;;; Guix Data Service -- Information about Guix over time
;;; Copyright © 2020 Christopher Baines <mail@cbaines.net>
;;;
;;; This program is free software: you can redistribute it and/or
;;; modify it under the terms of the GNU Affero General Public License
;;; as published by the Free Software Foundation, either version 3 of
;;; the License, or (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;; Affero General Public License for more details.
;;;
;;; You should have received a copy of the GNU Affero General Public
;;; License along with this program.  If not, see
;;; <http://www.gnu.org/licenses/>.

(define-module (guix-data-service utils)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-11)
  #:use-module (ice-9 ftw)
  #:use-module (ice-9 match)
  #:use-module (ice-9 atomic)
  #:use-module (ice-9 format)
  #:use-module (ice-9 threads)
  #:use-module (ice-9 exceptions)
  #:use-module (ice-9 ports internal)
  #:use-module (ice-9 suspendable-ports)
  #:use-module (lzlib)
  #:use-module (fibers)
  #:use-module (fibers channels)
  #:use-module (fibers operations)
  #:use-module (fibers timers)
  #:use-module (fibers conditions)
  #:use-module (fibers scheduler)
  #:use-module (prometheus)
  #:export (call-with-time-logging
            with-time-logging
            prevent-inlining-for-tests

            resource-pool-default-timeout
            %resource-pool-timeout-handler
            make-resource-pool
            destroy-resource-pool
            call-with-resource-from-pool
            with-resource-from-pool
            resource-pool-stats

            parallel-via-fibers
            par-map&
            letpar&

            chunk
            chunk!
            chunk-for-each!

            delete-duplicates/sort!

            get-guix-metrics-updater

            call-with-sigint
            run-server/patched

            spawn-port-monitoring-fiber))

(define (call-with-time-logging action thunk)
  (simple-format #t "debug: Starting ~A\n" action)
  (let ((start-time (current-time)))
    (let-values
        ((result (thunk)))
      (let ((time-taken (- (current-time) start-time)))
        (simple-format #t "debug: Finished ~A, took ~A seconds\n"
                       action time-taken))
      (apply values result))))

(define-syntax-rule (with-time-logging action exp ...)
  "Log under NAME the time taken to evaluate EXP."
  (call-with-time-logging action (lambda () exp ...)))

(define-syntax-rule (prevent-inlining-for-tests var)
  (set! var var))

(define* (make-resource-pool initializer max-size
                             #:key (min-size max-size)
                             (idle-seconds #f)
                             (delay-logger (const #f))
                             (duration-logger (const #f))
                             destructor
                             lifetime
                             (name "unnamed"))
  (define (initializer/safe)
    (with-exception-handler
        (lambda (exn)
          (simple-format
           (current-error-port)
           "exception running ~A resource pool initializer: ~A:\n  ~A\n"
           name
           initializer
           exn)
          #f)
      (lambda ()
        (with-throw-handler #t
          initializer
          (lambda args
            (backtrace))))
      #:unwind? #t))

  (define (destructor/safe args)
    (let ((success?
           (with-exception-handler
               (lambda (exn)
                 (simple-format
                  (current-error-port)
                  "exception running resource pool destructor (~A): ~A:\n  ~A\n"
                  name
                  destructor
                  exn)
                 #f)
             (lambda ()
               (with-throw-handler #t
                 (lambda ()
                   (destructor args)
                   #t)
                 (lambda _
                   (backtrace))))
             #:unwind? #t)))

      (or success?
          #t
          (begin
            (sleep 5)
            (destructor/safe args)))))

  (let ((channel (make-channel))
        (checkout-failure-count 0))
    (spawn-fiber
     (lambda ()
       (while #t
         (with-exception-handler
             (lambda (exn)
               (simple-format
                (current-error-port)
                "exception in the ~A pool fiber: ~A\n"
                name
                exn))
           (lambda ()
             (let loop ((resources '())
                        (available '())
                        (waiters '())
                        (resources-last-used '()))

               (match (if idle-seconds
                          (perform-operation
                           (choice-operation
                            (get-operation channel)
                            (wrap-operation
                             ;; TODO Do something smarter
                             (sleep-operation 10)
                             (const '(check-for-idle-resources)))))
                          (get-message channel))
                 (('checkout reply)
                  (if (null? available)
                      (if (= (length resources) max-size)
                          (loop resources
                                available
                                (cons reply waiters)
                                resources-last-used)
                          (let ((new-resource (initializer/safe)))
                            (if new-resource
                                (let ((checkout-success?
                                       (perform-operation
                                        (choice-operation
                                         (wrap-operation
                                          (put-operation reply new-resource)
                                          (const #t))
                                         (wrap-operation (sleep-operation 1)
                                                         (const #f))))))
                                  (unless checkout-success?
                                    (set! checkout-failure-count
                                          (+ 1 checkout-failure-count)))

                                  (loop (cons new-resource resources)
                                        (if checkout-success?
                                            available
                                            (cons new-resource available))
                                        waiters
                                        (cons (get-internal-real-time)
                                              resources-last-used)))
                                (loop resources
                                      available
                                      (cons reply waiters)
                                      resources-last-used))))
                      (let ((checkout-success?
                             (perform-operation
                              (choice-operation
                               (wrap-operation
                                (put-operation reply (car available))
                                (const #t))
                               (wrap-operation (sleep-operation 1)
                                               (const #f))))))
                        (unless checkout-success?
                          (set! checkout-failure-count
                                (+ 1 checkout-failure-count)))

                        (if checkout-success?
                            (loop resources
                                  (cdr available)
                                  waiters
                                  resources-last-used)
                            (loop resources
                                  available
                                  waiters
                                  resources-last-used)))))
                 (('return resource)
                  ;; When a resource is returned, prompt all the waiters to request
                  ;; again.  This is to avoid the pool waiting on channels that may
                  ;; be dead.
                  (for-each
                   (lambda (waiter)
                     (spawn-fiber
                      (lambda ()
                        (perform-operation
                         (choice-operation
                          (put-operation waiter 'resource-pool-retry-checkout)
                          (sleep-operation 0.2))))))
                   waiters)

                  (loop resources
                        (cons resource available)
                        ;; clear waiters, as they've been notified
                        '()
                        (begin
                          (list-set!
                           resources-last-used
                           (list-index (lambda (x)
                                         (eq? x resource))
                                       resources)
                           (get-internal-real-time))
                          resources-last-used)))
                 (('stats reply)
                  (let ((stats
                         `((resources              . ,(length resources))
                           (available              . ,(length available))
                           (waiters                . ,(length waiters))
                           (checkout-failure-count . ,checkout-failure-count))))

                    (spawn-fiber
                     (lambda ()
                       (perform-operation
                        (choice-operation
                         (wrap-operation
                          (put-operation reply stats)
                          (const #t))
                         (wrap-operation (sleep-operation 1)
                                         (const #f)))))))

                  (loop resources
                        available
                        waiters
                        resources-last-used))
                 (('check-for-idle-resources)
                  (let* ((resources-last-used-seconds
                          (map
                           (lambda (internal-time)
                             (/ (- (get-internal-real-time) internal-time)
                                internal-time-units-per-second))
                           resources-last-used))
                         (resources-to-destroy
                          (filter-map
                           (lambda (resource last-used-seconds)
                             (if (and (member resource available)
                                      (> last-used-seconds idle-seconds))
                                 resource
                                 #f))
                           resources
                           resources-last-used-seconds)))

                    (for-each
                     (lambda (resource)
                       (destructor/safe resource))
                     resources-to-destroy)

                    (loop (lset-difference eq? resources resources-to-destroy)
                          (lset-difference eq? available resources-to-destroy)
                          waiters
                          (filter-map
                           (lambda (resource last-used)
                             (if (memq resource resources-to-destroy)
                                 #f
                                 last-used))
                           resources
                           resources-last-used))))
                 (('destroy reply)
                  (if (= (length resources) (length available))
                      (begin
                        (for-each
                         (lambda (resource)
                           (destructor/safe resource))
                         resources)
                        (put-message reply 'destroy-success))
                      (begin
                        (spawn-fiber
                         (lambda ()
                           (perform-operation
                            (choice-operation
                             (put-operation reply 'resource-pool-destroy-failed)
                             (sleep-operation 10)))))
                        (loop resources
                              available
                              waiters
                              resources-last-used))))
                 (unknown
                  (simple-format
                   (current-error-port)
                   "unrecognised message to ~A resource pool channel: ~A\n"
                   name
                   unknown)
                  (loop resources
                        available
                        waiters
                        resources-last-used)))))
           #:unwind? #t))))

    channel))

(define (destroy-resource-pool pool)
  (let ((reply (make-channel)))
    (put-message pool (list 'destroy reply))
    (let ((msg (get-message reply)))
      (unless (eq? msg 'destroy-success)
        (error msg)))))

(define resource-pool-default-timeout
  (make-parameter #f))

(define &resource-pool-timeout
  (make-exception-type '&recource-pool-timeout
                       &error
                       '()))

(define make-resource-pool-timeout-error
  (record-constructor &resource-pool-timeout))

(define resource-pool-timeout-error?
  (record-predicate &resource-pool-timeout))

(define %resource-pool-timeout-handler
  (make-parameter #f))

(define* (call-with-resource-from-pool pool proc #:key (timeout 'default)
                                       (timeout-handler (%resource-pool-timeout-handler)))
  "Call PROC with a resource from POOL, blocking until a resource becomes
available.  Return the resource once PROC has returned."

  (define timeout-or-default
    (if (eq? timeout 'default)
        (resource-pool-default-timeout)
        timeout))

  (let ((resource
         (let ((reply (make-channel)))
           (if timeout-or-default
               (let loop ((start-time (get-internal-real-time)))
                 (perform-operation
                  (choice-operation
                   (wrap-operation
                    (put-operation pool `(checkout ,reply))
                    (const #t))
                   (wrap-operation (sleep-operation timeout-or-default)
                                   (const #f))))

                 (let ((time-remaining
                        (- timeout-or-default
                           (/ (- (get-internal-real-time)
                                 start-time)
                              internal-time-units-per-second))))
                   (if (> time-remaining 0)
                       (let ((response
                              (perform-operation
                               (choice-operation
                                (get-operation reply)
                                (wrap-operation (sleep-operation time-remaining)
                                                (const #f))))))
                         (if (or (not response)
                                 (eq? response 'resource-pool-retry-checkout))
                             (if (> (- timeout-or-default
                                       (/ (- (get-internal-real-time)
                                             start-time)
                                          internal-time-units-per-second))
                                    0)
                                 (loop start-time)
                                 #f)
                             response))
                       #f)))
               (let loop ()
                 (put-message pool `(checkout ,reply))
                 (let ((response (get-message reply)))
                   (if (eq? response 'resource-pool-retry-checkout)
                       (loop)
                       response)))))))

    (when (or (not resource)
              (eq? resource 'resource-pool-retry-checkout))
      (when timeout-handler
        (timeout-handler pool proc timeout))

      (raise-exception
       (make-resource-pool-timeout-error)))

    (with-exception-handler
        (lambda (exception)
          (put-message pool `(return ,resource))
          (raise-exception exception))
      (lambda ()
        (call-with-values
            (lambda ()
              (with-throw-handler #t
                (lambda ()
                  (proc resource))
                (lambda _
                  (backtrace))))
          (lambda vals
            (put-message pool `(return ,resource))
            (apply values vals))))
      #:unwind? #t)))

(define-syntax-rule (with-resource-from-pool pool resource exp ...)
  (call-with-resource-from-pool
   pool
   (lambda (resource) exp ...)))

(define* (resource-pool-stats pool #:key (timeout 5))
  (let ((reply (make-channel))
        (start-time (get-internal-real-time)))
    (perform-operation
     (choice-operation
      (wrap-operation
       (put-operation pool `(stats ,reply))
       (const #t))
      (wrap-operation (sleep-operation timeout)
                      (const #f))))

    (let ((time-remaining
           (- timeout
              (/ (- (get-internal-real-time)
                    start-time)
                 internal-time-units-per-second))))
      (if (> time-remaining 0)
          (let ((response
                 (perform-operation
                  (choice-operation
                   (get-operation reply)
                   (wrap-operation (sleep-operation time-remaining)
                                   (const #f))))))
            response)
          (raise-exception
           (make-resource-pool-timeout-error))))))

(define (defer-to-parallel-fiber thunk)
  (let ((reply (make-channel)))
    (spawn-fiber
     (lambda ()
       (with-exception-handler
           (lambda (exn)
             (put-message reply (cons 'exception exn)))
         (lambda ()
           (call-with-values
               (lambda ()
                 (with-throw-handler #t
                   thunk
                   (lambda _
                     (backtrace))))
             (lambda vals
               (put-message reply vals))))
         #:unwind? #t))
     #:parallel? #t)
    reply))

(define (fetch-result-of-defered-thunks . reply-channels)
  (let ((responses (map get-message
                        reply-channels)))
    (map
     (match-lambda
       (('exception . exn)
        (raise-exception exn))
       (result
        (apply values result)))
     responses)))

(define-syntax parallel-via-fibers
  (lambda (x)
    (syntax-case x ()
      ((_ e0 ...)
       (with-syntax (((tmp0 ...) (generate-temporaries (syntax (e0 ...)))))
         #'(let ((tmp0 (defer-to-parallel-fiber
                         (lambda ()
                           e0)))
                 ...)
             (apply values (fetch-result-of-defered-thunks tmp0 ...))))))))

(define-syntax-rule (letpar& ((v e) ...) b0 b1 ...)
  (call-with-values
      (lambda () (parallel-via-fibers e ...))
    (lambda (v ...)
      b0 b1 ...)))

(define (par-mapper' mapper cons)
  (lambda (proc . lists)
    (apply
     fetch-result-of-defered-thunks
     (let loop ((lists lists))
       (match lists
         (((heads tails ...) ...)
          (let ((tail (loop tails))
                (head (defer-to-parallel-fiber
                        (lambda ()
                          (apply proc heads)))))
            (cons head tail)))
         (_
          '()))))))

(define par-map& (par-mapper' map cons))

(define (chunk lst max-length)
  (if (> (length lst)
         max-length)
      (call-with-values (lambda ()
                          (split-at lst max-length))
        (lambda (first-lst rest)
          (cons first-lst
                (chunk rest max-length))))
      (list lst)))

(define (chunk! lst max-length)
  (if (> (length lst)
         max-length)
      (call-with-values (lambda ()
                          (split-at! lst max-length))
        (lambda (first-lst rest)
          (cons first-lst
                (chunk! rest max-length))))
      (list lst)))

(define* (chunk-for-each! proc chunk-size #:rest lsts)
  (define (do-one-iteration lsts)
    (if (> (length (car lsts))
           chunk-size)
        (let ((chunks-and-rest
               (map (lambda (lst)
                      (call-with-values (lambda ()
                                          (split-at! lst chunk-size))
                        (lambda (first-lst rest)
                          (cons first-lst
                                rest))))
                    lsts)))
          (apply proc
                 (map car chunks-and-rest))
          (do-one-iteration
           (map cdr chunks-and-rest)))
        (apply proc lsts)))

  (let ((list-lengths (map length lsts)))
    (unless (eq? 1 (length (delete-duplicates list-lengths)))
      (error "lists not equal length"))

    (unless (eq? 0 (first list-lengths))
      (do-one-iteration lsts)))

  #t)

(define* (delete-duplicates/sort! unsorted-lst less #:optional (equal? equal?))
  (if (null? unsorted-lst)
      unsorted-lst
      (let ((sorted-lst (sort! unsorted-lst less)))

        (let loop ((lst (cdr sorted-lst))
                   (last-element (car sorted-lst))
                   (result (list (car sorted-lst))))
          (if (null? lst)
              result
              (let ((current-element (car lst)))
                (if (equal? current-element last-element)
                    (loop (cdr lst)
                          last-element
                          result)
                    (loop (cdr lst)
                          current-element
                          (cons current-element
                                result)))))))))

(define (get-guix-metrics-updater registry)
  (define guix-db "/var/guix/db/db.sqlite")
  (define guix-db-wal (string-append guix-db "-wal"))

  (let ((guix-db-bytes-metric
         (make-gauge-metric registry "guix_db_bytes"))
        (guix-db-wal-bytes-metric
         (make-gauge-metric registry "guix_db_wal_bytes")))
    (lambda ()
      (with-exception-handler
          (lambda _
            #f)
        (lambda ()
          (metric-set guix-db-bytes-metric (stat:size (stat guix-db)))
          (metric-set guix-db-wal-bytes-metric
                      (if (file-exists? guix-db-wal)
                          (stat:size (stat guix-db-wal))
                          0)))
        #:unwind? #t))))

;; This variant of run-server from the fibers library supports running
;; multiple servers within one process.
(define run-server/patched
  (let ((fibers-web-server-module
         (resolve-module '(fibers web server))))

    (define set-nonblocking!
      (module-ref fibers-web-server-module 'set-nonblocking!))

    (define make-default-socket
      (module-ref fibers-web-server-module 'make-default-socket))

    (define socket-loop
      (module-ref fibers-web-server-module 'socket-loop))

    (lambda* (handler
              #:key
              (host #f)
              (family AF_INET)
              (addr (if host
                        (inet-pton family host)
                        INADDR_LOOPBACK))
              (port 8080)
              (socket (make-default-socket family addr port)))
      ;; We use a large backlog by default.  If the server is suddenly hit
      ;; with a number of connections on a small backlog, clients won't
      ;; receive confirmation for their SYN, leading them to retry --
      ;; probably successfully, but with a large latency.
      (listen socket 1024)
      (set-nonblocking! socket)
      (sigaction SIGPIPE SIG_IGN)
      (spawn-fiber (lambda () (socket-loop socket handler))))))

(define &port-timeout
  (make-exception-type '&port-timeout
                       &external-error
                       '(port)))

(define make-port-timeout-error
  (record-constructor &port-timeout))

(define port-timeout-error?
  (record-predicate &port-timeout))

(define &port-read-timeout
  (make-exception-type '&port-read-timeout
                       &port-timeout
                       '()))

(define make-port-read-timeout-error
  (record-constructor &port-read-timeout))

(define port-read-timeout-error?
  (record-predicate &port-read-timeout))

(define &port-write-timeout
  (make-exception-type '&port-write-timeout
                       &port-timeout
                       '()))

(define make-port-write-timeout-error
  (record-constructor &port-write-timeout))

(define port-write-timeout-error?
  (record-predicate &port-write-timeout))

;; These procedure are subject to spurious wakeups.

(define (readable? port)
  "Test if PORT is writable."
  (match (select (vector port) #() #() 0)
    ((#() #() #()) #f)
    ((#(_) #() #()) #t)))

(define (writable? port)
  "Test if PORT is writable."
  (match (select #() (vector port) #() 0)
    ((#() #() #()) #f)
    ((#() #(_) #()) #t)))

(define (make-wait-operation ready? schedule-when-ready port
                             port-ready-fd this-procedure)
  (make-base-operation
   #f
   (lambda _
     (and (ready? (port-ready-fd port)) values))
   (lambda (flag sched resume)
     (define (commit)
       (match (atomic-box-compare-and-swap! flag 'W 'S)
         ('W (resume values))
         ('C (commit))
         ('S #f)))
     (schedule-when-ready
      sched (port-ready-fd port) commit))))

(define (wait-until-port-readable-operation port)
  "Make an operation that will succeed when PORT is readable."
  (unless (input-port? port)
    (error "refusing to wait forever for input on non-input port"))
  (make-wait-operation readable? schedule-task-when-fd-readable port
                       port-read-wait-fd
                       wait-until-port-readable-operation))

(define (wait-until-port-writable-operation port)
  "Make an operation that will succeed when PORT is writable."
  (unless (output-port? port)
    (error "refusing to wait forever for output on non-output port"))
  (make-wait-operation writable? schedule-task-when-fd-writable port
                       port-write-wait-fd
                       wait-until-port-writable-operation))

(define* (with-fibers-port-timeouts thunk
                                    #:key timeout
                                    (read-timeout timeout)
                                    (write-timeout timeout))
  (define (no-fibers-wait port mode timeout)
    (define poll-timeout-ms 200)

    ;; When the GC runs, it restarts the poll syscall, but the timeout
    ;; remains unchanged! When the timeout is longer than the time
    ;; between the syscall restarting, I think this renders the
    ;; timeout useless. Therefore, this code uses a short timeout, and
    ;; repeatedly calls poll while watching the clock to see if it has
    ;; timed out overall.
    (let ((timeout-internal
           (+ (get-internal-real-time)
              (* internal-time-units-per-second
                 (/ timeout 1000)))))
      (let loop ((poll-value
                  (port-poll port mode poll-timeout-ms)))
        (if (= poll-value 0)
            (if (> (get-internal-real-time)
                   timeout-internal)
                (raise-exception
                 (if (string=? mode "r")
                     (make-port-read-timeout-error port)
                     (make-port-write-timeout-error port)))
                (loop (port-poll port mode poll-timeout-ms)))
            poll-value))))

  (parameterize
      ((current-read-waiter
        (lambda (port)
          (if (current-scheduler)
              (perform-operation
               (choice-operation
                (wait-until-port-readable-operation port)
                (wrap-operation
                 (sleep-operation read-timeout)
                 (lambda ()
                   (raise-exception
                    (make-port-read-timeout-error thunk port))))))
              (no-fibers-wait port "r" read-timeout))))
       (current-write-waiter
        (lambda (port)
          (if (current-scheduler)
              (perform-operation
               (choice-operation
                (wait-until-port-writable-operation port)
                (wrap-operation
                 (sleep-operation write-timeout)
                 (lambda ()
                   (raise-exception
                    (make-port-write-timeout-error thunk port))))))
              (no-fibers-wait port "w" write-timeout)))))
    (thunk)))

(define (spawn-port-monitoring-fiber port error-condition)
  (spawn-fiber
   (lambda ()
     (while #t
       (sleep 20)
       (with-exception-handler
           (lambda (exn)
             (simple-format (current-error-port)
                            "port monitoring fiber failed to connect to ~A: ~A\n"
                            port exn)
             (signal-condition! error-condition))
         (lambda ()
           (with-fibers-port-timeouts
            (lambda ()
              (let ((sock (socket PF_INET SOCK_STREAM 0)))
                (connect sock AF_INET INADDR_LOOPBACK port)
                (close-port sock)))
            #:timeout 20))
         #:unwind? #t)))))

;; Copied from (fibers web server)
(define (call-with-sigint thunk cvar)
  (let ((handler #f))
    (dynamic-wind
      (lambda ()
        (set! handler
          (sigaction SIGINT (lambda (sig) (signal-condition! cvar)))))
      thunk
      (lambda ()
        (if handler
            ;; restore Scheme handler, SIG_IGN or SIG_DFL.
            (sigaction SIGINT (car handler) (cdr handler))
            ;; restore original C handler.
            (sigaction SIGINT #f))))))