;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2017-2022 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2018, 2019 Ricardo Wurmus <rekado@elephly.net>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix 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 General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (guix status)
  #:use-module (guix records)
  #:use-module (guix i18n)
  #:use-module (guix colors)
  #:use-module (guix progress)
  #:autoload   (guix build syscalls) (terminal-columns)
  #:autoload   (guix build download) (nar-uri-abbreviation)
  #:use-module (guix store)
  #:use-module (guix derivations)
  #:use-module (guix memoization)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-9 gnu)
  #:use-module (srfi srfi-19)
  #:use-module (srfi srfi-26)
  #:use-module (ice-9 regex)
  #:use-module (ice-9 match)
  #:use-module (ice-9 format)
  #:use-module (ice-9 binary-ports)
  #:autoload   (ice-9 rdelim) (read-string)
  #:use-module (rnrs bytevectors)
  #:use-module ((system foreign)
                #:select (bytevector->pointer pointer->bytevector))
  #:export (build-event-output-port
            compute-status

            build-status
            build-status?
            build-status-building
            build-status-downloading
            build-status-builds-completed
            build-status-downloads-completed

            build?
            build
            build-derivation
            build-system
            build-log-file
            build-phase
            build-completion

            download?
            download
            download-item
            download-uri
            download-size
            download-start
            download-end
            download-transferred

            build-status-updater
            print-build-event
            print-build-event/quiet
            print-build-status

            with-status-report
            with-status-verbosity))

;;; Commentary:
;;;
;;; This module provides facilities to track the status of ongoing builds and
;;; downloads in a given session, as well as tools to report about the current
;;; status to user interfaces.  It does so by analyzing the output of
;;; 'current-build-output-port'.  The build status is maintained in a
;;; <build-status> record.
;;;
;;; Code:


;;;
;;; Build status tracking.
;;;

;; Builds and substitutions performed by the daemon.
(define-record-type* <build-status> build-status make-build-status
  build-status?
  (building     build-status-building             ;list of <build>
                (default '()))
  (downloading  build-status-downloading          ;list of <download>
                (default '()))
  (builds-completed build-status-builds-completed ;list of <build>
                    (default '()))
  (downloads-completed build-status-downloads-completed ;list of <download>
                       (default '())))

;; On-going or completed build.
(define-immutable-record-type <build>
  (%build derivation id system log-file phase completion)
  build?
  (derivation  build-derivation)                ;string (.drv file name)
  (id          build-id)                        ;#f | integer
  (system      build-system)                    ;string
  (log-file    build-log-file)                  ;#f | string
  (phase       build-phase                      ;#f | symbol
               set-build-phase)
  (completion  build-completion                 ;#f | integer (percentage)
               set-build-completion))

(define* (build derivation system #:key id log-file phase completion)
  "Return a new build."
  (%build derivation id system log-file phase completion))

;; On-going or completed downloads.  Downloads can be stem from substitutes
;; and from "builtin:download" fixed-output derivations.
(define-record-type <download>
  (%download item uri size start end transferred)
  download?
  (item         download-item)            ;store item
  (uri          download-uri)             ;string | #f
  (size         download-size)            ;integer | #f
  (start        download-start)           ;<time>
  (end          download-end)             ;#f | <time>
  (transferred  download-transferred))    ;integer

(define* (download item uri
                   #:key size
                   (start (current-time time-monotonic)) end
                   (transferred 0))
  "Return a new download."
  (%download item uri size start end transferred))

(define (matching-build drv)
  "Return a predicate that matches builds of DRV."
  (lambda (build)
    (string=? drv (build-derivation build))))

(define (matching-download item)
  "Return a predicate that matches downloads of ITEM."
  (lambda (download)
    (string=? item (download-item download))))

(define %phase-start-rx
  ;; Match the "starting phase" message emitted by 'gnu-build-system'.
  (make-regexp "^starting phase [`']([^']+)'"))

(define %percentage-line-rx
  ;; Things like CMake write lines like "[ 10%] gcc -c …".  This regexp
  ;; matches them.
  (make-regexp "^[[:space:]]*\\[ *([0-9]+)%\\]"))

(define %fraction-line-rx
  ;; The 'compiled-modules' derivations and Ninja produce reports like
  ;; "[ 1/32]" at the beginning of each line, while GHC prints "[ 6 of 45]".
  ;; This regexp matches these.
  (make-regexp "^[[:space:]]*\\[ *([0-9]+) *(/|of) *([0-9]+)\\]"))

(define (update-build status id line)
  "Update STATUS based on LINE, a build output line for ID that might contain
a completion indication."
  (define (find-build)
    (find (lambda (build)
            (and (build-id build)
                 (= (build-id build) id)))
          (build-status-building status)))

  (define (update %)
    (let ((build (find-build)))
      (build-status
       (inherit status)
       (building (cons (set-build-completion build %)
                       (delq build (build-status-building status)))))))

  (cond ((string-any #\nul line)
         ;; Don't try to match a regexp here.
         status)
        ((regexp-exec %percentage-line-rx line)
         =>
         (lambda (match)
           (let ((% (string->number (match:substring match 1))))
             (update %))))
        ((regexp-exec %fraction-line-rx line)
         =>
         (lambda (match)
           (let ((done  (string->number (match:substring match 1)))
                 (total (string->number (match:substring match 3))))
             (update (* 100. (/ done total))))))
        ((regexp-exec %phase-start-rx line)
         =>
         (lambda (match)
           (let ((phase (match:substring match 1))
                 (build (find-build)))
             (if build
                 (build-status
                  (inherit status)
                  (building
                   (cons (set-build-phase (set-build-completion build #f)
                                          (string->symbol phase))
                         (delq build (build-status-building status)))))
                 status))))
        (else
         status)))

(define* (compute-status event status
                         #:key
                         (current-time current-time)
                         (derivation-path->output-path
                          derivation-path->output-path))
  "Given EVENT, a tuple like (build-started \"/gnu/store/...-foo.drv\" ...),
compute a new status based on STATUS."
  (match event
    (('build-started drv "-" system log-file . rest)
     (let ((build (build drv system
                         #:id (match rest
                                ((pid . _) (string->number pid))
                                (_ #f))
                         #:log-file (if (string-null? log-file)
                                        #f
                                        log-file))))
       (build-status
        (inherit status)
        (building (cons build (build-status-building status))))))
    (((or 'build-succeeded 'build-failed) drv _ ...)
     (let ((build (find (matching-build drv)
                        (build-status-building status))))
       ;; If BUILD is #f, this may be because DRV corresponds to a
       ;; fixed-output derivation that is listed as a download.
       (if build
           (build-status
            (inherit status)
            (building (delq build (build-status-building status)))
            (builds-completed
             (cons build (build-status-builds-completed status))))
           status)))

    ;; Note: Ignore 'substituter-started' and 'substituter-succeeded' because
    ;; they're not as informative as 'download-started' and
    ;; 'download-succeeded'.

    (('download-started item uri (= string->number size))
     ;; This is presumably a fixed-output derivation so move it from
     ;; 'building' to 'downloading'.  XXX: This doesn't work in 'check' mode
     ;; because ITEM is different from DRV's output.
     (build-status
      (inherit status)
      (building (remove (lambda (build)
                          (let ((drv (build-derivation build)))
                            (equal? (false-if-exception
                                     (derivation-path->output-path drv))
                                    item)))
                        (build-status-building status)))
      (downloading (cons (download item uri #:size size
                                   #:start (current-time time-monotonic))
                         (build-status-downloading status)))))
    (('download-succeeded item uri (= string->number size))
     (let ((current (find (matching-download item)
                          (build-status-downloading status))))
       (build-status
        (inherit status)
        (downloading (delq current (build-status-downloading status)))
        (downloads-completed
         (cons (download item uri
                         #:size size
                         #:start (download-start current)
                         #:transferred size
                         #:end (current-time time-monotonic))
               (build-status-downloads-completed status))))))
    (('substituter-succeeded item _ ...)
     (match (find (matching-download item)
                  (build-status-downloading status))
       (#f
        ;; Presumably we already got a 'download-succeeded' event for ITEM,
        ;; everything is fine.
        status)
       (current
        ;; Maybe the build process didn't emit a 'download-succeeded' event
        ;; for ITEM, so remove CURRENT from the queue now.
        (build-status
         (inherit status)
         (downloading (delq current (build-status-downloading status)))
         (downloads-completed
          (cons (download item (download-uri current)
                          #:size (download-size current)
                          #:start (download-start current)
                          #:transferred (download-size current)
                          #:end (current-time time-monotonic))
                (build-status-downloads-completed status)))))))
    (('download-progress item uri
                         (= string->number size)
                         (= string->number transferred))
     (let ((downloads (remove (matching-download item)
                              (build-status-downloading status)))
           (current   (find (matching-download item)
                            (build-status-downloading status))))
       (build-status
        (inherit status)
        (downloading (cons (download item uri
                                     #:size size
                                     #:start
                                     (or (and current
                                              (download-start current))
                                         (current-time time-monotonic))
                                     #:transferred transferred)
                           downloads)))))
    (('build-log (? integer? pid) line)
     (update-build status pid line))
    (_
     status)))

(define (simultaneous-jobs status)
  "Return the number of on-going builds and downloads for STATUS."
  (+ (length (build-status-building status))
     (length (build-status-downloading status))))


;;;
;;; Rendering.
;;;

(define (extended-build-trace-supported?)
  "Return true if the currently used store is known to support \"extended
build traces\" such as \"@ download-progress\" traces."
  ;; Support for extended build traces was added in protocol version #x162.
  (and (current-store-protocol-version)
       (>= (current-store-protocol-version) #x162)))

(define (multiplexed-output-supported?)
  "Return true if the daemon supports \"multiplexed output\"--i.e., \"@
build-log\" traces."
  (and (current-store-protocol-version)
       (>= (current-store-protocol-version) #x163)))

(define spin!
  (let ((steps (circular-list "\\" "|" "/" "-")))
    (lambda (phase port)
      "Display a spinner on PORT.  If PHASE is true, display it as a hint of
the current build phase."
      (when (isatty?* port)
        (match steps
          ((first . rest)
           (set! steps rest)
           (display "\r\x1b[K" port)
           (display first port)
           (when phase
             (display " " port)
             ;; TRANSLATORS: The word "phase" here denotes a "build phase";
             ;; "~a" is a placeholder for the untranslated name of the current
             ;; build phase--e.g., 'configure' or 'build'.
             (format port (G_ "'~a' phase") phase))
           (force-output port)))))))

(define colorize-log-line
  ;; Take a string and return a possibly colorized string according to the
  ;; rules below.
  (color-rules
   ("^(phase)(.*)(succeeded after)(.*)(seconds)(.*)"
    GREEN    BOLD GREEN          RESET  GREEN  BLUE)
   ("^(phase)(.*)(failed after)(.*)(seconds)(.*)"
    RED BLUE RED BLUE RED BLUE)
   ("^(.*)(error|fail|failed|\\<FAIL|FAILED)([[:blank:]]*)(:)(.*)"
    RESET  RED                           BOLD         BOLD BOLD)
   ("^(.*)(warning)([[:blank:]]*)(:)(.*)"
    RESET  MAGENTA   BOLD        BOLD BOLD)))

(define (hook-message hook-type)
  "Return a human-readable string for the profile hook type HOOK-TYPE."
  (match hook-type
    ('info-dir
     (G_ "building directory of Info manuals..."))
    ('ghc-package-cache
     (G_ "building GHC package cache..."))
    ('ca-certificate-bundle
     (G_ "building CA certificate bundle..."))
    ('emacs-subdirs
     (G_ "listing Emacs sub-directories..."))
    ('gdk-pixbuf-loaders-cache-file
     (G_ "generating GdkPixbuf loaders cache..."))
    ('glib-schemas
     (G_ "generating GLib schema cache..."))
    ('gtk-icon-themes
     (G_ "creating GTK+ icon theme cache..."))
    ('gtk-im-modules
     (G_ "building cache files for GTK+ input methods..."))
    ('xdg-desktop-database
     (G_ "building XDG desktop file cache..."))
    ('xdg-mime-database
     (G_ "building XDG MIME database..."))
    ('fonts-dir
     (G_ "building fonts directory..."))
    ('texlive-font-maps
     (G_ "building TeX Live font maps..."))
    ('manual-database
     (G_ "building database for manual pages..."))
    ('package-cache                    ;package cache generated by 'guix pull'
     (G_ "building package cache..."))
    (_ #f)))

(define* (print-build-event event old-status status
                            #:optional (port (current-error-port))
                            #:key
                            (colorize? (color-output? port))
                            (print-urls? #t)
                            (print-log? #t))
  "Print information about EVENT and STATUS to PORT.  When COLORIZE? is true,
produce colorful output.  When PRINT-LOG? is true, display the build log in
addition to build events.  When PRINT-URLS? is true, display the URL of
substitutes being downloaded."
  (define info
    (if (and colorize? (or print-urls? print-log?))
        (cute colorize-string <> (color BOLD))
        identity))

  (define emph
    (if colorize?
        (cute colorize-string <> (color BOLD))
        identity))

  (define success
    (if colorize?
        (cute colorize-string <> (color GREEN BOLD))
        identity))

  (define failure
    (if colorize?
        (cute colorize-string <> (color RED BOLD))
        identity))

  (define tty?
    (isatty?* port))

  (define (report-build-progress phase %)
    (let ((% (min (max % 0) 100)))                ;sanitize
      (erase-current-line port)
      (let* ((prefix (format #f "~3d% ~@['~a' ~]"
                            (inexact->exact (round %))
                            (case phase
                              ((build) #f)        ;not useful to display it
                              (else phase))))
             (length (string-length prefix)))
        (display prefix port)
        (display (progress-bar % (- (current-terminal-columns) length))
                 port))
      (force-output port)))

  (define print-log-line
    (if print-log?
        (if colorize?
            (lambda (id line)
              (display (colorize-log-line line) port))
            (lambda (id line)
              (display line port)))
        (lambda (id line)
          (match (build-status-building status)
            ((build)                              ;single job
             (match (build-completion build)
               ((? number? %)
                (report-build-progress (build-phase build) %))
               (_
                (spin! (build-phase build) port))))
            (_
             (spin! #f port))))))

  (define erase-current-line*
    (if (and (not print-log?) (isatty?* port))
        (lambda ()
          (erase-current-line port)
          (force-output port))
        (const #t)))

  (match event
    (('build-started drv . _)
     (erase-current-line*)
     (let ((properties (derivation-properties
                        (read-derivation-from-file drv))))
       (match (assq-ref properties 'type)
         ('graft
           (let ((count (match (assq-ref properties 'graft)
                          (#f  0)
                          (lst (or (assq-ref lst 'count) 0)))))
             (format port (info (N_ "applying ~a graft for ~a ..."
                                    "applying ~a grafts for ~a ..."
                                    count))
                     count
                     (string-drop-right (store-path-package-name drv)
                                        (string-length ".drv")))))
         ('profile
          (let ((count (match (assq-ref properties 'profile)
                         (#f  0)
                         (lst (or (assq-ref lst 'count) 0)))))
            (format port (info (N_ "building profile with ~a package..."
                                   "building profile with ~a packages..."
                                   count))
                    count)))
         ('profile-hook
          (let ((hook-type (assq-ref properties 'hook)))
            (or (and=> (hook-message hook-type)
                       (lambda (msg)
                         (display (info msg) port)))
                (format port (info (G_ "running profile hook of type '~a'..."))
                        hook-type))))
         (_
          (format port (info (G_ "building ~a...")) drv))))
     (newline port))
    (('build-succeeded drv . _)
     (erase-current-line*)                      ;erase spinner or progress bar
     (when (or print-log? (not (extended-build-trace-supported?)))
       (format port (success (G_ "successfully built ~a")) drv)
       (newline port))
     (match (build-status-building status)
       (() #t)
       (ongoing                                   ;when max-jobs > 1
        (format port
                (N_ "The following build is still in progress:~%~{  ~a~%~}~%"
                    "The following builds are still in progress:~%~{  ~a~%~}~%"
                    (length ongoing))
                (map build-derivation ongoing)))))
    (('build-failed drv . _)
     (erase-current-line*)                      ;erase spinner or progress bar
     (format port (failure (G_ "build of ~a failed")) drv)
     (newline port)
     (match (derivation-log-file drv)
       (#f
        (format port (failure (G_ "Could not find build log for '~a'."))
                drv))
       (log
        (format port (emph (G_ "View build log at '~a'.")) log)))
     (newline port))
    (('substituter-started item _ ...)
     (erase-current-line*)
     (when (or print-log? (not (extended-build-trace-supported?)))
       (format port (info (G_ "substituting ~a...")) item)
       (newline port)))
    (('download-started item uri _ ...)
     (when print-urls?
       (erase-current-line*)
       (format port (info (G_ "downloading from ~a ...")) uri)
       (newline port)))
    (('download-progress item uri
                         (= string->number size)
                         (= string->number transferred))
     ;; Print a progress bar, but only if there's only one on-going
     ;; job--otherwise the output would be intermingled.
     (when (= 1 (simultaneous-jobs status))
       (match (find (matching-download item)
                    (build-status-downloading status))
         (#f #f)                                  ;shouldn't happen!
         (download
          ;; XXX: It would be nice to memoize the abbreviation.
          (let ((uri (if (string-contains uri "/nar/")
                         (nar-uri-abbreviation uri)
                         (basename uri))))
            (display-download-progress uri size
                                       #:tty? tty?
                                       #:start-time
                                       (download-start download)
                                       #:transferred transferred))))))
    (('substituter-succeeded item _ ...)
     (when (extended-build-trace-supported?)
       ;; If there are no jobs running, we already reported download completion
       ;; so there's nothing left to do.
       (unless (zero? (simultaneous-jobs status))
         (format port (success (G_ "substitution of ~a complete")) item)
         (newline port))

       (when (and print-urls? (zero? (simultaneous-jobs status)))
         ;; Leave a blank line after the "downloading ..." line and the
         ;; progress bar (that's three lines in total).
         (newline port))))
    (('substituter-failed item _ ...)
     (format port (failure (G_ "substitution of ~a failed")) item)
     (newline port))
    (('hash-mismatch item algo expected actual _ ...)
     ;; TRANSLATORS: The final string looks like "sha256 hash mismatch for
     ;; /gnu/store/…-sth:", where "sha256" is the hash algorithm.
     (format port (failure (G_ "~a hash mismatch for ~a:")) algo item)
     (newline port)
     (format port (emph (G_ "\
  expected hash: ~a
  actual hash:   ~a~%"))
             expected actual))
    (('build-remote drv host _ ...)
     (format port (emph (G_ "offloading build of ~a to '~a'")) drv host)
     (newline port))
    (('build-log pid line)
     (if (multiplexed-output-supported?)
         (if (not pid)
             (begin
               ;; LINE comes from the daemon, not from builders.  Let it
               ;; through.
               (display line port)
               (force-output port))
             (print-log-line pid line))
         (cond ((string-prefix? "substitute: " line)
                ;; The daemon prefixes early messages coming with 'guix
                ;; substitute' with "substitute:".  These are useful ("updating
                ;; substitutes from URL"), so let them through.
                (display line port)
                (force-output port))
               ((string-prefix? "waiting for locks" line)
                ;; This is when a derivation is already being built and we're just
                ;; waiting for the build to complete.
                (display (info (string-trim-right line)) port)
                (newline))
               (else
                (print-log-line pid line)))))
    (_
     event)))

(define* (print-build-event/quiet event old-status status
                                  #:optional
                                  (port (current-error-port))
                                  #:key
                                  (colorize? (color-output? port)))
  (print-build-event event old-status status port
                     #:colorize? colorize?
                     #:print-urls? #f
                     #:print-log? #f))

(define* (print-build-event/quiet-with-urls event old-status status
                                            #:optional
                                            (port (current-error-port))
                                            #:key
                                            (colorize? (color-output? port)))
  (print-build-event event old-status status port
                     #:colorize? colorize?
                     #:print-urls? #t             ;show download URLs
                     #:print-log? #f))

(define* (build-status-updater #:optional (on-change (const #t)))
  "Return a procedure that can be passed to 'build-event-output-port'.  That
procedure computes the new build status upon each event and calls ON-CHANGE:

  (ON-CHANGE event status new-status)

ON-CHANGE can display the build status, build events, etc."
  (lambda (event status)
    (let ((new (compute-status event status)))
      (on-change event status new)
      new)))


;;;
;;; Build port.
;;;

(define (maybe-utf8->string bv)
  "Attempt to decode BV as UTF-8 string and return it.  Gracefully handle the
case where BV does not contain only valid UTF-8."
  (catch 'decoding-error
    (lambda ()
      (utf8->string bv))
    (lambda _
      ;; This is the sledgehammer but it's the only safe way we have to
      ;; properly handle this.  It's expensive but it's rarely needed.
      (let ((port (open-bytevector-input-port bv)))
        (set-port-encoding! port "UTF-8")
        (set-port-conversion-strategy! port 'substitute)
        (let ((str (read-string port)))
          (close-port port)
          str)))))

(define (bytevector-index bv number offset count)
  "Search for NUMBER in BV starting from OFFSET and reading up to COUNT bytes;
return the offset where NUMBER first occurs or #f if it could not be found."
  (let loop ((offset offset)
             (count count))
    (cond ((zero? count) #f)
          ((= (bytevector-u8-ref bv offset) number) offset)
          (else (loop (+ 1 offset) (- count 1))))))

(define (split-lines str)
  "Split STR into lines in a way that preserves newline characters."
  (let loop ((str str)
             (result '()))
    (if (string-null? str)
        (reverse result)
        (match (string-index str #\newline)
          (#f
           (loop "" (cons str result)))
          (index
           (loop (string-drop str (+ index 1))
                 (cons (string-take str (+ index 1)) result)))))))

(define* (build-event-output-port proc #:optional (seed (build-status)))
  "Return an output port for use as 'current-build-output-port' that calls
PROC with its current state value, initialized with SEED, on every build
event.  Build events passed to PROC are tuples corresponding to the \"build
traces\" produced by the daemon:

  (build-started \"/gnu/store/...-foo.drv\" ...)
  (substituter-started \"/gnu/store/...-foo\" ...)

and so on.

The second return value is a thunk to retrieve the current state."
  (define %fragments
    ;; Line fragments received so far.
    '())

  (define %state
    ;; Current state for PROC.
    seed)

  ;; When true, this represents the current state while reading a
  ;; "@ build-log" trace: the current builder PID, the previously-read
  ;; bytevectors, and the number of bytes that remain to be read.
  (define %build-output-pid #f)
  (define %build-output '())
  (define %build-output-left #f)

  (define (process-line line)
    (cond ((string-prefix? "@ " line)
           ;; Note: Drop the trailing \n, and use 'string-split' to preserve
           ;; spaces (the log file part of 'build-started' events can be the
           ;; empty string.)
           (match (string-split (string-drop (string-drop-right line 1) 2)
                                #\space)
             (("build-log" (= string->number pid) (= string->number len))
              (set! %build-output-pid pid)
              (set! %build-output '())
              (set! %build-output-left len))
             (((= string->symbol event-name) args ...)
              (set! %state
                (proc (cons event-name args)
                      %state)))))
          (else
           (set! %state (proc (list 'build-log #f line)
                              %state)))))

  (define (process-build-output pid output)
    ;; Transform OUTPUT in 'build-log' events or download events as generated
    ;; by extended build traces.
    (define (line->event line)
      (match (and (string-prefix? "@ " line)
                  (string-tokenize (string-drop line 2)))
        ((type . args)
         (if (or (string-prefix? "download-" type)
                 (string=? "build-remote" type))
             (cons (string->symbol type) args)
             `(build-log ,pid ,line)))
        (_
         `(build-log ,pid ,line))))

    (let* ((lines  (split-lines output))
           (events (map line->event lines)))
      (set! %state (fold proc %state events))))

  (define (bytevector-range bv offset count)
    (let ((ptr (bytevector->pointer bv offset)))
      (pointer->bytevector ptr count)))

  (define (write! bv offset count)
    (if %build-output-pid
        (let ((keep (min count %build-output-left)))
          (set! %build-output
            (let ((bv* (make-bytevector keep)))
              (bytevector-copy! bv offset bv* 0 keep)
              (cons bv* %build-output)))
          (set! %build-output-left
            (- %build-output-left keep))

          (when (zero? %build-output-left)
            (process-build-output %build-output-pid
                                  (string-concatenate-reverse
                                   (map maybe-utf8->string %build-output))) ;XXX
            (set! %build-output '())
            (set! %build-output-pid #f))
          keep)
        (match (bytevector-index bv (char->integer #\newline)
                                 offset count)
          ((? integer? cr)
           (let* ((tail (maybe-utf8->string
                         (bytevector-range bv offset (- cr -1 offset))))
                  (line (string-concatenate-reverse
                         (cons tail %fragments))))
             (process-line line)
             (set! %fragments '())
             (- cr -1 offset)))
          (#f
           (unless (zero? count)
             (let ((str (maybe-utf8->string
                         (bytevector-range bv offset count))))
               (set! %fragments (cons str %fragments))))
           count))))

  (define port
    (make-custom-binary-output-port "filtering-input-port"
                                    write!
                                    #f #f
                                    #f))

  ;; The build port actually receives Unicode strings.
  (set-port-encoding! port "UTF-8")
  (setvbuf port 'line)
  (values port (lambda () %state)))

(define (call-with-status-report on-event thunk)
  (parameterize ((current-terminal-columns (terminal-columns))
                 (current-build-output-port
                  (build-event-output-port (build-status-updater on-event))))
    (thunk)))

(define-syntax-rule (with-status-report on-event exp ...)
  "Set up build status reporting to the user using the ON-EVENT procedure;
evaluate EXP... in that context."
  (call-with-status-report on-event (lambda () exp ...)))

(define (logger-for-level level)
  "Return the logging procedure that corresponds to LEVEL."
  (cond ((<= level 0) (const #t))
        ((= level 1)  print-build-event/quiet)
        ((= level 2)  print-build-event/quiet-with-urls)
        (else         print-build-event)))

(define (call-with-status-verbosity level thunk)
  (call-with-status-report (logger-for-level level) thunk))

(define-syntax-rule (with-status-verbosity level exp ...)
  "Set up build status reporting to the user at the given LEVEL: 0 means
silent, 1 means quiet, 2 means verbose.  Evaluate EXP... in that context."
  (call-with-status-verbosity level (lambda () exp ...)))
