4fc71951ee
Keep track of whether any Racket-managed subprocesses are pending, and use waitpid(0, ...) only if there is one, to better cooperate with an embedding environment. Also, add a chapter to the "Inside" manual to explain the issues.
40 lines
1.9 KiB
Racket
40 lines
1.9 KiB
Racket
#lang scribble/doc
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@(require "utils.rkt"
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(for-label racket/system))
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@title{Subprocesses}
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On Unix and Mac OS X, subprocess handling involves
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@as-index[@cpp{fork}], @as-index[@cpp{waitpid}], and
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@as-index[@cpp{SIGCHLD}], which creates a variety of issues within an
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embedding application. On Windows, subprocess handling is more
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straightforward, since no @cpp{fork} is required, and since Windows
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provides an abstraction that is a close fit to Racket's subprocess
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values.
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After Racket creates a subprocess via @racket[subprocess] (or
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@racket[system], @racket[process], etc.), it periodically polls the
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process status using @cpp{waitpid}. If the process is created as its
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own group, then the call to @cpp{waitpid} uses the created
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subprocess's process ID; for all other subprocesses, polling uses a
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single call to @cpp{waitpid} with the first argument as @cpp{0}. Using
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@cpp{0}, in particular, can interfere with other libraries in an
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embedding context, so Racket refrains from calling @cpp{waitpid} if no
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subprocesses are pending.
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Racket may or may not rely on a @cpp{SIGCHLD} handler, and it may or
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may not block @cpp{SIGCHLD}. Currently, when Racket is compiled to
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support @|tech-place|s, Racket blocks @cpp{SIGCHLD} on start up with
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the expectation that all created threads have @cpp{SIGCHLD} blocked.
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When Racket is not compiled to support @|tech-place|s, then a
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@cpp{SIGCHLD} handler is installed.
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Using @cpp{fork} in an application that embeds Racket is problematic
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for several reasons: Racket may install a @cpp{SIGALRM} handler and
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schedule alarms to implement context switches, it may have file
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descriptors open that should be closed in a child process, and it may
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have changed the disposition of signals such as
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@cpp{SIGCHLD}. Consequently, embedding Racket in a process that
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@cpp{fork}s is technically not supported; in the future, Racket may
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provide better support for such applications.
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