175 lines
6.4 KiB
Racket
175 lines
6.4 KiB
Racket
#lang scribble/doc
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@(require "mz.ss"
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(for-label scheme/cmdline))
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@title[#:tag "logging"]{Logging}
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A @deftech{logger} accepts events that contain information to be
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logged for interested parties. A @deftech{log receiver} represents an
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interested party that receives logged events asynchronously. Each
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event has a level of importance, and a @tech{log receiver} subscribes to
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logging events at a certain level of importance and higher. The
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levels, in decreasing order of importance, are @scheme['fatal],
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@scheme['error], @scheme['warning], @scheme['info], and
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@scheme['debug].
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To help organize logged events, @tech{loggers} can be named and
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hierarchical. Every event reported to a logger is also propagated to
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its parent (if any), but the event message is prefixed with the name
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(if any) of the logger to which is was originally reported. A logger
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is not required to have a parent or name.
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On start-up, PLT Scheme creates an initial logger that is used to
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record events from the core run-time system. For example, an
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@scheme['info] event is reported for each garbage collection (see
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@secref["gc-model"]). For this initial logger, two log receivers are
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also created: one that writes events to the process's original error
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output port, and one that writes events to the system log. The level
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of written events in each case is system-specific, and the default can
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be changed through command-line flags (see @secref["mz-cmdline"]) or
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through environment variables:
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@itemize{
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@item{If the @indexed-envvar{PLTSTDERR} environment variable is
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defined and is not overridden by a command-line flag, it
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determines the level of the @tech{log receiver} that propagates
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events to the original error port. The environment variable's value
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should be @scheme["none"], @scheme["fatal"], @scheme["error"],
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@scheme["warning"], @scheme["info"], or @scheme["debug"].
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The default is @scheme["error"].}
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@item{If the @indexed-envvar{PLTSYSLOG} environment variable is
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defined and is not overridden by a command-line flag, it
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determines the level of the @tech{log receiver} that propagates
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events to the system log. The possible values are the
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same as for @envvar{PLTSYSLOG}.
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The default is @scheme["none"] for Unix or @scheme["error"] for
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Windows and Mac OS X.}
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}
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The @scheme[current-logger] @tech{parameter} determines the
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@deftech{current logger} that is used by forms such as
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@scheme[log-warning]. On start-up, the initial value of this parameter
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is the initial logger. The run-time system sometimes uses the current
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logger to report events. For example, the bytecode compiler sometimes
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reports @scheme['warning] events when it detects an expression that
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would produce a run-time error if evaluated.
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@; ----------------------------------------
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@section{Creating Loggers}
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@defproc[(logger? [v any/c]) boolean?]{
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Returns @scheme[#t] if @scheme[v] is a @tech{logger}, @scheme[#f]
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otherwise.}
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@defproc[(make-logger [name (or/c symbol? #f) #f]
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[parent (or/c logger? #f) #f])
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logger?]{
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Creates a new logger with an optional name and parent.}
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@defproc[(logger-name [logger logger?]) (or/c symbol? #f)]{
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Reports @scheme[logger]'s name, if any.}
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@defparam[current-logger logger logger?]{
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A @tech{parameter} that determines the @tech{current logger}.}
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@; ----------------------------------------
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@section{Logging Events}
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@defproc[(log-message [logger logger?]
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[level (or/c 'fatal 'error 'warning 'info 'debug)]
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[message string?]
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[data any/c])
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void?]{
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Reports an event to @scheme[logger], which in turn distributes the
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information to any @tech{log receivers} attached to @scheme[logger] or
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its ancestors that are interested in events at @scheme[level] or
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higher.
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If @scheme[logger] has a name, then @scheme[message] is prefixed with
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the logger's name followed by @scheme[": "] before it is sent to
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receivers.}
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@defproc[(log-level? [logger logger?]
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[level (or/c 'fatal 'error 'warning 'info 'debug)])
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boolean?]{
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Reports whether any @tech{log receiver} attached to @scheme[logger] or
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one of its ancestors is interested in @scheme[level] events (or
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potentially lower). Use this function to avoid work generating an
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event for @scheme[log-message] if no receiver is interested in the
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information; this shortcut is built into @scheme[log-fatal],
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@scheme[log-error], @scheme[log-warning], @scheme[log-info], and
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@scheme[log-debug], however, so it should not be used with those
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forms.
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The result of this function can change if a garbage collection
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determines that a log receiver is no longer accessible (and therefore
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that any event information it receives will never become accessible).}
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@deftogether[(
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@defform[(log-fatal string-expr)]
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@defform[(log-error string-expr)]
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@defform[(log-warning string-expr)]
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@defform[(log-info string-expr)]
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@defform[(log-debug string-expr)]
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)]{
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Log an event with the @tech{current logger}, evaluating
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@scheme[string-expr] only if the logger has receivers that are
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interested in the event. In addition, the current continuation's
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@tech{continuation marks} are sent to the logger with the message
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string.
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For each @schemekeywordfont{log-}@scheme[_level],
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@schemeblock[
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(#, @schemekeywordfont{log-}_level string-expr)
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]
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is equivalent to
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@schemeblock[
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(let ([l (current-logger)])
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(when (log-level? l '#, @scheme[_level])
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(log-message l '#, @scheme[_level] string-expr
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(current-continuation-marks))))
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]}
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@; ----------------------------------------
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@section{Receiving Logged Events}
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@defproc[(log-receiver? [v any/c]) boolean?]{
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Returns @scheme[#t] if @scheme[v] is a @tech{log receiver}, @scheme[#f]
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otherwise.}
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@defproc[(make-log-receiver [logger logger?]
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[level (or/c 'fatal 'error 'warning 'info 'debug)])
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log-receiver?]{
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Creates a @tech{log receiver} to receive events of importance
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@scheme[level] and higher as reported to @scheme[logger] and its
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descendants.
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A @tech{log receiver} is a @tech{synchronizable event}. It becomes
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ready as an @tech{synchronizable event} when a logging event is
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received, so use @scheme[sync] to receive an logged event. The
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@tech{log receiver}'s synchronization value is a vector containing
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three values: the level of the event as a symbol, an immutable string
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for the event message, and an arbitrary value that was supplied as the
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last argument to @scheme[log-message] when the event was logged.}
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