},
				}
			case COUNTER:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.CounterValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, variableLabels, serverLabels),
					conversion: func(in interface{}) (float64, bool) {
						return dbToFloat64(in, logger)
					},
				}
			case GAUGE:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.GaugeValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, variableLabels, serverLabels),
					conversion: func(in interface{}) (float64, bool) {
						return dbToFloat64(in, logger)
					},
				}
			case HISTOGRAM:
				thisMap[columnName] = MetricMap{
					histogram: true,
					vtype:     prometheus.UntypedValue,
					desc:      prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, variableLabels, serverLabels),
					conversion: func(in interface{}) (float64, bool) {
						return dbToFloat64(in, logger)
					},
				}
				thisMap[columnName+"_bucket"] = MetricMap{
					histogram: true,
					discard:   true,
				}
				thisMap[columnName+"_sum"] = MetricMap{
					histogram: true,
					discard:   true,
				}
				thisMap[columnName+"_count"] = MetricMap{
					histogram: true,
					discard:   true,
				}
			case MAPPEDMETRIC:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.GaugeValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, variableLabels, serverLabels),
					conversion: func(in interface{}) (float64, bool) {
						text, ok := in.(string)
						if !ok {
							return math.NaN(), false
						}

						val, ok := columnMapping.mapping[text]
						if !ok {
							return math.NaN(), false
						}
						return val, true
					},
				}
			case DURATION:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.GaugeValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s_milliseconds", namespace, columnName), columnMapping.description, variableLabels, serverLabels),
					conversion: func(in interface{}) (float64, bool) {
						var durationString string
						switch t := in.(type) {
						case []byte:
							durationString = string(t)
						case string:
							durationString = t
						default:
							logger.Error("Duration conversion metric was not a string")
							return math.NaN(), false
						}

						if durationString == "-1" {
							return math.NaN(), false
						}

						d, err := time.ParseDuration(durationString)
						if err != nil {
							logger.Error("Failed converting result to metric", "column", columnName, "in", in, "err", err)
							return math.NaN(), false
						}
						return float64(d / time.Millisecond), true
					},
				}
			}
		}

		metricMap[namespace] = MetricMapNamespace{variableLabels, thisMap, intermediateMappings.master, intermediateMappings.cacheSeconds}
	}

	return metricMap
}

type cachedMetrics struct {
	metrics    []prometheus.Metric
	lastScrape time.Time
}

// Exporter collects Postgres metrics. It implements prometheus.Collector.
type Exporter struct {
	// Holds a reference to the build in column mappings. Currently this is for testing purposes
	// only, since it just points to the global.
	builtinMetricMaps map[string]intermediateMetricMap

	disableDefaultMetrics, disableSettingsMetrics, autoDiscoverDatabases bool

	excludeDatabases []string
	includeDatabases []string
	dsn              []string
	userQueriesPath  string
	constantLabels   prometheus.Labels
	duration         prometheus.Gauge
	error            prometheus.Gauge
	psqlUp           prometheus.Gauge
	userQueriesError *prometheus.GaugeVec
	totalScrapes     prometheus.Counter

	// servers are used to allow re-using the DB connection between scrapes.
	// servers contains metrics map and query overrides.
	servers *Servers

	logger       *slog.Logger
	metricPrefix string
}

// ExporterOpt configures Exporter.
type ExporterOpt func(*Exporter)

// DisableDefaultMetrics configures default metrics export.
func DisableDefaultMetrics(b bool) ExporterOpt {
	return func(e *Exporter) {
		e.disableDefaultMetrics = b
	}
}

// DisableSettingsMetrics configures pg_settings export.
func DisableSettingsMetrics(b bool) ExporterOpt {
	return func(e *Exporter) {
		e.disableSettingsMetrics = b
	}
}

// AutoDiscoverDatabases allows scraping all databases on a database server.
func AutoDiscoverDatabases(b bool) ExporterOpt {
	return func(e *Exporter) {
		e.autoDiscoverDatabases = b
	}
}

// ExcludeDatabases allows to filter out result from AutoDiscoverDatabases
func ExcludeDatabases(s []string) ExporterOpt {
	return func(e *Exporter) {
		e.excludeDatabases = s
	}
}

// IncludeDatabases allows to filter result from AutoDiscoverDatabases
func IncludeDatabases(s string) ExporterOpt {
	return func(e *Exporter) {
		if len(s) > 0 {
			e.includeDatabases = strings.Split(s, ",")
		}
	}
}

// WithUserQueriesPath configures user's queries path.
func WithUserQueriesPath(p string) ExporterOpt {
	return func(e *Exporter) {
		e.userQueriesPath = p
	}
}

// WithConstantLabels configures constant labels.
func WithConstantLabels(s string) ExporterOpt {
	return func(e *Exporter) {
		e.constantLabels = parseConstLabels(s, e.logger)
	}
}

// WithMetricPrefix configures metric prefix.
func WithMetricPrefix(prefix string) ExporterOpt {
	return func(e *Exporter) {
		e.metricPrefix = prefix
	}
}

func parseConstLabels(s string, logger *slog.Logger) prometheus.Labels {
	labels := make(prometheus.Labels)

	s = strings.TrimSpace(s)
	if len(s) == 0 {
		return labels
	}

	parts := strings.Split(s, ",")
	for _, p := range parts {
		keyValue := strings.Split(strings.TrimSpace(p), "=")
		if len(keyValue) != 2 {
			logger.Error(`Wrong constant labels format, should be "key=value"`, "input", p)
			continue
		}
		key := strings.TrimSpace(keyValue[0])
		value := strings.TrimSpace(keyValue[1])
		if key == "" || value == "" {
			continue
		}
		labels[key] = value
	}

	return labels
}

// NewExporter returns a new PostgreSQL exporter for the provided DSN.
func NewExporter(dsn []string, logger *slog.Logger, opts ...ExporterOpt) *Exporter {
	e := &Exporter{
		dsn:               dsn,
		builtinMetricMaps: builtinMetricMaps,
		logger:            logger,
	}

	for _, opt := range opts {
		opt(e)
	}

	e.setupInternalMetrics()
	e.servers = NewServers(ServerWithLabels(e.constantLabels), ServerWithLogger(e.logger))

	return e
}

func (e *Exporter) setupInternalMetrics() {
	e.duration = prometheus.NewGauge(prometheus.GaugeOpts{
		Namespace:   namespace,
		Subsystem:   exporter,
		Name:        "last_scrape_duration_seconds",
		Help:        "Duration of the last scrape of metrics from PostgreSQL.",
		ConstLabels: e.constantLabels,
	})
	e.totalScrapes = prometheus.NewCounter(prometheus.CounterOpts{
		Namespace:   namespace,
		Subsystem:   exporter,
		Name:        "scrapes_total",
		Help:        "Total number of times PostgreSQL was scraped for metrics.",
		ConstLabels: e.constantLabels,
	})
	e.error = prometheus.NewGauge(prometheus.GaugeOpts{
		Namespace:   namespace,
		Subsystem:   exporter,
		Name:        "last_scrape_error",
		Help:        "Whether the last scrape of metrics from PostgreSQL resulted in an error (1 for error, 0 for success).",
		ConstLabels: e.constantLabels,
	})
	e.psqlUp = prometheus.NewGauge(prometheus.GaugeOpts{
		Namespace:   namespace,
		Name:        "up",
		Help:        "Whether the last scrape of metrics from PostgreSQL was able to connect to the server (1 for yes, 0 for no).",
		ConstLabels: e.constantLabels,
	})
	e.userQueriesError = prometheus.NewGaugeVec(prometheus.GaugeOpts{
		Namespace:   namespace,
		Subsystem:   exporter,
		Name:        "user_queries_load_error",
		Help:        "Whether the user queries file was loaded and parsed successfully (1 for error, 0 for success).",
		ConstLabels: e.constantLabels,
	}, []string{"filename", "hashsum"})
}

// Describe implements prometheus.Collector.
func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
}

// Collect implements prometheus.Collector.
func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
	e.scrape(ch)

	ch <- e.duration
	ch <- e.totalScrapes
	ch <- e.error
	ch <- e.psqlUp
	e.userQueriesError.Collect(ch)
}

func (e *Exporter) CloseServers() {
	e.servers.Close()
}

func newDesc(subsystem, name, help string, labels prometheus.Labels) *prometheus.Desc {
	return prometheus.NewDesc(
		prometheus.BuildFQName(namespace, subsystem, name),
		help, nil, labels,
	)
}

func checkPostgresVersion(db *sql.DB, server string, logger *slog.Logger) (semver.Version, string, error) {
	logger.Debug("Querying PostgreSQL version", "server", server)
	versionRow := db.QueryRow("SELECT version();")
	var versionString string
	err := versionRow.Scan(&versionString)
	if err != nil {
		return semver.Version{}, "", fmt.Errorf("Error scanning version string on %q: %v", server, err)
	}
	semanticVersion, err := parseVersion(versionString)
	if err != nil {
		return semver.Version{}, "", fmt.Errorf("Error parsing version string on %q: %v", server, err)
	}

	return semanticVersion, versionString, nil
}

// Check and update the exporters query maps if the version has changed.
func (e *Exporter) checkMapVersions(ch chan<- prometheus.Metric, server *Server) error {
	semanticVersion, versionString, err := checkPostgresVersion(server.db, server.String(), e.logger)
	if err != nil {
		return fmt.Errorf("Error fetching version string on %q: %v", server, err)
	}

	if !e.disableDefaultMetrics && semanticVersion.LT(lowestSupportedVersion) {
		e.logger.Warn("PostgreSQL version is lower than our lowest supported version", "server", server, "version", semanticVersion, "lowest_supported_version", lowestSupportedVersion)
	}

	// Check if semantic version changed and recalculate maps if needed.
	if semanticVersion.NE(server.lastMapVersion) || server.metricMap == nil {
		e.logger.Info("Semantic version changed", "server", server, "from", server.lastMapVersion, "to", semanticVersion)
		server.mappingMtx.Lock()

		// Get Default Metrics only for master database
		if !e.disableDefaultMetrics && server.master {
			server.metricMap = makeDescMap(semanticVersion, server.labels, e.builtinMetricMaps, server.logger, e.metricPrefix)
			server.queryOverrides = makeQueryOverrideMap(semanticVersion, queryOverrides, server.logger)
		} else {
			server.metricMap = make(map[string]MetricMapNamespace)
			server.queryOverrides = make(map[string]string)
		}

		server.lastMapVersion = semanticVersion

		if e.userQueriesPath != "" {
			// Clear the metric while a reload is happening
			e.userQueriesError.Reset()

			// Calculate the hashsum of the useQueries
			userQueriesData, err := os.ReadFile(e.userQueriesPath)
			if err != nil {
				e.logger.Error("Failed to reload user queries", "path", e.userQueriesPath, "err", err)
				e.userQueriesError.WithLabelValues(e.userQueriesPath, "").Set(1)
			} else {
				hashsumStr := fmt.Sprintf("%x", sha256.Sum256(userQueriesData))

				if err := addQueries(userQueriesData, semanticVersion, server, e.metricPrefix); err != nil {
					e.logger.Error("Failed to reload user queries", "path", e.userQueriesPath, "err", err)
					e.userQueriesError.WithLabelValues(e.userQueriesPath, hashsumStr).Set(1)
				} else {
					// Mark user queries as successfully loaded
					e.userQueriesError.WithLabelValues(e.userQueriesPath, hashsumStr).Set(0)
				}
			}
		}

		server.mappingMtx.Unlock()
	}

	// Output the version as a special metric only for master database
	versionDesc := prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, staticLabelName),
		"Version string as reported by postgres", []string{"version", "short_version"}, server.labels)

	if !e.disableDefaultMetrics && server.master {
		ch <- prometheus.MustNewConstMetric(versionDesc,
			prometheus.UntypedValue, 1, versionString, semanticVersion.String())
	}
	return nil
}

func (e *Exporter) scrape(ch chan<- prometheus.Metric) {
	defer func(begun time.Time) {
		e.duration.Set(time.Since(begun).Seconds())
	}(time.Now())

	e.totalScrapes.Inc()

	dsns := e.dsn
	if e.autoDiscoverDatabases {
		dsns = e.discoverDatabaseDSNs()
	}

	var errorsCount int
	var connectionErrorsCount int

	for _, dsn := range dsns {
		if err := e.scrapeDSN(ch, dsn); err != nil {
			errorsCount++

			e.logger.Error("error scraping dsn", "err", err, "dsn", loggableDSN(dsn))

			if _, ok := err.(*ErrorConnectToServer); ok {
				connectionErrorsCount++
			}
		}
	}

	switch {
	case connectionErrorsCount >= len(dsns):
		e.psqlUp.Set(0)
	default:
		e.psqlUp.Set(1) // Didn't fail, can mark connection as up for this scrape.
	}

	switch errorsCount {
	case 0:
		e.error.Set(0)
	default:
		e.error.Set(1)
	}
}